inch. The lead then would equal 1 / 1-1/4 = 0.8 inch, and if the
carriage is moved back 0.8 / 2 = 0.4 inch, the tool will be properly
located for the second cut; but the lock-nuts could not be re-engaged
unless the lead-screw had ten threads per inch, which is finer than the
pitch found on the lead-screws of ordinary engine lathes. However, if
the movement were 0.4 + 0.8 x 2 = 2 inches, the lock-nuts could be
re-engaged regardless of the number of threads per inch on the
lead-screw. The rule then, is as follows:
_Divide the lead of the thread by 2 for a double thread, 3 for a triple
thread, 4 for a quadruple thread, etc., thus obtaining the pitch; then
add the pitch to any multiple of the lead, which will give a movement,
in inches, that will enable the lock-nuts to be re-engaged with the
lead-screw._
Whenever the number obtained by this rule is a whole number, obviously,
the movement can be obtained with a lead-screw of any pitch. If the
number is fractional, the number of threads per inch on the lead-screw
must be divisible by the denominator of the fraction.
To illustrate the application of the foregoing rule, suppose a quadruple
thread is to be cut having 1-1/2 single threads per inch (which would be
the number the lathe would be geared to cut). Then the lead of the
thread = 1 / 1-1/2 = 0.6666 inch and the pitch = 0.6666 / 4 = 0.1666
inch; adding the pitch to twice the lead we have 0.1666 + 2 x 0.6666 =
1.499 inch. Hence, if the carriage is moved 1-1/2 inch (which will
require a lead-screw having an even number of threads per inch), the
tool will be located accurately enough for practical purposes. When the
tool is set in this way, if it does not clear the end of the part being
threaded, the lathe can be turned backward to place the tool in the
proper position.
[Illustration: Fig. 13. Indexing Faceplate used for Multiple Thread
Cutting]
The foregoing rule, as applied to triple threads or those of a higher
number, does not always give the only distance that the carriage can be
moved. To illustrate, in the preceding example the carriage movement
could be equal to 0.499, or what is practically one-half inch, instead
of 1-1/2 inch, and the tool would be properly located. The rule,
however, has the merit of simplicity and can be used in most cases.
Special faceplates are sometimes used for multiple thread cutting, that
enable work to be easily and accurately indexed. One of these is
illustrated in Fig. 13; it consists of two parts _A_ and _B_, part _A_
being free to rotate in relation to _B_ when bolts _C_ are loosened. The
driving pin for the lathe dog is attached to plate _A_. When one groove
of a multiple thread is finished, bolts _C_ are loosened and plate _A_
is turned around an amount corresponding to the type of thread being
cut. The periphery of plate _A_ is graduated in degrees, as shown, and
for a double thread it would be turned one-half revolution or 180
degrees, for a triple thread, 120 degrees, etc. This is a very good
arrangement where multiple thread cutting is done frequently.
[Illustration: Fig. 14. Correct and Incorrect Positions of Tool for
Taper Thread Cutting]
=Taper Threading.=--When a taper thread is to be cut, the tool should be
set square with axis _a--a_ as at _A_, Fig. 14, and not by the tapering
surface as at _B_. If there is a cylindrical part, the tool can be set
as indicated by the dotted lines. All taper threads should be cut by the
use of taper attachments. If the tailstock is set over to get the
required taper, and an ordinary bent-tail dog is used for driving, the
curve of the thread will not be true, or in other words the thread will
not advance at a uniform rate; this is referred to by machinists as a
"drunken thread." This error in the thread is due to the angularity
between the driving dog and the faceplate, which causes the work to be
rotated at a varying velocity. The pitch of a taper thread that is cut
with the tailstock set over will also be slightly finer than the pitch
for which the lathe is geared. The amount of these errors depends upon
the angle of the taper and the distance that the center must be offset.
=Internal Threading.=--Internal threading, or cutting threads in holes,
is an operation performed on work held in the chuck or on a faceplate,
as for boring. The tool used is similar to a boring tool except that the
working end is shaped to conform to the thread to be cut. The method of
procedure, when cutting an internal thread, is similar to that for
outside work, as far as handling the lathe is concerned. The hole to be
threaded is first bored to the root diameter _D_, Fig. 15, of the screw
that is to fit into it. The tool-point (of a tool for a U. S. standard
or V-thread) is then set square by holding a gage _G_ against the true
side of the work and adjusting the point to fit the notch in the gage as
shown. The view to the right shows the tool taking the first cut.
[Illustration: Fig. 15. Method of setting and using Inside Thread Tool]
Very often the size of a threaded hole can be tested by using as a gage
the threaded part that is to fit into it. When making such a test, the
tool is, of course, moved back out of the way. It is rather difficult to
cut an accurate thread in a small hole, especially when the hole is
quite deep, owing to the flexibility of the tool; for this reason
threads are sometimes cut slightly under size with the tool, after which
a tap with its shank end held straight by the tailstock center is run
through the hole. In such a case, the tap should be calipered and the
thread made just small enough with the tool to give the tap a light cut.
Small square-threaded holes are often finished in this way, and if a
number of pieces are to be threaded, the use of a tap makes the holes
uniform in size.
=Stop for Thread Tools.=--When cutting a thread, it is rather difficult
to feed in the tool just the right amount for each successive cut,
because the tool is moved in before it feeds up to the work. A stop is
sometimes used for threading which overcomes this difficulty. This stop
consists of a screw _S_, Fig. 16, which enters the tool slide and passes
through a block _B_ clamped in front of the slide. The hole in the block
through which the stop-screw passes is not threaded, but is large enough
to permit the screw to move freely. When cutting a thread, the tool is
set for the first cut and the screw is adjusted until the head is
against the fixed block. After taking the first cut, the stop-screw is
backed out, say one-half revolution, which allows the tool to be fed in
far enough for a second cut. If this cut is about right for depth, the
screw is again turned about one-half revolution for the next cut and
this is continued for each successive cut until the thread is finished.
By using a stop of this kind, there is no danger of feeding the tool in
too far as is often done when the tool is set by guess. If this form of
stop is used for internal threading, the screw, instead of passing
through the fixed block, is placed in the slide so that the end or head
will come against the stop _B_. This change is made because the tool is
fed outward when cutting an internal thread.
[Illustration: Fig. 16. Cross-slide equipped with Stop for Regulating
Depth of Cut when Threading]
=The Acme Standard Thread.=--The Acme thread is often used, at the
present time, in place of a square thread. The angle between the sides
of the Acme thread is 29 degrees (see Fig. 21) and the depth is made
equal to one-half the pitch plus 0.010 inch to provide clearance and
insure a bearing upon the sides. The thread tool is ordinarily ground to
fit a gage having notches representing different pitches. An improved
form of Acme thread gage is shown in Fig. 17. The tool point is first
ground to the correct angle by fitting it to the 29-degree notch in the
end of the gage, as at _A_. The end is then ground to the proper width
for the pitch to be cut, by testing it, as at _B_. The numbers opposite
the shallow notches for gaging the width represent the number of threads
per inch. With this particular gage, the tool can be set square by
placing edge _D_ against the turned surface to be threaded, and
adjusting the tool until the end is in line with the gage, as at _C_. By
placing the tool in this position, the angle between the side and the
end can also be tested.
[Illustration: Fig. 17. Gage for grinding and setting Acme Thread Tools]
In case it should be necessary to measure the end width of an Acme
thread tool, for a pitch not on the regular gage, this can be done by
using a vernier gear-tooth caliper, as indicated in Fig. 18. If we
assume that the caliper jaws bear on the sides of the tool at a distance
_A_ from the top, equal to 1/4 inch, then the width of the tool point
equals the caliper reading (as shown by the horizontal scale) minus
0.1293 inch. For example, if the caliper reading was 0.315 inch, the
width at the point would equal 0.315 - 0.1293 = 0.1857 inch, assuming that
the sides were ground to the standard angle of 29 degrees. The constant
to be subtracted from the caliper reading equals 2 _A_ tan 14 deg. 30' or,
in this case, 2 x 0.25 x 0.2586 = 0.1293.
[Illustration: Fig. 18. Measuring Width of Acme Thread Tool with Vernier
Gear-tooth Caliper]
=The Whitworth Thread.=--The Whitworth (or British Standard Whitworth)
thread, which is used principally in Great Britain, has an included
angle of 55 degrees, and the threads are rounded at the top and at the
root, as shown in Fig. 23. The shape of the tool used for cutting this
thread is also shown in this illustration. The end is rounded to form
the fillet at the root of the thread, and the round corners on the sides
give the top of the thread the required curvature. Every pitch requires
a different tool, and the cutting end is given the curved form by
milling or hobbing. The hob used for this purpose is accurately threaded
to correspond with the pitch for which the tool is required, and then it
is fluted to form cutting edges, and is hardened. The hob is then used
like a milling cutter for forming the end of the thread tool. The tool
is sharpened by grinding on the top. The method of cutting a Whitworth
thread is, of course, similar to that followed for a U. S. standard or
V-thread, in that the tool is set square with the unthreaded blank and
at the same height as the lathe centers, in order to secure a thread of
the proper form. Care should be taken to turn the blank to the right
diameter so that the top of the thread will be fully rounded when the
screw is the required size.
[Illustration: Fig. 19. United States Standard Thread]
[Illustration: Fig. 20. Standard Sharp V-thread]
[Illustration: Fig. 21. Acme Standard Thread]
[Illustration: Fig. 22. Square Thread]
[Illustration: Fig. 23. Whitworth Standard Thread]
[Illustration: Fig. 24. Standard Worm Thread]
=Worm Threads.=--The standard worm thread has an angle of 29 degrees
between the sides, the same as an Acme thread, but the depth of a worm
thread and the width of the flat at the top and bottom differ from the
Acme standard, as will be seen by comparing Figs. 21 and 24. The whole
depth of the thread equals the linear pitch multiplied by 0.6866, and
the width of the thread tool at the end equals the linear pitch
multiplied by 0.31. Gages notched for threads of different pitch are
ordinarily used when grinding worm thread tools.
When it is necessary to cut multiple-threaded worms of large lead in an
ordinary lathe, difficulty is sometimes experienced because the
lead-screw must be geared to run much faster than the spindle, thus
imposing excessive strains on the gearing. This difficulty is sometimes
overcome by mounting a belt pulley on the lead-screw, beside the change
gear, and connecting it to the countershaft by a belt; the spindle is
then driven through the change gearing from the lead-screw, instead of
_vice versa_.
=Coarse Threading Attachment.=--To avoid the difficulties connected with
cutting threads of large lead, some lathes are equipped with a coarse
screw-cutting attachment. The arrangement of this attachment, as made by
the Bradford Machine Tool Co., is as follows: On the usual reversing
shaft, and inside of the headstock, there is a sliding double gear, so
arranged as to be engaged with either the usual gear on the spindle, or
with a small pinion at the end of the cone. The gears are so
proportioned that the ratio of the two engagements is as 10 to 1; that
is, when engaged with the cone gear (the back-gears being thrown in) the
mating gear will make ten revolutions to one of the spindle, so that
when the lathe is ordinarily geared to cut one thread per inch, it will,
when driven by the cone pinion, cut one thread in ten inches. This
construction dispenses with the extra strain on the reverse gears due to
moving the carriage at the rapid rate that would be necessary for such a
large lead, when not using an attachment. These attachments are not only
extensively used for the cutting of coarse screws but for cutting oil
grooves on cylindrical parts.
When cutting a thread of large lead or "steep pitch," the top of the
thread tool should be ground so that it is at right angles to the
thread; then the thread groove will be cut to the same width as the
tool.
=Testing the Size of a Thread.=--When the thread tool has been fed in
far enough to form a complete thread, the screw is then tested for size.
If we assume that a bolt is being threaded for a standard nut, it would
be removed from the lathe and the test made by screwing a nut on the
end. If the thread were too large, the nut might screw on very tightly
or not at all; in either case, the work would again be placed in the
lathe and a light cut taken over it to reduce the thread to the proper
size. When replacing a threaded part between the centers, it should be
put back in the original position, that is, with the "tail" of the
driving dog in the same slot of the faceplate it previously occupied.
[Illustration: Fig. 25. Testing Diameter of Thread with Calipers and
Micrometer]
As it is difficult to tell just when a thread is cut to the exact size,
special thread calipers having wedge-shaped ends are sometimes used for
measuring the diameter of a V-thread or a U. S. standard thread, at the
bottom of the grooves or the root diameter, as shown at _A_ in Fig. 25.
These calipers can be set from a tap corresponding to the size of the
thread being cut, or from a previously threaded piece of the right
size.
=The Thread Micrometer.=--Another form of caliper for testing threads is
shown at _B_. This is one of the micrometer type and is intended for
very accurate work. The spindle of this micrometer has a conical end and
the "anvil" is V-shaped, and these ends bear on the sides of the thread
or the surfaces which form the bearing when the screw is inserted in a
nut or threaded hole. The cone-shaped point is slightly rounded so that
it will not bear in the bottom of the thread. There is also sufficient
clearance at the bottom of the V-shaped anvil to prevent it from bearing
on top of the thread. The diameter as indicated by this micrometer is
the "pitch diameter" of the thread and is equal to the outside diameter
minus the depth of one thread. This depth may be determined as follows:
Depth of a V-thread = 0.866 / No. of threads per inch;
Depth of a U. S. standard thread = 0.6495 / No. of threads per inch;
Depth of Whitworth thread = 0.6403 / No. of threads per inch.
