The first requisite for the above work is a well made and sharp tool,
for, strange as it may appear, a keen edge is as necessary for making
good work in metal as in turning wood. The principle of this cutting
edge must be well understood, and this has been well explained by
Nasmyth and others.
The remarks of the above eminent mechanic upon this subject, as also
those of Professor Willis and Mr. Babbage have been embodied in a
very excellent paper by Dodsworth Haydon, Esq., an amateur, and will
be found in the Appendix to this work. The whole principle of the
formation and application of cutting tools is explained in that paper,
so that it only remains to treat briefly of a few special _forms_ of
tools which are required for metal-turning in the lathe, whether by
hand or by the aid of the slide-rest. In the first place, however,
a word or two may be necessary as to the kind or quality of steel
required for such tools. What is called Blister steel may be at once
passed over as unfit for the formation of tools--it is, however, the
raw material (so to speak) from which, by the process of reheating
and welding, the next quality, called Shear steel, is made. When bars
of this are similarly heated and again welded into a homogeneous mass
under the tilt-hammer or between rollers, double shear-steel is made,
which is of extensive use for cutting tools, and must, moreover, of
necessity be used in almost every case where there is to be an iron
shank, for economy's sake, the steel being then welded to the iron,
and forming that part of the tool intended for the edge. The third
and best kind of all is Cast-steel, formed of blister steel, melted
at an intense heat and run into iron moulds. This, however, can be
welded only with great difficulty, and hence the whole tool, whatever
its length, must be of the same material. This can be purchased in
bars of a convenient size of round, triangular, square, or other
section, and needs only the careful use of the hammer, file, and
grindstone to become a tool of any required pattern. It would be very
advantageous to an amateur to master the art of forging in a small
way to enable him to make his own tools, for he may sometimes require
them of unusual form, and if he lives far from a manufacturing town
he will find it very difficult to get them fashioned to his liking.
Cast-steel will not allow of the welding heat applied to iron--it will
burn, and cannot then be made to recover its proper consistency, and
is for ever useless for the purpose in question. Double shear will
take a moderate white heat, while cast-steel must not be brought to
a higher temperature than that indicated by bright red--a point never
to be forgotten when shaping a tool at the forge. There are in every
workshop a number of files laid aside as worn out. These being made of
the best cast-steel, are invaluable to the turner in metal, as they
supply the best material for his tools at no cost whatever. To begin
with the saw files (called, by a horrible perversion of mathematical
definition, "three square"). Here you possess a tool at once for the
mere trouble of grinding off the teeth and reducing the sides to a
smooth surface. Each angle is equally useful--each 60°, which, as the
paper above alluded to demonstrates, is the best angle for cutting
iron. On brass, however, its use is by no means to be recommended,
being, as Holtzapffel remarks, "too penetrative and disposed to dig
into the work." It is to be used upon iron in the position shown in
Fig. 110, where A is the rest, B the cross section of the tool, C the
diametrical line. The side, of which D is a continuation, is to form
very nearly a tangent to the circumference of the work, being, as
explained in the Appendix, only 3° from that position. Worn out square
files being of rectangular section, are exactly suited for brass
turning, for which metal a cutting edge is required of 80° to 90°, the
former for the first or roughing down cut, the latter for finishing.
The position of such tool is shown in Fig. 111.
[Illustration: FIG. 110.]
[Illustration: FIG. 111.]
The flat files are not altogether so useful for making turning tools
as those alluded to; they are too thin in proportion to their breadth.
Their sides, moreover, generally speaking, are not rectilineal, but
curved, so that they are more fit for grinding off at the ends to
form brass-turning tools with rectangular edges. They may, however,
by careful forging, be made to assume a square section, and thence
be formed as desired; but a rectangular bar of steel, ready-made, is
then to be preferred. A round or rat's tail file is of far greater
service than the last-named--not, indeed, in its own shape, for the
reasons stated by Mr. Haydon, but as being capable, with very little
labour at the forge, of being converted into a bar of square section.
The first tool to be made from such a bar is the graver, _the_ tool
of the watchmaker, and not less useful to the general mechanician.
This is formed by grinding the end of a square bar diagonally, so as
to produce a lozenge-shaped face. The angle to be preferred in this
operation is 45°, which will give two cutting edges of 60°. The latter
may be varied at pleasure by varying the angle at which this face is
ground, as explained in the chapter which treats upon this question.
The graver will do all kinds of outside work, light or heavy, as it
may be made of any size. It is represented in Fig. 112. Fig. 113 is
the heel-tool shown in position for work; 114 and 115, two forms of
nail-head tool very commonly used, but both requiring great attention
to the angles of the cutting edge to become effective. All the above
are for outside work, and are to be so held that the side next to
the work--the _sole_ of the tool in Fig. 113--forms very nearly a
tangent to the work--a position, as Holtzapffel remarks, strangely
similar to that required by the soft wood chisel and similar tools.
