It now becomes necessary to speak of another addition to the lathe,
by means of which the use of the slide rest is considerably extended.
We mean the overhead motion. Of this there are several patterns, and
we have sketched three of these. As to their respective merits we can
hardly venture to speak. They all answer equally well the purpose for
which they have been designed, and the turner must select according to
his fancy, or, if he please, design a better for himself. A, Fig. 144,
represents the lathe bed. From the left-hand standard rises a round
iron rod, not less than one inch in diameter. This is not generally
fixed, but is attached to the standard by two staples, _a, b_, which
hold it securely in an upright position, but allow it to turn with
its projecting bar F, F, after the manner of a crane. It may thus be
turned back, out of the way, or brought into any desired position. The
part F, F is made to slide up and down on the part B, and is fixed by
a clamping screw D. Thus, if the cord should break and require to be
shortened, the arm can be brought nearer to the bed of the lathe. Upon
F, F slide two rings, or rather short pieces of tube, from which depend
two India rubber springs (door springs), E, E, now procurable at any
ironmonger's at one shilling each. From these hang double pulleys, or
better still two single ones. These pulleys, with their attachments,
are adjustable at any position on the arm F, F, which may be round or
square. If considered desirable, a second standard can be added, so as
to uphold both ends of this bar; but it is hardly necessary, as the
latter is seldom required of greater length than half that of this
lathe-bed. It is evident that the above addition to the lathe can be
made complete for a few shillings. The following are more expensive,
but more general, the writer having devised the above to suit his own
fancy, and for his own use. In Fig. 145 A represents the standard as
before, the top of which is forked, as shown at E, and sustains the
ring, free to revolve in its arms, as seen in the sketch. Through
this passes a bar, B, B, with a heavy ball C cast on its end to act
as a counterbalance to the longer arm and its connections, and to
keep the cord stretched. By sliding this bar either way through the
ring which supports it, the tension of the cord can be increased or
lessened.[11] When in position it must be very securely fixed by the
screw T, which should not simply press against it, but enter one of a
row of depressions made for the purpose. The pulleys C, D are double,
as in the previous plan of overhead, and are likewise adjustable at
any position on the bar B, B. The only drawback to this pattern is the
danger of the heavy ball slipping out and falling. We prefer to hang
a weight from the end of the lever, as shown by the dotted line. This
may be within a few inches of the floor, and if it should fall no
harm can ensue. The third pattern, Fig. 146, is the most expensive,
but although it is of a more finished appearance, and wears an aspect
more stiff and stable, it is not practically any better than the last.
From a standard A, with overhanging bar F, F, is suspended a frame H,
by means of two coiled springs in brass boxes B, B, which keeps up the
necessary tension on the cord, or rather cords, for in this case two
are needed--one from the flywheel to the small pulley, and a second
from the roller to the slide rest.
[11] Sometimes the bar is merely hung on pivots, and the weight is
made to slide upon it.
[Illustration: FIG. 144.]
[Illustration: FIG. 145.]
[Illustration: FIG. 146.]
[Illustration: FIG. 145.]
[Illustration: FIG. 143]
This roller, which may be grooved or plain, may be replaced by a
second small pulley, which is capable of being slid along the round
bar which forms its axle and turns with it between the centre screws.
In this case the bar is made with a groove or channel along its
length, Fig. 147, and a pin projecting from the central hole in the
pulley enters this groove. Thus the two will turn together, and at
the same time the position of the pulley is adjustable at pleasure.
The tension of the springs B, B, is increased or lessened by a turn
or two of the nuts C, C, just in the same manner as the spring of
the safety valve is adjusted on the boiler of a locomotive.[12] The
application of the overhead motion is plainly shown in Figs. 143 and
144. The work is fixed in the lathe, and the mandrel kept stationary.
The cord passes to the overhead directly from the fly wheel, and
thence to a pulley on the screw of the slide rest, as in 143, to a
drill, as in 144, or to any other apparatus at pleasure. To fit the
work in the lathe in such a manner as to enable any point in the face
or side to be operated upon, a division plate and index are required.