The movable point measures all pitches, but the fixed anvil is limited
in its capacity, for if made large enough to measure a thread of, say,
1/4-inch pitch, it would be too wide at the top to measure a thread of
1/24-inch pitch, hence each caliper is limited in the range of threads
that the anvil can measure. When measuring the "angle diameter" of a
thread, the micrometer should be passed back and forth across the
thread, in order to make sure that the largest dimension or the actual
diameter is being measured. If the micrometer is placed over what seems
to be the center of the screw and the reading is taken by simply
adjusting in the anvil or point against the thread, without moving the
micrometer back and forth across it, an incorrect reading may be
obtained.
If standard threaded reference gages are available, the size of the
thread being cut can be tested by comparing it with the gage.
Micrometers having small spherical measuring ends (see sketch _A_, Fig.
26) are sometimes used for this purpose. The ball points are small
enough to bear against the sides of the thread and the diameter, as
compared with the reference gage, can be determined with great
accuracy.
[Illustration: Fig. 26. (A) Testing Size of Thread with Ball-point
Micrometer. (B) Testing Size of V-thread by the Three-wire System. (C)
Testing the Size of a U. S. Standard Thread]
=Three-wire System of Measuring Threads.=--A method of measuring threads
by using an ordinary micrometer and three wires of equal diameter is
illustrated at _B_ and _C_, Fig. 26. Two wires are placed between the
threads on one side and one on the opposite side of the screw. The
dimension _M_ over the wires is then measured with an ordinary
micrometer. When the thread is cut to a standard size, the dimension _M_
for different threads is as follows:
For a U. S. standard thread:
_m_ = _d_ - 1.5155_p_ + 3_w_
For a sharp V-thread:
_m_ = _d_ - 1.732_p_ + 3_w_
For a Whitworth standard thread:
_m_ = _d_ - 1.6008_p_ + 3.1657_w_
In these formulas, _d_ = standard outside diameter of screw; _m_ =
measurement over wires; _w_ = diameter of wires; _p_ = pitch of thread =
1 / number of threads per inch.
To illustrate the use of the formula for the U. S. standard thread, let
us assume that a screw having 6 threads per inch (1/6-inch pitch) is to
be cut to a diameter of 1-1/2 inch, and that wires 0.140 inch diameter
are to be used in conjunction with a micrometer for measurement. Then
the micrometer reading _m_ should be
1-1/2 - 1.5155 x 1/6 + 3 x 0.140 = 1.6674 inch
If the micrometer reading were 1.670 inch, it would indicate that the
pitch diameter of the screw was too large, the error being equal to
difference between 1.667 and the actual reading.
[Illustration: Fig. 27. Rivett-Dock Circular Threading Tool in Working
Position]
=Rivett-Dock Threading Tool.=--A special form of thread tool, which
overcomes a number of disadvantages common to an ordinary single-point
thread tool, is shown in Fig. 27. This tool has a circular-shaped cutter
_C_, having ten teeth around its circumference, which, beginning with
tooth No. 1, gradually increase in height, cutter No. 2 being higher
than No. 1, etc. This cutter is mounted on a slide _S_, that is fitted
to the frame _F_, and can be moved in or out by lever _L_. The hub of
this lever has an eccentric stud which moves slide _S_ and locks it when
in the forward or cutting position. The action of the lever in moving
the slide engages the cutter with pawl _P_, thus rotating the cutter one
tooth at a time and presenting a different tooth to the work for each
movement of the lever. When the slide is moved forward, the heel or
underside of the tooth which is in the working position rests on a stop
that takes the thrust of the cut.
When the tool is in use, it is mounted on the tool-block of the lathe as
shown in the illustration. The cutter is set for height by placing a
tooth in the working position and setting the top level with the lathe
center. The cutter is also set square with the work by using an ordinary
square, and it is tilted slightly from the vertical to correspond with
the angle of the thread to be cut, by adjusting frame _F_. At first a
light cut is taken with lever _L_ moved forward and tooth No. 1 on the
stop. After this cut is completed, the lever is reversed which rotates
the cutter one tooth, and the return movement places tooth No. 2 in the
working position. This operation is repeated until the tenth tooth
finishes the thread. It is often necessary, when using a single-point
thread tool, to re-sharpen it before taking the finishing cut, but with
a circular tool this is not necessary, for by using the different teeth
successively, the last tooth, which only takes finishing cuts, is kept
in good condition.
=Cutting Screws to Compensate for Shrinkage.=--Some tool steels are
liable to shrink more or less when they are hardened; consequently if a
very accurate hardened screw is required, it is sometimes cut so that
the pitch is slightly greater than standard, to compensate for the
shrinkage due to the hardening operation. As the amount of contraction
incident to hardening is very little, it is not practicable to use
change gears that will give the exact pitch required. A well-known
method of obtaining this increase of pitch is by the use of a taper
attachment.
For example, suppose a tap having 8 threads per inch is to be threaded,
and, owing to the contraction of the steel, the pitch must be 0.12502
inch instead of 0.125 inch. The lathe is geared to cut 8 threads per
inch or 0.125 inch pitch, and then the taper attachment is set to an
angle _a_, Fig. 28, the cosine of which equals 0.125/0.12502; that is,
the cosine of angle _a_ equals _the pitch required after hardening_,
divided by the _pitch necessary to compensate for shrinkage_. The angle
is then found by referring to a table of cosines. The tap blank is also
set to the same angle a by adjusting the tailstock center, thus locating
the axis of the work parallel with the slide of the taper attachment.
When the carriage moves a distance _x_, the tool point will have moved a
greater distance _y_ along the work, the difference between x and y
depending upon angle _a_; hence the tool will cut a thread of slightly
greater pitch than the lathe is geared to cut.
To illustrate by using the preceding example, cosine of angle _a_ =
0.125/0.12502 = 0.99984. By referring to a table of cosines, we find
that 0.99984 is the cosine of 1 degree, approximately; hence, the taper
attachment slide and the work should be set to this angle. (The angle
_a_ in Fig. 28 has been exaggerated in order to more clearly illustrate
the principle.)
[Illustration: Fig. 28. Diagram Illustrating Method of Cutting a Thread
to Compensate for the Error in Pitch due to Shrinkage in Hardening]
As is well known, it is objectionable to cut a thread with the tailstock
center offset, because the work is not rotated at a uniform velocity,
owing to the fact that the driving dog is at an angle with the
faceplate. For a small angle such as 1 degree, however, the error
resulting from this cause would be very small.
If a thread having a pitch slightly less than standard is needed to fit
a threaded part which has contracted in hardening, the taper attachment
can also be used provided the lathe is equipped with special gears to
cut a little less than the required pitch. Suppose a screw having a
pitch of 0.198 inch is required to fit the thread of a nut the pitch of
which has been reduced from 0.200 inch to 0.198 inch. If gears having 83
and 84 teeth are available, these can be inserted in a compound train,
so as to reduce the 0.200 inch pitch that would be obtained with the
regular gearing, to 83/84 of 0.200 or 0.19762 inch. This pitch, which is
less than the 0.198 inch pitch required, is then increased by using the
taper attachment as previously described. (This method was described by
Mr. G. H. Gardner in MACHINERY, February, 1914.)
=Calculating Change Gears for Thread Cutting.=--As previously mentioned,
the change gears for cutting threads of various pitches are shown by a
table or "index plate" attached to the lathe. The proper gears to be
used can be calculated, but the use of the table saves time and tends to
avoid mistakes. Every machinist, however, should know how to determine
the size of gears used for cutting any number of threads to the inch.
Before referring to any rules, let us first consider why a lathe cuts a
certain number of threads to the inch and how this number is changed by
the use of different gears.
As the carriage _C_ and the tool are moved by the lead-screw _S_ (see
Fig. 2), which is geared to the spindle, the number of threads to the
inch that are cut depends, in every case, upon the number of turns the
work makes while the lead-screw is moving the carriage one inch. If the
lead-screw has six threads per inch, it will make six revolutions while
the carriage and the thread tool travel one inch along the piece to be
threaded. Now if the change gears _a_ and _c_ (see also sketch _A_, Fig.
29) are so proportioned that the spindle makes the same number of
revolutions as the lead-screw, in a given time, it is evident that the
tool will cut six threads per inch. If the spindle revolved twice as
fast as the lead-screw, it would make twelve turns while the tool moved
one inch, and, consequently, twelve threads per inch would be cut; but
to get this difference in speeds it is necessary to use a combination of
gearing that will cause the lead-screw to revolve once while the lathe
spindle and work make two revolutions.
[Illustration: Fig. 29. (A) Lathe with Simple Gearing for Thread
Cutting. (B) Compound Geared Lathe]
Suppose that nine threads to the inch are to be cut and the lead-screw
has six threads per inch. In this case the work must make nine
revolutions while the lead-screw makes six and causes the carriage and
thread tool to move one inch, or in other words, one revolution of the
lead-screw corresponds to one and one-half revolution of the spindle;
therefore, if the lead-screw gear _c_ has 36 teeth, the gear _a_ on the
spindle stud should have 24 teeth. The spindle will then revolve one and
one-half times faster than the lead-screw, provided the stud rotates at
the same rate of speed as the main lathe spindle. The number of teeth in
the change gears that is required for a certain pitch can be found by
multiplying the number of threads per inch of the lead-screw, and the
number of threads per inch to be cut, by the same trial multiplier. The
formula which expresses the relation between threads per inch of
lead-screw, threads per inch to be cut, and the number of teeth in the
change gears, is as follows:
threads per inch of lead-screw teeth in gear on spindle stud
------------------------------ = -----------------------------
threads per inch to be cut teeth in gear on lead-screw
Applying this to the example given, we have 6/9 = 24/36. The values of
36 and 24 are obtained by multiplying 6 and 9, respectively, by 4,
which, of course, does not change the proportion. Any other number could
be used as a multiplier, and if gears having 24 and 36 teeth were not
available, this might be necessary. For example, if there were no gears
of this size, some other multiplier as 5 or 6 might be used.
Suppose the number of teeth in the change gears supplied with the lathe
are 24, 28, 32, 36, etc., increasing by four teeth up to 100, and assume
that the lead-screw has 6 threads per inch and that 10 threads per inch
are to be cut. Then,
6 6 x 4 24
-- = ------ = --
10 10 x 4 40
By multiplying both numerator and denominator by 4, we obtain two
available gears having 24 and 40 teeth, respectively. The 24-tooth gear
goes on the spindle stud and, the 40-tooth gear on the lead-screw. The
number of teeth in the intermediate or "idler" gear _b_, which connects
the stud and lead-screw gears, is not considered as it does not affect
the ratios between gears _a_ and _c_, but is used simply to transmit
motion from one gear to the other.
We have assumed in the foregoing that the spindle stud (on which gear
_a_ is mounted) and the main spindle of the lathe are geared in the
ratio of one to one and make the same number of revolutions. In some
lathes, however, these two members do not rotate at the same speed, so
that if equal gears were placed on the lead-screw and spindle stud, the
spindle would not make the same number of revolutions as the lead-screw.
In that case if the actual number of threads per inch in the lead-screw
were used when calculating the change gears, the result would be
incorrect; hence, to avoid mistakes, the following general rule should
be used as it gives the correct result, regardless of the ratios of the
gears which connect the spindle and spindle stud:
_Rule.--First find the number of threads per inch that is cut when gears
of the same size are placed on the lead-screw and spindle, either by
actual trial or by referring to the index plate. Then place this number
as the numerator of a fraction and the number of threads per inch to be
cut, as the denominator; multiply both numerator and denominator by some
trial number, until numbers are obtained which correspond to numbers of
teeth in gears that are available._ The product of the trial number and
the numerator (or "lathe screw constant") represents the gear _a_ for
the spindle stud, and the product of the trial number and the
denominator, the gear for the lead-screw.
=Lathes with Compound Gearing.=--When gearing is arranged as shown at
_A_, Fig. 29, it is referred to as simple gearing, but sometimes it is
necessary to introduce two gears between the stud and screw as at _B_,
which is termed compound gearing. The method of figuring compound
gearing is practically the same as that for simple gearing. To find the
change gears used in compound gearing, place the "screw constant"
obtained by the foregoing rule, as the numerator, and the number of
threads per inch to be cut as the denominator of a fraction; resolve
both numerator and denominator into two factors each, and multiply each
"pair" of factors by the same number, until values are obtained
representing numbers of teeth in available change gears. (One factor in
the numerator and one in the denominator make a "pair" of factors.)