The heel-tool, indeed, if more keenly sharpened, will cut soft wood
(on the face) with great rapidity, and is in principle similar to the
broads used for that purpose. It is, however, an unsafe tool, owing
to its great tendency to dig into the work. It is a good plan to make
extensive use of various shapes of hand-tool before passing to the
slide-rest, because the hand feels exactly the _resistance_ which the
tool meets with, and the best form and position is thus _practically
tested_, and will be found to bear out to the utmost the theory
advanced in this work, and founded on mathematical truths worked out
and applied by Willis, Babbage, and others.
[Illustration: FIG. 112.]
[Illustration: FIG. 113.]
[Illustration: FIG. 114.]
[Illustration: FIG. 115.]
Inside tools must of course be made upon the same principles as the
last, the particular form alone being modified to enable the cutting
edges to penetrate into the various nooks and corners that may occur
in such work. The inside tool for iron (Fig. 116), with cutting angles
of 60°, is of a general and useful pattern. It must be so curved and
so placed that _both_ cutting edges come into action, one on the
face and one on the side of the cut, a condition explained in the
Appendix as _essential_ to all good work. It being quite impossible
to cut metal like wood, and necessary to allow sufficient time for
such work, a pointed tool is in most cases preferable to one of a
semicircular or rectangular form of edge, and the greater part of
the heavy work done in large factories is thus executed. Inasmuch,
however, as such a tool, well formed on correct principles, may be
made to take a tolerably deep cut, the shaving detached will be of
sufficient _thickness_, and consequently sufficient width to reduce
work a satisfactory quantity at each cut. The cut, too, must be
continuous wherever possible, the tool being slowly and steadily
advanced the whole of the acquired distance without being removed,
and then re-entered for a second cut. The result should be a smooth,
even surface, and that great exertion is not required when thus
working with hand-tools is sufficiently evidenced by a remark of Mr.
Haydon to the writer, "I have often detached, with a graver alone, a
tolerably thick shaving of iron, two feet and more in length." From
what has been said, it will be understood that in hollowing out a
piece of metal such as a chuck, the tool should not be made with a
_long_ cutting edge, such as would be used if it were intended to
scrape the whole depth of the side. A _broad_ shaving is not to be
thus aimed at, nor is the _inside only_ to be thus attacked, but
the tool advanced gradually inwards from the face of the work till
it reaches the bottom, thus (to repeat the important point again)
cutting at the same time the front and side of the shaving. The
half-round or cylinder-boring bit already described is, of course, an
inside turning tool, but is used with the aid of the back centre. In
principle it follows other inside tools, the end being bevelled or
sloped off 3°, and the side being 90°. The latter is to be regarded
as a blunt cutting tool, being the largest angle that can be used;
but, nevertheless, this bit must be regarded as cutting in the two
required directions--forward and sideways. If a regular Goniometer for
measuring angles is not to be obtained, nor any apparatus for grinding
a tool to the required bevel, an addition to Nasmyth's tool-gauge,
described in the Appendix, may be made by constructing in tin a set
of templates, with the angles marked upon them. The easiest way is to
mount on the lathe a few round sheets of tin, and mark the degrees
by the division-plate, the outer circle of which contains 360°.
The tin may then have pieces cut out, as shown in Fig. 117, to be
applied as gauges over the ends of the tool, or _solid_ pieces of the
required sections (those which are removed in forming the above) may
be retained. It will, perhaps, be as well to finish up both neatly,
taking care to mark the angles on each. The degrees most required are,
as explained, 90°, 60°, 80°, and 3°; but intermediate numbers may be
prepared, and will often be found convenient. An inside tool for brass
must retain the angles 80° to 90°, the latter acting as a scraping
tool to put a finish to the interior of work roughed out by a tool
of the lesser angle. There are not many forms in general vogue, for
brass work, whether internal or external. The round, the flat, and the
point tools (Figs. 118 to 121) are more or less capable of hollowing
out work, as well as surfacing. With the first ground to an angle
of 80°, brass chucks can be hollowed, and, with the second at 90°,
finished; and if there chance to be any internal angles out of reach
by the point-tool, it is only necessary to use a similar one bent
round at the end towards the left. Much of the turner's success in
brass work depends upon the quality of the metal, which is often very
hard and unequal in texture and perhaps blown and honey-combed. It is
always the best plan to send patterns of any work of importance to
some well-known firm, instead of trusting to a country foundry, whose
business, if worthy the name, is generally iron work, and who only run
brass once in a way, and make a terrible mess of it, too. The same may
be said respecting iron castings. It is worth while, as an experiment,
to test with turning tools the quality of a country casting, pulley,
or what not, by the side of a similar work really made of malleable
iron, such as is now so extensively used by the sewing-machine makers.
Tools that will stand the first and make good work deserve a place in
the British Museum, with a portrait of the turner!
[Illustration: FIG. 116.]
[Illustration: FIG. 117.]
[Illustration: FIG. 118, 119, 120, 121.]
