HORIZONTAL BORING MACHINES
Horizontal Boring Machine with Vertical Table
Adjustment--Drilling and Boring--Cutters Used--Cutter-heads
for Boring Large Holes--Cylinder Boring--Boring a Duplex
Gasoline Engine Cylinder--Examples of Boring, Radial Facing
and Milling--Fixture for Cylinder Lining or
Bushing--Horizontal Boring Machine of Floor Type 275-297
INDEX 299-307
TURNING AND BORING
CHAPTER I
THE ENGINE LATHE--TURNING AND BORING OPERATIONS
The standard "engine" lathe, which is the type commonly used by
machinists for doing general work, is one of the most important tools in
a machine shop, because it is adapted to a great variety of operations,
such as turning all sorts of cylindrical and taper parts, boring holes,
cutting threads, etc. The illustration Fig. 1 shows a lathe which, in
many respects, represents a typical design, and while some of the parts
are arranged differently on other makes, the general construction is
practically the same as on the machine illustrated.
The principal parts are the bed _B_, the headstock _H_, the tailstock
_T_, and the carriage _C_. The headstock contains a spindle which is
rotated by a belt that passes over the cone-pulley _P_, and this spindle
rotates the work, which is usually held between pointed or conical
centers _h_ and _h_{1}_ in the headstock and tailstock, or in a chuck
screwed onto the spindle instead of the faceplate _F_. The carriage _C_
can be moved lengthwise along the bed by turning handle _d_, and it can
also be moved by power, the movement being transmitted from the
headstock spindle either through gears _a_, _b_, _c_, and lead-screw
_S_, or by a belt operating on pulleys _p_ and _p_{1}_, which drive the
feed-rod _R_. The lead-screw _S_ is used when cutting threads, and the
feed-rod _R_ for ordinary turning operations; in this way the wear on
the lead-screw is reduced and its accuracy is preserved.
[Illustration: Fig. 1. Bradford Belt-driven Lathe--View of Front or
Operating Side]
On the carriage, there is a cross-slide _D_ which can be moved at right
angles to the lathe bed by handle _e_, and on _D_ there is an upper or
compound slide _E_ which can be swiveled to different positions. The
tool _t_, that does the turning, is clamped to the upper slide, as
shown, and it can be moved with relation to the work by the movement of
the carriage _C_ along the bed, or by moving slide _D_ crosswise. The
lengthwise movement is used to feed the tool along the work when
turning, boring or cutting a screw, and the crosswise movement for
facing the ends of shafts, etc., or for radial turning. When the tool is
to be fed at an angle, other than at right angles to the bed, slide _E_,
which can be set to the required angle, is used. The lengthwise and
crosswise feeding movements can be effected by power, the lengthwise
feed being engaged by tightening knob _k_, and the cross-feed by
tightening knob _l_. The direction of either of these movements can also
be reversed by shifting lever _r_. Ordinarily the carriage and slide are
adjusted by hand to bring the tool into the proper position for turning
to the required diameter, and then the power feed (operating in the
desired direction) is engaged. The tailstock _T_ can be clamped in
different positions along the bed, to suit the length of the work, and
its center _h_{1}_ can be moved in or out for a short distance, when
adjusting it to the work, by turning handle _n_.
[Illustration: Fig. 2. Plan View of Lathe Headstock showing Back-gears]
[Illustration: Fig. 3. Feed Mechanism of Lathe Apron]
As some metals are much harder than others, and as the diameters of
parts to be turned also vary considerably, speed changes are necessary,
because if the speed is excessive, the turning tool will become dull in
too short a time. These speed changes (with a belt-driven lathe) are
obtained by placing the driving belt on different steps of cone-pulley
_P_, and also by the use of back-gears. The cone-pulley can be connected
directly with the spindle or be disengaged from it by means of bolt _m_.
When the pulley and spindle are connected, five speeds (with this
particular lathe) are obtained by simply shifting the driving belt to
different steps of the cone. When a slower speed is required than can be
obtained with the belt on the largest step of the cone, the latter is
disconnected from the spindle, and the back-gears _G_ and _G_{1}_ (shown
in the plan view Fig. 2) are moved forward into mesh by turning handle
_O_; the drive is then from cone-pulley _P_ and gear _L_ to gear _G_,
and from gear _G_{1}_ to the large gear _J_ on the spindle. When driving
through the back-gears, five more speed changes are obtained by shifting
the position of the driving belt, as before. The fastest speed with the
back-gears in mesh is somewhat slower than the slowest speed when
driving direct or with the back-gears out of mesh; hence, with this
particular lathe, a series of ten gradually increasing speeds is
obtained. Changes of feed for the turning tool are also required, and
these are obtained by shifting the belt operating on pulleys _p_ and
_p_{1}_ to different-sized steps. On some lathes these feed changes are
obtained through gears which can be shifted to give different ratios.
Many lathes also have gears in the headstock for changing the speeds.
[Illustration: Fig. 4. Rear View of Lathe Apron]
Front and rear views of the carriage apron, which contains the feeding
mechanism, are shown in Figs. 3 and 4, to indicate how the feeds are
engaged and reversed. The feed-rod _R_ (Fig. 1) drives the small bevel
gears _A_ and _A_{1}_ (Figs. 3 and 4), which are mounted on a slide _S_
that can be moved by lever _r_ to bring either bevel gear into mesh with
gear _B_. Gear _B_ is attached to pinion _b_ (see Fig. 3) meshing with
gear _C_, which, when knob _k_ (Fig. 1) is tightened, is locked by a
friction clutch to pinion _c_. The latter pinion drives gear _D_ which
rotates shaft _E_. A pinion cut on the end of shaft _E_ engages rack _K_
(Fig. 1) attached to the bed, so that the rotation of _E_ (which is
controlled by knob _k_) moves the carriage along the bed. To reverse the
direction of the movement, it is only necessary to throw gear _A_ into
mesh and gear _A_{1}_ out, or _vice versa_, by operating lever _r_. When
the carriage is traversed by hand, shaft _E_ and gear _D_ are rotated by
pinion _d_{1}_ connected with handle _d_ (Fig. 1).
The drive for the cross-feed is from gear _C_ to gear _F_ which can be
engaged through a friction clutch (operated by knob _l_, Fig. 1) with
gear _G_ meshing with a pinion _H_. The latter rotates the cross-feed
screw, which passes through a nut attached to slide _D_ (Fig. 1), thus
moving the latter at right angles to the ways of the bed. The cross-feed
is also reversed by means of lever _r_. As previously explained,
lead-screw _S_ is only used for feeding the carriage when cutting
threads. The carriage is engaged with this screw by means of two
half-nuts _N_ (Fig. 4) that are free to slide vertically and are closed
around the screw by operating lever _u_. These half-nuts can only be
closed when lever _r_ is in a central or neutral position, so that the
screw feed and the regular turning feed cannot be engaged at the same
time. As previously mentioned, lead-screw _S_, Fig. 1, is rotated from
the lathe spindle, through gears _a_, _b_ and _c_, called change gears.
An assortment of these gears, of various sizes, is provided with the
lathe, for cutting screws of different pitch. The gears to use for any
pitch within the range of the lathe are given on the plate _I_.
=Example of Cylindrical Turning.=--Having now considered the principal
features of what might be called a standard lathe, the method of using
it in the production of machine parts will be explained. To begin with a
simple example of work, suppose a steel shaft is to be turned to a
diameter of 2-1/4 inches and a length of 14-1/2 inches, these being the
finished dimensions. We will assume that the rough stock is cut off to a
length of 14-5/8 inches and has a diameter of 2-5/8 inches. The first
step in this operation is to form conically shaped center-holes in each
end of the piece as indicated at _c_ in Fig. 5. As all work of this kind
is held, while being turned, between the centers _h_ and _h_{1}_, holes
corresponding in shape to these centers are necessary to keep the work
in place. There are several methods of forming these center-holes, as
explained later.
After the work is centered, a dog _A_ is clamped to one end by
tightening screw _s_; it is then placed between the centers of the
lathe. The dog has a projecting end or "tail," as it is commonly called,
which enters a slot in the faceplate _F_ and thereby drives or rotates
the work, when power is applied to the lathe spindle onto which the
faceplate is screwed. The tailstock center _h_1_, after being oiled,
should be set up just tight enough to eliminate all play, without
interfering with a free rotary movement of the work. This is done by
turning handle _n_, and when the center is properly adjusted, the
tailstock spindle containing the center is locked by tightening handle
_p_. (Ordinary machine oil is commonly used for lubricating lathe
centers, but a lubricant having more "body" should be used, especially
when turning heavy parts. The following mixtures are recommended: 1. Dry
or powdered red lead mixed with a good grade of mineral oil to the
consistency of cream. 2. White lead mixed with sperm oil with enough
graphite added to give the mixture a dark lead color.)
[Illustration: Fig. 5. Plan View showing Work Mounted between Centers of
Lathe]
=Facing the Ends Square with a Side-tool.=--Everything is now ready for
the turning operation. The ends of the piece should be faced square
before turning the body to size, and the tool for this squaring
operation is shown in Fig. 6; this is known as a side-tool. It has a
cutting edge _e_ which shaves off the metal as indicated in the end view
by the dotted lines. The side _f_ is ground to an angle so that when
the tool is moved in the direction shown by the arrow, the cutting edge
will come in contact with the part to be turned; in other words, side
_f_ is ground so as to provide clearance for the cutting edge. In
addition, the top surface against which the chip bears, is beveled to
give the tool keenness so that it will cut easily. As the principles of
tool grinding are treated separately in Chapter II we shall for the
present consider the tool's use rather than its form.
[Illustration: Fig. 6. Lathe Side-tool for Facing Ends of Shafts, etc.]
For facing the end, the side tool is clamped in the toolpost by
tightening the screw _u_, Fig. 5, and it should be set with the cutting
edge slightly inclined from a right-angled position, the point being in
advance so that it will first come into contact with the work. The
cutting edge should also be about the same height as the center of the
work. When the tool is set, the lathe (if belt-driven) is started by
shifting an overhead belt and the tool is then moved in until the point
is in the position shown at _A_, Fig. 7. The tool-point is then fed
against the end by handle _d_, Fig. 5, until a light chip is being
turned off, and then it is moved outward by handle _e_ (as indicated by
the arrow at _B_, Fig. 7), the carriage remaining stationary. As the
movement of the tool-point is guided by the cross-slide _D_, which is at
right angles with the axis of the work, the end will be faced square.
For short turning operations of this kind, the power feeds ordinarily
are not used as they are intended for comparatively long cuts. If it
were necessary to remove much metal from the end, a number of cuts would
be taken across it; in this case, however, the rough stock is only 1/8
inch too long so that this end need only be made true.
[Illustration: Fig. 7. Facing End with Side-tool and Turning Work
Cylindrical]
After taking a cut as described, the surface, if left rough by the
tool-point, should be made smooth by a second or finishing cut. If the
tool is ground slightly round at the point and the cutting edge is set
almost square, as at _C_, Fig. 7, a smooth finish can be obtained; the
cut, however, should be light and the outward feed uniform. The work is
next reversed in the centers and the driving dog is placed on the end
just finished; the other end is then faced, enough metal being removed
to make the piece 14-1/2 inches long, as required in this particular
case. This completes the facing operation. If the end of the work does
not need to be perfectly square, the facing operation can be performed
by setting the tool in a right-angled position and then feeding it
sidewise, thus removing a chip equal to the width of one side. Evidently
this method is confined to comparatively small diameters and the
squareness of the turned end will be determined by the position of the
tool's cutting edge.
=Turning Tool--Turning Work Cylindrical.=--The tool used to turn the
body to the required diameter is shaped differently from the side-tool,
the cutting edge _E_ of most tools used for plain cylindrical turning
being curved as shown in Fig. 8. A tool of this shape can be used for a
variety of cylindrical turning operations. As most of the work is done
by that part of the edge marked by arrow _a_, the top of the tool is
ground to slope back from this part to give it keenness. The end _F_, or
the flank, is also ground to an angle to provide clearance for the
cutting edge. If the tool did not have this clearance, the flank would
rub against the work and prevent the cutting edge from entering the
metal. This type of tool is placed about square with the work, for
turning, and with the cutting end a little above the center.
[Illustration: Fig. 8. Tool used for Cylindrical Turning]
Before beginning to turn, a pair of outside calipers or a micrometer
should be set to 2-1/4 inches, which, in this case, is the finished
diameter of the work. Calipers are sometimes set by using a graduated
scale as at _A_, Fig. 9, or they can be adjusted to fit a standard
cylindrical gage of the required size as at _B_. Very often fixed
caliper gages _C_ are used instead of the adjustable spring calipers.
These fixed gages, sometimes called "snap" gages, are accurately made to
different sizes, and they are particularly useful when a number of
pieces have to be turned to exactly the same size.