Suppose the lathe cuts 6 threads per inch when gears of equal size are
used, and that the number of teeth in the gears available are 30, 35, 40
and so on, increasing by 5 up to 100. If 24 threads per inch are to be
cut, the screw constant 6 is placed in the numerator and 24 in the
denominator. The numerator and denominator are then divided into factors
and each pair of factors is multiplied by the same number to find the
gears, thus:
6 2 x 3 (2 x 20) x (3 x 10) 40 x 30
-- = ----- = ------------------- = -------
24 4 x 6 (4 x 20) x (6 x 10) 80 x 60
The last four numbers indicate the gears which should be used. The upper
two having 40 and 30 teeth are the _driving_ gears and the lower two
having 80 and 60 teeth are the _driven_ gears. The driving gears are
gear _a_ on the spindle stud and gear _c_ on the intermediate stud,
meshing with the lead-screw gear, and the driven gears are gears _b_ and
_d_. It makes no difference which of the driving gears is placed on the
spindle stud, or which of the driven is placed on the lead-screw.
=Fractional Threads.=--Sometimes the lead of a thread is given as a
fraction of an inch instead of stating the number of threads per inch.
For example, a thread may be required to be cut, having 3/8-inch lead.
The expression "3/8-inch lead" should first be transformed to "number of
threads per inch." The number of threads per inch (the thread being
single) equals:
1 3 8
--- = 1 / - = - = 2-2/3
3/8 8 3
To find the change gears to cut 2-2/3 threads per inch in a lathe having
a screw constant of 8 and change gears varying from 24 to 100 teeth,
increasing by 4, proceed as follows:
8 2 x 4 (2 x 36) x (4 x 24) 72 x 96
----- = --------- = ----------------------- = -------
2-2/3 1 x 2-2/3 (1 x 36) x (2-2/3 x 24) 36 x 64
As another illustration, suppose we are to cut 1-3/4 thread per inch on
a lathe having a screw constant of 8, and that the gears have 24, 28,
32, 36, 40 teeth, etc., increasing by four up to one hundred. Following
the rule:
8 2 x 4 (2 x 36) x (4 x 16) 72 x 64
----- = --------- = ----------------------- = -------
1-3/4 1 x 1-3/4 (1 x 36) x (1-3/4 x 16) 36 x 28
The gears having 72 and 64 teeth are the _driving_ gears, and those with
36 and 28 teeth are the _driven_ gears.
=Change Gears for Metric Pitches.=--When screws are cut in accordance
with the metric system, it is the usual practice to give the lead of the
thread in millimeters, instead of the number of threads per unit of
measurement. To find the change gears for cutting metric threads, when
using a lathe having an English lead-screw, first determine the number
of threads per inch corresponding to the given lead in millimeters.
Suppose a thread of 3 millimeters lead is to be cut in a lathe having an
English lead-screw and a screw constant of 6. As there are 25.4
millimeters per inch, the number of threads per inch will equal 25.4 /
3. Place the screw constant as the numerator, and the number of threads
per inch to be cut as the denominator:
6 25.4 6 x 3
------- = 6 / ---- = -----
25.4 3 25.4
----
3
The numerator and denominator of this fractional expression of the
change-gear ratio are next multiplied by some trial number to determine
the size of the gears. The first whole number by which 25.4 can be
multiplied so as to get a whole number as the result is 5. Thus, 25.4 x
5 = 127; hence, one gear having 127 teeth is always used when cutting
metric threads with an English lead-screw. The other gear required in
this case has 90 teeth. Thus:
6 x 3 x 5 90
--------- = ---
25.4 x 5 127
Therefore, the following rule can be used to find the change gears for
cutting metric pitches with an English lead-screw:
_Rule.--Place the lathe screw constant multiplied by the lead of the
required thread in millimeters multiplied by 5, as the numerator of the
fraction, and 127 as the denominator. The product of the numbers in the
numerator equals the number of teeth for the spindle-stud gear, and 127
is the number of teeth for the lead-screw gear._
If the lathe has a metric pitch lead-screw, and a screw having a given
number of threads per inch is to be cut, first find the "metric screw
constant" of the lathe or the lead of thread in millimeters that would
be cut with change gears of equal size on the lead-screw and spindle
stud; then the method of determining the change gears is simply the
reverse of the one already explained for cutting a metric thread with an
English lead-screw.
_Rule.--To find the change gears for cutting English threads with a
metric lead-screw, place 127 in the numerator and the threads per inch
to be cut, multiplied by the metric screw constant multiplied by 5, in
the denominator; 127 is the number of teeth on the spindle-stud gear and
the product of the numbers in the denominator equals the number of teeth
in the lead-screw gear._
=Quick Change-gear Type of Lathe.=--A type of lathe that is much used at
the present time is shown in Fig. 30. This is known as the quick
change-gear type, because it has a system of gearing which makes it
unnecessary to remove the change gears and replace them with different
sizes for cutting threads of various pitches. Changes of feed are also
obtained by the same mechanism, but the feeding movement is transmitted
to the carriage by the rod _R_, whereas the screw _S_{1}_ is used for
screw cutting. As previously explained, the idea of using the screw
exclusively for threading is to prevent it from being worn excessively,
as it would be if continually used in place of rod _R_, for feeding the
carriage when turning.
[Illustration: Fig. 30. Lathe having Quick Change-gear Mechanism]
[Illustration: Fig. 31. End and Side Views of Quick Change-gear
Mechanism]
The general construction of this quick change gear mechanism and the
way the changes are made for cutting threads of different pitch, will be
explained in connection with Figs. 30, 31 and 32, which are marked with
the same reference letters for corresponding parts. Referring to Fig.
30, the movement is transmitted from gear _s_ on the spindle stud
through idler gear _I_, which can be moved sidewise to mesh with either
of the three gears _a_, _b_ or _c_, Fig. 31. This cone of three gears
engages gears _d_, _e_ and _f_, any one of which can be locked with
shaft _T_ (Fig. 32) by changing the position of knob _K_. On shaft _T_
there is a gear _S_ which can be moved along the shaft by hand lever _L_
and, owing to the spline or key _t_, both the sliding gear and shaft
rotate together. Shaft _T_, carrying gears _d_, _e_ and _f_ and the
sliding gear _S_, is mounted in a yoke _Y_, which can be turned about
shaft _N_, thus making it possible to lower sliding gear _S_ into mesh
with any one of a cone of eight gears _C_, Fig. 31. The shaft on which
the eight gears are mounted has at the end a small gear _m_ meshing with
gear _n_ on the feed-rod, and the latter, in turn, drives the
lead-screw, unless gear _o_ is shifted to the right out of engagement,
which is its position except when cutting threads.
[Illustration: Fig. 32. Sectional Views of Quick Change-gear Mechanism]
With this mechanism, eight changes for different threads or feeds are
obtained by simply placing gear _S_ into mesh with the various sized
gears in cone _C_. As the speed of shaft _T_ depends on which of the
three gears _d_, _e_ and _f_ are locked to it, the eight changes are
tripled by changing the position of knob _K_, making twenty-four. Now by
shifting idler gear _I_, three speed changes may be obtained for gears
_a_, _b_ and _c_, which rotate together, so that the twenty-four changes
are also tripled, giving a total of seventy-two variations without
removing any gears, and if a different sized gear _s_ were placed on the
spindle stud, an entirely different range could be obtained, but such a
change would rarely be necessary. As shown in Fig. 30, there are eight
hardened steel buttons _B_, or one for each gear of the cone _C_, placed
at different heights in the casing. When lever _L_ is shifted sidewise
to change the position of sliding gear _S_, it is lowered onto one of
these buttons (which enters a pocket on the under side) and in this way
gear _S_ is brought into proper mesh with any gear of the cone _C_. To
shift lever _L_, the handle is pulled outward against the tension of
spring _r_ (Fig. 32), which disengages latch _l_ and enables the lever
to be lifted clear of the button; yoke _Y_ is then raised or lowered, as
the case may be, and lever _L_ with the sliding gear is shifted
laterally to the required position.
[Illustration: Fig. 33. Index Plate showing Position of Control Levers
for Cutting Threads of Different Pitch]
The position of lever _L_ and knob _K_ for cutting threads of different
pitches is shown by an index plate or table attached to the lathe and
arranged as shown in Fig. 33. The upper section _a_ of this table shows
the different numbers of threads to the inch that can be obtained when
idler gear _I_ is in the position shown by the diagram _A_. Section _b_
gives the changes when the idler gear is moved, as shown at _B_, and,
similarly, section _c_ gives the changes for position _C_ of the idler.
The horizontal row of figures from 1 to 8 below the word "stops"
represents the eight positions for lever _L_, which has a plate _p_
(Fig. 30) just beneath it with corresponding numbers, and the column to
the left shows whether knob _K_ should be out, in a central position, or
in.
In order to find what the position of lever _L_ and knob _K_ should be
for cutting any given number of threads to the inch, find what "stop"
number is directly above the number of threads to be cut, which will
indicate the location of lever _L_, and also what position should be
occupied by knob _K_, as shown in the column to the left. For example,
suppose the lathe is to be geared for cutting eight threads to the inch.
By referring to section a we see that lever _L_ should be in position 4
and knob _K_ in the center, provided the idler gear _I_ were in position
_A_, as it would be ordinarily, because all standard numbers of threads
per inch (U. S. standard) from 1/4 inch up to and including 4 inches in
diameter can be cut with the idler gear in that position. As another
illustration, suppose we want to cut twenty-eight threads per inch. This
is listed in section _c_, which shows that lever _L_ must be placed in
position 3 with knob _K_ pushed in and the idler gear shifted to the
left as at _C_.
The simplicity of this method as compared with the time-consuming
operation of removing and changing gears is apparent. The diagram _D_ to
the right shows an arrangement of gearing for cutting nineteen threads
per inch. A 20-tooth gear is placed on the spindle stud (in place of the
regular one having 16 teeth) and one with 95 teeth on the lead-screw,
thus driving the latter direct as with ordinary change gears. Of course
it will be understood that the arrangement of a quick change-gear
mechanism varies somewhat on lathes of different make.
CHAPTER V
TURRET LATHE PRACTICE
Turret lathes are adapted for turning duplicate parts in quantity. The
characteristic feature of a turret lathe is the turret which is mounted
upon a carriage and contains the tools which are successively brought
into the working position by indexing or rotating the turret. In many
instances, all the tools required can be held in the turret, although it
is often necessary to use other tools, held on a cross-slide, for
cutting off the finished part, facing a radial surface, knurling, or for
some other operation. After a turret lathe is equipped with the tools
needed for machining a certain part, it produces the finished work much
more rapidly than would be possible by using an ordinary engine lathe,
principally because each tool is carefully set for turning or boring to
whatever size is required and the turret makes it possible to quickly
place any tool in the working position. Turret lathes also have systems
of stops or gages for controlling the travel of the turret carriage and
cross-slide, in order to regulate the depth of a bored hole, the length
of a cylindrical part or its diameter; hence, turning machines of this
type are much more efficient than ordinary lathes for turning duplicate
parts, unless the quantity is small, in which case, the advantage of the
turret lathe might be much more than offset by the cost of the special
tool equipment and the time required for "setting up" the machine. (See
"Selecting Type of Turning Machine.")
[Illustration: Fig. 1. Bardons & Oliver Turret Lathe of Motor-driven
Geared-head Type]
=General Description of a Turret Lathe.=--The turret lathe shown in Fig.
1 has a hexagonal shaped turret _A_ with a hole in each side in which
the tools are held. This turret is mounted on a slide _B_ which is
carried by a saddle _C_ that can be moved along the bed to locate the
turret slide with reference to the length of the tools in the turret and
the room required for indexing. The turret slide can be moved
longitudinally by turning the pilot wheel or turnstile _D_, or it can
be fed by power. Ordinarily, the hand adjustment is used for quickly
moving the carriage when the tools are not cutting, although sometimes
the hand feed is preferable to a power feed when the tools are at work,
especially if the cuts are short. After a turret tool has finished its
cut, the turnstile is used to return the slide to the starting point,
and at the end of this backward movement the turret is automatically
indexed or turned one-sixth of a revolution, thus bringing the next tool
into the working position. The turret is accurately located in each of
its six positions by a lock bolt which engages notches formed in a large
index ring at the turret base. A binder lever _E_ at the top of the
turret stud is used to clamp the turret rigidly to the slide when the
tools are cutting.
The forward movement of the slide for each position of the turret is
controlled by stops at _F_, which are set to suit the work being turned.
When parts are being turned from bar stock, the latter passes through
the hollow spindle of the headstock and extends just far enough beyond
the end of the spindle to permit turning one of the parts. The bar is
held while the turning tools are at work, by a chuck of the collet type
at _G_. This chuck is opened or closed around the bar by turning
handwheel _H_. After a finished part has been cut off by a tool held in
cross-slide _J_, the chuck is released and further movement of wheel _H_
causes ratchet feed dog _K_, and the bar which passes through it, to be
drawn forward. This forward movement is continued until the end of the
bar comes against a stop gage held in one of the turret holes, to insure
feeding the bar out just the right amount for turning the next piece. On
some turret lathes, the lever which operates the chuck also controls a
power feed for the bar stock, the latter being pushed through the
spindle against the stop.
