The first requisite for fitting up a lathe for screw-cutting and
plain turning is the fitting a guide-screw, and adding a saddle to
the slide-rest. But it must be observed that the ordinary mandrel of
small foot-lathes, which works in a collar and back centre, is not
convertible, and must be replaced by one working through two collars,
so that a part may project at the opposite end to that intended for
the chucks. On this projection various cogwheels have to be fitted.
Now there are divers patterns of mandrel suitable for the above
purpose; some are perfectly cylindrical, some have one conical part,
some are fitted with a second cone, independent of the mandrel, and
which slides upon it, and can be adjusted by means of regulating
nuts. This latter being a good pattern, when well constructed, we
shall first describe in detail. In Fig. 162 is shown A, the mandrel
complete, with fittings in section, but without any of the change
wheels. The cone _a_ is forged on the mandrel, which then becomes
cylindrical. The cone is figured too large in proportion to size of
mandrel; the latter should be represented as much more substantial.
At _b_ the second cone slides on, the latter shown again at B. It is
bored truly, and a slot cut to fit a feather on the mandrel. Thus it
will slide along it, but must necessarily turn with it. Beyond D on
the left are two screwed parts, one slightly larger than the other,
one being made with a left-handed, the other with a right-handed
thread. On these screw the two nuts, _d, e_, which drive the movable
cone towards the right, causing it to fit more tightly into its
collar, and also by the same movement tightening the fixed cone in
its bearings. The mandrel is thus readily adjustable in the collars,
and can be made to run very truly and easily without shake or endlong
movement. Any pressure, however, against the front, such as would
be caused by drilling, would jam the front cone in its collar, and
tend to loosen the other. This is counteracted by the part _f, g_,
with its screw, _h_, against which the end of the mandrel bears. That
this form requires good workmanship is evident, for there are three
points, or bearing surfaces, to be brought into a correct line, and
the slightest deviation will cause the mandrel to jam in some part of
its revolution. The nuts screw up in opposite directions to counteract
the tendency in either to screw up more tightly, or to become looser
by the revolution of the mandrel. On the whole the above is a good
form of mandrel; if it has a fault, it is a slight tendency to work
heavily. The next form is that of the ordinary mandrel, with single
cone, but a second collar is added, which is cylindrical, through
which the mandrel passes, and it then abuts on the sustaining screw
as before. This is shown in Fig. 163, with the addition of the wheel,
which is to be connected by intermediate wheels and pinions with the
leading screw. If the conical collar is replaced by a cylindrical
one, so that two similar bearings are made use of, a shoulder becomes
necessary to prevent the mandrel from slipping endwise. The collars
must also be split as in Fig. 164, so that they can be tightened up as
they become worn. Either of the above forms of mandrel can be used;
each has its advocates, and not unfrequently all may be found in
different machines in the same manufactory. The bed of a self-acting
lathe requires to be accurately surfaced, and formed by the planing
machine with two V's or edges bevelled underneath, as 165 _b, b_. The
saddle of which we have spoken is a flat plate of cast iron fitted
with V pieces to match the bevelled edges of the lathe bed, along
which it slides truly, its under-edge being planed. To this plate the
slide rest is attached securely, either turning when required on a
central pin, and being clamped at any desired angle, as before stated
when treating of the compound slide rest, or, when this movement
is preferred to be given to the upper slide, fixed permanently by
nuts and screws. The saddle is represented detached in Fig. 166. The
principle of the self-acting lathe is very simple. Motion is given
to the screw by means of cog wheels geared with the mandrel, a nut
fitting the screw is attached to a hanging bracket of the saddle, and
this with the rest is thereby carried along the bed. It is necessary,
however, to add some contrivance for instantly throwing the screw out
of gear, without the necessity of stopping the lathe itself. There
are many ways of effecting this, the most common being the use of a
split nut, which embraces the leading screw when the two halves are
brought together upon it, but which is instantly freed by separating
them through the action of levers and cams, or other simple mechanical
contrivance. In Fig. 167, _a_ is the bottom of the saddle from which
depend the brackets E, E. The nut B, B, which is divided across the
middle, slides up and down between these brackets, D being the leading
screw which they embrace when closed. The movement of the halves of
the nut is effected by the lever _c_, in the form of the letter T
moving on a centre pin at K, and having two links, D, D, attached to
the halves of the nut at one end, and to the ends of the cross lever
at the other. A connecting bar pivoted to the part, L, is attached
to a lever and handle, by which motion is communicated to the lever.