The turning tool is started at the right-hand end of the work and the
tool should be adjusted with the left hand when beginning a cut, as
shown in Fig. 10, in order to have the right hand free for calipering. A
short space is first turned by hand feeding, as at _D_, Fig. 7, and when
the calipers show that the diameter is slightly greater than the
finished size (to allow for a light finishing cut, either in the lathe
or grinding machine) the power feed for the carriage is engaged; the
tool then moves along the work, reducing it as at _E_. Evidently, if the
movement is along a line _b--b_, parallel with the axis _a--a_, the
diameter _d_ will be the same at all points, and a true cylindrical
piece will be turned. On the other hand, if the axis _a--a_ is inclined
one way or the other, the work will be made tapering; in fact, the
tailstock center _h_1_ can be adjusted laterally for turning tapers, but
for straight turning, both centers must be in alignment with the
carriage travel. Most lathes have lines on the stationary and movable
parts of the tailstock base which show when the centers are set for
straight turning. These lines, however, may not be absolutely correct,
and it is good practice to test the alignment of the centers before
beginning to turn. This can be done by taking trial cuts, at each end of
the work (without disturbing the tool's crosswise position), and then
comparing the diameters, or by testing the carriage travel with a true
cylindrical piece held between the centers as explained later.
[Illustration: Fig. 9. Setting Calipers by Scale--Setting by Gage--Fixed
Gage]
If the relative positions of the lathe centers are not known, the work
should be calipered as the cut progresses to see if the diameter _d_ is
the same at all points. In case the diameter gradually increases, the
tailstock center should be shifted slightly to the rear before taking
the next cut, but if the diameter gradually diminishes, the adjustment
would, of course, be made in the opposite direction. The diameter is
tested by attempting to pass the calipers over the work. When the
measuring points just touch the work as they are gently passed across
it, the diameter being turned is evidently the same as the size to which
the calipers are set.
[Illustration: Fig. 10. Views showing how the Cross-slide and Carriage
are Manipulated by Hand when Starting a Cut--View to Left, Feeding Tool
Laterally; View to Right, Feeding Tool in a Lengthwise Direction]
As the driving dog is on one end, the cut cannot be taken over the
entire length, and when the tool has arrived at say position _x_, Fig.
5, it is returned to the starting point and the work is reversed in the
centers, the dog being placed upon the other end. The unfinished part is
then turned, and if the cross-slide is not moved, the tool will meet the
first cut. It is not likely that the two cuts will be joined or blended
together perfectly, however, and for this reason a cut should be
continuous when this is possible.
=Roughing and Finishing Cuts.=--Ordinarily in lathe work, as well as in
other machine work, there are two classes of cuts, known as "roughing"
and "finishing" cuts. Roughing cuts are for reducing the work as
quickly as possible almost to the required size, whereas finishing cuts,
as the name implies, are intended to leave the part smooth and of the
proper size. When the rough stock is only a little larger than the
finished diameter, a single cut is sufficient, but if there is
considerable metal to turn away, one or more deep roughing cuts would
have to be taken, and, finally, a light cut for finishing. In this
particular case, one roughing and one finishing cut would doubtless be
taken, as the diameter has to be reduced 3/8 inch. Ordinarily the
roughing cut would be deep enough to leave the work about 1/32 or
perhaps 1/16 inch above the finished size. When there is considerable
metal to remove and a number of roughing cuts have to be taken, the
depth of each cut and the feed of the tool are governed largely by the
pulling power of the lathe and the strength of the work to withstand the
strain of a heavy cut. The depth of roughing cuts often has to be
reduced considerably because the part being turned is so flexible that a
heavy cut would spring the work and cause the tool to gouge in. Of
course, just as few cuts as possible should be taken in order to save
time. The speed of the work should also be as fast as the conditions
will allow for the same reason, but as there are many things which
govern the speed, the feed of the tool, and the depth of the cut, these
important points are referred to separately in Chapter II.
=Filing and Finishing.=--In many cases the last or finishing cut does
not leave as smooth a surface as is required and it is necessary to
resort to other means. The method commonly employed for finishing in the
lathe is by the use of a file and emery cloth. The work is rotated
considerably faster for filing than for turning, and the entire surface
is filed by a flat, single-cut file, held as shown in Fig. 11. The file
is passed across the work and advanced sidewise for each forward stroke,
until the entire surface is finished. The file should be kept in contact
with the work continually, but on the return stroke the pressure should
be relieved. The movement of the file during the forward or cutting
stroke should be much slower than when filing in a vise. By moving the
file slowly, the work can make a number of revolutions for each stroke,
which tends to keep it round, as practically the same amount of metal is
removed from the entire circumference. On the other hand, short rapid
strokes tend to produce flat spots, or at least an irregular surface,
especially if the work can only make part of a revolution for each
cutting stroke. The pressure on the file during the forward stroke
should also be kept as nearly uniform as possible.
[Illustration: Fig. 11. Filing Work after Finishing Cut is taken]
It is very difficult to file a part smooth and at the same time to keep
it round and cylindrical, and the more filing that has to be done, the
greater the chance of error. For this reason, the amount left for filing
should be very small; in fact, the metal removed by filing should be
just enough to take out the tool marks and give a smooth finish. Very
often a satisfactory finish can be obtained with a turning tool, and
filing is not necessary at all. The file generally used for lathe work
is a "single-cut bastard" of "mill" section, having a length of from 12
to 14 inches.
Sometimes particles of metal collect between the teeth of a file and
make deep scratches as the file is passed across the work. When this
occurs, the teeth should be cleaned by using a wire brush or a file
card, which is drawn across the file in the direction of the teeth. This
forming of tiny particles between the teeth is known as "pinning" and it
can sometimes be avoided by rubbing chalk on the file. Filing is not
only done to obtain a smooth finish, but also to reduce the work to an
exact diameter, as a very slight reduction can be made in this way.
[Illustration: Fig. 12. Two Methods of Aligning Centers for Cylindrical
Turning]
If a polish is desired, this can be obtained by holding a piece of emery
cloth tightly around the work as it revolves. The coarseness of emery
cloth is indicated by letters and numbers corresponding to the grain
number of loose emery. The letters and numbers for grits ranging from
fine to coarse are as follows: _FF_, _F_, 120, 100, 90, 80, 70, 60, 54,
46, 40. For large work roughly filed, use coarse cloth such as Nos. 46
or 54, and then finer grades to obtain the required polish. If the work
has been carefully filed, a good polish can be obtained with Nos. 60 and
90 cloth, and a brilliant polish by finishing with No. 120 and
flour-emery.
Most cylindrical parts can be finished more quickly and accurately in
the grinder than in the lathe, and many classes of work are, at the
present time, simply rough-turned in the lathe and then ground to size
in a cylindrical grinding machine.
=Aligning Centers for Cylindrical Turning.=--When a rod or shaft must be
turned cylindrical or to the same diameter throughout its entire length,
it is good practice to test the alignment of the centers, before
inserting the work. The position of the tailstock center for cylindrical
turning may be indicated by the coincidence of graduation marks on the
base, but if accuracy is necessary, the relative position of the two
centers should be determined in a more positive way. A very simple and
convenient method of testing the alignment is shown at _A_ in Fig. 12.
The work is first turned for a short distance, near the dogged end, as
shown, and the tool is left as set for this cut; then the tailstock
center is withdrawn and the work is moved sufficiently to permit running
the tool back to the tailstock end without changing its original
setting. A short cut is then taken at this end and the diameters _d_ and
_d_{1}_ are carefully compared. In case there is any variation, the
tailstock center is adjusted laterally, other trial cuts are taken, and
the test repeated.
Another method is illustrated at _B_, which requires the use of a
test-bar _t_. This bar should have accurately made centers and the ends
finished to exactly the same diameter. The lathe centers are aligned by
placing the bar between them and then testing the position of the ends.
This can be done by comparing each end with a tool held in the toolpost
and moved from one to the other by shifting the carriage, but a better
method is to clamp a test indicator _i_ in the toolpost and bring it in
contact with first one end of the bar and then the other. If the dial
does not register the same at each end, it shows that the lathe centers
are not in line. Even when centers are correctly set, lathes that have
been in use a long time do not always turn cylindrical or straight,
because if the ways that guide the carriage are worn unevenly, the tool
as it moves along does not remain in the same plane and this causes a
variation in the diameter of the part being turned.
=Application of Drivers or Dogs.=--Work that is turned between centers
is sometimes driven by a dog which is so short for the faceplate that
the bent driving end bears against the bottom _a_ of the faceplate slot,
as shown at _A_, Fig. 13. If the dog is nearly the right length, it may
allow the headstock center to enter the center in the work part way,
with the result that the turned surface is not true with the centers.
When a driving dog of this type is used, care should be taken to see
that it moves freely in the faceplate slot and does not bind against the
bottom. By using a straight dog (_B_), which is driven by a pin _b_
bolted to the faceplate, all danger from this source is eliminated. The
straight dog, however, is used more particularly to do away with the
leverage _l_ of a bent dog, as this leverage tends to spring a flexible
part when a cut is being taken.
[Illustration: Fig. 13. (A) Dog that is too Short for Faceplate. (B)
Straight Driving Dog]
Straight dogs are also made with two driving ends which engage pins on
opposite sides of the faceplate. This type is preferable because it
applies the power required for turning, evenly to the work, which still
further reduces the tendency to spring it out of shape. The principal
objection to the double-ended type lies in the difficulty of adjusting
the driving pins so that each bears with equal pressure against the dog.
The double-ended driver is often used for large work especially if deep
roughing cuts are necessary.
=Lathe Arbors or Mandrels.=--When it is necessary to turn the outside of
a part having a hole through it, centers cannot, of course, be drilled
in the ends and other means must be resorted to. We shall assume that
the bushing _B_, Fig. 14, has a finished hole through the center, and
it is desired to turn the outside cylindrical and concentric with the
hole. This could be done by forcing a tightly-fitted arbor _M_, having
accurately-centered ends, into the bushing and inserting the mandrel and
work between the lathe centers _h_ and _h_{1}_ as shown. Evidently, if
the arbor runs true on its centers, the hole in the bushing will also
run true and the outside can be turned the same as though the arbor and
bushing were a solid piece. From this it will be seen that an arbor
simply forms a temporary support for parts that are bored and therefore
cannot be centered.
[Illustration: Fig. 14. Bushing mounted on Arbor for Turning]
Another example of work that would be turned on an arbor is shown in
Fig. 15. This is a small cast-iron wheel having a finished hole through
the hub, and the outer surface and sides of the rim are to be turned
true with this hole. In this case, the casting would also be held by
pressing a mandrel through the hub; as shown. This method, however,
would only apply to comparatively small wheels because it would be
difficult, if not impossible, to prevent a large wheel from turning on
the arbor when taking a cut, and even if it could be driven, large work
could be done to better advantage on another type of machine. (The
vertical boring mill is used extensively for turning large wheels, as
explained in Chapter VI.) When turning the outside of the rim, a tool
similar to that shown at _t_ should be used, but for facing or turning
the sides, it might be better, if not necessary, to use tools having
bent ends as shown by the dotted lines; in fact, turning tools of
various kinds are made with the ends bent to the right or left, as this
enables them to be used on surfaces that could not be reached very well
with a straight tool. If a comparatively large pulley is mounted near
the end of the arbor, it can be driven directly by pins attached to the
faceplate and engaging the pulley arms. This method of driving is often
employed when the diameter to be turned is large and the hole for the
arbor is so small that there will not be sufficient friction for
driving.
[Illustration: Fig. 15. Turning Pulley Held on an Arbor]
=Different Types of Lathe Arbors.=--Three different types of lathe
arbors are shown in Fig. 16. The kind shown at _A_ is usually made of
tool steel and the body is finished to a standard size. The ends are
somewhat reduced and flat spots are milled, as shown, to give the
clamping screw of the dog a good grip. The body of the arbor is usually
tapered about 0.006 inch per foot. This taper makes it easier to insert
the arbor in a close-fitting hole, and it also permits slight variations
in the diameter of different holes. As to hardening, the practice at the
present time among manufacturers is to harden arbors all over, but for
extremely accurate work, an arbor having hardened ends and a soft body
is generally considered superior, as there is less tendency of
distortion from internal stresses. Hardened arbors are "seasoned" before
finish-grinding to relieve these internal stresses.
The solid type _A_, Fig. 16, is used very extensively, but in shops
where a great variety of work is being done and there are many odd-sized
holes, some form of expanding arbor _B_ can be used to advantage. This
type, instead of being solid, consists of a tapering inner arbor _M_ on
which is placed a split bushing that can be expanded, within certain
limits, by driving in the tapering member. The advantage of this type is
that a comparatively small stock of arbors is required, as
different-sized bushings can be used. This type can also be fitted to
holes of odd sizes, whereas a solid arbor must be provided for each
different size hole, unless the variation is very slight. The latter
are, however, more accurate than the expanding type.
[Illustration: Fig. 16. Different Types of Lathe Arbors]
Another form of expanding arbor is shown at _C_. This type has a
straight body _N_ in which four tapering grooves are cut lengthwise, as
shown, and there is a sleeve _S_, containing four slots that are located
to correspond with the tapering grooves. Strips s are fitted into these
slots, and as the part _N_ is driven in, the strips are moved outward as
they ascend the tapering grooves. By having different sets of these
strips of various heights, one arbor of this type can be made to cover
quite a range of sizes. It is not suited, however, to thin work, as the
pressure, being concentrated in four places, would spring a flexible
part out of shape.