The machine illustrated has a power feed for the cross-slide as well as
for the turret. The motion is obtained from the same shaft _L_ which
actuates the turret slide, but the feed changes are independent. The
cross-slide feed changes are varied by levers _M_ and those for the
turret by levers _N_. For many turret lathe operations, such as turning
castings, etc., a jawed chuck is screwed onto the spindle and the work
is held the same as when a chuck is used on an engine lathe. Sometimes
chucks are used having special jaws for holding castings of irregular
shape, or special work-holding fixtures which are bolted to the
faceplate. The small handle at _O_ is for moving the cross-slide along
the bed when this is necessary in order to feed a tool sidewise.
This particular machine is driven by a motor at the rear of the
headstock, connection being made with the spindle through gearing. The
necessary speed changes are obtained both by varying the speed of the
motor and by shifting gears in the headstock. The motor is controlled by
the turnstile _P_ and the gears are shifted by the vertical levers
shown.
While many of the features referred to are common to turret lathes in
general, it will be understood that the details such as the control
levers, arrangement of stops, etc., vary on turret lathes of different
make.
[Illustration: Figs. 2 and 3. Diagrams showing Turret Lathe Tool
Equipment for Machining Automobile Hub Casting]
=Example of Turret Lathe Work.=--The diagrams Figs. 2 and 3 show a
turret lathe operation which is typical in many respects. The part to be
turned is a hub casting for an automobile and it is machined in two
series of operations. The first series is shown by the plan view, Fig.
2. The casting _A_ is held in a three-jaw chuck _B_. Tool No. 1 on the
cross-slide is equipped with two cutters and rough faces the flange and
end, while the inner and outer surfaces of the cylindrical part are
rough bored and turned by combination boring and turning tool No. 2.
This tool has, in addition to a regular boring-bar, a bracket or
tool-holder which projects above the work and carries cutters that
operate on the top surface. Tools Nos. 3 and 4 next come into action,
No. 3 finishing the surfaces roughed out by No. 2, and No. 4
finish-facing the flange and end of the hub. The detailed side view of
Tool No. 3 (which is practically the same as No. 2), shows the
arrangement of the cutters _C_ and _D_, one of which turns the
cylindrical surface and the other bevels the end of the hub. The hole in
the hub is next finished by tool No. 5 which is a stepped reamer that
machines the bore and counterbore to the required size within very
close limits. The surfaces machined by the different tools referred to
are indicated by the sectional view _E_ of the hub, which shows by the
numbers what tools are used on each surface.
For the second series of operations, the position of the hub is reversed
and it is held in a spring or collet type of chuck as shown by the plan
view Fig. 3. The finished cylindrical end of the hub is inserted in the
split collet _F_ which is drawn back into the tapering collet ring by
rod _G_ (operated by turnstile _H_, Fig. 1) thus closing the collet
tightly around the casting. The first operation is that of facing the
side of the flange and end of the hub with tool No. 6 on the
cross-slide, which is shown in the working position. A broad cutter _H_
is used for facing the flange and finishing the large fillet, and the
end is faced by a smaller cutter _I_. When these tools are withdrawn,
tool No. 7 is moved up for rough turning the outside of the cylindrical
end (preparatory to cutting a thread) and rough boring the hole. These
same surfaces are then finished by tool No. 8. The arrangement of tools
Nos. 7 and 8 is shown by the detailed view. Tool _J_ turns the part to
be threaded; tool _K_ turns the end beyond the threaded part; and tool
_L_ bevels the corner or edge. The reaming tool No. 9 is next indexed to
the working position for finishing the hole and beveling the outer edge
slightly. At the same time, the form tool No. 10, held at the rear of
the cross-slide, is fed up for beveling the flange to an angle of 60
degrees. The final operation is that of threading the end, which is done
with die No. 11. The boring-bars of tools Nos. 2, 3, 7 and 8 are all
provided with pilots _N_ which enter close fitting bushings held in the
spindle, to steady the bar while taking the cut. This is a common method
of supporting turret lathe tools.
The feed of the turret for both the first and second series of
operations is 1/27 inch per revolution and the speeds 60 revolutions per
minute for the roughing cuts and 90 revolutions per minute for the
finishing cuts. The total time for machining one of these castings
complete is about 7-1/2 minutes, which includes the time required for
placing the work in the chuck.
=Machining Flywheels in Turret Lathe.=--Figs. 4 to 6, inclusive,
illustrate how a gasoline engine flywheel is finished all over in two
cycles of operations. First the flywheel is turned complete on one side,
the hole bored and reamed, and the outside of the rim finished; in the
second cycle the other side of the flywheel is completed.
[Illustration: Fig. 4. First Cycle of Operations in Finishing Gasoline
Engine Flywheels on a Pond Turret Lathe]
During the first operation, the work is held by the inside of the rim by
means of a four-jaw chuck equipped with hard jaws. The side of the rim,
the tapering circumference of the recess, the web, and the hub are first
rough-turned, using tools held in the carriage toolpost. The hole is
then rough-bored by bar _C_, which is supported in a bushing in the
chuck, as shown in Fig. 4. The outside of the wheel rim is rough-turned
at the same time by a cutter held in the extension turret tool-holder
_T_ (Fig. 5), and the taper fit on the inside of the flywheel is turned
by means of cutter _A_ (Fig. 4) held in a tool-holder attached to the
turret.
The outside of the wheel rim is next finish-turned with cutter _V_ (Fig.
5) held in an extension turret tool-holder the same as the roughing
tool _T_. At the same time, the bore is finished by a cutter in
boring-bar _D_ (Fig. 4). The side of the rim and the hub of the wheel
are also finished at this time by two facing cutters _H_ and _K_, held
in tool-holders on the face of the turret. When the finishing cuts on
the rim and hub are being taken, the work is supported by a bushing on
the boring-bar which enters the bore of the wheel, the boring cutter and
facing tools being set in such relation to each other that the final
boring of the hole is completed before the facing cuts are taken.
[Illustration: Fig. 5. Elevation of Turret and Tools for Finishing
Flywheels--First Operation]
The web of the wheel is next finish-faced with the facing cutter held in
the holder _E_, and the taper surface on the inside of the rim is
finished by the tool _L_, at the same time. While these last operations
are performed, the work is supported by a bushing on a supporting arbor
_J_, which enters the bore of the wheel. The bore is finally reamed to
size by a reamer _F_ held in a "floating" reamer-holder. When the
reaming operation is completed, a clearance groove _N_ is cut on the
inside of the rim, using a tool _G_ held in the carriage toolpost. The
first cycle of operations on the flywheel is now completed.
The flywheel is then removed from the chuck, turned around, and held in
"soft" jaws for the second cycle of operations, the jaws fitting the
outside of the wheel rim. (Soft unhardened jaws are used to prevent
marring the finished surface of the rim.) The operations on this side
are very similar to those performed on the other side. First, the side
of the rim, the inside of the rim, the web, and hub are rough-turned,
using tools held in the carriage toolpost. The inside of the rim and the
web are then finished by a cutter held in a tool-holder at _P_, Fig. 6,
which is bolted to the face of the turret. The work is supported during
this operation by a bushing held on a supporting arbor _U_, having a
pilot which enters a bushing in the chuck. Finally, the rim and hub are
finished, by the facing cutters _R_ and _S_, the work being supported by
an arbor, as before.
[Illustration: Fig. 6. Second Cycle of Operations on Flywheel]
These operations illustrate the methods employed in automobile
factories, and other shops where large numbers of engine flywheels,
etc., must be machined.
=Finishing a Flywheel at One Setting in Turret Lathe.=--The plan view
_A_, Fig. 7, shows an arrangement of tools for finishing a flywheel
complete at one setting. The hole for the shaft has to be bored and
reamed and the hub faced on both sides. The sides and periphery of the
rim also have to be finished and all four corners of the rim rounded.
The tools for doing this work consist of boring-bars, a reamer, facing
heads on the main turret, a turret toolpost on the slide rest
(carrying, in this case, three tools) and a special supplementary wing
rest attached to the front of the carriage at the extreme left.
The casting is held by three special hardened jaws _b_ in a universal
chuck. These jaws grip the work on the inner side of the rim, leaving
room for a tool to finish the rear face without striking the chuck body
or jaws. Three rests _c_ are provided between the chuck jaws. The work
is pressed against these rests while being tightened in the chuck, and
they serve to locate it so that the arms will run true so far as
sidewise movement is concerned. These rests also locate the casting with
relation to the stops for the turret and carriage movements. The chuck
carries a bushing _r_ of suitable diameter to support the boring-bars in
the main turret, as will be described.
In the first operation, boring-bar _m_ is brought in line with the
spindle and is entered in bushing _r_ in the chuck. Double-ended cutter
_n_ is then fed through the hub of the pulley to true up the cored hole.
While boring the hole, the scale on the front face of the rim and hub is
removed by tool _j_. Tool _k_ is then brought into action to rough turn
the periphery, after which tool _e_, in the wing rest, is fed down to
clean up the back face of the rim. As soon as the scale is removed, the
hole is bored nearly to size by cutter _n_{1}_ in bar _m_{1}_, and it is
finally finished with reamer _q_ mounted on a floating arbor.
The cutters _f_, _g_ and _h_, in the facing head, are next brought up to
rough face the hub and rim, and round the corners of the rim on the
front side. This operation is all done by broad shaving cuts. The facing
head in which the tools are held is provided with a pilot bar _t_ which
fits the finished hole in the flywheel hub, and steadies the head during
the operation. The cutters _f_, _g_ and _h_ are mounted in holders which
may be so adjusted as to bring them to the proper setting for the
desired dimensions. This completes the roughing operations.
[Illustration: Fig. 7. Turret Lathe Tool Equipment for Machining
Flywheels]
The periphery of the rim is now finished by cutter _l_ in the turret
toolpost which is indexed to the proper position for this operation. The
rear face of the rim is finished by the same tool _e_ with which the
roughing was done. Tool _e_ is then removed and replaced with _d_
which rounds the inner corner of the rim. Tool _d_ is also replaced with
a third tool for rounding the outer corner of the rear side. For
finishing the front faces of the rim and hub and rounding the corners of
the rim, a second facing head, identical with the first one, is
employed. This is shown in position in the illustration. Cutters
_f_{1}_, _g_{1}_ and _h_{1}_ correspond with the cutters _f_, _g_ and
_h_, previously referred to, and perform the same operations.
The remaining operation of finishing the back of the hub is effected by
cutter _p_. This cutter is removed from the bar, which is then inserted
through the bore; the cutter is then replaced in its slot and the rear
end of the hub is faced by feeding the carriage away from the headstock.
This completes the operations, the flywheel being finished at one
setting.
=Finishing a Webbed Flywheel in Two Settings.=--The plan views _B_ and
_C_, Fig. 7, show the arrangement of tools for finishing a webbed
flywheel which has to be machined all over. This, of course, requires
two operations. In the first of these (see sketch _B_) the rough casting
is chucked on the inside of the rim with regular inside hard chuck jaws
_b_. The cored hole is first rough bored with cutter _n_ attached to the
end of boring-bar _m_, and guided by the drill support _d_ pivoted to
the carriage. Next, the boring-bar _m_{1}_ is brought into position, the
drill support being swung back out of the way. This bar is steadied by
its bearing in bushing _r_ in the chuck. Two cutters, _n_{1}_ and
_n_{2}_, are used to roughly shape the hole to the desired taper, the
small end being finished to within 0.002 inch of the required diameter.
While boring with the bar _m_{1}_, the scale is broken on the web and
hub of the casting by the tool _k_ in the turret toolpost. The latter is
then shifted to bring the tool _j_ into position for removing the scale
on the periphery of the wheel. Next, the hole is reamed with taper
reamer _q_, the pilot of which is supported by bushing _r_.
The first of the facing heads is now brought into action. This facing
head carries a guide _t_ which is steadied in a taper bushing _c_,
driven into the taper hole of the hub for that purpose. The top cutter
_f_ turns the periphery, cutter _g_ turns the hub and faces the web,
and cutter _h_ faces the rim. A fourth cutter _e_ on the under side of
the head faces the hub. This casting is now machined approximately to
size.
For finishing, similar cutters, _e_{1}_, _f_{1}_, _g_{1}_ and _h_{1}_,
in the other facing head are used, the latter being supported by the
taper bushing _c_ in the same way. A very light cut is taken for
finishing. Tool _l_ in the carriage turret is used to round the outer
and inner corners of the rim, which completes the work on this face of
the casting.
In the second cycle of operations, shown at _C_, the casting is chucked
on the outside with the soft jaws _b_, which are bored to the exact
diameter of the finished rim. The work is further supported and centered
by sliding bushing _c_, which is tapered to fit the finished hole in the
hub, and has an accurate bearing in bushing _r_ in the chuck. This
bushing is provided with a threaded collar for forcing it into the work
and withdrawing it. The scale on the web and the inside and face of the
rim is first broken with the tool _k_ in the turret toolpost. These
surfaces are then roughed off with cutters _f_, _g_ and _h_, in the
facing head. This latter is steadied by a pilot _t_ which enters the
hole in the sliding bushing _c_ on which the work is supported. A light
cut is next taken with cutters _f_{1}_, _g_{1}_ and _h_{1}_, in the
finishing facing head, which completes the operation.