The first is a round plate of brass or gun metal, 1-16th of an inch
thick, drilled with holes in concentric circles. The index is a steel
spring with a projecting point, which, entering any one of the holes,
retains the plate, and with it the pulley, on the face of which it is
fixed immovable. There are generally four circles of holes, the number
in each selected with reference to its divisibility by the greatest
possible number of divisions--thus 360, which is usually on the
outside or largest circle, can be divided without a remainder by 2, 3,
4, 5, 6, 8, or 9--144 is a good number for the second circle, being
divisible by 2, 3, 4, 6, 8, 9. The other two may be 112 and 96. The
uses of the division plate are many. Eccentric cutting and drilling
could not be done without its aid, and wheel cutting for clocks, or
for making cycloidal and other chucks, is entirely dependent on this
contrivance.
[12] The frontispiece shows another superior form of overhead, with
balance-weights hanging close to the ground behind the lathe.
[Illustration: FIG. 147.]
The division plate and index are shown in Figs. 147, A, B, C. The part
B is a knob with a round hole through it, to take the tail of the
spring C. This is shown again at C on a larger scale, to bring to view
a slot which allows of some slight adjustment of the point, to suit
the different-sized circles. The milled-headed screw clamps the point
at any desired part of this slot. When not in use, the spring is drawn
back so as to release it from the division plate, and turned down in
the position shown by the dotted line.
The overhead apparatus is applicable not only to revolving cutters in
the slide rest, but to other contrivances available where the latter
is not possessed. There are many cases in which the back poppet may be
brought into use to hold revolving drills, or stationary tools, if the
resistance to their action is not too great. The only damage that can
well happen in this method is to the pin working in the slot on the
outside of the spindle. If this is of steel and tolerably stout and
strong (being very short it cannot be subject to any very injurious
strain), no harm is likely to result. With this it is possible to
drill very neatly, and also to do a little eccentric cutter work,
under certain conditions to be described. Screwing, or rather chasing,
may likewise be done very passably. We do not, of course, advise this
course when the more perfect slide rest is at hand, but there are many
who are obliged to put up with all sorts of homemade contrivances, and
to press into service for divers operations apparatus and tools not
precisely meant to be thus used, and it is as well to learn how to act
under an emergency, even if the practice is not intended to be carried
out generally. The drill stock is shown in Fig. 148, in which the
screw A fits into the spindle of the back poppet. If the hole in the
latter is conical instead of cut with a screw thread, the drill must
be made accordingly.
[Illustration: FIG. 148.]
The pulley of brass has a hole drilled through it, and the screw is
also drilled, the hole in the latter being rather larger than that
in the pulley (which is tapped). The steel pin H, shown white in
the sectional drawing, passes truly through the centre of the screw
in which it revolves, and is screwed into the pulley. The socket
of steel to hold the drill is then screwed or soldered into the
opposite face of the pulley. The black part shows a flange on the
screw which abuts against the cylinder of the poppet into which it is
screwed. Thus the pulley, being attached to the cord from the overhead
apparatus, is free to revolve upon the steel pin with great rapidity,
and will carry round any drills or cutters placed in the socket B. The
simple straight drill being thus worked and advanced by the leading
screw of the poppet, will suffice for holes in any work held on the
mandrel. On page 335, vol. ii., of the _English Mechanic_, this method
is mentioned, and also a similar pattern of slide tool to be used for
boring, being, in fact, the head and spindle of the back poppet, but
instead of the standard and sole, a pin, like that of a =T=, to
fit the socket of the rest. This has the advantage of the back poppet,
but must be made on purpose, and if any special slide tool is added,
a proper slide rest is by far the best. Our present purpose is to
describe the use of the back poppet as a substitute for a better tool.
With the following modification of the eccentric cutter, a fair amount
of good ornamental work may be done. Fig. 149 represents the back
poppet with the apparatus in position. The various parts are shown
separately in A, B, C, D. A is a small frame, two or three inches
long, and 3/4 to 1 inch wide, cast in iron, with a circular piece
at the back, 1-1/2 or 2 inches diameter. This circular piece is to
be accurately divided on the edge with any even number, which is
divisible as above explained. This is turned and drilled through the
centre. B is a flange-shaped piece to screw to the poppet as before,
and this is also accurately drilled, and the two are attached by a
central steel pin, so that the two flanges can turn face to face, the
outer one with frame revolving against the other. The pin must be
put in its place in B, and turned at the same time as the face of B,
that it may be truly central. The divisions on the edge of the outer
flange are to be drilled like the division plate of the lathe, or cut
into cogs, and in either case can be held at any point by a spring
detent or index attached. C shows the front of the frame, with a screw
down its centre and traversing slide _f_. The head of this screw is
divided, and a small brass index marks its position; one side of the
frame may likewise be divided. The small brass pulley and drill socket
are fixed to revolve in the traversing nut or slide _f_, as shown in a
side view at D. Drills of all shapes, as E 1, 2, 3, 4, may be fixed in
the socket at pleasure.