The cone arbor or mandrel shown at _A_, in Fig. 17, is convenient for
holding parts having comparatively large holes, as it can be adjusted
for quite a range of diameters. The work is gripped between the two
cones _c_ and _c_{1}_ which are forced together by nut _n_. The cones
are prevented from turning upon the arbor by keys. This style of arbor
should not be used for accurate work. The threaded arbor _B_ is used for
facing the sides of nuts square with the tapped hole. When a nut is
first put upon the arbor, the rough side comes against an equalizing
washer _w_. This washer rests against a spherical seat so that it can
shift to provide a uniform bearing for the rough side of the nut, even
though it is not square with the tapped hole. This feature prevents the
nut from being canted on the arbor and insures an accurately faced nut.
The revolving conical center shown at _C_ is often used for holding a
pipe or tube while turning the outside. The cone is adjusted to fit into
the hole of the pipe, by means of the tailstock spindle, and the
opposite end is usually held in a chuck.
[Illustration: Fig. 17. (A) Cone Arbor. (B) Nut Arbor. (C) Pipe Center]
Particular care should be taken to preserve the accuracy of the centers
of lathe arbors by keeping them clean and well-oiled while in use.
=Mandrel or Arbor Press.=--The best method of inserting an arbor of the
solid type in a hole is by using a press, Fig. 18, designed for that
purpose, but if such a press is not available and it is necessary to
drive the mandrel in, a "soft" hammer, made of copper, lead or other
soft material, should be used to protect the centered end of the arbor.
In either case, the arbor should not be forced in too tightly, for if it
fits properly, this will not be necessary in order to hold the work
securely. On the other hand, the work might easily be broken by
attempting to force the arbor in as far and as tightly as possible. In
using the arbor press, the work is placed on the base _B_ with the hole
in a vertical position, and the arbor (which should be oiled slightly)
is forced down into it by ram _R_, operated by lever _L_. Slots are
provided in the base, as shown, so that the end of the arbor can come
through at the bottom of the hole. The lever of this particular press is
counter-weighted so that it rises to a vertical position when released.
The ram can then be adjusted quickly to any required height by the
handwheel seen at the left.
[Illustration: Fig. 18. Press for Forcing Arbors into Work]
Some shops are equipped with power-driven mandrel or arbor presses.
This type is particularly desirable for large work, owing to the greater
pressure required for inserting mandrels that are comparatively large in
diameter. One well-known type of power press is driven by a belt, and
the downward pressure of the ram is controlled by a handwheel. The ram
is raised or lowered by turning this handwheel in one direction or the
other, and a gage shows how much pressure is being applied. This type of
press can also be used for other purposes, such as forcing bushings or
pins into or out of holes, bending or straightening parts, or for
similar work.
[Illustration: Fig. 19. Steadyrest and Follow-rest for Supporting
Flexible Parts]
=Steadyrest for Supporting Flexible Parts.=--Occasionally long slender
shafts, rods, etc., which have to be turned, are so flexible that it is
necessary to support them at some point between the lathe centers. An
attachment for the lathe known as a steadyrest is often used for this
purpose. A steadyrest is composed of a frame containing three jaws _J_
(Fig. 19), that can be adjusted in or out radially by turning screws
_S_. The frame is hinged at _h_, thus allowing the upper half to be
swung back (as shown by the dotted lines) for inserting or removing the
work. The bolt-clamp _c_ holds the hinged part in the closed position.
The base of the frame has V-grooves in it that fit the ways of the lathe
bed. When the steadyrest is in use, it is secured to the bed by clamp
_C_, and the jaws _J_ are set in against the work, thus supporting or
steadying it during the turning operation. The steadyrest must, of
course, be located at a point where it will not interfere with the
turning tool.
[Illustration: Fig. 20. Application of Steadyrest to a Flexible Rod]
Fig. 20 shows the application of the steadyrest to a long forged rod,
having one small end, which makes it too flexible to be turned without
support. As this forging is rough, a true surface _n_ a little wider
than the jaws _J_ (Fig. 19) is first turned as a bearing for the jaws.
This should be done very carefully to prevent the work from mounting the
tool. A sharp pointed tool should be used and very light cuts taken. The
steadyrest is next clamped to the lathe bed opposite the turned surface,
and the jaws are adjusted in against this surface, thus forming a
bearing. Care should be taken not to set up the jaws too tightly, as the
work should turn freely but without play. The large part of the rod and
central collar are then turned to size, this half being machined while
the small part is in the rough and as stiff as possible. The rod is then
reversed and the steadyrest is applied to the part just finished, as
shown at _B_, thus supporting the work while the small end is being
turned. That part against which the jaws bear should be kept well oiled,
and if the surface is finished it should be protected by placing a strip
of emery cloth beneath the jaws with the emery side out; a strip of
belt leather is also used for this purpose, the object in each case
being to prevent the jaws from scratching and marring the finished
surface, as they tend to do, especially if at all rough.
If the work were too flexible to permit turning a spot at _n_, this
could be done by first "spotting" it at some point _o_, and placing the
steadyrest at that point while turning another spot at _n_.
Sometimes it is desirable to apply a steadyrest to a surface that does
not run true and one which is not to be turned; in such a case a device
called a "cat-head" is used. This is simply a sleeve _S_ (Fig. 21) which
is placed over the untrue surface to serve as a bearing for the
steadyrest. The sleeve is made to run true by adjusting the four
set-screws at each end, and the jaws of the steadyrest are set against
it, thus supporting the work.
[Illustration: Fig. 21. Cat-head which is sometimes used as Bearing for
Steadyrest]
=Application of Steadyrest when Boring.=--Another example illustrating
the use of the steadyrest is shown in Fig. 22. The rod _R_ is turned on
the outside and a hole is to be bored in the end (as shown by dotted
lines) true with the outer surface. If the centers used for turning the
rod are still in the ends, as they would be ordinarily, this work could
be done very accurately by the following method: The rod is first placed
between the centers as for turning, with a driving dog _D_ attached, and
the steadyrest jaws _J_ are set against it near the outer end, as shown.
Before any machine work is done, means must be provided for holding the
rod back against the headstock center _h_, because, for an operation of
this kind, the outer end cannot be supported by the tailstock center;
consequently the work tends to shift to the right. One method of
accomplishing this is shown in the illustration. A hardwood piece _w_,
having a hole somewhat larger than the work, is clamped against the dog,
in a crosswise position, by the swinging bolts and thumb-screws shown.
If the dog is not square with the work, the wood piece should be canted
so that the bearing will not be all on one side. For large heavy parts a
similar "bridle" or "hold-back"--as this is commonly called--is made by
using steel instead of wood for the part _w_. Another very common method
which requires no special equipment is illustrated in Fig. 23. An
ordinary leather belt lacing _L_ is attached to the work and faceplate
while the latter is screwed off a few turns as shown. Then the lacing is
drawn up by hand and tied, and the faceplate is screwed onto the
spindle, thus tightening the lacing and drawing the work against the
headstock center. The method of applying the lacing is quite clearly
indicated in the illustration. If a small driving faceplate is used, it
may be necessary to drill holes for the belt lacing, as shown.
[Illustration: Fig. 22. Shaft supported by Steadyrest for Drilling and
Boring End]
A hole is next drilled in the end of the rod by using a twist drill in
the tailstock. If the hole is finished by boring, a depth mark should
be made on the tool shank that will warn the workman of the cutting
end's approach to the bottom. A chuck can also be used in connection
with a steadyrest for doing work of this kind, as shown in Fig. 24, the
end of the rod being held and driven by the chuck _C_. If the piece is
centered, it can be held on these centers while setting the steadyrest
and adjusting the chuck, but if the ends are without centers, a very
good way is to make light centers in the ends with a punch; after these
are properly located they are used for holding the work until the
steadyrest and chuck jaws have been adjusted. In case it is necessary to
have the end hole very accurate with the outside of the finished rod, a
test indicator _I_ should be applied to the shaft as shown. This is an
instrument which shows with great accuracy whether a rotating part runs
true and it is also used for many other purposes in machine shops. The
indicator is held in the lathe toolpost and the contact point beneath
the dial is brought against the work. If the latter does not run true,
the hand of the indicator vibrates and the graduations on the dial show
how much the work is out in thousandths of an inch.
[Illustration: Fig. 23. Hold-back used when Outer End of Work is held in
Steadyrest]
=The Follow-rest.=--When turning long slender parts, such as shafts,
etc., a follow-rest is often used for supporting the work. The
follow-rest differs from the steadyrest in that it is attached to and
travels with the lathe carriage. The type illustrated to the right in
Fig. 19 has two adjustable jaws which are located nearly opposite the
turning tool, thus providing support where it is most needed. In using
this rest, a cut is started at the end and the jaws are adjusted to this
turned part. The tool is then fed across the shaft, which cannot spring
away from the cut because of the supporting jaws. Some follow-rests
have, instead of jaws, a bushing bored to fit the diameter being turned,
different bushings being used for different diameters. The bushing forms
a bearing for the work and holds it rigidly. Whether a bushing or jaws
are used, the turning tool is slightly in advance of the supporting
member.
[Illustration: Fig. 24. Testing Work with Dial Indicator]
=Centering Parts to be Turned.=--As previously mentioned, there are a
number of different methods of forming center-holes in the ends of parts
that have to be turned while held between lathe centers. A method of
centering light work, and one that requires few special tools, is first
to locate a central point on the end and then drill and ream the
center-hole by using the lathe itself. Hermaphrodite dividers are useful
for finding the center, as illustrated at _A_, Fig. 25, but if the work
is fairly round, a center-square _B_ is preferable. A line is scribed
across the end and then another line at right angles to the first by
changing the position of the square; the intersection of these two lines
will be the center, which should be marked by striking a pointed punch
_C_ with a hammer. If a cup or bell center-punch _D_ is available, it
will not be necessary to first make center lines, as the conical part
shown locates the punch in a central position. This style of punch
should only be used on work which is fairly round.
[Illustration: Fig. 25. Centering End with Punch preparatory to
Drilling]
After small centers have been located in both ends, their position can
be tested by placing the work between the lathe centers and rotating it
rapidly by drawing the hand quickly across it. By holding a piece of
chalk close to the work as it spins around, a mark will be made on the
"high" side if the centers are not accurate; the centers are then
shifted toward these marks. If the work is close to the finished
diameter, the centers should, of course, be located quite accurately in
order that the entire surface of the work will be turned true when it is
reduced to the finished size.
One method of forming these center-holes is indicated in Fig. 26. A
chuck _C_ is screwed onto the spindle in place of the faceplate, and a
combination center drill and reamer _R_ is gripped by the chuck jaws and
set to run true. The center is then drilled and reamed at one end by
pressing the work against the revolving drill with the tailstock
spindle, which is fed out by turning handle _n_. The piece is then
reversed for drilling the opposite end. The work may be kept from
revolving while the centers are being drilled and reamed, by attaching a
dog to it close to the tailstock end and then adjusting the cross-slide
until the dog rests upon the slide. Many parts can be held by simply
gripping them with one hand. From the foregoing it will be seen that the
small centers made by punch _C_, Fig. 25, serve as a starting point for
the drill and also as a support for the outer end of the work while the
first hole is being drilled.
[Illustration: Fig. 26. Drilling Centers in the Lathe]
The form of center-hole produced by a combination drill and reamer is
shown by the lower left-hand view in Fig. 27. A small straight hole a in
the bottom prevents the point of the lathe center from coming in contact
with the work and insures a good bearing on the conical surface _c_. The
standard angle for lathe centers is sixty degrees, as the illustration
shows, and the tapering part of all center-holes should be made to this
angle.
[Illustration: Fig. 27. Centers of Incorrect and Correct Form]
[Illustration: Fig. 28. Special Machine for Centering Parts to be
Turned]
=Centering Machine.=--Many shops have a special machine for forming
centers which enables the operation to be performed quickly. One type
of centering machine is shown in Fig. 28. The work is gripped in a chuck
_C_ that automatically locates it in a central position so that it is
not necessary to lay out the end before drilling. There are two spindles
_s_, one of which holds the drill and the other the countersink, and
these are rotated by a belt passing over pulley _P_. Each of these
spindles is advanced by lever _L_ and either of them can be moved to a
position central with the work, as they are mounted in a swiveling
frame. In operating this machine, a small straight hole is first made by
a twist drill held in one of the spindles; the other spindle is then
moved over to the center and the hole is reamed tapering. The
arrangement is such that neither spindle can be advanced by the feeding
lever except when in a central position. The amount that each spindle
can be advanced is limited by a fixed collar inside the head, and there
is also a swinging adjustable stop against which the end of the work
should be placed before tightening the chuck. These two features make it
possible to ream center holes of the same size or depth in any number of
pieces.