=Tools for Turret Lathes.=--The operation of a turret lathe after the
tools have been properly arranged is not particularly difficult, but
designing and making the tools, determining what order of operations
will give the most efficient and accurate results, and setting the tools
on the machine, requires both skill and experience. For some classes of
work, especially if of a rather complicated nature, many of the tools
must be specially designed, although there are certain standard types
used on turret lathes which are adapted to general turning operations.
Some of the principal types are referred to in the following.
=Box-tools.=--Tools of this type are used for turning bar stock. There
are many different designs, some of which are shown in Figs. 8, 9 and
10. Box-tools are held in the turret and they have back-rests opposite
the turning tools, for supporting the part being turned. The box-tool
shown at _A_, Fig. 8, is for roughing. The cutter _a_ is a piece of
high-speed steel beveled on the cutting end to produce a keen edge. It
takes a shearing tangent cut on top of the bar and the latter is kept
from springing away by means of the adjustable, hardened tool-steel
back-rest _b_. This tool is considered superior to a hollow mill
whenever a fair amount of stock must be removed. If considerable
smoothness and accuracy are necessary, the finishing box-tool shown at
_B_ should follow the roughing box tool, but in most cases, especially
if the part is to be threaded by a die, a finishing cut is unnecessary.
[Illustration: Fig. 8. Different Types of Box-tools for Turret Lathe]
The finishing box-tool _B_ is also used to follow a hollow mill if
special accuracy or smoothness is desired. This tool is only intended
for light finishing cuts, the allowances varying from 0.005 inch to
0.015 inch in diameter. The cutters are made of square tool steel of
commercial size, and are ground and set to take a scraping end cut. This
particular tool has two tool-holders which permit finishing two
diameters at once. If a larger number of sizes must be turned, extra
tool-holders can be applied.
The single-cutter box-tool shown at _C_ is bolted directly to the face
of the turret instead of being held by a shank in the turret hole, and
it is adapted for heavy cuts such as are necessary when turning
comparatively large bar stock. The tool-holder _a_ swivels on a stud,
thus allowing the cutter to be withdrawn from the work while being
returned, which prevents marring the turned surface. The high-speed
steel cutter is ground to take a side cut on the end of the bar. The
latter is supported by hardened and ground tool-steel rolls _b_ which
revolve on hardened and ground studs. These rolls are mounted on
swinging arms which have a screw adjustment for different diameters.
They can also be adjusted parallel to the bar, thus enabling them to be
set either in advance of or back of the cutter. The opening in the base
allows the stock to pass into the turret when it is not larger than the
turret hole.
The box-tool shown at _D_ is similar to the one just described, except
that it has two or more cutters and roller back-rests, thus enabling
different diameters to be turned simultaneously. The cutters are ground
to take a side cut. Ordinarily this gives a satisfactory finish, but if
special accuracy and smoothness are desired, two tools should be used,
one for roughing and one for finishing, the latter being ground to take
a light scraping end cut.
The taper-turning box-tool shown at _E_ is designed for accurately
turning tapers on brass or cast-iron parts, when there is a small amount
of stock to be removed. The taper is obtained by cross motion imparted
to the cutter slide as the turret advances. The taper-turning box-tool
shown at _F_, instead of having a single-point cutter, is provided with
a wide cutter _a_. This tool is designed to turn tapering parts of
small or medium diameter, requiring the use of a support which cannot be
provided with a straight forming tool and holder mounted on the cut-off
slide. The cutter is backed up by the screws shown, which also provide
adjustment for different tapers within a limited range. The bar is
supported by the three back-rests shown, which also have screw
adjustment.
=Examples of Box-tool Turning.=--Box-tools are not only used for
cylindrical and taper turning on the end of a bar, but for many other
operations. Figs. 9 and 10 show a number of box-tools of different
designs, with examples of the work for which each is intended. While
these tools are designed for some specific part, they can, of course,
with slight modifications be adapted to other work.
[Illustration: Fig. 9. Box-tools and Work for which they are Intended]
A box-tool of the pilot type that is used for finishing, after the
surplus stock has been removed by roughing tools, is shown at _A_, Fig.
9. The work, which is the cone for a ball bearing, is shown at _a_ by
the dotted lines and also by the detail view to the right. The pilot _b_
enters the work before either of the cutters begins to operate on its
respective surface. The inverted cutter _c_, which sizes the flange of
the cone, is held in position by a clamp _d_, which is forced down by a
collar-head screw. The cutter is further secured against a beveled
shoulder at _g_ by the set-screws _f_, and it is adjusted forward by the
screw _e_. By loosening the screws _f_ and the collar-head screw, the
cutter may be removed for sharpening. The cutter _h_ is adjusted to cut
to the proper diameter, by the screws _l_, after which the clamp _k_ is
made level by the screw _j_. The collar-screw _m_ is then used to secure
the tool in place. The cutter is made from drill rod and it is slightly
cupped out on the cutting end to give keenness to the cutting edge. The
adjusting screw _o_, which passes through plate _p_, prevents the cutter
from backing away from the work. This adjusting screw plate has its
screw holes slotted to avoid removing the screws when it becomes
necessary to remove the plate and cutter for sharpening. Pilot _b_ is
held firmly to the tool body by set-screw _r_. The hole _s_ through the
shank makes it easy to remove the pilot, in case this is necessary.
A pilot box-tool for finishing another type of ball bearing cone is
shown at _B_. The shape of the work itself is indicated by the dotted
lines _a_ and by the detail view. This tool is somewhat similar in its
construction to the one just described. The cutters _b_ and _c_ are
inverted and are used to face the flange at _d_ and to turn it to the
proper diameter. These cutters are held by the clamp _f_ and screws _g_
and are adjusted forward by the screw _h_. The cutter _j_, which
operates on top of the stock, rests on a bolster, of the proper angle
and is adjusted up or down by the screws _k_. The clamp _l_, which binds
against this tool, is beveled to correspond with the angle of the tool.
This clamp is secured by the collar-screw shown and it is leveled by
set-screws _s_. The adjusting screw _p_ prevents the cutter from
slipping back. The holes in the adjusting-screw plate are also slotted
in this case so that it will not be necessary to remove any screws when
the cutter has to be taken out of the holder.
A box-tool for finishing a treadle-rod cone for a sewing machine is
shown at _C_. This tool is also of the pilot type. The cutters in it
operate on opposite sides of the cone _a_. The inverted cutter _b_ sizes
the cylindrical part of the cone, while the front cutter _d_ is set at
the proper angle to finish the tapered part. The rear cutter _b_ is held
in place by the clamp _g_ and a collar screw. It is adjusted forward by
the screw _h_ in the plate _i_ which is held by screws as shown. The
pilot is retained by a set-screw, and it is easily removed by inserting
a small rod in the hole _l_ which passes through the shank. The cutter
_d_ is held by clamp _m_ and is adjusted by screw _n_ which passes
through a tapped hole in plate _o_. The screw holes in both the
adjusting plates _i_ and _o_ are slotted to facilitate their removal.
The box-tool illustrated at _A_, Fig. 10, is used for finishing the
bushing of a double-taper cone bearing _a_. The cutters are so arranged
that they all cut on the center; that is, the cutting edges lie in a
horizontal plane. The inverted cutter _b_ at the rear forms the short
angular surface, and the cutter _c_ in front forms the long tapering
part of the bearing. The large diameter is turned, to size by cutter
_d_. The pilot _e_ has a bearing in the bore nearly equal to the length
of the work and it is provided with oil grooves, as shown. The taper
shank of this pilot is tapped for the screw _i_ which extends the whole
length of the shank and is used to draw the pilot back to its seat. It
is not necessary to remove adjusting-screw plate _k_ to take out the
cutter _b_, as the latter can be drawn out from the front after the
collar-screw _m_ is loosened. The cutter _c_ is removed by taking off
the adjusting-screw plate _s_ after loosening the collar-screw _n_. The
cutter _d_ is held in a dove-tailed slot by two headless set-screws _q_.
It is also backed up by an adjusting screw in the plate _s_. These
adjusting screws should all have fine threads, say from 32 to 40 per
inch, and be nicely fitted so they will not loosen after being adjusted.
The box-tools shown at _B_ and _C_, Fig. 10, are for turning the sides
of a loose pulley for a sewing machine. This pulley (shown by the dotted
lines) is finished in two operations. The box-tool for finishing the
side of the pulley on which the hub projects beyond the rim, is shown at
_B_. The inverted cutter _a_, which faces the end of the hub, is held by
a clamp _c_ (clearly shown in the end view) from the under side and it
has no adjustment. The collar-screw _d_ is tapped into this clamp, which
is prevented from getting out of place by the dowel-pin _f_. The pilot
_g_ is made small in the shank, so that tool _a_ can be so placed as to
insure the removal of all burrs around the bore of the hub. The pilot is
held by a set-screw and it is provided with oil grooves. The cutter _j_
sizes the outside of the hub, and the cutter _k_ faces the side of the
pulley rim. These cutters are both held by the clamp _l_ and the
collar-screw _m_. No side plates are used on this tool, and the cutters
are all easily removed.
[Illustration: Fig. 10. Examples of Box-tool Designs]
Sketch _C_ shows the box-tool used for the second operation. As the hub
is flush with the rim on the side for which this tool is intended, it
needs only one cutter to face both. This is done by the wide cutter _a_
which is held in a dove-tailed slot in the front of the tool and is
fastened by the clamp _b_ and collar-screw _c_. The bushing _d_, in
which the end of the work arbor is supported, is held by the
collar-screw _e_, and to obtain the necessary compression, the body of
the tool is slotted as far back as _f_. This bushing is provided with
oil grooves and one side is cut away to clear the cutter _a_. The
pilot end of the arbor on which the work is mounted is 1/16 inch smaller
than the bore of the pulley, which allows the cutter to be set in far
enough to prevent any burr which might form at the edge of the bore. A
disk _i_ is inserted back of bushing _d_, so that the latter may be
easily removed by passing a rod through the hollow shank. The special
chuck used for this second operation on the loose pulley is screwed onto
the spindle, and the work is mounted on a projecting arbor and driven by
the pins engaging holes in the pulley web. The arbor is made a driving
fit for the work, and the end or pilot is a running fit in the bushing
of the box-tool. A counterbore in the arbor hub provides clearance for
the hub of the pulley which projects beyond the rim on one side.
[Illustration: Fig. 11. (A) Hollow Mill and Holder. (B) Spring
Screw-threading Die and Releasing Die-holder]
=Hollow Mills.=--A hollow mill such as is shown at _A_ in Fig. 11 is
sometimes used in place of a box-tool (especially when turning brass)
for short roughing cuts preceding a threading operation. The turning is
done by the cutting edges _e_, and the turned part enters the mill and
is steadied by it. If this type of tool is used for long, straight cuts,
especially on square stock and when making screws with large heads from
the bar, it should always be followed by a finishing box-tool to insure
accurate work. A hollow mill can be sharpened readily by grinding the
ends without materially changing the cutting size. A slight adjustment
can be obtained by means of the clamp collar shown to the left, although
this is not generally used. When making these mills, they should be
reamed out tapering from the rear to give clearance to the cutting
edges. For turning steel, the cutting edge should be about 1/10 of the
diameter ahead of the center, whereas for brass, it should be on the
center-line.
[Illustration: Fig. 12. Geometric Adjustable Hollow Milling Tool]
Hollow mills are also made adjustable. The design shown in Fig. 12 is
especially adapted for brass finishing. It can also be used for taking
light cuts on cast iron or steel but its use in place of roughing or
finishing box-tools for general use is not recommended. With the
exception of the cutters and screws, the complete tool consists of three
parts, _viz._, the holder, cam, and ring. The cam serves to adjust the
cutters for different diameters. The adjustment is made by the two
screws shown, the amount being indicated by a micrometer scale. When
adjusting the cutters for a given diameter, the use of a hardened steel
plug of the required size is advisable, the cutters being adjusted
against the plug.
=Releasing Die and Tap Holders.=--Threads are cut in the turret lathe by
means of dies for external threading, and taps for internal threading,
the die or tap being held in a holder attached to the turret. A simple
form of releasing die holder is shown at _B_, Fig. 11. This holder was
designed for the spring-screw type of threading die shown to the left.
The die is clamped in the holder _a_ by the set-screw shown, and the
shank _b_ of the holder is inserted in the turret hole. Holder _a_ has
an extension _c_ which passes through the hollow shank. When the die is
pressed against the end of the work, holder _a_ and its extension moves
back until lug _d_ on the holder engages lug _e_ on the shank. The die
and holder are then prevented from rotating with the work and the die
begins to cut a thread. It continues to screw itself onto the work with
the turret following, until the thread has been cut to the required
length; the turret is then stopped and as the die and holder _a_ are
drawn forward, lugs _d_ and _e_ disengage so that the die simply rotates
with the work without continuing to advance. The lathe spindle is then
reversed and as the turret is moved back by hand, pin _f_ comes around
and enters notch _g_, thus holding the die stationary; the die then
backs off from the threaded end. Some tap holders are also constructed
the same as this die holder, so far as the releasing mechanism is
concerned. There are also many other designs in use, some of which
operate on this same principle.