When this rod is moved in the direction of the arrow the links will
cause the nut to close and _vice versa_. The next form, 168 and 169,
represents the split nut attached to two arms, A, A, hinged together
at E. B, B, are slots in which work the pins attached to the cross
head of the levers C, C. A heavy knob of iron keeps the latter in
the position to which it may be moved. In 168 the pins keep the arms
and nuts apart. When the lever is thrown over, as in 169, the nut
is securely closed and held in gear. This form requires to be fixed
to the lower part of a bracket attached to the saddle of the slide
rest, such as is shown at B, C, Fig. 170. In this figure, A is the
bottom of the saddle E, E, the V piece, that on the left, having an
adjusting screw to tighten it on the lathe bed when necessary. At F
is a projecting piece of the saddle fitting accurately between the
lathe bed, and kept down by a screw with bed plate underneath. This
serves to steady the movement of the saddle and relieve the pressure
and strain upon the V pieces. The bed-plate may be cut out to fit the
lower part of the bed on which it slides, both being planed for the
purpose, as shown at H.
[Illustration: FIG. 162.]
[Illustration: FIG. 163.]
[Illustration: FIG. 164.]
[Illustration: FIG. 165.]
[Illustration: FIG. 166.]
[Illustration: FIG. 167.]
[Illustration: FIG. 168.]
[Illustration: FIG. 169.]
[Illustration: FIG. 170.]
[Illustration: FIG. 171.]
In Fig. 171 the half nuts slide up and down on the front of the
saddle, and are moved by a circular plate on the outside. The action
is very similar to that of Fig. 167, intervening links, attached to
the plate and to the halves of the nuts, giving motion to the latter
as required. Cams and eccentrics of varied forms are also made use
of for the same purpose, every lathe maker devising new and improved
movements from time to time. There is indeed little difficulty in
arranging a satisfactory method, the best being that which is under
easy control, of adequate strength, and not likely to be easily
disarranged by the rough treatment it is liable to receive at the
hands of the workman. The leading screw may either be placed on the
outside, in front of the bed next to the workman, or inside between
the bearers. The former is preferable as being more accessible, but
it is rather more exposed to injury. When between the bearers the
large saddle of the slide rest serves to protect it from the falling
chips and shavings of metal which, mixing with the oil, are apt
to clog the threads and add to the friction of the parts, besides
increasing considerably the wear and tear, and no accidental blow is
likely to reach the screw thus protected on both sides by the lathe
bed. Nevertheless, all the various parts of a machine should be made
as accessible as possible, and, with the exercise of ordinary care in
its preservation, the position outside is on the whole the best. If
the lathe bed is very long, it may be desirable to support the screw
by allowing it to rest on friction wheels fixed to the saddle at both
ends, or by allowing it to pass through a pair of brasses. Thus, as
the saddle is often two feet in length, such bearings at each end,
with the split nut between the two, form points of support which go
far to prevent the screw from jagging or bending in the middle from
its own weight.