[Illustration: Fig. 29. The Imperfect Center Bearing is the Result of
Centering before Straightening]
=Different Forms of Centers.=--In some poorly equipped shops it is
necessary to form centers by the use of a center-punch only, as there is
no better tool. If the end of the punch has a sixty-degree taper, a fair
center can be formed in this way, but it is not a method to be
recommended, especially when accurate work is required. Sometimes
centers are made with punches that are too blunt, producing a shallow
center, such as the one shown in the upper left-hand view, Fig. 27. In
this case all the bearing is on the point of the lathe center, which is
the worst possible place for it. Another way is to simply drill a
straight hole as in the upper view to the right; this is also bad
practice in more than one respect. The lower view to the right shows a
form of center which is often found in the ends of lathe arbors, the
mouth of the center being rounded, at _r_, and the arbor end recessed as
shown. The rounded corner prevents the point of the lathe center from
catching when it is moved rapidly towards work which is not being held
quite centrally (as shown by the illustration), and the end is recessed
to protect the center against bruises. Stock that is bent should always
be straightened before the centers are drilled and reamed. If the work
is first centered and then straightened the bearing on the lathe center
would be as shown in Fig. 29. The center will then wear unevenly with
the result that the surfaces last turned will not be concentric with
those which were finished first.
[Illustration: Fig. 30. Tool Steel should be centered Concentric, in
order to remove the Decarbonized Outer Surface]
=Precaution When Centering Tool Steel.=--Ordinarily centers are so
located that the stock runs approximately true before being turned, but
when centering tool steel to be used in making tools, such as reamers,
mills, etc., which need to be hardened, particular care should be taken
to have the rough surface run fairly true. This is not merely to insure
that the piece will "true-up," as there is a more important
consideration, the disregard of which often affects the quality of the
finished tool. As is well known, the degree of hardness of a piece of
tool steel that has been heated and then suddenly cooled depends upon
the amount of carbon that it contains, steel that is high in carbon
becoming much harder than that which contains less carbon. Furthermore,
the amount of carbon found at the surface, and to some little depth
below the surface of a bar of steel, is less than the carbon content in
the rest of the bar. This is illustrated diagrammatically in Fig. 30 by
the shaded area in the view to the left. (This decarbonization is
probably due to the action of the oxygen of the air on the bar during
the process of manufacture.) If stock for a reamer is so centered that
the tool removes the decarbonized surface only on one side, as
illustrated to the right, evidently when the reamer is finished and
hardened the teeth on the side _A_ will be harder than those on the
opposite side, which would not have been the case if the rough bar had
been centered true. To avoid any trouble of this kind, stock that is to
be used for hardened tools should be enough larger than the finished
diameter and so centered that this decarbonized surface will be entirely
removed in turning.
[Illustration: Fig. 31. Three Methods of Facing the Ends Square]
=Facing the Ends of Centered Stock.=--As a piece of work is not properly
centered until the ends are faced square, we will consider this
operation in connection with centering. Some machinists prefer lathe
centers that are cut away as shown at _A_, Fig. 31, so that the point of
the side tool can be fed in far enough to face the end right up to the
center hole. Others, instead of using a special center, simply loosen
the regular one slightly and then, with the tool in a position as at
_B_, face the projecting teat by feeding both tool and center inward as
shown by the arrow. Whenever this method is employed, care should be
taken to remove any chips from the center hole which may have entered. A
method which makes it unnecessary to loosen the regular center, or to
use a special one, is to provide clearance for the tool-point by
grinding it to an angle of approximately forty-five degrees, as shown at
_C_. If the tool is not set too high, it can then be fed right up to the
lathe center and the end squared without difficulty. As for the special
center _A_, the use of special tools and appliances should always be
avoided unless they effect a saving in time or their use makes it
possible to accomplish the same end with less work.
=Truing Lathe Centers.=--The lathe centers should receive careful
attention especially when accurate work must be turned. If the headstock
center does not run true as it revolves with the work, a round surface
may be turned, but if the position of the driving dog with reference to
the faceplate is changed, the turned surface will not run true because
the turned surface is not true with the work centers. Furthermore, if
it is necessary to reverse the work for finishing the dogged or driving
end, the last part turned will be eccentric to the first. Therefore, the
lathe centers should be kept true in order to produce turned surfaces
that are true or concentric with the centered ends, as it is often
necessary to change the part being turned "end for end" for finishing,
and any eccentricity between the different surfaces would, in many
cases, spoil the work.
[Illustration: Fig. 32. Grinder for Truing Lathe Centers]
Some lathes are equipped with hardened centers in both the head-and
tailstock and others have only one hardened center which is in the
tailstock. The object in having a soft or unhardened headstock center is
to permit its being trued by turning, but as a soft center is quite
easily bruised and requires truing oftener than one that is hard, it is
better to have both centers hardened. Special grinders are used for
truing these hardened centers. One type that is very simple and easily
applied to a lathe is shown in Fig. 32. This grinder is held in the
lathe toolpost and is driven by a wheel _A_ that is held in contact with
the cone-pulley. The emery wheel _B_ is moved to a position for grinding
by adjusting the carriage and cross-slide, and it is traversed across
the conical surface of the center by handle _C_. As the grinding
proceeds, the wheel is fed inward slightly by manipulating the
cross-slide.
This grinder is set to the proper angle by placing the two centered ends
_D_ and _D_{1}_ between the lathe centers, which should be aligned as
for straight turning. The grinding spindle will then be 30 degrees from
the axis of the lathe spindle. The grinder should be carefully clamped
in the toolpost so that it will remain as located by the centered ends.
After the tailstock center is withdrawn, the emery wheel is adjusted for
grinding. As the wheel spindle is 30 degrees from the axis of the lathe
spindle, the lathe center is not only ground true but to an included
angle of 60 degrees, which is the standard angle for lathe centers.
There are many other styles of center grinders on the market, some of
which are driven by a small belt from the cone-pulley and others by
electric motors which are connected with ordinary lighting circuits. The
tailstock center is ground by inserting it in the spindle in place of
the headstock center. Before a center is replaced in its spindle, the
hole should be perfectly clean as even a small particle of dirt may
affect the alignment. The center in the headstock is usually referred to
as the "live center" because it turns around when the lathe is in use,
and the center in the tailstock as the "dead center," because it remains
stationary.
=Universal, Independent and Combination Chucks.=--Many parts that are
turned in the lathe are so shaped that they cannot be held between the
lathe centers like shafts and other similar pieces and it is often
necessary to hold them in a chuck _A_, Fig. 33, which is screwed onto
the lathe spindle instead of the faceplate. The work is gripped by the
jaws _J_ which can be moved in or out to accommodate various diameters.
There are three classes of chucks ordinarily used on the lathe, known as
the independent, universal and combination types. The independent chuck
is so named because each jaw can be adjusted in or out independently of
the others by turning the jaw screws S with a wrench. The jaws of the
universal chuck all move together and keep the same distance from the
center, and they can be adjusted by turning any one of the screws _S_,
whereas with the independent type the chuck wrench must be applied to
each jaw screw. The combination chuck, as the name implies, may be
changed to operate either as an independent or universal type. The
advantage of the universal chuck is that round and other parts of a
uniform shape are located in a central position for turning without any
adjustment. The independent type is, however, preferable in some
respects as it is usually stronger and adapted for holding odd-shaped
pieces because each jaw can be set to any required position.
[Illustration: Fig. 33. (A) Lathe Chuck. (B) Faceplate Jaw]
=Application of Chucks.=--As an example of chuck work, we shall assume
that the sides of disk _D_, Fig. 34, are to be turned flat and parallel
with each other and that an independent chuck is to be used. First the
chuck is screwed onto the lathe spindle after removing the faceplate.
The chuck jaws are then moved out or in, as the case may be, far enough
to receive the disk and each jaw is set about the same distance from the
center by the aid of concentric circles on the face of the chuck. The
jaws are then tightened while the disk is held back against them to
bring the rough inner surface in a vertical plane. If the work is quite
heavy, it can be held against the chuck, before the jaws are tightened,
by inserting a piece of wood between it and the tailstock center; the
latter is then run out far enough to force the work back. The outside
or periphery of the disk should run nearly true and it may be necessary
to move the jaws in on one side and out on the other to bring the disk
to a central position. To test its location, the lathe is run at a
moderate speed and a piece of chalk is held near the outer surface. If
the latter runs out, the "high" side will be marked by the chalk, and
this mark can be used as a guide in adjusting the jaws. It should be
remembered that the jaws are moved only one-half the amount that the
work runs out.
[Illustration: Fig. 34. (A) Radial Facing. (B) Boring Pulley Held in
Chuck]
A round-nosed tool _t_ of the shape shown can be used for radial facing
or turning operations of the kind illustrated. This tool is similar to
the form used when turning between centers, the principal difference
being in the direction of the top slope. The radial facing tool should
be ground to slope downward toward _a_ (see Fig. 35) whereas the regular
turning tool slopes toward _b_, the inclination in each case being away
from that part of the cutting edge which does the work. The cutting edge
should be the same height as the lathe centers, and the cut is taken by
feeding the tool from the outside in to the center. The cut is started
by hand and then the power feed is engaged, except for small surfaces.
The first cut should, if possible, be deep enough to get beneath the
scale, especially if turning cast iron, as a tool which just grazes the
hard outer surface will be dulled in a comparatively short time.
If it were simply necessary to turn a true flat surface and the
thickness of the disk were immaterial, two cuts would be sufficient,
unless the surface were very uneven, the first or roughing cut being
followed by a light finishing cut. For a finishing cut, the same tool
could be used, but if there were a number of disks to be faced, a
square-nosed tool _F_, Fig. 35, could probably be used to better
advantage. This type has a broad flat cutting edge that is set parallel
with the rough-turned surface and this broad edge enables a coarse feed
to be taken, thus reducing the time required for the finishing cut. If a
coarse feed were taken with the round tool, the turned surface would
have spiral grooves in it, whereas with the broad cutting edge, a smooth
surface is obtained even though the feed is coarse. The amount of feed
per revolution of the work, however, should always be less than the
width _w_ of the cutting edge. Very often broad tools cannot be used for
finishing cuts, especially when turning steel, because their greater
contact causes chattering and results in a rough surface. An old and
worn lathe is more liable to chatter than one that is heavy and
well-built, and as the diameter of the work also makes a difference, a
broad tool cannot always be used for finishing, even though,
theoretically, it would be preferable. After one side of the disk is
finished, it is reversed in the chuck, the finished surface being placed
against the jaws. The remaining rough side is then turned, care being
taken when starting the first cut to caliper the width of the disk at
several points to make sure that the two sides are parallel.
[Illustration: Fig. 35. Tools Ground so that Top Slopes away from
Working Part of Cutting Edge]
=Example of Boring.=--Another example of chuck work is shown at _B_,
Fig. 34. In this case a cast-iron pulley is to have a true hole _h_
bored through the hub. (The finishing of internal cylindrical surfaces
in a lathe is referred to as boring rather than turning.) The casting
should be set true by the rim instead of by the rough-cored hole in the
hub; this can be done by the use of chalk as previously explained. Even
though a universal type of chuck were used, the jaws of which, as will
be recalled, are self-centering, it might be necessary to turn the
pulley relative to the chuck as a casting sometimes runs out because of
rough spots or lumps which happen to come beneath one or more of the
jaws.
[Illustration: Fig. 36. Boring Tool]
The shape of tool _t_ for boring is quite different from one used for
outside turning, as shown by Fig. 36. The cutting end of a solid type of
tool is forged approximately at right angles to the body or shank, and
the top surface is ground to slope away from the working part _w_ of the
cutting edge, as with practically all turning tools. The front part or
flank, _f_ is also ground away to give the edge clearance. This type of
tool is clamped in the toolpost with the body about parallel with the
lathe spindle, and ordinarily the cutting edge would be about as high as
the center of the hole, or a little below, if anything. When starting a
cut, the tool is brought up to the work by moving the carriage and it is
then adjusted radially to get the right depth of cut, by shifting the
cross-slide. The power feed for the carriage is then used, the tool
feeding back through the hole as indicated by the arrow, Fig. 34. In
this case, as with all turning operations, the first cut should be deep
enough to remove the hard outer scale at every part of the hole. Usually
a rough-cored hole is so much smaller than the finished size that
several cuts are necessary; in any case, the last or finishing cut
should be very light to prevent the tool from springing away from the
work, so that the hole will be as true as possible. Boring tools,
particularly for small holes, are not as rigid as those used for outside
turning, as the tool has to be small enough to enter the hole and for
this reason comparatively light cuts have to be taken. When boring a
small hole, the largest tool that will enter it without interference
should be used to get the greatest rigidity possible.
[Illustration: Fig. 37 (A) Setting Outside Calipers. (B) Transferring
Measurements to Inside Calipers. (C) Micrometer Gage]
=Measuring Bored Holes.=--The diameters of small holes that are being
bored are usually measured with inside calipers or standard gages. If
the pulley were being bored to fit over some shaft, the diameter of the
shaft would first be measured by using outside calipers, as shown at
_A_, Fig. 37, the measuring points of the calipers being adjusted until
they just made contact with the shaft when passed over it. The inside
calipers are then set as at _B_ to correspond with the size of the
shaft, and the hole is bored just large enough to admit the inside
calipers easily. Very accurate measurements can be made with calipers,
but to become expert in their use requires experience. Some mechanics
never become proficient in the art of calipering because their hands are
"heavy" and they lack the sensitiveness and delicacy of touch that is
necessary. For large holes, a gage _C_ is often used, the length _l_
being adjusted to the diameter desired. Small holes are often bored to
fit hardened steel plug gages (Fig. 38), the cylindrical measuring ends
of which are made with great accuracy to standard sizes. This type of
gage is particularly useful when a number of holes have to be bored to
the same size, all holes being made just large enough to fit the gage
without any perceptible play.