[Illustration: Fig. 13. Geometric Self-opening and Adjustable
Screw-cutting Die Head]
=Self-opening Die Heads.=--The type of die holder shown at _B_ in Fig.
11 is objectionable because of the time required for backing the die off
the threaded end; hence, self-opening dies are extensively used in
turret lathe work. As the name implies, this type of die, instead of
being solid, has several chasers which are opened automatically when the
thread has been cut to the required length. The turret can then be
returned without reversing the lathe spindle. The dies are opened by
simply stopping the travel of the turret slide, the stop-rod for the
feed of the turret being adjusted to give the proper amount of travel.
[Illustration: Fig. 14. Geometric Collapsing Tap]
A well-known die head of the self-opening type is shown in Fig. 13. The
dies open automatically as soon as the travel of the head is retarded,
or they can be opened at any point by simply holding back on the
turnstile or lever by which the turret slide is moved. The die is closed
again by means of the small handle seen projecting at right-angles from
the side of the head. The closing may be done by hand or automatically
by screwing a pin into a threaded hole opposite the handle and attaching
a small piece of flat steel to the back edge of the turret slide. The
latter will then engage the pin as the turret revolves, thus closing the
die head. This die head has a roughing and finishing attachment which is
operated by handle _A_. When this handle is moved forward, the dies are
adjusted outward 0.01 inch for the roughing cut, whereas returning the
handle closes and locks the dies for the finishing cut. The die head has
a micrometer scale which is used when making slight adjustments to
compensate for the wear of the chasers or to make either a tight-or a
loose-fitting thread.
=Collapsing Taps.=--The collapsing tap shown in Fig. 14 is one of many
different designs that are manufactured. They are often used in turret
lathe practice in place of solid taps. When using this particular style
of collapsing tap, the adjustable gage _A_ is set for the length of
thread required. When the tap has been fed to this depth, the gage comes
into contact with the end of the work, which causes the chasers to
collapse automatically. The tool is then withdrawn, after which the
chasers are again expanded and locked in position by the handle seen at
the side of the holder. In all threading operations, whether using taps
or dies, a suitable lubricant should be used, as a better thread is
obtained and there is less wear on the tools. Lard oil is a good
lubricant, although cheaper compounds give satisfactory results on many
classes of work.
=Miscellaneous Turret Lathe Tools.=--The chamfering tool shown at _A_,
Fig. 15, is used for pointing the end of a bar before running on a
roughing box-tool. This not only finishes the end of the bar but
provides an even surface for the box-tool to start on. The cutter is
beveled on the end to form a cutting edge and it is held at an angle.
The back-rest consists of a bell-mouthed, hardened tool-steel bushing
which supports the bar while the cut is being taken.
The stop gages _B_ and _C_ are used in the turret to govern the length
of stock that is fed through the spindle. When a finished piece has been
cut off, the rough bar is fed through the spindle and up against the
stop gage, thus locating it for another operation. This gage may be a
plain cylindrical piece of hardened steel, as at _B_, or it may have an
adjusting screw as at _C_; for special work, different forms or shapes
are also required. The stop gages on some machines, instead of being
held in the turret, are attached to a swinging arm or bracket that is
fastened to the turret slide and is swung up in line with the spindle
when the stock is fed forward.
The center drilling tool _D_ is designed to hold a standard combination
center drill and reamer. This type of tool is often used when turning
parts that must be finished afterwards by grinding, to form a center for
the grinding machine. The adjustable turning tool _E_ is used for
turning the outside of gear blanks, pulley hubs or the rims of small
pulleys. The pilot _a_ enters the finished bore to steady the tool, and
cutter _b_ is adjusted to turn to the required diameter.
[Illustration: Fig. 15. Various Types of Tools for the Turret Lathe]
The cutting-off tool-holder _F_ (which is held on the cross-slide of the
turret lathe) is usually more convenient than a regular toolpost, as the
blade can be set closer to the chuck. The blade is held in an inclined
position, as shown, to provide rake for the cutting edge; the inclined
blade can also be adjusted vertically, a limited amount, by moving it in
or out. The multiple cutting-off tool _G_ holds two or more blades and
is used for cutting off several washers, collars, etc., simultaneously.
By changing the distance pieces between the cutters, the latter are
spaced for work of different widths. The flat drill holder _H_ is used
for drilling short holes, and also to form a true "spot" or starting
point for other drills.
Knurling tools are shown at _I_ and _J_. The former is intended for
knurling short lengths and is sometimes clamped on top of the cut-off
tool on the cross-slide, the end being swung back after knurling (as
shown by the dotted lines) to prevent interference with the work when
the cutting-off tool is in operation. The knurling tool _J_ has a shank
and is held in the turret. The two knurls are on opposite sides of the
work so that the pressure of knurling is equalized. By adjusting the
arms which hold the knurls, the tool can be set for different diameters.
Three styles of drill holders are shown at _K_, _L_ and _M_. Holder _K_
is provided with a split collet (seen to the left) which is tightened on
the drill shank by a set-screw in the holder. This holder requires a
separate collet for each size drill. The taper shank drill holder _L_
has a standard taper hole into which the shank of the drill is inserted.
The adjustable type of holder _M_ is extensively used, especially on
small and medium sized machines when several sizes of drills are
necessary. This holder is simply a drill chuck fitted with a special
shank. For large drills the plain style of holder _K_ is recommended,
and if only a few sizes of drills are required, it is more satisfactory
and economical than the adjustable type.
The various types of small turret lathe tools referred to in the
foregoing for turning, threading, tapping, knurling, etc., are a few of
the many different designs of tools used in turret lathe practice.
Naturally, the tool equipment for each particular job must be changed
somewhat to suit the conditions governing each case. The tools referred
to, however, represent in a general way, the principal types used in
ordinary practice. Some of the more special tools are shown in
connection with examples of turret lathe work, which are referred to in
the following.
=Turning Gasoline Engine Pistons in Turret Lathe.=--The making of
pistons for gas engines, especially in automobile factories, is done on
such a large scale that rapid methods of machining them are necessary.
The plan view _A_, Fig. 16, shows the turret lathe tools used in one
shop for doing this work. As is often advisable with work done in large
quantities, the rough castings are made with extra projections so
arranged as to assist in holding them. These projections are, of course,
removed when the piece is completed. In this case the piston casting _a_
has a ring about 1-1/4 inch long and a little less in diameter than the
piston, at the chucking end. The piston is held in suitable chuck jaws
_b_ which are tightened against the inside of this ring. The set-screws
in these special jaws are then tightened, thus clamping the casting
between the points of the screws and the jaws. This method of holding
permits the whole exterior of the piston to be turned, since it projects
beyond the chuck jaws. This is the object in providing the piston with
the projecting ring by which it is held.
[Illustration: Fig. 16. (A) Method of Boring and Turning Pistons in
Gisholt Lathe. (B) Special Chuck and Tools for Turning, Boring and
Cutting Off Eccentric Piston Rings]
The first operation consists in rough-boring the front end of the
piston. The double-ended cutter _n_ is held in boring-bar _m_, which is,
in turn, supported by a drill-holder, clamped to one of the faces of
the turret. This bar is steadied by a bushing in the drill support _c_
which is attached to the carriage, and may be swung into or out of the
operating position, as required. After this cut is completed, the turret
is revolved half way around and the casting is finish-bored in a similar
manner, with double-ended cutter _n_{1}_ held in bar _m_{1}_, the drill
support being used as in the previous case. The support is then turned
back out of the way to allow the turning tools in the turret toolpost to
be used.
The outside of the piston is next rough-turned with tool _k_ in the
turret toolpost, which is revolved to bring this cutter into action. The
toolpost is then turned to the position shown, and the outside is
finish-turned by tool _j_, which takes a broad shaving cut. The turret
tool-holder is again revolved to bring form tool _l_ into position. This
tool cuts the grooves for the piston rings. Suitable positive stops are,
of course, provided for both the longitudinal and cross movements of the
turret toolpost.
In the second operation, the piston _a_ is reversed and held in soft
jaws, which are used in place of the hardened jaws _b_ shown in the
illustration. These jaws are bored to the outside diameter of the
piston, so that when closed, they hold the work true or concentric with
the lathe spindle. In this operation the chucking ring by which the
piston was previously held is cut off, and the end of the piston is
faced true. If the crank-pin hole is to be finished, a third operation
is necessary, a self-centering chuck-plate and boring and reaming tools
being used. (These are not shown in the illustration.)
=Turning Piston Rings in Turret Lathe.=--One method of turning piston
rings is shown at _B_ in Fig. 16. The piston rings are cut from a
cast-iron cylindrical piece which has three lugs _b_ cast on one end and
so arranged that they may be held in a three-jawed chuck. This
cylindrical casting is about 10 inches long, and when the rings are to
have their inside and outside surfaces concentric, the casting is held
by the lugs in the regular jaws furnished with the chuck. (The
arrangement used for turning and boring eccentric rings, which is that
shown in the illustration, will be described later.)
The casting _a_, from which the rings are made, is first rough-bored
with double-ended cutter _n_ in boring-bar _m_, after which it is
finish-bored with cutter _n_{1}_ in bar _m_{1}_. While taking these
cuts, the bars _m_ and _m_{1}_ are supported by their extension ends
which enter bushing _r_ located in the central hole of the chuck. This
furnishes a rigid support so that a heavy cut can be taken.
The outside of the casting is next rough-turned with tool _k_, held in
the turret toolpost. This toolpost is then revolved to bring tool _j_
into position, by which the outside is turned true to size, a broad
shaving chip being taken. The toolpost is again swung around, to bring
the cutting-off tool-holder _l_ into position. This holder contains four
blades set the proper distance apart to give rings of the desired width.
Each blade, from right to left, is set a little back of the preceding
one, so that the rings are cut off one after the other, the outer rings
being supported until they are completely severed. After the first four
rings are cut off, the carriage is moved ahead to a second stop, and
four more rings are severed, this operation being continued until the
casting has been entirely cut up into rings.
When the bore of the ring is to be eccentric with the outside, the
holding arrangement shown in the illustration is used. The casting a is
bolted to a sliding chuck-plate _c_, and the outside is rough-turned
with tool _k_ in the toolpost. Finishing tool _j_ is then brought into
action, and the outside diameter is turned accurately to size. Then the
sliding chuck-plate _c_, carrying the work, is moved over a distance
equal to the eccentricity desired, and the work is bored with cutters
_n_ and _n_{1}_ as in the previous case. The turret toolpost is next
revolved and the tools _l_ are used for cutting off the rings. The
reason for finishing the outside first is to secure smooth rings in
cutting off, as this operation should be done when the work is running
concentric with the bore, rather than with the exterior surface.
It will be evident that this method gives a far greater output of rings
than is possible by finishing them in the more primitive way on engine
lathes. The faces of the rings may be finished in a second operation if
desired, or they may be ground, depending on the method used in the
shop where the work is being done, and the accuracy required.
[Illustration: Fig. 17. Turning Gasoline Engine Pistons in Pratt &
Whitney Turret Lathe]
=Piston Turning in Pratt and Whitney Turret Lathe.=--A turret lathe
equipped with tools for turning, facing and grooving automobile gasoline
engine pistons is shown in Fig. 17. The piston is held on an expanding
pin chuck which is so constructed that all of the pins are forced
outward with equal pressure and automatically conform to any
irregularities on the inside of the piston. Tool _A_ rough-turns the
outside, and just as this tool completes its cut, a center hole is
drilled and reamed in the end of the piston by combination drill and
reamer _B_. The turret is then indexed one-half a revolution and a
finishing cut is taken by tool _C_. After the cylindrical body of the
piston has been turned, tools held in a special holder _E_ attached to
the cut-off slide are used to face the ends of the piston and cut the
packing-ring grooves. While the grooves are being cut, the outer end of
the piston is supported by center _D_. The center hole in the end also
serves to support the piston while being ground to the required diameter
in a cylindrical grinding machine. The edge at the open end of the
piston may also be faced square and the inner corner beveled by a hook
tool mounted on the rear cross-slide, although this is usually done in a
separate operation. (This provides a true surface by which to hold this
end when grinding.)
[Illustration: Fig. 18. Pratt & Whitney Turret Lathe equipped with
Special Attachment for Turning Eccentric Piston Rings]
This illustration (Fig. 17) shows very clearly the stops which
automatically disengage the turret feed. A bracket _F_ is bolted to the
front of the bed and contains six stop-rods _G_ (one for each position
or side of the turret). When one of these stop-rods strikes lever _H_,
the feed is disengaged, the stop being adjusted to throw out the feed
when the tool has completed its cut. Lever _H_ is automatically aligned
with the stop-rods for different sides of the turret by a cam _J_ on the
turret base. A roller _K_ bears against this cam and, through the
connecting shaft and lever shown, causes lever _H_ to move opposite the
stop-rod for whatever turret face is in the working position. Lever _L_
is used for engaging the feed and lever _R_ for disengaging it by hand.