For light brass work, such as small model engines now so generally
sold, a description of lathe may be used of which no mention has yet
been made--namely, a bar lathe Fig. 122. Such a tool may be made for
£3 or £4, and will be found sufficient for such work as specified.
[Illustration: FIG. 122.]
In place of the double bed a triangular bar of cast iron is used, on
which the poppets slide, and are clamped. D shows one of two sockets
with feet which hold the ends of the bar, and by which the lathe can
be mounted on any stout plank or on the window sill. The pulley A is
made for a strap, because this allows of a plain flywheel, which is
much cheaper than those which are bevelled and turned. The rest is
shown at E and F. The part E slides upon the bar like the poppet--at
the top is a dovetailed groove to receive the slide F, which carries
the socket for the tee, and is fixed by a turn of the screw seen on
the top of it. There is much to be said in favour of triangle bar
lathes, they are very stiff, and can be fixed anywhere. The flywheel
can be supported on a separate frame, which is an excellent plan, for
the jerk of the treadle and crank is not communicated to the poppets
and mandrel. In Maudslay's triangle bar lathe, which was made for
amateurs, a slide rest was attached, and the whole work was first
class--of late they have gone out of fashion, but are nevertheless
good tools.
Fig. 123 represents another very simple form of lathe for turning
small articles of brass. It may be said to be one remove from the
watchmaker's bow lathe, as it has no true mandrel or treadle; the
small flywheel being attached to an arm at the back and worked by
hand. The left hand is used for the latter purpose while the tool
is held in the right. Through a tapped hole in the left hand poppet
passes a steel pin shown at E, on a larger scale; this screws into
the poppet after passing through a brass pulley B; this bolt ends
in a point, and an arm fixed into the pulley becomes a dog to act
against a carrier screwed on to the work as in turning iron. Thus
the mandrel and point are fixtures, and the pulley only turns when
motion is communicated to it by a catgut from the flywheel behind
it. The back poppet and rest, which last is shown separately, slide
on the rectangular bar; the latter is about two inches wide, and
three eighths thick, and is made with feet to screw to any convenient
support. This lathe is especially adapted for work of small size which
can be centred at both ends, and on which a carrier can be fixed
to bear against the pin, E, of the pulley; nevertheless it is even
possible to use chucks, if cast in metal with a pulley to each like G.
A spindle must in this case be made like H, on which the chuck must
be slipped, and fixed by the nut in the hollow of the chuck. Although
however this form of lathe is sometimes met with, and may be used as a
makeshift, a small triangle bar or 3in. lathe of the usual form is far
preferable.
[Illustration: FIG. 123.]
In centering a bar of iron in the lathe too much care cannot be
exercised in causing it to run evenly. The ends should be drilled,
first with a small, and afterwards with a larger drill, so that a
countersunk hole may be obtained in order _to keep the point of the
lathe-centre from touching the bottom of the hole_. If this is not
done the friction of the work upon its bearing will soon spoil the
lathe-centre, and the work itself will speedily get out of truth, and
it will not be possible to screw up the spindle of the back poppet
so as to correct this. Of course in turning up very small work this
drilling cannot be done. A simple hole must then be made, sufficient
for the safe support of the work while being turned, but even in this
case the angle of the drill point should be less than that of the
conical centre of the lathe, the point of which will then run free.
To mark the true centres of a round bar of metal a punch has been
devised like Fig. 124. This is figured in Bergeron's work, and is
very serviceable. To insure its working truly be careful that the bar
of metal is filed flat on the ends, and that the surface of the latter
is at right angles to the length of the bar. It is then only necessary
to place the end of the piece in the conical part of the cup (which
will be best effected by fixing the bar in a vice) and by raising the
spring and letting it go sudden by a sufficient mark will be made to
guide the point of the drill.
[Illustration: FIG. 124]
The proper place at which to commence turning an iron bar is at one
end, the rest being placed so as to bring the tool just upon the line
of centres; if applied lower the tool would take too deep a hold, and
would either be broken or lift the bar out of the lathe, damaging the
centre points. The ends of the bar should be squared up before the
circumference is turned, and on no account must a file be used after
the turning tool has done its work. It is only at the commencement
before a cutting tool has been applied, that an old file may be made
use of to take off the scale and roughness left from the forging or
casting.
Brass may be attacked upon or just below the line of centres, because
the form of tool is such as cannot penetrate deeply. This metal is
perhaps easier in some respects to turn, but the tool is apt to form
undulations on its surface (is apt to chatter). This is due, partly
to the impossibility of obtaining continuous shavings, and partly to
the vibration of the tool, and when once this has taken place there
is a tendency to deepen these channels, which makes it difficult to
produce a plane and even surface. If a rectangular tool is used in
the position already shown and described under the head of tools for
metal turning, this chattering will be avoided. If it takes place, the
undulations should be worked off by gentle usage of the angle of the
tool, the rest being placed close to the work, and only a light cut
taken.