[Illustration: Fig. 38. Standard Plug Gage]
_Setting Work in the Chuck._--When setting a part in a chuck, care
should be taken to so locate it that every surface to be turned will be
true when machined to the finished size. As a simple illustration, let
us assume that the hole through the cast-iron disk, Fig. 39, has been
cored considerably out of center, as shown. If the work is set by the
outside surface _S_, as it would be ordinarily, the hole is so much out
of center that it will not be true when bored to the finished size, as
indicated by the dotted lines. On the other hand, if the rough hole is
set true, the outside cannot be finished all over, without making the
diameter too small, when it is finally turned. In such a case, the
casting should be shifted, as shown by the arrow, to divide the error
between the two surfaces, both of which can then be turned as shown by
the dotted lines in the view to the right. This principle of dividing
the error when setting work can often be applied in connection with
turning and boring. After a casting or other part has been set true by
the most important surface, all other surfaces which require machining
should be tested to make sure that they all can be finished to the
proper size.
=Inaccuracy from Pressure of Chuck Jaws.=--Work that is held in a chuck
is sometimes sprung out of shape by the pressure of the chuck jaws so
that when the part is bored or turned, the finished surfaces are untrue
after the jaws are released and the work has resumed its normal shape.
This applies more particularly to frail parts, such as rings, thin
cylindrical parts, etc. Occasionally the distortion can be prevented by
so locating the work with relation to the chuck jaws that the latter
bear against a rigid part. When the work cannot be held tightly enough
for the roughing cuts without springing it, the jaws should be released
somewhat before taking the finishing cut, to permit the part to spring
back to its natural shape.
[Illustration: Fig. 39. Diagram Illustrating Importance of Setting Work
with Reference to Surfaces to be Turned]
[Illustration: Fig. 40. Drilling in the Lathe]
=Drilling and Reaming.=--When a hole is to be bored from the solid, it
is necessary to drill a hole before a boring tool can be used. One
method of drilling in the lathe is to insert an ordinary twist drill in
a holder or socket _S_, Fig. 40, which is inserted in the tailstock
spindle in place of the center. The drill is then fed through the work
by turning the handle _n_ and feeding the spindle outward as shown by
the arrow. Before beginning to drill, it is well to turn a conical spot
or center for the drill point so the latter will start true. This is
often done by using a special tool having a point like a flat drill.
This tool is clamped in the toolpost with the point at the same height
as the lathe centers. It is then fed against the center of the work and
a conical center is turned. If the drill were not given this true
starting point, it probably would enter the work more or less off
center. Drills can also be started without turning a center by bringing
the square end or butt of a tool-shank held in the toolpost in contact
with the drill near the cutting end. If the point starts off center,
thus causing the drill to wobble, the stationary tool-shank will
gradually force or bump it over to the center.
[Illustration: Fig. 41. Flat Drill and Holder]
Small holes are often finished in the lathe by drilling and reaming
without the use of a boring tool. The form of drill that is used quite
extensively for drilling cored holes in castings is shown in Fig. 41, at
_A_. This drill is flat and the right end has a large center hole for
receiving the center of the tailstock. To prevent the drill from
turning, a holder _B_, having a slot _s_ in its end through which the
drill passes, is clamped in the toolpost, as at _C_. This slot should be
set central with the lathe centers, and the drill, when being started,
should be held tightly in the slot by turning or twisting it with a
wrench as indicated in the end view at _D_; this steadies the drill and
causes it to start fairly true even though the cored hole runs out
considerably.
Another style of tool for enlarging cored holes is shown in Fig. 42, at
_A_. This is a rose chucking reamer, having beveled cutting edges on the
end and a cylindrical body, which fits closely in the reamed hole, thus
supporting and guiding the cutting end. The reamer shown at _B_ is a
fluted type with cutting edges that extend from _a_ to _b_; it is used
for finishing holes and the drill or rose reamer preceding it should
leave the hole very close to the required size. These reamers are held
while in use in a socket inserted in the tailstock spindle, as when
using a twist drill.
[Illustration: Fig. 42. Rose and Fluted Reamers]
=Holding Work on Faceplate.=--Some castings or forgings are so shaped
that they cannot be held in a chuck very well, or perhaps not at all,
and work of this kind is often clamped to a faceplate which is usually
larger than the faceplate used for driving parts that are turned between
the centers. An example of faceplate work is shown in Fig. 43. This is a
rectangular-shaped casting having a round boss or projection, the end
_e_ of which is to be turned parallel with the back face of the casting
previously finished on a planer. A rough cored hole through the center
of the boss also needs to be bored true.
The best way to perform this operation in the lathe would be to clamp
the finished surface of the casting directly against the faceplate by
bolts and clamps _a_, _b_, _c_, and _d_, as shown; the work would then
be turned just as though it were held in a chuck. By holding the casting
in this way, face _e_ will be finished parallel with the back surface
because the latter is clamped directly against the true-running surface
of the faceplate. If a casting of this shape were small enough it could
also be held in the jaws of an independent chuck, but if the surface e
needs to be exactly parallel with the back face, it is better to clamp
the work to the faceplate. Most lathes have two faceplates: One of small
diameter used principally for driving work turned between centers, and a
large one for holding heavy or irregularly shaped pieces; either of
these can be screwed onto the spindle, and the large faceplate has a
number of slots through which clamping bolts can be inserted.
[Illustration: Fig. 43. Casting Clamped to Faceplate for Turning and
Boring]
The proper way to clamp a piece to the faceplate depends, of course,
largely on its shape and the location of the surface to be machined, but
in any case it is necessary to hold it securely to prevent any shifting
after a cut is started. Sometimes castings can be held by inserting
bolts through previously drilled holes, but when clamps are used in
connection with the bolts, their outer ends are supported by hardwood or
metal blocks which should be just high enough to make the clamp bear
evenly on the work. When deep roughing cuts have to be taken,
especially on large diameters, it is well to bolt a piece to the
faceplate and against one side of the casting, as at _D_, to act as a
driver and prevent the work from shifting; but a driver would not be
needed in this particular case. Of course a faceplate driver is always
placed to the rear, as determined by the direction of rotation, because
the work tends to shift backward when a cut is being taken. If the
surface which is clamped against the faceplate is finished as in this
case, the work will be less likely to shift if a piece of paper is
placed between it and the faceplate.
[Illustration: Fig. 44. Cast Elbow held on Angle-plate attached to
Faceplate]
Work mounted on the faceplate is generally set true by some surface
before turning. As the hole in this casting should be true with the
round boss, the casting is shifted on the faceplate until the rough
outer surface of the boss runs true; the clamps which were previously
set up lightly are then tightened. The face e is first turned by using a
round-nosed tool. This tool is then replaced by a boring tool and the
hole is finished to the required diameter. If the hole being bored is
larger than the central hole in the faceplate, the casting should be
clamped against parallel pieces, and not directly against the faceplate,
to provide clearance for the tool when it reaches the inner end of the
hole and prevent it from cutting the faceplate. The parallel pieces
should be of the same thickness and be located near the clamps to
prevent springing the casting.
=Application of Angle-plate to Faceplate.=--Another example of faceplate
work is shown in Fig. 44. This is a cast-iron elbow _E_, the two flanges
of which are to be faced true and square with each other. The shape of
this casting is such that it would be very difficult to clamp it
directly to the faceplate, but it is easily held on an angle-plate _P_,
which is bolted to the faceplate. The two surfaces of this angle-plate
are square with each other so that when one flange of the elbow is
finished and bolted against the angle-plate, the other will be faced
square. When setting up an angle-plate for work of this kind, the
distance from its work-holding side to the center of the faceplate is
made equal to the distance _d_ between the center of one flange and the
face of the other, so that the flange to be faced will run about true
when bolted in place. As the angle-plate and work are almost entirely on
one side of the faceplate, a weight _W_ is attached to the opposite side
for counterbalancing. Very often weights are also needed to
counterbalance offset parts that are bolted directly to the faceplate.
The necessity of counterbalancing depends somewhat upon the speed to be
used for turning. If the surface to be machined is small in diameter so
that the lathe can be run quite rapidly, any unbalanced part should
always be counterbalanced.
Sometimes it is rather difficult to hold heavy pieces against the
vertical surface of the faceplate while applying the clamps, and
occasionally the faceplate is removed and placed in a horizontal
position on the bench; the work can then be located about right, and
after it is clamped, the faceplate is placed on the lathe spindle by the
assistance of a crane.
Special faceplate jaws, such as the one shown to the right in Fig. 33,
can often be used to advantage for holding work on large faceplates.
Three or four of these jaws are bolted to the faceplate which is
converted into a kind of independent chuck. These faceplate jaws are
especially useful for holding irregularly shaped parts, as the different
jaws can be located in any position.
=Supporting Outer End of Chucked Work.=--Fig. 45 shows how the tailstock
center is sometimes used for supporting the outer end of a long casting,
the opposite end of which is held in a chuck. This particular casting
is to be turned and bored to make a lining for the cylinder of a
locomotive in order to reduce the diameter of the cylinder which has
been considerably enlarged by re-boring a number of times. These
bushings are rough-turned on the outside while the outer end is
supported by the cross-shaped piece or "spider" which forms a
center-bearing for the tailstock. This spider has set screws in the
flanged ends of the arms, which are tightened against the inner surface
of the casting and are adjusted one way or the other in order to locate
it in a concentric position. After roughing the outside, the inside is
bored to the finish size; then centered disks, which fit into the bore,
are placed in the ends of the bushing and the latter is finish-turned.
The object in rough turning the outside prior to boring is to avoid the
distortion which might occur if this hard outer surface were removed
last.
[Illustration: Fig. 45. Rough Turning a Cylinder Lining--Note Method of
Supporting Outer End]
=Boring Large Castings in the Lathe.=--An ordinary engine lathe is
sometimes used for boring engine or pump cylinders, linings, etc., which
are too large to be held in the chuck or on a faceplate, and must be
attached to the lathe carriage. As a rule, work of this class is done in
a special boring machine (see "Horizontal Boring Machines"), but if
such a machine is not available, it may be necessary to use a lathe.
There are two general methods of boring.
Fig. 46 shows how the lining illustrated in Fig. 45 is bored in a large
engine lathe. The casting is held in special fixtures which are attached
to the lathe carriage, and the boring-bar is rotated by the lathe
spindle. The tool-head of this boring-bar carries two tools located 180
degrees apart and it is fed along the bar by a star-feed mechanism shown
attached to the bar and the tailstock spindle. Each time the bar
revolves, the star wheel strikes a stationary pin and turns the
feed-screw which, as the illustration shows, extends along a groove cut
in one side of the bar. This feed-screw passes through a nut attached to
the tool-head so that the latter is slowly fed through the bore. When
using a bar of this type, the carriage, of course, remains stationary.
[Illustration: Fig. 46. Boring a Cylinder Lining in an Ordinary Engine
Lathe]
Cylindrical parts attached to the carriage can also be bored by using a
plain solid bar mounted between the centers. The bar must be provided
with a cutter for small holes or a tool-head for larger diameters
(preferably holding two or more tools) and the boring is done by
feeding the carriage along the bed by using the regular power feed of
the lathe. A symmetrically shaped casting like a bushing or lining is
often held upon wooden blocks bolted across the carriage. These are
first cut away to form a circular seat of the required radius, by using
the boring-bar and a special tool having a thin curved edge. The casting
is then clamped upon these blocks by the use of straps and bolts, and if
the curved seats were cut to the correct radius, the work will be
located concentric with the boring-bar. When using a boring-bar of this
type, the bar must be long enough to allow the part being bored to feed
from one side of the cutter-head to the other, the cutter-head being
approximately in a central location.
[Illustration: Fig. 47. Method of Setting Circle on Work Concentric with
Lathe Spindle]
=Boring Holes to a Given Center Distance.=--In connection with faceplate
work, it is often necessary to bore two or more holes at a given
distance apart. The best method of doing this may depend upon the
accuracy required. For ordinary work sometimes two or more circles _A_
and _B_ (Fig. 47) are drawn upon the part to be bored, in the position
for the holes; the piece is then clamped to the faceplate and one of the
circles is centered with the lathe spindle by testing it with a pointer
C held in the toolpost; that is, when the pointer follows the circle as
the work is turned, evidently the circle is concentric with the spindle.
The hole is then drilled and bored. The other circle is then centered
in the same way for boring the second hole. As will be seen, the
accuracy of this method depends first, upon the accuracy with which the
circles were laid out, and second; upon the care taken in setting them
concentric. For a more accurate way of locating parts for boring, see
"Use of Center Indicator" and "Locating Work by the Button Method."