[Illustration: E]
[Illustration: FIG. 149.]
[Illustration: Fig. 150.]
[Illustration: Fig. 151.]
[Illustration: Fig. 152.]
[Illustration: Fig. 153.]
It is evident that the cutter frame last described having a division
plate of its own dispenses with the necessity of one on the pulley of
the mandrel. Indeed it was the impossibility of supplying the latter
on the face of the cogwheel of the author's back-geared lathe that
necessitated the substitution of that alluded to. The latter may
consequently be omitted if applied to a lathe already fitted with
division plate and index. With crank-form drills the cutting edges
of which may be as E 1, 2, 3, 4, or other form, such patterns can be
cut, as shown in Figs. 149, 150, 151, 152, which are merely samples
of the simplest combinations of circles intersecting in straight,
spiral, or other lines. Although not now treating of eccentric work,
we may state in passing that to produce the pattern round the edge
of 150 the division plate of the instrument only is required. Select
a crank-form drill which by its revolution will produce the required
size of circle. Turn the divided screw head until you find upon trial
that the circle will come at the required distance from the edge,
and putting the lathe in motion cut the first circle. The work on
the mandrel is understood to be stationary, the mandrel being fixed
so as not to turn. The cutter frame is now turned on its axis, one,
two, or more of its divisions, and fixed by its detent, and a second
circle cut intersecting the first. This is repeated till the whole
circle is completed. The straight line of circles in Fig. 151 is cut
as follows:--The first circle is made as before, but the cutter-frame
should be placed so as to stand either horizontally or vertically.
Between each cut the divided screw-head is moved so many points
according to the proposed fineness of the pattern, and no movement
of the cutter frame on its axis is to be made. The spiral, 152, is
simply the result of a combination of the two movements. Start from
the centre by turning the screw head until the shank of the crank-form
drill is coincident with the axis of the mandrel. Cut the central
circle. Thence for each successive circle turn the screw head and the
frame an equal number of points, and the pattern will be formed. If
the divided screw head is made to take off and a handle substituted,
stars formed of channelled lines, or radial flutes can be made as Fig
153, but in this case the crank form drill is replaced by a plain
straight tool with a rounded end like 149 E 4. Set the tool as for
pattern 150 straight across the face of the work, put the drill in
rapid motion, let it be advanced by the leading screw of the poppet so
as to penetrate slightly, and with the handle that has been attached
to the screw of the cutter frame cause it to traverse the face of the
work as it revolves. When one flute is thus cut, turn the frame on its
axis as many points as required, and proceed with number two, and so
on to the completion of the star. It is not necessary at present to
describe other patterns. Fig. 153 is therefore merely given without
special details of the method of producing it.
We must now return from this digression to speak of other applications
of the slide rest. It is evident that when connection is made between
the overhead and a pulley on the screw of the slide rest, the latter
becomes self-acting. The speed is, however, far too great, and in
addition, the mandrel is stationary. The above connection, therefore,
is not practically possible, and the overhead is only connected with
the pulleys of revolving drills and cutters fixed in the tool-holders
of the rest. It is, however, very important to be able to form some
such connection between the lathe mandrel and the screw of the rest,
for the purpose of cutting screws or spirals. A little consideration
will show the principle of this arrangement, from which some practical
plan is not difficult to design. If, for instance, the tool simply
remains in contact with a cylinder while the latter revolves with the
mandrel, a simple line will be cut round its circumference; but if,
while the mandrel revolves once, motion is given to the screw of the
rest by which the tool is made to traverse a distance of one-eighth
of an inch, the commencement of a spiral having that pitch will be
made. A perfectly smooth surface, as it leaves the lathe, in which a
slide rest has been used with a point tool, is in reality cut with a
very fine screw thread readily discernable under the microscope. We
have, therefore, only to devise some method of giving regular motion
to the screw of the rest while the work revolves as usual, in order
to turn plain surfaces, screws, or spirals. For the purpose of plain
turning a plan is sketched by Nasmyth in the last chapter of "Baker's
Mechanics," in Weale's series. A spur wheel is represented fixed to
the slide rest screw, the teeth of which are alternately caught at
every turn of the work, by an arm fixed to the latter, after the
manner of a lathe carrier. This plan is simple, and might be to some
extent used, but for one defect, due to the fact that the slides
of ordinary rests are the reverse of what is required to make this
plan available. The screw which advances the tool towards the work
is generally underneath that which moves the tool along the surface
of the work. The result is that, when the tool-holder is advanced
to take the deeper cut the spur wheel is brought nearer to the arm
which acts upon it, and greater traverse is thus given to the screw.