With regard to the method of mounting a piece of brass in the lathe,
any convenient chuck may be used, but sometimes the piece is short
or irregular and requires to be bored out. In this case use solder
and firmly attach the piece to a face plate of brass. The easiest way
is to smear the two faces to be joined with sal-ammoniac made into a
paste with water, and laying a piece of tin-foil between the surfaces,
which must be quite clean and bright, apply heat. The tin will melt
and a perfect union will be effected. When the piece is finished it
is re-heated and detached. This may be considered a wrinkle worth
knowing. The flat flanges of brass spoken of under the head of chucks
are just suited for this method of working, and they are not damaged
by the process, as the solder can be wiped off quite clean when
the chuck is made hot. This is a good way to mount small cylinders
of brass for model engines, as they can be bored and turned on the
outside at one operation with great ease and certainty. If a piece is
to be drilled or bored in the lathe, the following is the arrangement
to be adopted. Fig. 125, A is the face plate, B, the piece to be
drilled, C the drill, which is advanced by screwing up the point, E,
of the back centre; D is a hand vice or similar article to prevent the
drill from revolving with the work. If it is more convenient to fix
the drill itself in a chuck, the point of the back centre is to be
removed, and a flange of brass or iron substituted, as A, Fig. 126. If
the drill is to penetrate quite through the work a piece of wood must
be interposed between the latter and the flange to receive the point
of the drill and protect it from injury. The pressure should be so
regulated as to be constant and equal without being excessive, or the
drill will be bent or broken. Boring is simply drilling on a larger
scale, and is of such general use as to require detailed notice.
In the first place there are several tools used for the purpose,
according to the size of the work. The first is the cylinder bit,
Fig. 127. This is a most excellent tool, as it will work very truly,
and can hardly get out of place if properly directed at starting. The
cutting part A is half a cylinder, the centre being just left visible,
the end is not quite at right angles with the length of the tool,
but is sloped off a little and bevelled also slightly below.[7] This
forms the cutting edge. The other end of the tool has a central hole,
drilled to receive the point of the back centre by which it is kept to
its work. To use this tool, let the piece to be drilled be placed in a
chuck, and a recess turned in it of the same diameter as the cylinder
bit, the latter is then placed in this recess B, and when screwed up
it cannot possibly rise or shift its position; a hand-vice or spanner
is then fixed as shown in Fig. 125, and the lathe put in slow motion,
oil or soap and water being freely used to lubricate the tool. Either
a solid piece of metal, or a hollow casting can be thus bored.
[7] To an angle of 3°. See Appendix.
[Illustration: FIG. 125.]
[Illustration: FIG. 126.]
[Illustration: FIG. 127.]
These cylinder bits can be had of all sizes, from one-eighth of an
inch upwards.
Pipe stems are bored with the smallest of these tools. For this
purpose they are made of round steel wire, which is sometimes merely
flattened with a hammer at one end to spread and enlarge it, this
part being afterwards rounded on the underside with a file, and with
the same tool finished on the upper flat face. These slender drills
require to be delicately used, and are conveniently held in a pin vice
or pair of pin pliers, the handle of which being hollow, allows the
greater part of the drill shank to lie within it, a small part only
being drawn out at a time for use; thus the drill will be kept from
bending, and will work quickly and well.
Of recent inventions in the matter of drills, the most important is
the Morse American twist drills sold, in sets, at 30s. on a neat stand
with a self-centering chuck, complete. The form is that of a cylinder
with spiral grooves cut round it of extended pitch. The cutting edge
is as difficult to draw as it is to describe, and must be seen to be
understood. It is chiefly formed by the meeting of the spiral grooves
and the solid end, the latter forming a blunt angular point rendered
cutting by the edge of the grooves. They should have a place in every
workshop.
The next tool to be described is also much used, especially in
agricultural implement manufactories, for boring out the brasses,
or bearings. It is called a rose bit, or grinder, and is shown in
Fig. 128 A and B. In this case also a recess is cut in the work as a
guide, and as the tool fills up the whole interior as it proceeds, no
change of position can occur. The rose bit is used as shown in 125. A
bit of this pattern is very useful for brass work of all kinds, such
as the cylinders of small engines, bosses of wheels, bearings and
collars, and one of these tools of the exact size for hollowing out
the sockets of brass chucks, previously to their being tapped, will be
found serviceable. The third kind of boring tool is made with movable
cutters, which can be removed at pleasure, to be sharpened or replaced
by more convenient ones. The simplest consists of a cutter bar, A,
Fig. 129, with a slot in it to hold the tool, which is fixed by
driving a wedge at the back of it. The tool here shown has two cutting
edges, _b_ and _c_ which should be shaped according to the principles
already enunciated respecting hand tools for iron and brass. The
cutter bar is usually fixed between the lathe centres, and turned by
a driver chuck and carrier, while the cylinder to be bored is clamped
to the slide rest, and thereby advanced against the cutting edge. This
form is chiefly used upon work that has been cast hollow, or drilled.