=Turning Brass, Bronze and Copper.=--When turning soft yellow brass, a
tool should be used having very little or no slope or rake on the top
surface against which the chip bears, and for plain cylindrical turning,
the point of the tool is drawn out quite thin and rounded, by grinding,
to a radius of about 1/8 or 3/16 inch. If a tool having very much top
slope is used for brass, there is danger of its gouging into the metal,
especially if the part being turned is at all flexible. The clearance
angle of a brass tool is usually about 12 or 14 degrees, which is 3 or 4
degrees greater than the clearance for steel turning tools. Most brass
is easily turned, as compared with steel, and for that reason this
increase in clearance is desirable, because it facilitates feeding the
tool into the metal, especially when the carriage and cross-slide
movements are being controlled by hand as when turning irregular shapes.
The speed for turning soft brass is much higher than for steel, being
ordinarily between 150 and 200 feet per minute. When turning phosphor,
tobin or other tough bronze compositions, the tool should be ground with
rake the same as for turning steel, and lard oil is sometimes used as a
lubricant. The cutting speed for bronzes varies from 35 or 40 to 80 feet
per minute, owing to the difference in the composition of bronze alloys.
Turning tools for copper are ground with a little more top rake than is
given steel turning tools, and the point should be slightly rounded. It
is important to have a keen edge, and a grindstone is recommended for
sharpening copper turning tools. Milk is generally considered the best
lubricant to use when turning copper. The speed can be nearly as fast as
for brass.
=Machining Aluminum.=--Tools for turning aluminum should have acute
cutting angles. After rough-grinding the tool, it is advisable to finish
sharpening the cutting edge on a grindstone or with an oilstone for fine
work, as a keen edge is very essential. High speeds and comparatively
light cuts are recommended. The principal difficulty in the machining of
aluminum and aluminum alloys is caused by the clogging of the chips,
especially when using such tools as counterbores and milling cutters.
This difficulty can be avoided largely by using the right kind of
cutting lubricant. Soap-water and kerosene are commonly employed. The
latter enables a fine finish to be obtained, provided the cutting tool
is properly ground.
The following information on this subject represents the experience of
the Brown-Lipe Gear Co., where aluminum parts are machined in large
quantities: For finishing bored holes, a bar equipped with cutters has
been found more practicable than reamers. The cutters used for machining
4-inch holes have a clearance of from 20 to 22 degrees and no rake or
slope on the front faces against which the chips bear. The roughing
cutters for this work have a rather sharp nose, being ground on the
point to a radius of about 3/32 inch, but for securing a smooth surface,
the finishing tools are rounded to a radius of about 3/4 inch. The
cutting speed, as well as the feed, for machining aluminum is from 50 to
60 per cent faster than the speeds and feeds for cast iron. The
lubricant used by this company is composed of one part "aqualine" and 20
parts water. This lubricant not only gives a smooth finish but preserves
a keen cutting edge and enables tools to be used much longer without
grinding. Formerly, a lubricant composed of one part of high-grade lard
oil and one part of kerosene was used. This mixture costs approximately
30 cents per gallon, whereas the aqualine and water mixture now being
used costs less than 4 cents per gallon, and has proved more effective
than the lubricant formerly employed.
CHAPTER II
LATHE TURNING TOOLS AND CUTTING SPEEDS
Notwithstanding the fact that a great variety of work can be done in the
lathe, the number of turning tools required is comparatively small. Fig.
1 shows the forms of tools that are used principally, and typical
examples of the application of these various tools are indicated in Fig.
2. The reference letters used in these two illustrations correspond for
tools of the same type, and both views should be referred to in
connection with the following description.
=Turning Tools for General Work.=--The tool shown at _A_ is the form
generally used for rough turning, that is for taking deep cuts when
considerable metal has to be removed. At _B_ a tool of the same type is
shown, having a bent end which enables it to be used close up to a
shoulder or surface _s_ that might come in contact with the tool-rest if
the straight form were employed. Tool _C_, which has a straight cutting
end, is used on certain classes of work for taking light finishing cuts,
with a coarse feed. This type of tool has a flat or straight cutting
edge at the end, and will leave a smooth finish even though the feed is
coarse, provided the cutting edge is set parallel with the tool's travel
so as to avoid ridges. Broad-nosed tools and wide feeds are better
adapted for finishing cast iron than steel. When turning steel, if the
work is at all flexible, a broad tool tends to gouge into it and for
this reason round-nosed tools and finer feeds are generally necessary. A
little experience in turning will teach more on this point than a whole
chapter on the subject.
[Illustration: Fig. 1. Set of Lathe Turning Tools for General Work]
[Illustration: Fig. 2. Views illustrating Use of Various Types of Lathe
Tools]
The side-tools shown at _D_ and _E_ are for facing the ends of shafts,
collars, etc. The first tool is known as a right side-tool because it
operates on the right end or side of a shaft or collar, whereas the left
side-tool _E_ is used on the opposite side, as shown in Fig. 2.
Side-tools are also bent to the right or left because the cutting edge
of a straight tool cannot always be located properly for facing certain
surfaces. A bent right side-tool is shown at _F_. A form of tool that is
frequently used is shown at _G_; this is known as a parting tool and is
used for severing pieces and for cutting grooves, squaring corners, etc.
The same type of tool having a bent end is shown at _H_ (Fig. 2)
severing a piece held in the chuck. Work that is held between centers
should not be entirely severed with a parting tool unless a steadyrest
is placed between the tool and faceplate, as otherwise the tool may be
broken by the springing of the work just before the piece is cut in two.
It should be noted that the sides of this tool slope inward back of the
cutting edge to provide clearance when cutting in a narrow groove.
At _I_ a thread tool is shown for cutting a U. S. standard thread. This
thread is the form most commonly used in this country at the present
time. A tool for cutting a square thread is shown at _J_. This is shaped
very much like a parting tool except that the cutting end is inclined
slightly to correspond with the helix angle of the thread, as explained
in Chapter IV, which contains descriptions of different thread forms and
methods of cutting them. Internal thread tools are shown at _K_ and _L_
for cutting U. S. standard and square threads in holes. It will be seen
that these tools are somewhat like boring tools excepting the ends which
are shaped to correspond with the thread which they are intended to cut.
[Illustration: Fig. 3. Turning Tool with Inserted Cutter]
A tool for turning brass is shown at _M_. Brass tools intended for
general work are drawn out quite thin and they are given a narrow
rounded point. The top of the brass tool is usually ground flat or
without slope as otherwise it tends to gouge into the work, especially
if the latter is at all flexible. The end of a brass tool is sometimes
ground with a straight cutting edge for turning large rigid work, such
as brass pump linings, etc., so that a coarse feed can be used without
leaving a rough surface. The tools at _N_ and _O_ are for boring or
finishing drilled or cored holes. Two sizes are shown, which are
intended for small and large holes, respectively.
The different tools referred to in the foregoing might be called the
standard types because they are the ones generally used, and as Fig. 2
indicates, they make it possible to turn an almost endless variety of
forms. Occasionally some special form of tool is needed for doing odd
jobs, having, perhaps, an end bent differently or a cutting edge shaped
to some particular form. Tools of the latter type, which are known as
"form tools," are sometimes used for finishing surfaces that are either
convex, concave, or irregular in shape. The cutting edges of these tools
are carefully filed or ground to the required shape, and the form given
the tool is reproduced in the part turned. Ornamental or other irregular
surfaces can be finished very neatly by the use of such tools. It is
very difficult, of course, to turn convex or concave surfaces with a
regular tool; in fact, it would not be possible to form a true spherical
surface, for instance, without special equipment, because the tool could
not be moved along a true curve by simply using the longitudinal and
cross feeds. Form tools should be sharpened by grinding entirely on the
top surface, as any grinding on the end or flank would alter the shape
of the tool.
[Illustration: Fig. 4. Heavy Inserted-cutter Turning Tool]
=Tool-holders with Inserted Cutters.=--All of the tools shown in Fig. 1
are forged from the bar, and when the cutting ends have been ground down
considerably it is necessary to forge a new end. To eliminate the
expense of this continual dressing of tools and also to effect a great
reduction in the amount of tool steel required, tool-holders having
small inserted cutters are used in many shops. A tool-holder of this
type, for outside turning, is shown in Fig. 3. The cutter _C_ is held in
a fixed position by the set-screw shown, and it is sharpened,
principally, by grinding the end, except when it is desired to give the
top of the cutter a different slope from that due to its angular
position. Another inserted-cutter turning tool is shown in Fig. 4, which
is a heavy type intended for roughing. The cutter in this case has teeth
on the rear side engaging with corresponding teeth cut in the clamping
block which is tightened by a set-screw on the side opposite that shown.
With this arrangement, the cutter can be adjusted upward as the top is
ground away.
[Illustration: Fig. 5. Parting Tool with Inserted Blade]
[Illustration: Fig. 6. Boring Tool with Inserted Cutter and Adjustable
Bar]
A parting tool of the inserted blade type is shown in Fig. 5. The blade
_B_ is clamped by screw _S_ and also by the spring of the holder when
the latter is clamped in the toolpost. The blade can, of course, be
moved outward when necessary. Fig. 6 shows a boring tool consisting of a
holder _H_, a bar _B_ that can be clamped in any position, and an
inserted cutter _C_. With this type of boring tool, the bar can be
extended beyond the holder just far enough to reach through the hole to
be bored, which makes the tool very rigid. A thread tool of the holder
type is shown in Fig. 7. The angular edge of the cutter _C_ is
accurately ground by the manufacturers, so that the tool is sharpened
by simply grinding it flat on the top. As the top is ground away, the
cutter is raised by turning screw _S_, which can also be used for
setting the tool to the proper height.
=The Position of Turning Tools.=--The production of accurate lathe work
depends partly on the condition of the lathe used and also on the care
and judgment exercised by the man operating it. Even though a lathe is
properly adjusted and in good condition otherwise, errors are often made
which are due to other causes which should be carefully avoided. If the
turning tool is clamped so that the cutting end extends too far from the
supporting block, the downward spring of the tool, owing to the thrust
of the cut, sometimes results in spoiled work, especially when an
attempt is made to turn close to the finished size by taking a heavy
roughing cut. Suppose the end of a cylindrical part is first reduced for
a short distance by taking several trial cuts until the diameter _d_,
Fig. 8, is slightly above the finished size and the power feed is then
engaged. When the tool begins to take the full depth _e_ of the cut, the
point, which ordinarily would be set a little above the center, tends to
spring downward into the work, and if there were considerable springing
action, the part would probably be turned below the finished size, the
increased reduction beginning at the point where the full cut started.
[Illustration: Fig. 7. Threading Tool]
This springing action, as far as the tool is concerned, can be
practically eliminated by locating the tool so that the distance _A_
between the tool-block and cutting end, or the "overhang," is as short
as possible. Even though the tool has little overhang it may tilt
downward because the toolslide is loose on its ways, and for this reason
the slide should have a snug adjustment that will permit an easy
movement without unnecessary play. The toolslides of all lathes are
provided with gibs which can be adjusted by screws to compensate for
wear, or to secure a more rigid bearing.
[Illustration: Fig. 8. To avoid springing, Overhang A of Tool should not
be Excessive]
When roughing cuts are to be taken, the tool should be located so that
any change in its position which might be caused by the pressure of the
cut will not spoil the work. This point is illustrated at _A_ in Fig. 9.
Suppose the end of a rod has been reduced by taking a number of trial
cuts, until it is 1/32 inch above the finished size. If the power feed
is then engaged with the tool clamped in an oblique position, as shown,
when the full cut is encountered at _c_, the tool, unless very tightly
clamped, may be shifted backward by the lateral thrust of the cut, as
indicated by the dotted lines. The point will then begin turning smaller
than the finished size and the work will be spoiled. To prevent any
change of position, it is good practice, especially when roughing, to
clamp the tool square with the surface being turned, or in other words,
at right angles to its direction of movement. Occasionally, however,
there is a decided advantage in having the tool set at an angle. For
example, if it is held about as shown at _B_, when turning the flange
casting _C_, the surfaces _s_ and _s_{1}_ can be finished without
changing the tool's position. Cylindrical and radial surfaces are often
turned in this way in order to avoid shifting the tool, especially when
machining parts in quantity.
=Tool Grinding.=--In the grinding of lathe tools there are three things
of importance to be considered: First, the cutting edge of the tool (as
viewed from the top) needs to be given a certain shape; second, there
must be a sufficient amount of clearance for the cutting edge; and
third, tools, with certain exceptions, are ground with a backward slope
or a side slope, or with a combination of these two slopes on that part
against which the chip bears when the tool is in use.
[Illustration: Fig. 9. (A) The Way in which Tool is sometimes displaced
by Thrust of Cut, when set at an Angle. (B) Tool Set for Finishing both
Cylindrical and Radial Surfaces]
In Fig. 10 a few of the different types of tools which are used in
connection with lathe work are shown. This illustration also indicates
the meaning of the various terms used in tool grinding. As shown, the
clearance of the tool is represented by the angle [alpha], the back
slope is represented by the angle [beta], and the side slope by the
angle [gamma]. The angle [delta] for a tool without side slope is known
as the lip angle or the angle of keenness. When, however, the tool has
both back and side slopes, this lip angle would more properly be the
angle between the flank _f_ and the top of the tool, measured diagonally
along a line _z--z_. It will be seen that the lines _A--B_ and _A--C_
from which the angles of clearance and back slope are measured are
parallel with the top and sides of the tool shank, respectively. For
lathe tools, however, these lines are not necessarily located in this
way when the tool is in use, as the height of the tool point with
relation to the work center determines the position of these lines, so
that the _effective_ angles of back slope, clearance and keenness are
changed as the tool point is lowered or raised. The way the position of
the tool affects these angles will be explained later.