This is shown in Fig. 154. A is the spur wheel, B the cylinder to be
turned, C the arm or carrier. The arrow shows the direction of the
movement. Now, if the lathe is put in motion, the arm will remain
in contact with one tooth of the wheel until both arrive at _b_,
giving a certain amount of motion to the screw, and thence to the
tool-holder. After one cut is thus taken, the lower screw of the rest
is turned to advance the tool nearer to the work, the effect of which
is to cause the arm to extend further over the wheel. Suppose its
position represented by the dotted line, it will remain in contact
with the tooth till both arrive at _c_, having thus traversed a
larger arc, and given more movement to the tool. Now if the frames
of the slide rest were made to cross in the contrary direction so
that the screw to advance the tool towards the work was above that
which gave the traverse in the direction of the bed, this objection
would no longer hold, and the above gearing would answer very well,
since the necessary advance of the tool would not affect the relative
position of the spur-wheel and carrier. In Fig. 5 of the same book,
in which the gearing is effected by two cogwheels, this alteration
in the rest appears in the drawing. In this case the work and rest
are connected for screw cutting, and the arrangement is satisfactory
and simple, and for the amateur especially is the simplest and best
that can be devised. The range of screw pitches is however limited,
and the rest must have a left-handed screw, or the result will be a
left-hand thread to that which is cut. Hence another device has been
arranged, represented in Fig. 155, A, B, C. A shows the apparatus
complete. B is an arm of iron or brass which is about 1/4 inch thick
or rather more. This is first slipped over the mandrel screw in front
of the poppet and fixed in any desired position by a screw passing
through the slot _a_, into the face of the poppet. This slot allows
the arm to be raised or lowered at pleasure and adjusted, as will be
presently described. In the slot formed in the long arm B, pins D with
nuts, fit, on the rounded part of which cogwheels, _b_, _c_, _d_,
are made to revolve and to gear with each other, and with a similar
wheel attached to the back of the chuck, C. The centre of the outside
wheel, whether one, two, or three are used, is connected to the screw
of the slide-rest. For the production of a right-handed screw, the
intermediate wheel comes into play, simply to reverse the direction
of the motion imparted to the screw of the slide rest. The number of
teeth which it may contain is of no importance, the calculation of
the change wheel teeth being only necessary with the first and last.
The central one is called an idle wheel, though its work is equal to
that of the rest. Thus, suppose the wheel on the chuck to contain 40
teeth, and the third wheel 20, while the former revolves once, the
third will, if in immediate contact with it, revolve twice, introduce
an idle wheel with 10 teeth between these two. The wheel, with 40
teeth, revolving once, the idle wheel will revolve 4 times--the third
wheel twice, just as if the idle wheel was not in use. In any train of
wheels, if we regard relative speed, any number between the first and
last become similarly idle wheels, and the ultimate result is the same
as if the first and last were in immediate contact.
[Illustration: FIG. 154.]
[Illustration: FIG. 155.]
Take, for example, the following train of five wheels, even
numbers being given for the sake of clearness, to represent the
circumferences or number of cogs in each, Fig. 156. We have here, as
the first and last, 6 in. and 120 in., and, if in contact, the first
must revolve 20 times, while the latter revolves once. Interpose the
three idle wheels of 10, 30, and 60 in. respectively. During one
revolution of the largest wheel, the second will revolve twice, the
third four times, the fourth twelve times, the fifth twenty times, the
same precisely as if the first and last had been in immediate contact.