[Illustration: FIG. 128.]
[Illustration: FIG. 129.]
Fig. 130 is another form of boring tool for large and heavy work. A
boss, A, is fixed to the cutter bar, having a series of dovetailed
grooves, or slots, on its surface, in which cutters are fixed by
wedges. In this and every similar form, it is expedient always to
complete the circle, or, at any rate, two-thirds or three-quarters
of it, by driving in blocks of wood in the slots not occupied by the
cutters. This preserves the concentricity of the tool. One edge of
these movable cutters should be radial to the centre of the bar, or
boss, the other rather less than a right angle, which will ensure a
good cutting edge. The best lubricant is oil for the first cut, and
soap and water, or pure water, for the finishing cut. The surface will
thus be left bright. It is not well to finish with emery any collar
in which an axle is to work (as the collar in which the mandrel of
the lathe runs). This substance imbeds itself in the pores of the
metal, and by forming a grinding surface, considerably increases
the friction and wear and tear of the parts.[8] Although boring and
drilling are capable of being done in the lathe, a far superior plan
is to employ an upright boring apparatus, as is now generally used in
making steam cylinders. The work is not then suspended between two
points, or carried on the slide rest, but takes up a firm bearing on
a fixed support, and the boring tool descends by a pressure screw, or
self-adjusting contrivance, as the work proceeds.
[8] Oilstone powder may be substituted, especially for the best brass
work.
[Illustration: FIG. 130.]
We have spoken of the slow motion as necessary for turning metal work.
This is represented in Fig. 131 A B C D. The first is a plan seen from
above. The poppet is cast double like F, so as to afford a bearing
for the mandrel, and a second for the back spindle seen at A. This
back spindle, it will be observed, passes through its two collars or
bearings, and can slide freely in them from side to side. This can,
however, be prevented by dropping a pin through a hole in the top of
the poppet, which falls into a semicircular groove in the spindle. The
pulley is securely attached to a small cog wheel, and can be firmly
united to a larger one, as seen at A2, and separately at C and D.
This pulley and small cog wheel run loosely on the mandrel, and do
not revolve with it until clamped to the wheel, C, which is itself
keyed to the mandrel. Suppose them to be thus free to revolve, and the
wheels in position shown in the plan, A. On putting the fly wheel in
motion, the pulley will revolve on the mandrel, carrying with it the
small cog wheel, which in turn will act on the large wheel on the back
spindle. The small cog wheel on the latter will thus put in motion the
large one geared with it, the which being keyed to the mandrel, will
put the latter in motion. There are many ways of clamping the pulley
to the large cog wheel, perhaps the following is as good as any. It
must be so clamped for wood turning when the back spindle is to be
slipped on one side out of gear.
[Illustration: FIG. 131.]
In the face of the pulley, which is concave, is a piece of brass flush
with the rim, and which forms a dovetailed groove, into which the head
of a clamping screw, E, fits. This screw projects through a slot in
the wheel D. When it is required to fix the pulley, this screw is slid
up towards the rim till the head rests in the dovetailed projection,
and it is clamped in that position by a nut. When it is desired to
put the back action into gear, this nut is loosened, the screw-bolt
dropped towards the axle (thus freeing the head from the dovetail),
and again fixed by the nut. The wheel and pulley are thus independent
of each other, the back spindle is slipped sideways into gear, and
held by the pin, and the slow motion will be obtained.
There is one fault in the arrangement of the back geared lathe that
with amateurs in a private house is especially disagreeable, and it is
questionable whether in large machinery establishments it might not
with great advantage be corrected. In the action of toothed wheels,
nuts and screws, and similar gearing, there occurs what is called
_back lash_. If, for instance, the tool holder of a slide rest is
advanced, and then the action is to be reversed, the movement of the
nut and tool holder does not commence simultaneously with the movement
of the screw. This is due to the play, or necessary looseness of the
working parts, the pressure coming on one side of the thread when the
screw is turned in one direction, and on the contrary side when the
motion is reversed. In toothed wheels a similar defect exists, and
gives rise to that disagreeable and ceaseless noise which assails the
ear on entering a building where machinery is in motion. This may
be avoided by the use of frictional gearing, a simple but excellent
mechanical contrivance which deserves far more extensive notice than
it has yet received. It is the invention of a Mr. Robertson, and
is patented. A lathe fitted with it would be almost noiseless, and
would work with a delicious smoothness, very conducive to the comfort
of the workman. This gearing, represented in Fig. 132, is merely
the substitution of V shaped or semicircular grooves for cogs, the
former running round the periphery of the wheel like the grooves in
an ordinary lathe pulley. In this method of gearing, it would be
necessary to move the back spindle to-and-fro, the usual horizontal
movement not being possible. This is easily effected by a screw or a
cam, either of which might be made to act on the frame which carries
the back spindle, and which may then work on a centre, as Fig. 133,
where A is the poppet, B, the support of the spindle, D, a cam; when
the handle of the latter is raised, the standard, B, is allowed to
fall back out of gear into the position shown by the dotted line, C.