[Illustration: Fig. 10. Illustration showing the Meaning of Terms used
in Tool Grinding as applied to Tools of Different Types]
While tools must, of necessity, be varied considerably in shape to adapt
them to various purposes, there are certain underlying principles
governing their shape which apply generally; so in what follows we shall
not attempt to explain in detail just what the form of each tool used on
the lathe should be, as it is more important to understand how the
cutting action of the tool and its efficiency is affected when it is
improperly ground. When the principle is understood, the grinding of
tools of various types and shapes is comparatively easy.
[Illustration: Fig. 11. Plan View of Lathe Turning and Threading Tools]
=Shape or Contour of Cutting Edge.=--In the first place we shall
consider the shape or contour of the cutting edge of the tool as viewed
from the top, and then take up the question of clearance and slope, the
different elements being considered separately to avoid confusion. The
contour of the cutting edge depends primarily upon the purpose for which
the tool is intended. For example, the tool _A_, in Fig. 11, where a
plan view of a number of different lathe tools is shown, has a very
different shape from that of, say, tool _D_, as the first tool is used
for rough turning, while tool _D_ is intended for cutting grooves or
severing a turned part. Similarly, tool _E_ is V-shaped because it is
used for cutting V-threads. Tools _A_, _B_ and _C_, however, are regular
turning tools; that is, they are all intended for turning plain
cylindrical surfaces, but the contour of the cutting edges varies
considerably, as shown. In this case it is the characteristics of the
work and the cut that are the factors which determine the shape. To
illustrate, tool _A_ is of a shape suitable for rough-turning large and
rigid work, while tool _B_ is adapted for smaller and more flexible
parts. The first tool is well shaped for roughing because experiments
have shown that a cutting edge of a large radius is capable of higher
cutting speed than could be used with a tool like _B_, which has a
smaller point. This increase in the cutting speed is due to the fact
that the tool _A_ removes a thinner chip for a given feed than tool _B_;
therefore, the speed may be increased without injuring the cutting edge
to the same extent. If, however, tool _A_ were to be used for turning a
long and flexible part, chattering might result; consequently, a tool
_B_ having a point with a smaller radius would be preferable, if not
absolutely necessary.
The character of the work also affects the shape of tools. The tool
shown at _C_ is used for taking light finishing cuts with a wide feed.
Obviously, if the straight or flat part of the cutting edge is in line
with the travel of the tool, the cut will be smooth and free from
ridges, even though the feed is coarse, and by using a coarse feed the
cut is taken in less time; but such a tool cannot be used on work that
is not rigid, as chattering would result. Therefore, a smaller cutting
point and a reduced feed would have to be employed. Tools with broad
flat cutting edges and coarse feeds are often used for taking finishing
cuts in cast iron, as this metal offers less resistance to cutting than
steel, and is less conducive to chattering.
The shape of a tool (as viewed from the top) which is intended for a
more specific purpose than regular turning, can be largely determined by
simply considering the tool under working conditions. This point may be
illustrated by the parting tool _D_ which, as previously stated, is used
for cutting grooves, squaring corners, etc. Evidently this tool should
be widest at the cutting edge; that is, the sides _d_ should have a
slight amount of clearance so that they will not bind as the tool is fed
into a groove. As the tool at _E_ is for cutting a V-thread, the angle
[alpha] between its cutting edges must equal the angle between the sides
of a V-thread, or 60 degrees. The tool illustrated at _F_ is for cutting
inside square threads. In this case the width _w_ should be made equal
to one-half the pitch of the thread (or slightly greater to provide
clearance for the screw), and the sides should be given a slight amount
of side clearance, the same as with the parting tool _D_. So we see that
the outline of the tool, as viewed from the top, must conform to and be
governed by its use.
=Direction of Top Slope for Turning Tools.=--Aside from the question of
the shape of the cutting edge as viewed from the top, there remains to
be determined the amount of clearance that the tool shall have, and also
the slope (and its direction) of the top of the tool. By the top is
meant that surface against which the chip bears while it is being
severed. It may be stated, in a general way, that the direction in which
the top of the tool should slope should be away from what is to be the
_working part_ of the cutting edge. For example, the working edge of a
roughing tool _A_ (Fig. 11), which is used for heavy cuts, would be,
practically speaking, between points _a_ and _b_, or, in other words,
most of the work would be done by this part of the cutting edge;
therefore the top should slope back from this part of the edge.
Obviously, a tool ground in this way will have both a back and a side
slope.
When most of the work is done on the point or nose of the tool, as, for
example, with the lathe finishing tool _C_ which takes light cuts, the
slope should be straight back from the point or cutting edge _a--b_. As
the side tool shown in Fig. 10 does its cutting along the edge _a--b_,
the top is given a slope back from this edge as shown in the end view.
This point should be remembered, for when the top slopes in the right
direction, less power is required for cutting. Tools for certain classes
of work, such as thread tools, or those for turning brass or chilled
iron, are ground flat on top, that is, without back or side slope.
=Clearance for the Cutting Edge.=--In order that the cutting edge may
work without interference, it must have clearance; that is, the flank
_f_ (Fig. 10) must be ground to a certain angle [alpha] so that it will
not rub against the work and prevent the cutting edge from entering the
metal. This clearance should be just enough to permit the tool to cut
freely. A clearance angle of eight or ten degrees is about right for
lathe turning tools.
The back slope of a tool is measured from a line _A--B_ which is
parallel to the shank, and the clearance angle, from a line _A--C_ at
right angles to line _A--B_. These lines do not, however, always occupy
this position with relation to the tool shank when the tool is in use.
As shown to the left in Fig. 12, the base line _A--B_ for a turning
tool in use intersects with the point of the tool and center of the
work, while the line _A--C_ remains at right angles to the first. It
will be seen, then, that by raising the tool, as shown to the right, the
_effective_ clearance angle [alpha] will be diminished, whereas lowering
it, as shown by the dotted lines, will have the opposite effect.
A turning tool for brass or other soft metal, particularly where
considerable hand manipulation is required, could advantageously have a
clearance of twelve or fourteen degrees, as it would then be easier to
feed the tool into the metal; but, generally speaking, the clearance for
turning tools should be just enough to permit them to cut freely.
Excessive clearance weakens the cutting edge and may cause it to crumble
under the pressure of the cut.
[Illustration: Fig. 12. Illustrations showing how Effective Angles of
Slope and Clearance change as Tool is raised or lowered]
=Angle of Tool-point and Amount of Top Slope.=--The lip angle or the
angle of keenness [delta] (Fig. 10) is another important consideration
in connection with tool grinding, for it is upon this angle that the
efficiency of the tool largely depends. By referring to the illustration
it will be seen that this angle is governed by the clearance and the
slope [beta], and as the clearance remains practically the same, it is
the slope which is varied to meet different conditions. Now, the amount
of slope a tool should have depends on the work for which it is
intended. If, for example, a turning tool is to be used for roughing
medium or soft steel, it should have a back slope of about eight degrees
and a side slope ranging from fourteen to twenty degrees, while a tool
for cutting very hard steel should have a back slope of about five
degrees and a side slope of nine degrees.
[Illustration: Fig. 13. (A) Blunt Tool for Turning Hard Steel. (B)
Tool-point Ground to give Keenness]
The reason for decreasing the slope and thus increasing the lip angle
for harder metals is to give the necessary increased strength to the
cutting edge to prevent it from crumbling under the pressure of the cut.
The tool illustrated at _A_, Fig. 13, is much stronger than it would be
if ground as shown at _B_, as the former is more blunt. If a tool ground
as at _A_, however, were used for cutting very soft steel, there would
be a greater chip pressure on the top and, consequently, a greater
resistance to cutting, than if a keener tool had been employed;
furthermore the cutting speed would have to be lower, which is of even
greater importance than the chip pressure; therefore, the lip angle, as
a general rule, should be as small as possible without weakening the
tool so that it cannot do the required work. In order to secure a strong
and well-supported cutting edge, tools used for turning very hard metal,
such as chilled rolls, etc., are ground with practically no slope and
with very little clearance. Brass tools, while given considerable
clearance, as previously stated, are ground flat on top or without
slope; this is not done, however, to give strength to the cutting edge,
but rather to prevent the tool from gouging into the work, which it is
likely to do if the part being turned is at all flexible and the tool
has top slope.
Experiments conducted by Mr. F. W. Taylor to determine the most
efficient form for lathe roughing tools showed that the nearer the lip
angle approached sixty-one degrees, the higher the cutting speed. This,
however, does not apply to tools for turning cast iron, as the latter
will work more efficiently with a lip angle of about sixty-eight
degrees. This is doubtless because the chip pressure, when turning cast
iron, comes closer to the cutting edge which should, therefore, be more
blunt to withstand the abrasive action and heat. Of course, the
foregoing remarks concerning lip angles apply more particularly to tools
used for roughing.
[Illustration: Fig. 14. Grinding the Top and Flank of a Turning Tool]
=Grinding a Lathe Tool.=--The way a turning tool is held while the top
surface is being ground is shown to the left in Fig. 14. By inclining
the tool with the wheel face, it will be seen that both the back and
side slopes may be ground at the same time. When grinding the flank of
the tool it should be held on the tool-rest of the emery wheel or
grindstone, as shown by the view to the right. In order to form a curved
cutting edge, the tool is turned about the face of the stone while it is
being ground. This rotary movement can be effected by supporting the
inner end of the tool with one hand while the shank is moved to and fro
with the other.
Often a tool which has been ground properly in the first place is
greatly misshapen after it has been sharpened a few times. This is
usually the result of attempts on the part of the workman to re-sharpen
it hurriedly; for example, it is easier to secure a sharp edge on the
turning tool shown to the left in Fig. 12, by grinding the flank as
indicated by the dotted line, than by grinding the entire flank. The
clearance is, however, reduced and the lip angle changed.
There is great danger when grinding a tool of burning it or drawing the
temper from the fine cutting edge, and, aside from the actual shape of
the cutting end, this is the most important point in connection with
tool grinding. If a tool is pressed hard against an emery or other
abrasive wheel, even though the latter has a copious supply of water,
the temper will sometimes be drawn. When grinding a flat surface, to
avoid burning, the tool should frequently be withdrawn from the stone so
that the cooling water (a copious supply of which should be provided)
can reach the surface being ground. A moderate pressure should also be
applied, as it is better to spend an extra minute or two in grinding
than to ruin the tool by burning, in an attempt to sharpen it quickly.
Of course, what has been said about burning applies more particularly to
carbon steel, but even self-hardening steels are not improved by being
over-heated at the stone. In some shops, tools are ground to the
theoretically correct shape in special machines instead of by hand. The
sharpened tools are then kept in the tool-room and are given out as they
are needed.
=Cutting Speeds and Feeds.=--The term cutting speed as applied to
turning operations is the speed in feet per minute of the surface being
turned, or, practically speaking, it is equivalent to the length of a
chip, in feet, which would be turned in one minute. The term cutting
speed should not be confused with revolutions per minute, because the
cutting speed depends not only upon the speed of the work but also upon
its diameter. The feed of a tool is the amount it moves across the
surface being turned for each revolution; that is, when turning a
cylindrical piece, the feed is the amount that the tool moves sidewise
for each revolution of the work. Evidently the time required for turning
is governed largely by the cutting speed, the feed, and the depth of the
cut; therefore, these elements should be carefully considered.