The range of a slide rest-screw is quite long enough for many purposes
of the amateur, and a connection thus made between the mandrel and
such screw is what may be termed a miniature of the arrangement
adopted in the large self-acting lathes. In the latter, however, a
leading screw is added the full length of the bed along which the
slide rest travels bodily. We may, therefore, consider the screw of
the slide-rest a leading screw, and make use of the rules applied in
the case of large lathes to decide the proportions of wheels required
to cut a given screw. It is plain that when the pitch of the required
screw is greater than that of the leading screw, the revolution of the
latter must be at a quicker rate than the former. If, for instance,
a spiral is to be cut, like the Elizabethan twist, containing but
one perfect thread in two inches, while the leading screw contains
twenty threads in the inch, or forty in the 2 in., the latter must be
arranged to make forty revolutions while the former makes one, because
it takes 40 revolutions to carry the tool along 2 in., which is the
pitch of the required spiral. The two outside wheels must therefore
bear that proportion to one another. Forty to one, however, would be
a practically difficult ratio, to place as described, even a pinion
of ten teeth on the leading screw requiring 400 teeth on the chuck.
Hence a different arrangement would be necessary if such very great
difference exist between the pitch of the leading or rest screw and
that to be cut. The same obvious difficulty would occur where a very
fine screw is required, and the pitch of the leading screw is coarse.
This will have to be referred to again. One example, therefore, of the
method of overcoming this difficulty will suffice. A train of wheels
is shown in Fig. 157, of which A has 60 teeth, B 10 teeth, C, _on the
same axle and united to_ C, 30 teeth, D 20 teeth. While A turns once,
B will turn six times, C necessarily six times also, D nine times. In
this case, if the first and last had geared together D would have made
but three turns, while A made one.
[Illustration: FIG. 156.]
[Illustration: FIG. 157.]
The following is an easy method of calculating a series of such change
wheels:--
Write down the number of threads in the screw to be cut, and also
the number of threads in the leading screw; multiply both by any
convenient number likely to give such results as to tally with the
cogs in the set of change wheels. Suppose it is desired to cut eight
threads to the inch, and that the leading screw has two threads in
that length. Then:
8 80 8 120 8 160
- × 10 -- or - × 15 --- or - × 20 = --- &c.
2 20 2 30 2 40
If you have either couple of these wheels, you can put one on
the leading screw, and another on the mandrel, and fill up the
intermediate space with dummies.
If the above is inconvenient, or the wheels are not to hand exactly
as required, proceed thus. If you have the wheel for the mandrel, and
the one you wish to use for the leading screw has only half the proper
amount of teeth, it is evident that the leading screw would revolve
twice as fast as required, for they are proportioned as two to one--or
if I have the proper wheel for the leading screw, and the wheel I
wish to use for the mandrel has twice the proper number of teeth, it
amounts to the same thing. You can get over the difficulty by using
any two wheels which are in the proportion of two to one (say 20 and
40, 30 and 60, 40 and 80, &c.), and coupling the two firmly together,
so that the larger wheel of the two works into the mandrel wheel (or
dummy working into the mandrel wheel) and the smallest into the screw
wheel (or its dummy); if the speed is wrong in the contrary way, so
that the case is reversed, the coupled wheels are made to gear in a
reversed direction; and whatever may be the amount of error, whether
such as to cause either mandrel or screw to revolve 1/8, 1/4, or
3/4 too slow or too fast, the same arrangement may be pursued, the
coupled wheels bearing that proportion to each other. The above method
was communicated to the _English Mechanic_ by a working man, James
Connor, and is perhaps as easy as any; but tables are published of
change wheels for any pitch, with any thread of leading screw. Where
it is not possible or inconvenient to apply the above arrangement,
and where only a few pitches are likely to be needed, another method
can be arranged by connecting the lathe pulley to the overhead motion
and thence to the screw of the rest. Such an arrangement is shown
in Fig. 158. A is the fly wheel, B mandrel pulley, C, D, pulleys on
the overhead, E pulley and screw of the slide rest. To facilitate
calculation, let diameter of C equal that of the part of the mandrel
pulley that drives it, by which it will revolve in the same time. The
calculation of the sizes of pulleys, D and E, will be the same as for
the cogwheels of the screw-cutting lathe, circumference and number
of cogs being, so far as calculation is concerned, the same thing.
Let the leading screw have eight threads to the inch, and let it be
required to cut a spiral of two threads to the inch. Proceed as before
by dividing the required number of threads to be cut by the number
on the leading screw 2-8 =·25. The pulley on the leading screw will
be therefore one quarter the size of that on the overhead (which is
virtually that on the mandrel as it revolves at an equal speed with
the latter). The overhead pulley may be conveniently twelve inches
diameter, and that on the screw three inches. While the mandrel makes
one revolution the screw will make four, advancing the tool half an
inch, and cutting one thread of a spiral in that distance. The next
revolution will advance the cutter a second half inch, cutting a
second complete thread of spiral. Two threads will, therefore, have
been cut in the space of one inch as desired. By the above method
short screws and spirals of divers pitches may be cut at pleasure.