A screw movement would have the advantage of enabling the workman to
regulate with greater precision the pressure of the friction pulleys
against each other. The drawing shows the grooves of these pulleys
larger and deeper than usually made. They are generally rather shallow
and numerous, and it is astonishing with what firm hold they grip each
other without that violent pressure which it might be imagined would
be necessary to prevent slipping when in use.
[Illustration: FIG. 132.]
[Illustration: FIG. 133.]
The ordinary slide rest for hand lathes is made as follows:--That for
ornamental turning will have a separate notice. Fig. 134, shows the
slide rest viewed from above, and it is evident if the tool is clamped
to the holder F on the top plate, it can be advanced from end to end
of the top slide B, and also (with the upper frame itself) along the
lower frame A, A, these movements being at right angles the one to the
other. For parallel work this is sufficient. In this compound rest a
third motion is arranged for turning cones or taper plugs like those
of stop cocks, taps for screw plates and such like articles. For this
purpose the upper frame is cast like Fig. 135, with a flat surface,
but with two ribs underneath, uniting the frame to a circular plate
with two concentric slots in it. This plate revolves on the plate G
G, turning on a central pin, and it can be clamped by the two screws
which pass through the slots into the plate in any desired position;
once clamped at the required angle a piece of metal can be bored
with a conical hole and a plug turned to fit it without possibility
of failure. The details of construction allow of considerable
variety, and different makers keep to their respective patterns; the
main desideratum is strength and solidity, combined with accurate
adjustment of the moving parts. The =V='s, underneath the frames,
and the edges of the latter, must fit, so as not to be tighter in one
place than another, and the upper and lower frames must cross each
other accurately at right angles. It is likewise essential that the
tool traverse the work in a perfectly horizontal line. Every part
must, therefore, be accurately made by means of the lathe and planing
machine, and the whole carefully put together. Notwithstanding the
above desiderata, a slide rest is not necessarily beyond the skill of
the amateur. We have, indeed, seen one thus made quite equal to the
work of a professed mechanic, though the file and scraper had to take
the place of the planing machine. The rough castings can be bought
for about half-a-crown, suitable for a five-inch centre lathe, and it
would be much better to try and fit up a set of these castings than
to attempt such a substitute as a wooden slide rest. The latter has
nevertheless been made, and we remember seeing one of mahogany edged
with brass, the work of a cabinet maker, which did good service in
turning and ornamenting wood.[9] This, however, was upwards of twenty
years ago, since which time the facilities for obtaining slide rests
of metal, properly constructed, have materially increased. As the
dovetailed edges of the slides wear away by use, it is necessary to
provide means for tightening up the =V=-pieces. This is shown
in Fig. 136. The holes in the =V=-pieces through which the
top screws pass are not round, but oval, so as to admit of lateral
movement.
[9] In the Paris Exhibition of this year (1867) are some slide-rests
made of hardwood and metal.
[Illustration: FIG. 134.]
[Illustration: FIG. 135.]
[Illustration: FIG. 136]
Two large headed screws, E E, are tapped into the place on which
the =V=-pieces rest, and when these are screwed up, their
heads (which are sunk for the purpose in two recesses in the lower
plate) press against the =V=-pieces, driving them closer to the
dovetailed slide. When thus adjusted the top screws are made use of
to fix the strips _c_, _c_. By this method the slides can be adjusted
to work with the utmost ease and accuracy, without shake or side
play. The edge of the circular plate and the heads of the leading
screws are very frequently marked in graduated divisions, so that
the advance of the tool or the angle to be made with the work by the
tool can be accurately measured and preserved. There should at any
rate be a mark on the circular plate to show when the rest is set for
parallel work. There are several patterns of tool holder, of which
the forms shewn are convenient for light work. The one shown in 138
& 139 on the rest is somewhat different. The plate F, Fig. 134, is
cast with a boss and socket, like that of an ordinary rest. In this
socket the tool holder fits, and can be not only turned round so as
to set the tool at any angle, but also slightly adjusted in height,
which is a great advantage. The tool is clamped by a single screw
as shown in the sketch. The drawback to this form, and that on the
rest, is this single screw, which will indeed hold the tool when the
work is easy, but will not always retain it with sufficient firmness
when the work is rough, or of tolerable size. In large workshops one
usually meets with the holder represented in Fig. 140. A plate, A,
with central block B, and slide C, are in one casting. Through A pass
eight screws. The tool lies on either side of the central square block
and is clamped with three screws, it has thus a fair bearing on two
sides, and the screws form a third above, so that accidental shifting
of the tool during the progress of the work is hardly possible. The
tool holder of Professor Willis, which is described in the Appendix,
is perhaps the best of all at present in use. It holds the tool firmly
at any desired angle.
[Illustration: FIG. 140.]
[Illustration: FIG. 138.]