The general arrangement of self-acting tools is similar with all the
makers. Fig. 172 is a drawing of such a tool from a sketch of Eades &
Son, an old firm in Lichfield-street, Birmingham, the screw being here
placed in front. Whitworth, the eminent mechanician of Manchester,
makes many lathes with the screw between the beds. He also uses a
double slide rest, with a tool working at each side of the piece to
be turned, which cannot thus spring away from the cut. This is called
a duplex slide rest. By this method double the cut is taken, and the
strain upon the work lessened. In addition to the screw there is
usually a rack attached to the lathe bed, on the furthest side, and a
cog wheel working upon this, attached to the back of the slide rest
saddle, is turned by a handle in front of the rest, Fig. 173. This
enables the workman, after having, by means of the screw, completed
the cut to the end of the work, to run the rest back again very
quickly, having first released the split nut from the screw. The tool
is thus brought into position for the next cut far more rapidly than
could be managed by reversing the motion of the screw. This is called,
therefore, the quick return motion, and is almost always attached to
the larger class of machine-lathes. Before entering upon the details
of other forms of gearing, and the arrangement of change-wheels for
cutting screws or for plain turning, we shall introduce a few remarks
upon the often-repeated question as to the advisability of self-acting
lathes for the purposes of the amateur. It must be remembered that
the chief object of such a lathe is the manufacture of large screws,
long shafts, and such work as the piston rods of engines, requiring
perfect accuracy from one end to the other. Boring cylinders and
similar work is now generally done in an upright or vertical boring
machine, which is for many reasons more convenient. This class of
work, of course, the amateur has nothing to do with. The traverse of
an ordinary slide rest is generally adequate for surfacing or boring
to a length of six inches, which is sufficient for most purposes, and
can readily be done by turning the leading screw of the rest by
hand. Then, as regards screw cutting in the lathe, it is questionable
whether it is not easier to turn up the blank and cut with stock and
dies. The thread thus formed is a copy of that of the tool, and the
latter, as made by Whitworth and other first-class makers, is as
perfect as machinery directed by talented workmen can produce. If
it is really desirable, however, to make use of the lathe for this
purpose, the plan already suggested is generally applicable, namely,
gearing the leading screw of the slide rest to the mandrel, either
by means of change-wheels attached to an arm, or by connection with
the overhead pulleys. There is, in addition, a plan of attaching a
pair of dies to the tool holder of the rest, after detachment of
the screw from its nut, which may be available in some cases. There
are also screw plates made in two halves, the plates being divided
lengthwise and clamped together at pleasure, admitting of application
to any screw-blank while revolving in the lathe; and, lastly, pairs
of dies, fitted like pliers, can be similarly used by hand without
any slide rest. On the whole, then, it will hardly pay to give £50,
or £60, for a screw-cutting lathe that is merely destined for small
work; that can be done sufficiently well in a common lathe with
hand motion to the rest. For cutting short screws, such as those of
boxes, a convenient and practically useful contrivance is still a
desideratum. The traversing mandrel is, no doubt, the best, but it is
an expensive pattern, if well made, and it is necessary to sacrifice,
in a measure, general utility to questionable and certainly partial
advantage. Turning is, moreover, an art, and to attain skill in it,
perseverance and practice are necessary--hence the zest experienced
in its pursuit by the real lover of it. This perseverance pays, and
to attain the skill required to trace a short screw with the chasing
tool is within the reach of any one who desires to accomplish it. The
art once acquired, the desire for a traversing mandrel ceases; for
no good workman would accept a contrivance of the kind when he can
more easily and quickly accomplish his end without it. Writing this
for the special benefit of amateurs, we would strongly advise them
not to throw away money to purchase the means of doing what after
all they will probably never, and certainly only seldom, carry into
practice. The pleasure afforded by all the mechanical arts is greatly
enhanced by meeting with and triumphantly surmounting all sorts of
difficulties. Let it be a standing rule of our readers to make use
of the appliances at hand instead of seeking others which only save
trouble and render skill unnecessary. What can be more aggravating
than for an amateur, with his hundred guinea lathe and chests of
tools, to be obliged to take his work to a mechanic, and to see him,
whose whole stock might be bought with perhaps a tenth part of the
money, take in hand and finish with ease what has baffled the skill
of his more wealthy patron? The common fault of the amateur is undue
hurry. To him time is seldom an object, yet the mechanic, to whom it
is so precious, readily spends more upon his work. He never hurries,
never compromises, but with lathe and file fits the several parts of
his work with the most patient care and practised skill. The result is
at once seen when his productions are placed side by side with those
of the zealous but too hasty amateur.