The indexing of the turret at the end of the backward movement of the
slide is controlled by stop _M_ against which rod _N_ strikes, thus
disengaging the lock bolt so that the turret can turn. This stop _M_ is
adjusted along the bed to a position depending upon the length of the
turret tools and the distance the turret must move back to allow the
tools to clear as they swing around.
[Illustration: Fig. 19. Tool Equipment for Machining Worm Gear
Blanks--Davis Turret Lathe]
=Attachment for Turning Piston Rings.=--Fig. 18 shows a special
attachment applied to a Pratt & Whitney turret lathe for turning
eccentric, gas-engine piston rings. The boring of the ring casting,
turning the outside and cutting off the rings, is done simultaneously.
The interior of the casting is turned concentric with the lathe spindle
by a heavy boring-bar, the end of which is rigidly supported by a
bushing in the spindle. The slide which carries the outside turning tool
is mounted on a heavy casting which straddles the turret. The outside of
the ring casting is turned eccentric to the bore as a result of an
in-and-out movement imparted to the tool by a cam on shaft _A_ which is
rotated from the lathe spindle through the gearing shown. For each
revolution of the work, the tool recedes from the center and advances
toward it an amount sufficient to give the required eccentricity. When
the turning and boring tools have fed forward about 2 inches, then the
cutting-off tools which are held in holder _B_ come into action. The end
of each cutting-off tool, from right to left, is set a little farther
away from the work than the preceding tool, so that the end rings are
always severed first as the tools are fed in by the cross-slide. A
number of the completed rings may be seen in the pan of the machine.
[Illustration: Fig. 20. Turning Bevel Gear Blanks in Davis Turret
Lathe--First Operation]
=Turning Worm-gear Blanks in Turret Lathe.=--This is a second operation,
the hub of worm-gear blank _G_ (Fig. 19) having previously been bored,
reamed, and faced on the rear side. The casting is mounted upon a
close-fitting arbor attached to a plate bolted to the faceplate of the
lathe, and is driven by two pins which engage holes on the rear side.
The rim is first rough-turned by a tool _A_ which operates on top, and
the side is rough-faced by a toothed or serrated cutter _B_. A similar
tool-holder having a tool _C_ and a smooth cutter _D_ is then used to
turn the rim to the required diameter and finish the side. The end of
the hub is faced by cutters mounted in the end of bars _E_ and _F_, one
being the roughing cutter and the other the finishing cutter. The work
arbor projects beyond the hub, as will be seen, and forms a pilot that
steadies these cutter bars. The curved rim of the gear is turned to the
required radius (preparatory to gashing and bobbing the worm-wheel
teeth) by a formed tool _H_ held on the cross-slide.
[Illustration: Fig. 21. Second Operation on Bevel Gear Blanks]
=Turning Bevel Gear Blanks.=--Fig. 20 shows a plan view of the tools
used for the first turning operation on bevel gear blanks (these gears
are used for driving drill press spindles). The cored hole is beveled
true at the end by flat drill _A_ to form a true starting surface for
the three-fluted drill _B_ which follows. The hole is bored close to the
required size by a tool (not shown) held in the end of bar _C_, and it
is finished by reamer _D_. The cylindrical end of the gear blank or hub
is rough-and finish-turned by tools held in holders _E_ and _F_,
respectively. (These holders were made to set at an angle of 45 degrees,
instead of being directly over the work, as usual, so that the cutters
would be in view when setting up the machine.) It will be noted that the
chuck is equipped with special jaws which fit the beveled part of the
casting.
[Illustration: Fig. 22. Sectional View of Tapering Mold Shell which is
turned in Hartness Flat Turret Lathe, as illustrated in Figs. 23 to 27,
Inclusive]
The second and final operation on this blank is shown in Fig. 21. The
work _A_ is held by a special driver plate attached to the faceplate of
the machine. This driver plate has two pins which engage holes drilled
in the gear blank and prevent it from rotating. The blank is also held
by a bolt _B_ which forces a bushing against the cylindrical end. First,
the broad beveled side which is to be the toothed part of the gear, is
rough-turned by toothed cutters _C_, and a recess is formed in the end
of the blank, by a turning tool in this same tool-holder. A similar
tool-holder _E_, having finishing cutters, is then used to finish the
bevel face and recess. The other tools seen in the turret are not used
for this second operation. The rear bevel is roughed and finished by
tools and held on the cross-slide.
=Shell Turning Operation in Flat Turret Lathe.=--The "flat turret lathe"
is so named because the turret is a flat circular plate mounted on a low
carriage to secure direct and rigid support from the lathe bed. The
tools, instead of being held by shanks inserted in holes in the turret,
are designed so that they can be clamped firmly onto the low circular
turret plate.
An interesting example of flat turret lathe work is shown in Fig. 22.
This is a steel shell which must be accurately finished to a slight
taper, both inside and out, threaded and plain recesses are required at
the ends, and, in addition, one or two minor operations are necessary.
This work is done in the Hartness flat turret lathe, built by the Jones
& Lamson Machine Co. The shells are turned from cold-drawn seamless
steel tubing, having a carbon content of 0.20 per cent, and they are
finished at the rate of one in nine minutes. The tubing comes to the
machine in 12-foot lengths, and the tube being operated upon is, of
course, fed forward through the hollow spindle as each successive shell
is severed.
[Illustration: Fig. 23. First Operation on Shell Illustrated in Fig.
22--Rough-turning and Boring]
In finishing this shell, five different operations are required. During
the first operation the shell is rough-bored and turned by one passage
of a box-tool, Fig. 23, and the recess _A_, Fig. 22, at the outer end,
is finished to size by a second cutter located in the boring-bar close
to the turret. The turret is then indexed to the second station which
brings the threading attachment _G_ into position, as shown in Fig. 24.
After the thread is finished, the recess _B_, Fig. 22, is turned by a
flat cutter _K_, Fig. 25. The inner and outer surfaces are then finished
to size by a box-tool mounted on the fourth station of the turret and
shown in position in Fig. 26. The final operation, Fig. 27, is performed
by three tools held on an auxiliary turret cross-slide, and consists in
rounding the corners at _b_ and _c_, Fig. 22, and severing the finished
shell.
[Illustration: Fig. 24. Second Operation--Cutting Internal Thread]
One of the interesting features connected with the machining of this
shell is the finishing of the inner and outer tapering surfaces. The
taper on the outside is 3/32 inch Per foot, while the bore has a taper
of only 1/64 inch per foot, and these surfaces are finished
simultaneously. The box-tool employed is of a standard type, with the
exception of an inserted boring-bar, and the taper on the outside is
obtained by the regular attachment which consists of a templet _D_ (Fig.
23) of the required taper, that causes the turning tool to recede at a
uniform rate as it feeds along. To secure the internal taper, the
headstock of the machine is swiveled slightly on its transverse ways by
the use of tapering gibs. By this simple method, the double taper is
finished to the required accuracy without special tools or equipment.
As those familiar with this machine know, the longitudinal movements of
the turret as well as the transverse movements of the headstock are
controlled by positive stops. The headstock of this machine has ten
stops which are mounted in a revolving holder and are brought into
position, as required, by manipulating a lever at the front. The stops
for length, or those controlling the turret travel, are divided into two
general groups, known as "A" and "B". Each of these groups has six stops
so that there are two stops for each of the six positions or stations of
the turret, and, in addition, five extra stops are available for any one
tool, by the engagement of a pin at the rear of the turret. The change
from the "A" to the "B" stops is made by adjusting lever _L_, Fig. 26,
which also has a neutral position.
[Illustration: Fig. 25. Third Operation--Turning Recess at Rear End;
Tool is shown withdrawn]
After the box-tool for the roughing cut, shown at work in Fig. 23, has
reached the end of its travel, further movement is arrested by a stop of
the "A" group. The outside turning tool is then withdrawn by operating
lever _E_ and the turret is run back and indexed to the second station,
thus bringing the threading attachment into position. The surface speed
of 130 feet per minute which is used for turning is reduced to about 30
feet per minute for threading by manipulating levers _H_, Fig. 24. After
the turret is located by another stop of the "A" group, the threading
attachment is made operative by depressing a small plunger _I_, which
connects a vertical driving shaft from the spindle with the splined
transmission shaft _J_. A reciprocating movement is then imparted to the
thread chaser _t_ which advances on the cutting stroke and then
automatically retreats to clear the thread on the return. This movement
is repeated until the thread is cut to the proper depth, as determined
by one of the stops for the headstock. While the thread is being cut,
the carriage is locked to the bed by the lever _N_, Fig. 26. It was
found necessary to perform the threading operation before taking the
outside finishing cut, owing to a slight distortion of the shell wall,
caused by the threading operation.
[Illustration: Fig. 26. Fourth Operation--Finishing the Bore and
Outside]
After the thread is finished, the turret is turned to the third station
as shown in Fig. 25, and tool _K_ for the inner recess _B_, Fig. 22, is
brought into position and fed to the proper depth, as determined by
another cross-stop. The turret is also locked in position for this
operation. The finishing cuts for the bore and the outside are next
taken by a box-tool which is shown near the end of its cut in Fig. 26.
This box-tool is similar to the one used for roughing, but it is
equipped with differently shaped cutters to obtain the required finish.
The outside turning tool has a straight cutting edge set tangent to the
cylindrical surface and at an angle, while the boring tool has a cutting
edge of large radius. An end view of this box-tool is shown in Fig. 27.
A reduced feed is employed for the finishing cut, and the speed is
increased to 130 feet per minute, which is the same as that used for
roughing.
[Illustration: Fig. 27. Fifth Operation--Rounding Ends, Scoring Large
End, and Cutting Off]
During the next and final operation, the turret, after being indexed to
the position shown in Fig. 27, is first located by a stop of the "A"
group so that the cutting-off tool _R_ in front can be used for rounding
the corner _b_, Fig. 22. The stop lever _L_ is then shifted and the
turret is moved to a second stop of the "B" group. The corner _c_ is
then rounded and the shell is scored at _d_ by two inverted tools _S_
and _T_ at the rear, after which the finished work is severed by the
cut-off tool at the front. The cross-movement of these three tools is
controlled by positive stops on the cross-slide, and the latter is moved
to and fro by hand lever _O_. After the shell is cut off, the stop _M_,
mounted on the turret, Fig. 26, is swung into position, and the tube is
automatically fed forward to the swinging stop by the roll feed, as soon
as the chuck is released by operating lever _Q_. This completes the
cycle of operations. A copious supply of lubricant is, of course,
furnished to the tools during these operations, and the two boring-tool
shanks are hollow so that lubricant can be forced through them and be
made to play directly upon the cutters.
=Chuck Work in Flat Turret Lathe.=--Two examples of chuck work on the
Acme combination flat turret lathe are shown in Figs. 28 and 29. Fig. 28
shows the tool equipment for turning a cylindrical part _A_ which is
held in a three-jaw universal chuck. The front flange is first
rough-turned by a bent turning tool _B_. The diameter is regulated by
one of the cross-stops at _D_ which has been previously set and controls
the movement of the turret cross-slide. The longitudinal feed is
disengaged when the flange has been turned, by an independent stop. This
machine has twelve longitudinal stops, there being one for each turret
face and six auxiliary stops, in addition to the stops for the
cross-slide.
[Illustration: Fig. 28. Tool Equipment for Turning Scroll Gear Blank on
Acme Flat Turret Lathe]
After roughing the flange, the turret carriage is locked or clamped
rigidly to the bed to prevent any lengthwise movement, and the back face
of the front flange is rough-turned by tool _B_ in to the diameter of
the hub which is indicated by a micrometer dial on the cross-feed screw.
The carriage is then unlocked and auxiliary stop No. 7 is engaged (by
turning a knob at the front of the slide) and the cylindrical hub is
turned back to the rear flange, the feed being disengaged by the
auxiliary stop just as the tool reaches the flange. The cross-slide is
now moved outward, longitudinal auxiliary stop No. 8 is engaged, the
turret slide is moved against the stop, the carriage is locked and the
front sides of both the front and rear flanges are rough-faced by tools
_B_ and _C_. The turret is next indexed and the hole rough-bored by
cutter _E_. After again indexing the turret, the hub and flanges are
finish-turned and faced by tools _F_ and _G_, as described for the
rough-turning operation. The final operation is that of finishing the
bore by cutter _H_.
[Illustration: Fig. 29. Acme Flat Turret Lathe Arranged for Turning
Roller Feed Body]
The operation shown in Fig. 29 is that of turning the body of a roller
feed mechanism for a turret lathe. The casting is held in a three-jaw
universal chuck and it is first rough-bored by tool _A_. The turret is
then indexed and the side of the body and end of the hub are rough-faced
by tools at _B_. The turret is again indexed for rough-turning the
outside of the hub and body, by tools _C_ and _D_. Similar tools _E_
and _F_ are then used to finish these same surfaces, after which the end
of the hub and side of the body are finished by tools _G_ and _H_
similar to those located at _B_. The final operation is that of
finishing the bore by tool _J_ and cutting a groove in the outside of
the hub by the bent tool _K_.