[Illustration: FIG. 139.]
It is quite possible that the novice who has seen mere boys working
with slide rests at manufactories will be disappointed at his own
first attempts to use this piece of machinery. All the difficulty lies
in the shape and set of the tool. When turning metal with hand-tools
it is easy to feel one's way. If the cut is not satisfactory, the hand
at once modifies the angle of the tool, and regulates its direction
to a nicety, but the slide rest cannot thus adapt itself to its
work. It must be set with its slides in position, and the tool once
fixed must pursue its own course. Hence it requires a very accurate
knowledge of the nature of cutting tools, such as we have given in the
Appendix to this work. If the tool is well placed upon the axis of
the work for iron, a little below it for brass--it will cut cleanly
and easily, without rubbing or jarring, both of which are proofs of
either a wrong angle of edge, or a wrong form of tool. The work should
proceed with as much apparent ease as if the metal were an apple, and
the shaving should curl off like its peel. Moreover, this case is not
merely apparent, it is perfectly easy to cut iron, and the strain on
the tool, whether held by the hand or by slide rest, is comparatively
slight, if the tool is properly made and held. Fig. 141 is quite the
best form of tool for surfacing cylinders with the slide rest. It
is to be so placed that both edges are made to cut near the point;
hence the crank should slightly curve away to the left. It is not
possible to cut metal quickly, be content with fine clean shavings
curling off freely. You will soon see whether you can take a deeper
cut with safety. The tool here sketched is not at all likely to dig
in and hitch in the work; if it is not properly placed it will spring
and jump, or its side will rub against it, and no cut will be made.
To describe the exact position is very difficult, but the principle
once grasped, little difficulty should be experienced in making and
setting to work any tool, whether for inside or outside work. The
_rule of thumb_ was all well enough in olden days and in the infancy
of the art of metal working, but it is time to discard it; to master
and man it is equally advantageous to do so. Indeed, in some of our
leading firms, the old system of follow-my-leader, when the leader
was as ignorant of his work as the follower, is waning, and the "how"
is now, as it ought to be, coupled with the "reason why." One of the
best papers ever written on this subject is to be found in Weale's
series, "Mechanism and Construction of Machines," by T. Baker, and
"Tools and Machines," by J. Nasmyth, 2s. 6d. The latter part is that
specially referred to, and is well worth the whole price of the work.
The remarks, however, of that eminent mechanic are embodied in the
paper in the Appendix, and therefore, after the reader has studied the
latter, he should make trial for himself of the principles laid down.
Expend a quarter or a half an hour experimenting thus, with keen and
obtuse tools, held at divers angles, and you will see and understand
what is meant by setting a slide rest, or hand tool to cut metal as it
ought to do.
[Illustration: Fig. 141 is too much cranked; half the length of the
hanging part would amply suffice.]
Supposing the tool fixed in the tool holder in the position indicated,
and just overlapping the circumference at one end (the right). Take
hold of the handles, one in each hand, and with that which advances
the tool from end to end of the bar try very carefully whether the
tool will cut cleanly by making a turn or two while the lathe is in
slow motion. If the tool bites too deeply, turn the other handle
and ease it. If you still find the tool sticking into or scraping
the work, instead of bringing off a fine shaving, look well to its
position, and observe whether the edge is well placed on the axis of
the piece. If it has been hitching in the work it is probably too low,
if rubbing it, too high, and touches at some point below its edge.
It is presupposed, of course, that the tool is made correctly as to
angle of cutting edge. Do not lower the point by packing the end of
the shank; _pack the whole length_ or none. It is astonishing what
a great difference is made to the cutting power of a tool by slight
adjustments of this kind, and how smoothly a tool will work with
proper attention to these details, which would otherwise be probably
cast aside as unfit for the work. Hence the greater ease in managing
a hand-tool. The hand _feels_ the error, and at once, if experienced,
corrects it by an almost imperceptible movement--slightly raising or
depressing the handle or gently varying the angle sideways on the
rest. When once the tool is found to cut as it ought to do, nothing
remains but to turn the handle in the right hand, and thus cause the
tool to progress steadily along the work from end to end. Then free
it by a half turn of the other handle, reverse the movement until the
tool has arrived at its old place, and having slightly advanced it to
take a fresh bite, repeat the process until the whole bar is reduced
to the required size. If the piece is slender and bends away from the
tool, add to the slide rest a support; let it be fixed opposite to
the tool, and it will keep the work steadily up against the cutting
edge. It can be fixed (if a hole is made for the purpose) anywhere
about the slide which traverses in the direction of the length of
the work. It is well to drill and tap a few holes about the slide
rest, and some along the side of the bed of the lathe. These will be
found very useful at various times for fixing apparatus. For, be it
observed, (and we shall recur to this with some practical hints by and
by) the lathe may and ought to do many kinds of work beyond ordinary
turning. It may become a machine for planing, slotting, drilling,
wheel cutting, &c., and is to be pressed into the service of the
jack-of-all-trades, without compunction.