_The practical difficulty in this plan is due regulation of the
various speeds._
[Illustration: FIG. 158.]
We here introduce a modification of self-acting lathe for cutting
Elizabethan twist described by Mr. Wilcox in his MS. before alluded
to. The work is here done by a leading screw and toothed gearing, the
principle being that of the ordinary machine lathe. A chuck, A, with
cogwheel attached holds the work as usual, the back centre being also
required. The cogwheel gears with one of less diameter attached to the
end of the guide-screw B. On the latter works the rest C, in which
is a nut of the same thread as that on the guide screw, and which
holds the tool in a notch or hollow upon its upper part. The tool is
then used by hand, but is guided in its course along the surface of
the work to be turned. This guide screw, with the rest and cogwheel,
is mounted on a board as a separate piece of apparatus, and is, when
used, clamped on the lathe bed. As the rest is after all little else
than a large nut, it must be prevented from turning round, and must be
arranged to bear the pressure of the tool, relieving the long screw
from the strain that would be thus caused. This is effected by a long
flat bar--like the rest of a chair maker's lathe--extending the full
length of the bed shown here at E, and supported by standards F, F. A
projecting part of the rest bears upon this, and slides along it with
the tool. The work is begun with the rest on the right hand, and some
care is necessary as it nears the cogwheel on the left, when the work
must be stopped and the whole run back by hand to its starting-place.
This is one chief defect in this apparatus, for the rest very quickly
traverses the length of the screw, and great delay is caused by having
thus constantly to stop the lathe and reverse the motion. The closest
attention is also necessary to prevent the rest from overrunning its
mark and striking the cogwheel. It is evident that by using different
pitches of cogwheels, many screws of varied threads can be cut in the
above lathe. There is, however, another defect in the above tool not
noticed by the writer of the MS. from which the description is taken,
namely, the difficulty (common to all such contrivances for turning
wood), of obtaining the requisite speed. If the work is put in rapid
motion, without which wood will not be cut clean, the movement of the
rest will be so rapid also, from the effect of the multiplying wheel,
that the tool will be carried from end to end in a few seconds. We
will therefore proceed to describe a modification of this and similar
apparatus, which allows the tool a slow traverse lengthwise of the
work, but gives it immense rapidity in the necessary direction. The
following is applicable to the lathe described above, to the ordinary
slide rest worked by hand, or to the large self-acting screw-cutting
lathes used in manufactories, and is specially adapted for cutting
spirals or other patterns in wood. In the Fig. 160, A represents a
shank, which may be made of any shape to fit particular patterns of
tool holder. This shank is turned up and becomes a cylinder at B, like
that of the ordinary revolving cutter. This part is bored, and fitted
with a steel spindle, which should be of strength proportionate to
the size of stuff likely to be operated on. One end of the spindle is
fitted with a brass pulley, from which a cord is to be attached to the
overhead apparatus, the other end terminates in a round or hexagonal
boss, D, round the margin of which are securely held, by means of
bridles B1 or other simple contrivance, a pair or more of small sharp
gouges. This apparatus is put in the tool holder of the slide rest,
set to the angle that corresponds with that of the screw or twist,
and put in rapid revolution by means of the overhead apparatus. The
whole rest, or merely the upper part, is then put in motion by one of
the before-named means, and the tool advanced to make a cut. However
slow the movement of the rest may be, the cutters move with such
velocity as to make clean and beautiful work.[13] This may be applied
to the slide-rest of the twist lathe, just described, or any similar
apparatus. In the overhead, a roller, supplying the place of the
second pulley, as described in a previous page, will allow the second
cord sufficient power of traverse to keep up a proper position in
reference to the pulley C. Revolving cutters on the same principle as
the above, have of late years come into extensive use in wood-cutting
and carving machinery. The steam planes now used in the preparation
of flooring boards, the spoke turning lathe, moulding and shaping
machines for wood are all thus fitted. The gouges or other cutters
used must not be placed radially, but as tangents to the circumference
of the boss in which they are fixed. An improvement upon the simple
bridle to hold the cutters would be the substitution of Babbage's
tool holder, four radial arms being substituted for the metal boss
above alluded to. This tool is described and figured in Holtzapffel's