It remains, as previously described, to arrange certain cogwheels
gearing with each other upon the spindle of the mandrel and upon the
end of the screw, to enable the workman at pleasure to vary their
ratio of speed, in order to give him the means of regulating the
pitch of thread to be cut. For this purpose a set of change wheels is
supplied with the screw-cutting lathe, the number of cogs in which
usually commencing at fifteen, increase by five, until the number
one hundred and twenty is reached, then increasing by ten up to the
complete set. Duplicates of some numbers are also convenient. The
method of finding the proper sized wheels is, of course, based upon
the proportion borne by the required pitch to that of the leading
screw and one form of calculation for the purpose has already been
given. Thus, supposing the leading screw to have two threads to the
inch, it must revolve twice to move the tool that distance, and if
we wish to cut a screw of ten threads to the inch, the work on the
mandrel must revolve ten times while the leading screw revolves twice,
or, which is the same thing, the mandrel must revolve five times while
the screw revolves once. The cogs in the wheels must therefore be in
the proportion of five to one, say fifty on the screw and ten on the
mandrel, with an idle wheel on the stud to cause the motion of the
tool to be such as will cut a right-handed thread, or to cause the
mandrel and screw to take the same direction. Now, it seldom happens
that two wheels and an idle one will give the requisite speed to the
tool, and the number of cogs required soon mounts to inconvenient
numbers. In this case, therefore, according to a principle already
laid down, a stud-wheel with a pinion attached to the same, is
placed in the train of wheels, and the object readily attained. A
modification of the rule of calculation may make the system of change
wheels still clearer. Let the pitch of guide screw be as before,
namely, two threads to the inch, and let five be required. Place the
numbers thus, 2-5, and divide both into their factors, twice one, or 2
× 1, and twice two and a half, or 2-1/2 × 2.
2 1
--------
2-1/2..2
Now, it is certain you can have no wheels containing half notches, nor
of such small numbers of cogs, therefore multiply by any convenient
numbers--ten, for instance, which give
20..10
------.
25..20
This is all that is needed. Put a wheel of twenty cogs on the
mandrel, and let it work into one of twenty-five on the stud, the
latter carrying a pinion of ten cogs, driving one of twenty cogs
on the screw. To prove this, as before, the screw must make two
revolutions to carry the tool one inch, and during that time the
mandrel (carrying screws to be cut) must make five revolutions. Let
the screw wheel revolve twice, the pinion of ten teeth will revolve
four times (twice 20 = 40 and four times 10 = 40); but as the stud
wheel and pinion are as one, and revolve together, the stud wheel
must also revolve four times (4 × 25 = 100). Thus the mandrel wheel
will revolve five times as required (20 × 5 = 100). No other method
is so easy to understand and work as the above. To give full details
and provide working drawings of the various screw cutting lathes
by different makers would be to extend the present series to an
unnecessary limit, but we shall nevertheless describe another method
by which these lathes with traversing rests are made self-acting when
screw cutting is not required. Instead of a leading screw extending
from one end of the lathe to the other, there extends a bar of steel
about the diameter of the screw with a key groove or slot from end
to end. This bar is supported in bearings at each end, and carries
upon its surface a ferrule of steel with a screw cut upon its outside
similar to Fig. 174, where A is the bar and B the ferrule. The pitch
of screw is coarse, being similar in its object to the guide screw
previously described. A pin fixed into this ferrule falling into the
slot permits it to travel along the bar but causes both to revolve
together when the bar is put in motion by means of a cog wheel or
strap pulley at one end. Along the same side of the lathe bed, and
level with the surface, is a rack, C, upon the face of which works a
pinion, D, carrying on the same axis a cog or rather worm wheel, E,
to gear with the screw ferrule. The bearings of this axis are secured
to the saddle of the slide rest. Consequently, when the bar revolves,
the screw is also put in motion, the wheel, A, Fig. 175, and pinion,
B, partake of the movement, and the latter traverses the face of the
rack, carrying the saddle with its rest and tool holder along the bed
of the lathe. By this movement the screw ferrule traverses the bar as
it revolves, thereby virtually becoming a long leading screw. In Fig.