[Illustration: Fig. 30. Turret and Head of Jones & Lamson Double-spindle
Flat Turret Lathe]
=Double-spindle Flat Turret Lathe.=--The extent to which modern turning
machines have been developed, especially for turning duplicate parts in
quantity, is illustrated by the design of turret lathe the turret and
head of which is shown in Fig. 30. This machine has two spindles and a
large flat turret which holds a double set of tools, so that two
duplicate castings or forgings can be turned at the same time. It was
designed primarily for chuck work and can be used as a single-spindle
machine if desirable. When two spindles are employed for machining two
duplicate parts simultaneously, considerably more time is required for
setting up the machine than is necessary for the regular single-spindle
type, but it is claimed that the increased rate of production obtained
with the two-spindle design more than offsets this initial handicap.
The manufacturers consider the single-spindle machine the best type for
ordinary machine building operations, regardless of whether the work is
turned from the bar or is of the chucking variety. On the other hand,
the double-spindle type is preferred when work is to be produced in such
quantities that the time for setting up the machine becomes a secondary
consideration.
[Illustration: Fig. 31. Diagram showing Tool Equipment and Successive
Steps in Machining Sprocket Blanks on Double-spindle Flat Turret Lathe]
When the double-spindle machine is used as a single-spindle type, a
chuck 17 inches in diameter is used, and when both spindles are in
operation, two 9-inch chucks are employed. The general outline of the
turret is square, and the tools are rigidly held, with a minimum amount
of overhang, by means of tool-blocks and binding screws connected with
the clamping plates. Two duplicate sets of tools are clamped to each
side of the turret and these operate simultaneously on the two pieces
held in the chucks or on faceplates. Primarily the turret is used in
but four positions, but when a 17-inch chuck or faceplate is employed,
corner blocks may be held by the clamping plates in which tools are
supported, giving, if necessary, four additional operations by indexing
the turret to eight positions.
A typical job to demonstrate the application of the double-spindle flat
turret lathe is illustrated in Fig. 31. The parts to be turned are
sprocket wheels which are held in the two 9-inch chucks. At the first
position of the turret (which is the one illustrated), the inside is
rough-bored by tools _A_. At the second position of the turret, tools
_B_ rough-face the inner sides of the flanges; tools _C_ face the outer
sides of the flanges, while tools _D_ turn the faces of the flanges. At
the third position of the turret, tools _E_ finish-turn the inside of
the flanges; tools _F_ finish-turn the outside of the flanges, while
tools _G_ finish the faces of the flanges. At the fourth position of the
turret, tools _H_ finish-bore the sprockets; tools _I_ complete the
turning on the outside of the flanges, while tools _J_ accurately size
the interior of the flanges.
With the double-spindle flat turret lathe, each operation is a double
operation, and the speeds are varied according to the nature of the cut;
thus, if at one position of the turret, the tools are required to rough
out the work, this may be done rapidly, for it has no bearing on the
other operations that are subsequently performed. Furthermore, if the
following operation has to be performed with great care, this may be
done without reducing the speed of the less exacting operations.
[Illustration: Fig. 32. Potter & Johnston Automatic Chucking and Turning
Machine]
=Automatic Chucking and Turning Machine.=--The chucking and turning
machine shown in Fig. 32 is automatic in its operation, the feeding of
the tools, indexing of the turret, etc., being done automatically after
the machine is properly arranged, and the work is placed in the chuck.
This machine is adapted to turning and boring a great variety of
castings, forgings or parts from bar stock, and it is often used in
preference to the hand-operated turret lathe, especially when a great
many duplicate parts are required. It is provided with mechanism for
operating the cross-slide, feeding the turret slide forward, returning
it rapidly, rotating the turret to a new position, and feeding it
forward quickly for taking a new cut. The cross-slide and turret-slide
movements are effected by cams mounted on the large drum _E_ seen
beneath the turret, while the various speed and feed changes are
effected by dogs and pins carried on disk _D_ which is keyed to the same
shaft that the cam drum is mounted upon. This shaft with the cam drum
and governing disk _D_, makes one revolution for each piece of work
completed. The cams for operating the turret slide are mounted upon the
periphery of drum _E_. The roll which engages the angular faces of these
cams and imparts movement to the turret is carried by an intermediate
slide which has rack teeth engaging a pinion on the square shaft _C_. By
turning this shaft with a crank, the position of the turret-slide, with
relation to the cam, may be adjusted for long or short work and long or
short tools, as may be required.
[Illustration: Fig. 33. Rear View of Machine showing the Cross-slide
Mechanism, Driving Gearing, etc.]
The cams which operate the cross-slide are mounted on the right-hand end
of drum _E_ and actuate the yoke _A_ (see Fig. 33) which extends
diagonally upward. The rear end of this yoke has rack teeth meshing with
the teeth of a segmental pinion, which is fastened to rock-shaft _B_.
At the headstock end, this rock-shaft carries another segmental pinion
meshing with rack teeth formed on the cross-slide. The movement imparted
to the yoke by the cams is thus transmitted through the pinions and
rock-shaft to the cross-slide.
[Illustration: Fig. 34. The Automatic Controlling Mechanism for Feeds
and Speeds]
The cam drum _E_ is driven by a pinion meshing with a gear attached to
its front side. This pinion is driven through a train of gearing from
pulley _L_ (see Fig. 34) which is belted to the spindle. The feeds are
thus always dependent on the spindle speed. By means of epicyclic
gearing and suitable clutches, the motion thus derived from the spindle
may be made rapid for returning the turret to be indexed and then
advancing it to the cutting position again, or very slow for the forward
feed when the tools are at work. These changes from slow to fast or
_vice versa_ are controlled by disk _D_. This disk carries pins which
strike a star wheel located back of the disk at the top, and as this
star wheel is turned, the speeds are changed by operation of the gearing
and clutches referred to. The first pin _M_ that strikes the star wheel
advances it one-sixth of a rotation, changing the feed from fast to
slow; the next pin that strikes it advances it another sixth of a
rotation, changing the feed from slow to fast and so on. By adjusting
the pins for each piece of work, the feed changes are made to take place
at the proper time. Handwheel _E_ is geared with the cam-shaft on which
the star wheel is mounted, so that the feeds may be changed by hand if
desired.
In addition to these feed-changing pins, disk _D_ has a dog which
operates a lever by which the feed movement is stopped when the work has
been completed. Four rates of feed are provided by quick change gearing
of the sliding gear type, operated by handle _K_. With this handle set
in the central position, the feed is disengaged. On the periphery of
disk _D_ are also clamped dogs or cams _N_, which operate a horizontal
swinging lever _P_ connected by a link with vertical lever _J_, which
controls the two spindle speeds with which the machine is provided.
Either one of these speeds can be automatically engaged at any time, by
adjusting the cams _N_ on disk _D_.
Lever _H_ connects or disconnects the driving pulley from the shaft on
which it is mounted, thus starting or stopping the machine. The square
shaft _G_ serves to operate the drums by hand and is turned with a
crank. The rotation of the turret, which takes place at the rear of its
travel, is, of course, effected automatically. A dog, which may be seen
in Fig. 32 at the side of the bed, is set to trip the turret revolving
mechanism at the proper point in the travel, to avoid interference
between the tools and the work. The turret is provided with an automatic
clamping device. The mechanism first withdraws the locking pin, unclamps
the turret, revolves it, then throws in the locking pin and clamps the
turret again.
=Example of Work on Automatic Turning Machine.=--The piece selected for
illustrating the "setting up" and operation of the automatic chucking
and turning machine is shown in Fig. 35. This is a second operation, and
a very simple one which will clearly illustrate the principles involved.
In the first operation, the hole was drilled, bored and reamed, the
small end of the bushing faced, and the outside diameter finished, as
indicated by the sketch to the left. (The enlarged diameter at the end
was used for holding the work in the chuck.) In the second operation
(illustrated to the right), the enlarged chucking end is cut off and, in
order to prevent wasting this piece, it is made into a collar for
another part of the machine for which the bushing is intended; hence,
the outside diameter is turned and the outside end faced, before cutting
off the collar. In addition, the bushing is recessed in the second
operation, and the outer end faced. In order to have the surfaces
finished in the second operation, concentric with those machined in the
first operation, the chuck is equipped with a set of soft "false jaws"
which have been carefully bored to exactly the diameter of the work to
be held.
[Illustration: Fig. 35. Simple Example of Work done in Automatic
Chucking and Turning Machine]
The first thing to determine when setting up a machine of this type is
the order of operations. In this particular case, the order is as
follows: At the first position of the turret, the outside collar is
rough-turned and the outer end rough-faced. At the second position, the
collar is turned to the required diameter and the outer face is
finished. The third face of the turret is not equipped with tools, this
part of the cycle being taken up in cutting off the collar with a
cut-off tool on the rear cross-slide. The fourth operation is that of
recessing the bushing, and the fifth operation, facing the end to remove
the rough surface left by the cutting-off tool.
The tools _A_ and _B_, Fig. 36, used for turning the outside of the
flange, are held in brackets _C_ bolted to the face of the turret.
These brackets are each provided with three holes for carrying turning
tool-holders. This arrangement provides for turning a number of
diameters at different positions, simultaneously, but for this
particular operation, a single cutting tool for each tool-holder is all
that is necessary. A special device is used for recessing and will be
described later.
[Illustration: Fig. 36. Front View of Machine set up for the Finishing
Operation on the Recessed Bushing and Collar shown in the Foreground and
in Fig. 35]
=Determining Speed and Feed Changes.=--As previously mentioned, the
particular machine illustrated in Fig. 32 can be arranged for two
automatic changes of speed to suit different diameters on the work. The
change gears that will give the required spindle speeds should first be
selected. These change gears for different speeds are listed on a speed
and feed plate attached to the headstock of the machine (see Fig. 37).
It is possible to use one speed from the list given for the fast train
of gears, and one from the list for the slow train, so long as the same
gears are not used in each case. The diameter of the collar on the work
shown in Fig. 35 is 2-1/2 inches, and the diameter of the body is 2
inches. Assuming that the surface speed for this job should be about 40
feet per minute, a little calculation shows that the 66 revolutions per
minute, given by the fast train of gears, is equivalent to a surface
speed of 43 feet per minute on a diameter of 2-1/2 inches. Moreover, the
78 revolutions per minute obtained from the slow train of gearing, gives
about 41 feet per minute on a diameter of 2 inches. The spindle gearing
indicated for these speeds is, therefore, placed in position on the
proper studs at the back of the machine.
[Illustration: Fig. 37. Plate on the Headstock of Machine Illustrated in
Fig. 32 giving the Speeds and Feeds]
Next we have to determine on which faces of the turret to place the
different tools. Each turret face is numbered to agree with the
corresponding feed cam on the drum. The speed and feed plate (Fig. 37)
gives the various feeds obtainable per revolution of the spindle. As
will be seen, the different cams give different feeds. Cam No. 1 has a
coarse feed suitable for roughing; cam No. 2 a finer feed adapted to
finishing, and so on. Since the first operation consists in
rough-turning, cam No. 1 is used. Cam No. 2, which gives a finer feed,
is used for the finish-turning operation. Cam No. 4, which is ordinarily
used for reaming, could, in this case, be used for recessing, as this
recess is for clearance only and may be bored with a coarse feed.
The final operation, which is that of facing, can be done with any cam
and cam No. 5 may be used. It will be understood that for facing
operations, the feeds given do not apply. As the roll passes over the
point of the feed cam at the extreme end of the movement, the feed of
the turret slide is gradually slowed down to zero; since the facing
takes place in the last eighth or sixteenth inch of this movement, it is
done at a feed which is gradually reduced to zero. This is, of course,
as it should be, and it is not necessary to pay any attention to the
tabulated feeds in facing operations.
=Setting the Turret Slide.=--The next adjustment is that of setting the
turret slide. In making this adjustment the turret is set in such
relation to the work that the tools will have but a small amount of
overhang, the cam-shaft being revolved by hand until the cam-roll is at
the extreme top of the forward feeding cam, so that the turret slide is
at the extreme of its forward movement. When this adjustment has been
made by the means provided, set the turret index tripping dog so as to
revolve the turret at the proper point. After a turning tool-holder and
tool is attached to the face of the turret, cam No. 1 is placed in its
operating position and is revolved by hand until the roll is on the
point of the cam and the turret at the forward extreme of its motion. At
this point the tool-holder is set so that the cutter will be far enough
forward to complete its turning operation. The feed cam is then turned
backward, thus returning the turret slide, and the cutter is set to turn
the flange to the proper diameter for the roughing cut. The turret slide
is fed forward and back while the cutter is adjusted, and when it is
properly set, the flange is turned, the cam-drum being fed by hand. This
is the first trial cut on the piece.