"Mechanical Manipulation," to which the reader is referred for an
accurate description. It chiefly consists of a shank turned up at the
end like Fig. 161, the outside, at B, being rounded to fit hollow
gouges such as Fig. 161H; against this the hollow of the gouge is laid,
and opposite to it, at C, a small piece like D. A band or hoop, E, is
now placed over both the above, and between the two a wedge-shaped
piece, F, which is intended to bring a strain upon the hoop and
tighten it round the gouge. This last piece is attached to the shank
or holder by a screw passing through it into the shank. The tighter
this screw is worked the lower the central wedge is drawn down, and
the tighter the hoop is made to embrace the tool. This holder is also
modified to suit flat chisels. The tool cannot possibly slip, but
can be released in a few seconds if desired. Such a termination of
two, three, or four arms revolving on a spindle in place of the boss
would form the best possible circular cutter for shaping lathes. The
above lathe for producing Elizabethan twist introduces the reader
to self-acting, screw-cutting, and machine lathes, such as are used
in all large manufactories. Hand-turning, indeed, except in such
light work as turning up the heads of small bolts, and finishing up
work which, from peculiarities of form, cannot easily be done by
self-acting tools, has become a thing of the past in factories of any
pretensions; hand labour, in fact, not only no longer pays, but is
quite insufficient to meet the requirements of the present age. Take,
for example, a piston-rod requiring to be as "true as a hair," to use
a common expression, from end to end. The traverse of an ordinary
slide-rest would only enable us to turn a short length at a time,
and the result, when accomplished, would not be satisfactory. With a
self-acting lathe the tool traverses in a perfectly straight line from
end to end, is returned to its starting-point by a quick traverse, and
the movement repeated until the proper dimensions are attained. The
process is not absolutely rapid, because time is requisite in cutting
iron and steel, but the work is executed as speedily as the nature of
the metal to be cut will allow, and the execution is perfect. Of late,
however, even the above has been improved upon, for two cutters are
used at once--one on each side of the bar, so that by one traverse
of the rest a cut of double depth is taken, and the tendency of the
work to spring away from one tool is counteracted by the operation of
the other. But we require something more than speed in the present
day. We must have work of absolute truth of measurement. What would
our ancestors, or the immortal Watt himself, think of measuring work
to the hundredth part of an inch, yet it can be and is done to the
thousandth part. I believe I am correct in saying that Whitworth
constantly gauges work to that or even a higher degree of nicety. It
is not too much to assert that the best engines of the present day
really work with the precision of clockwork, and even the bore of
an Armstrong or Whitworth gun is executed with no less accuracy and
precision. Look again at that ponderous affair, the steam hammer,
so ponderous as to require a depth of solid masonry and timbers to
sustain the force of its terrific blows. In a few minutes a solid
mass of metal is reduced to a flat plate such as would have taken the
united strength of a dozen men wielding the heaviest sledge-hammers
for an hour at least. The ground beneath the feet of the spectator
trembles at every blow of the machine. The work is done, and behold!
with a touch the same ponderous concern becomes a nutcracker, not even
injuring the kernel when it breaks the shell. Such is one of a hundred
specimens of accurate workmanship carrying out in practice the clever
designs of the mechanical engineer. Sweep away self-acting machinery,
and such work would become a simple impossibility. Again, so long as
turning, boring, planing, and such work was performed by hand alone
(even after the introduction of the slide principle), an attendant was
required at each machine. Now that the latter is contrived to regulate
its own movements, one man at two or three lathes is sufficient. Thus
the same article that was once imperfect and costly, owing to the
demand on skilled labour, which was difficult to procure and at best
inefficient when procured, has now become cheap, and to all intents
and purposes perfect; and although the demand for such work increases
year by year, self-acting machinery being constantly improved and
simplified, enables the manufacturer to keep pace with the demand.
[13] The spiral chuck for fine work in ivory and costly woods is
described in a later page.
[Illustration: FIG. 159.]
[Illustration: FIG. 160.]
[Illustration: FIG. 161.]
The advantages of self-acting machinery are of course chiefly confined
to the trade, and it is not often that the mere amateur requires such
aid. Indeed, the expense necessarily attendant on the fabrication of
these machines deters the great majority from making such a purchase.