176, which is a view from above, looking down upon the lathe bed, A
is the rack, B, B, the saddle cut away to show the rack and pinion C;
E is the worm wheel, D the long bar, the screw ferrule, being under
the worm wheel, is not visible. As above arranged it is evident that
the ferrule might escape from the worm wheel instead of proceeding on
its proper course. This is prevented by its lying in the hollow of
a bracket attached to the rest, as Fig. 177, A and B. This retains
it in contact with the worm wheel, and also becomes a support to
the long bar. Fig. 174 shows the side of the lathe that is furthest
from the operator. The axle of the worm wheel and pinion carries a
handle on the near side to give the workman power to use the rack by
hand as a quick return movement. The above is frequently attached to
those lathes provided with a long screw, the latter being on the near
side and the bar on the other. Thus, the same lathe can be used for
ordinary or screwed work. Whitworth, however, commonly uses the long
screw placed between the beds or bearers of the lathe for screwing
and surfacing, instead of adding the apparatus just described. A long
screw being, however, an expensive affair, ought to be carefully
cherished, and when the work is such as the bar will suffice to
accomplish, it may be well made use of to preserve the screw. There is
an arrangement for forward or cross-feed in the above apparatus, the
principle of which is the connection of the cross screw of the rest
by means of a pinion, either with the worm wheel or with a cogwheel
on the same axle. When this is put into gear the rack and pinion are
disconnected. It is also necessary to provide a method of reversing
the motion of the long screw bar, especially when the cross feed is
used in surfacing. There are several modes of accomplishing this,
the best being the following, Fig. 178, which is a simple expedient
applicable to lathes or other machines. A is the end of the screw rod,
with bevel wheel attached, B, C, are similar wheels on the axle, D,
the latter being movable endwise in its bearings by means of the lever
handle; E, D, is the driving axle. By moving the lever to the right
B is geared to A. A movement to the left brings C into connection.
Between the two, both wheels are thrown out of gear, and though they
may continue to revolve, the screw bar will remain still. By this
contrivance the motion of the leading screw is reversed or stopped
in a second, with the advantage of its being unnecessary at the same
time to stop the working of the lathe altogether. It has probably
struck the reader that as the size of the change wheels are various,
there would be in some cases an impossibility of their touching so
as to gear together. This is partly remedied by the interposition of
dummies, or idle wheels, and partly by the following arrangement. The
stud wheel, or dummies, as the case may be, are not upon axles fixed
to the lathe-head or end standard, but upon such an arm as Fig. 179,
which turns upon a pin at A, and carries in the slot the pins upon
which the different wheels centre. These pins being made similar to B,
C, can be placed at any position in this slot, and are fixed by a nut
underneath. This arrangement gives considerable power of adjustment,
and enables the workman to place together wheels of various sizes
according to his need. It would not be by any means difficult to
arrange the above lathe for screw-cutting, especially if the pitch
of the required screw is not of great importance. An amateur's lathe
might be thus fitted to serve a good many purposes, although a leading
screw is to be preferred as the more complete and perfect arrangement.
It cannot, however, be denied that there is great friction produced
by the worm wheel and endless screw, which soon tells its own tale by
the wear and tear produced, and the power is not so economically used
as it is when the screw works in a nut. Expediency, however, in this,
as in many similar cases, must decide for or against the arrangement
in any particular case. It is, at any rate, a good addition to a
lathe provided with an ordinary leading screw, more especially in the
facility with which it can be arranged as a self-acting cross-feed to
the rest when used for surfacing.
[Illustration: FIG. 172.]
[Illustration: FIG. 173.]
[Illustration: FIG. 174.]
[Illustration: FIG. 175.]
[Illustration: FIG. 177.]
[Illustration: FIG. 176.]
[Illustration: FIG. 178.]
[Illustration: FIG. 179.]