Cutting Speeds and Feeds for Turning Tools[1]
+---------------------------------++---------------------------------+
| Steel--Standard 7/8-inch Tool ||Cast Iron--Standard 7/8-inch Tool|
+-----+-----+---------------------++-----+-----+---------------------+
| | | Speed in Feet per || | | Speed in Feet per |
|Depth|Feed | Minute for a Tool ||Depth|Feed | Minute for a Tool |
| of | in | which is to last || of | in | which is to last |
| Cut | In- | 1-1/2 Hour before || Cut | In- | 1-1/2 Hour before |
| in |ches | Re-grinding || in |ches | Re-grinding |
| In- | +------+------+-------++ In- | +-------+------+------+
|ches | | Soft |Medium| Hard ||ches | | Soft |Medium| Hard |
| | |Steel |Steel |Steel || | | Cast | Cast | Cast |
| | | | | || | | Iron | Iron | Iron |
+-----+-----+------+------+-------++-----+-----+-------+------+------+
| | 1/64| 476 | 238 | 108 || | 1/16| 122 | 61.2 | 35.7 |
|3/32 | 1/32| 325 | 162 | 73.8 ||3/32 | 1/8 | 86.4 | 43.2 | 25.2 |
| | 1/16| 222 | 111 | 50.4 || | 3/16| 70.1 | 35.1 | 20.5 |
| | 3/32| 177 | 88.4| 40.2 ++-----+-----+-------+------+------+
+-----+-----+------+------+-------++ | 1/32| 156 | 77.8 | 45.4 |
| | 1/64| 420 | 210 | 95.5 || 1/8 | 1/16| 112 | 56.2 | 32.8 |
| 1/8 | 1/32| 286 | 143 | 65.0 || | 1/8 | 79.3 | 39.7 | 23.2 |
| | 1/16| 195 | 97.6| 44.4 || | 3/16| 64.3 | 32.2 | 18.8 |
| | 1/8 | 133 | 66.4| 30.2 ++-----+-----+-------+------+------+
+-----+-----+------+------+-------++ | 1/32| 137 | 68.6 | 40.1 |
| | 1/64| 352 | 176 | 80.0 ||3/16 | 1/16| 99.4 | 49.7 | 29.0 |
|3/16 | 1/32| 240 | 120 | 54.5 || | 1/8 | 70.1 | 35.0 | 20.5 |
| | 1/16| 164 | 82 | 37.3 || | 3/16| 56.8 | 28.4 | 16.6 |
| | 1/8 | 112 | 56 | 25.5 ++-----+-----+-------+------+------+
+-----+-----+------+------+-------++ | 1/32| 126 | 62.9 | 36.7 |
| | 1/64| 312 | 156 | 70.9 || 1/4 | 1/16| 90.8 | 45.4 | 26.5 |
| 1/4 | 1/32| 213 | 107 | 48.4 || | 1/8 | 64.1 | 32.0 | 18.7 |
| | 1/16| 145 | 72.6| 33.0 || | 3/16| 52 | 26.0 | 15.2 |
| | 3/32| 116 | 58.1| 26.4 ++-----+-----+-------+------+------+
+-----+-----+------+------+-------++ | 1/32| 111 | 55.4 | 32.3 |
| | 1/64| 264 | 132 | 60.0 || 3/8 | 1/16| 80 | 40.0 | 23.4 |
| 3/8 | 1/32| 180 | 90.2| 41.0 || | 1/8 | 56.4 | 28.2 | 16.5 |
| | 1/16| 122 | 61.1| 27.8 ++-----+-----+-------+------+------+
+-----+-----+------+------+-------++ | 1/32| 104 | 52.1 | 30.4 |
| 1/2 | 1/64| 237 | 118 | 53.8 || 1/2 | 1/16| 75.2 | 37.6 | 22.0 |
| | 1/32| 162 | 80.8| 36.7 || | 1/8 | 43.1 | 21.6 | 12.6 |
+-----+-----+------+------+-------++-----+-----+-------+------+------+
|Steel--Standard 5/8-inch Tool ||Cast Iron--Standard 5/8-inch Tool|
+-----+-----+------+------+-------++-----+-----+-------+------+------+
|Depth|Feed | Soft |Medium| Hard ||Depth|Feed | Soft |Medium| Hard |
| of | |Steel |Steel |Steel || of | | Cast | Cast | Cast |
| Cut | | | | || Cut | | Iron | Iron | Iron |
+-----+-----+------+------+-------++-----+-----+-------+------+------+
| | 1/64| 548 | 274 | 125 || | 1/32| 160 | 80.0 | 46.6 |
|1/16 | 1/32| 358 | 179 | 81.6 ||3/32 | 1/16| 110 | 55.0 | 32.2 |
| | 1/16| 235 | 117 | 53.3 || | 1/8 | 75.4 | 37.7 | 22.0 |
+-----+-----+------+------+-------++-----+-----+-------+------+------+
| | 1/64| 467 | 234 | 106 || | 1/32| 148 | 74.0 | 43.3 |
|3/32 | 1/32| 306 | 153 | 69.5 || 1/8 | 1/16| 104 | 51.8 | 32.0 |
| | 1/16| 200 | 100 | 45.5 || | 1/8 | 69.6 | 34.8 | 20.3 |
| | 3/32| 156 | 78 | 35.5 ++-----+-----+-------+------+------+
+-----+-----+------+------+-------++ | 1/64| 183 | 91.6 | 68.0 |
| | 1/64| 417 | 209 | 94.8 ||3/16 | 1/32| 135 | 67.5 | 39.4 |
| 1/8 | 1/32| 273 | 136 | 62.0 || | 1/16| 94 | 47.0 | 27.4 |
| | 1/16| 179 | 89.3| 40.6 || | 1/8 | 64.3 | 32.2 | 18.8 |
| | 3/32| 140 | 69.8| 31.7 ++-----+-----+-------+------+------+
+-----+-----+------+------+-------++ | 1/64| 171 | 85.7 | 50.1 |
| | 1/64| 362 | 181 | 82.2 || 1/4 | 1/32| 126 | 63.2 | 36.9 |
|3/16 | 1/32| 236 | 118 | 53.8 || | 1/16| 87.8 | 43.9 | 25.6 |
| | 1/16| 155 | 77.4| 35.2 || | 3/32| 70.4 | 35.2 | 20.6 |
+-----+-----+------+------+-------++-----+-----+-------+------+------+
| 1/4 | 1/64| 328 | 164 | 74.5 || 3/8 | 1/64| 156 | 77.8 | 45.4 |
| | 1/32| 215 | 107 | 48.8 || | 1/32| 116 | 57.8 | 33.8 |
+-----+-----+------+------+-------++ | 1/16| 79.7 | 39.9 | 23.3 |
| 3/8 | 1/64| 286 | 143 | 65.0 || | | | | |
+-----+-----+------+------+-------++-----+-----+-------+------+------+
[1] Cutting speeds for tools of a good grade of high-speed steel,
properly ground and heat-treated.--From MACHINERY'S HANDBOOK.
=Average Cutting Speeds for Turning.=--The cutting speed is governed
principally by the hardness of the metal to be turned; the kind of steel
of which the turning tool is made; the shape of the tool and its
heat-treatment; the feed and depth of cut; whether or not a cooling
lubricant is used on the tool; the power of the lathe and also its
construction; hence it is impossible to give any definite rule for
determining either the speed, feed, or depth of cut, because these must
be varied to suit existing conditions. A general idea of the speeds used
in ordinary machine shop practice may be obtained from the following
figures:
Ordinary machine steel is generally turned at a speed varying between 45
and 65 feet per minute. For ordinary gray cast iron, the speed usually
varies from 40 to 50 feet per minute; for annealed tool steel, from 25
to 35 feet per minute; for soft yellow brass, from 150 to 200 feet per
minute; for hard bronze, from 35 to 80 feet per minute, the speed
depending upon the composition of the alloy. While these speeds
correspond closely to general practice, they can be exceeded for many
machining operations.
The most economical speeds for a given feed and depth of cut, as
determined by the experiments conducted by Mr. F. W. Taylor, are given
in the table, "Cutting Speeds and Feeds for Turning Tools." The speeds
given in this table represent results obtained with tools made of a good
grade of high-speed steel properly heat-treated and correctly ground. It
will be noted that the cutting speed is much slower for cast iron than
for steel. Cast iron is cut with less pressure or resistance than soft
steel, but the slower speed required for cast iron is probably due to
the fact that the pressure of the chip is concentrated closer to the
cutting edge, combined with the fact that cast iron wears the tool
faster than steel. The speeds given are higher than those ordinarily
used, and, in many cases, a slower rate would be necessary to prevent
chattering or because of some other limiting condition.
=Factors which limit the Cutting Speed.=--It is the durability of the
turning tool or the length of time that it will turn effectively
without grinding, that limits the cutting speed; and the hardness of the
metal being turned combined with the quality of the tool are the two
factors which largely govern the time that a tool can be used before
grinding is necessary. The cutting speed for very soft steel or cast
iron can be three or four times faster than the speed for hard steel or
hard castings, but whether the material is hard or soft, the kind and
quality of the tool used must also be considered, as the speed for a
tool made of ordinary carbon steel will have to be much slower than for
a tool made of modern "high-speed" steel.
When the cutting speed is too high, even though high-speed steel is
used, the point of the tool is softened to such an extent by the heat
resulting from the pressure and friction of the chip, that the cutting
edge is ruined in too short a time. On the other hand, when the speed is
too slow, the heat generated is so slight as to have little effect and
the tool point is dulled by being slowly worn or ground away by the
action of the chip. While a tool operating at such a low speed can be
used a comparatively long time without re-sharpening, this advantage is
more than offset by the fact that too much time is required for removing
a given amount of metal when the work is revolving so slowly.
Generally speaking, the speed should be such that a fair amount of work
can be done before the tool requires re-grinding. Evidently, it would
not pay to grind a tool every few minutes in order to maintain a high
cutting speed; neither would it be economical to use a very slow speed
and waste considerable time in turning, just to save the few minutes
required for grinding. For example, if a number of roughing cuts had to
be taken over a heavy rod or shaft, time might be saved by running at
such a speed that the tool would have to be sharpened (or be replaced by
a tool previously sharpened) when it had traversed half-way across the
work; that is, the time required for sharpening or changing the tool
would be short as compared with the gain effected by the higher work
speed. On the other hand, it might be more economical to run a little
slower and take a continuous cut across the work with one tool.
The experiments of Mr. Taylor led to the conclusion that, as a rule, it
is not economical to use roughing tools at a speed so slow as to cause
them to last more than 1-1/2 hour without being re-ground; hence the
speeds given in the table previously referred to are based upon this
length of time between grindings. Sometimes the work speed cannot be as
high as the tool will permit, because of the chattering that often
results when the lathe is old and not massive enough to absorb the
vibrations, or when there is unnecessary play in the working parts. The
shape of the tool used also affects the work speed, and as there are so
many things to be considered, the proper cutting speed is best
determined by experiment.
=Rules for Calculating Cutting Speeds.=--The number of revolutions
required to give any desired cutting speed can be found by multiplying
the cutting speed, in feet per minute, by 12 and dividing the product by
the circumference of the work in inches. Expressing this as a formula we
have
_C_ x 12
_R_ = --------
[pi]_d_
in which
_R_ = revolutions per minute;
_C_ = the cutting speed in feet per minute;
[pi] = 3.1416;
_d_ = the diameter in inches.
For example if a cutting speed of 60 feet per minute is wanted and the
diameter of the work is 5 inches, the required speed would be found as
follows:
60 x 12
_R_ = ---------- = 46 revolutions per minute.
3.1416 x 5
If the diameter is simply multiplied by 3 and the fractional part is
omitted, the calculation can easily be made, and the result will be
close enough for practical purposes. In case the cutting speed, for a
given number of revolutions and diameter, is wanted, the following
formula can be used:
_R_[pi]_d_
_C_ = ----------
12
Machinists who operate lathes do not know, ordinarily, what cutting
speeds, in feet per minute, are used for different classes of work, but
are guided entirely by past experience.
=Feed of Tool and Depth of Cut.=--The amount of feed and depth of cut
also vary like the cutting speed, for different conditions. When turning
soft machine steel the feed under ordinary conditions would vary between
1/32 and 1/16 inch per revolution. For turning soft cast iron the feed
might be increased to from 1/16 to 1/8 inch per revolution. These feeds
apply to fairly deep roughing cuts. Coarser feeds might be used in many
cases especially when turning large rigid parts in a powerful lathe. The
depth of a roughing cut in machine steel might vary from 1/8 to 3/8
inch, and in cast iron from 3/16 to 1/2 inch. These figures are intended
simply to give the reader a general idea of feeds and cuts that are
feasible under average conditions.
Ordinarily coarser feeds and a greater depth of cut can be used for cast
iron than for soft steel, because cast iron offers less resistance to
turning, but in any case, with a given depth of cut, metal can be
removed more quickly by using a coarse feed and the necessary slower
speed, than by using a fine feed and the higher speed which is possible
when the feed is reduced. When the turning operation is simply to remove
metal, the feed should be coarse, and the cut as deep as practicable.
Sometimes the cut must be comparatively light, either because the work
is too fragile and springy to withstand the strain of a heavy cut, or
the lathe has not sufficient pulling power. The difficulty with light
slender work is that a heavy cut may cause the part being turned to bend
under the strain, thus causing the tool to gouge in, which would
probably result in spoiling the work. Steadyrests can often be used to
prevent flexible parts from springing, as previously explained, but
there are many kinds of light work to which the steadyrest cannot be
applied to advantage.
The amount of feed to use for a finishing cut might, properly, be either
fine or coarse. Ordinarily, fine feeds are used for finishing steel,
especially if the work is at all flexible, whereas finishing cuts in
cast iron are often accompanied by a coarse feed. Fig. 15 illustrates
the feeds that are often used when turning cast iron. The view to the
left shows a deep roughing cut and the one to the right, a finishing
cut. By using a broad flat cutting edge set parallel to the tool's
travel, and a coarse feed for finishing, a smooth cut can be taken in a
comparatively short time. Castings which are close to the finished size
in the rough can often be finished to advantage by taking a single cut
with a broad tool, provided the work is sufficiently rigid. It is not
always practicable to use these broad tools and coarse feeds, as they
sometimes cause chattering, and when used on steel, a broad tool tends
to gouge or "dig in" unless the part being turned is rigid. Heavy steel
parts, however, are sometimes finished in this way. The modern method of
finishing many steel parts is to simply rough them out in a lathe to
within, say, 1/32 inch of the required diameter and take the finishing
cut in a cylindrical grinding machine.
[Illustration: Fig. 15. Roughing Cut--Light Finishing Cut and Coarse
Feed]