When ordering a slide rest let steel screws and nuts be specified, and
gun metal V-pieces, and let the parts be strongly made (too strong
for the supposed work); for the latter may unexpectedly turn out to
be sometimes rather heavy. We have found the top plate of a 3in.
slide-rest so weak that when the tool was clamped on the top of it,
by the screw of the tool holder, the slide itself became jammed; a
defect quite beyond remedy, except by the substitution of a new and
stiffer plate. The same advice may be given respecting the tools. Let
these also be strong; neat and pretty tools are all very well, but
you seldom see them in a workshop; you don't require pretty tools,
but good and serviceable ones. Nevertheless, let the material be of
the best quality possible; and that you may not be ever at a loss,
you should learn to make tools yourself. Procure some small square
and round steel bars; save up as directed your old files, and you
need but heat them red hot (not on any account white hot), and with
hammer and file shape them to your mind. Then temper to a deep straw
colour, and after being accurately ground and finished on the stone,
they will be fit to use upon any metal. The form of tool given as the
best for slide rest work may be exchanged, when the bar is nearly
turned, to the required size, for a fresh one, keen, sharp, and of
an almost flat edge instead of point. A tool, of which the edge is a
segment of a very large circle, will serve the best as a finishing
tool, just to take off the lines left by the pointed tool. With
regard to lubricating the work, we may observe that the chief object
is to prevent the point or edge of the tool from heating and losing
temper; oil, water, or soap and water will therefore answer, but it
is a curious fact that oil will not produce so polished a surface as
water will. We should advise in all cases soap and water. Soft soap
is best, boiled in water, and allowed to cool. The drills with which
the huge armour plates of ships six inches thick are drilled are thus
lubricated, and instead of throwing out dust in a wet state as usual,
these large drills fairly turn out curled shavings similar to those
produced by the planing tool. It is by no means a bad plan to lay the
shank of the tool which falls upon the top plate of the rest, upon a
piece of leather, wood, or sheet lead. The surface of the iron, when
planed and finished, is often too smooth, and the tool will sometimes
slip from this cause, unless screwed unduly tight, to the detriment of
the rest. By the above plan this annoyance will cease at once.
We will now say a word about hollowed work. Finishing a chuck will
serve our purpose, and here be it advised not to go to much expense
about chucks--get those which must be of brass in the rough, and
practice metal turning by finishing them yourself. If no slide rest
is available do it by hand. However, we are supposing the slide rest
to have been procured, and may therefore proceed to use it. First
you must drill the back part of your chuck as directed in a previous
chapter. The drill is to be the size of the diameter of the hollows
in the mandrel screw, that is, smaller by the depth of a thread, than
the full size of the nose. Having drilled it, proceed with the most
tapering of your taps, which we suppose to be provided to form the
internal thread (external if your mandrel has female screw, in which
case, instead of a drilled hole, the chuck will have a projection to
be turned truly cylindrical, and a screw cut outside with stock and
dies or chasing tool).
Follow up with the intermediate and finish with the plug tap. If you
were careful to square up the shoulder, the drill having been likewise
placed perpendicularly to the face of the chuck, the latter will fit
truly up to the collar or shoulder on the mandrel. If not, you must go
to work again, and square up the back of the chuck till a good fit is
produced.
Now, if you have a compound slide rest--that is, one in which the
slides turn on a centre pin--you can loosen the screws and turn it
a quarter of a circle. If not, you simply put the tool into the
holder, at right angles to its former position, so that the movement
of that slide which is parallel to the lathe bed will become that
requisite to advance the tool into the hollow of the chuck. Whichever
way you set to work put in a side tool, like Fig. 142, and, as it
is a brass chuck, remember that the bevel underneath is to be very
slight. Introduce the tool so as to take a light cut at first, until
the roughness is taken off, after which you may cause it to bite more
freely. Repeat this until the chuck is sufficiently hollowed out, when
you may substitute a similar shaped tool, but with a flat or slightly
rounded edge, to take off the marks left by the point tool. To finish
the bottom of the inside you will require a tool which cuts on the
end, but it should not have a perfectly flat end--at any rate, not
until, by means of a pointed or small round-ended tool, you have cut
away the roughness left from the process of casting. This has always
in its interstices a number of grains of sand fused, and very hard and
detrimental to cutting edges of all kinds. The point tools dig these
out very effectually, and should always precede those of other forms.
[Illustration: FIG. 142.]
The outside must be turned in a similar manner, and a hole drilled
to receive a pointed bar or wrench, for the purpose of unscrewing it
when screwed up tightly. To turn up a face plate of iron or brass
proceed in a similar way, but commence from the centre with a point
tool. This tool is the best for taking off the rough outside of hard
wood, instead of the gouge, as well as for removing the roughness of a
fresh casting. It is absolutely necessary that the faces of these flat
chucks, or surface chucks, be truly at right angles with the mandrel;
hence it is very difficult to turn and finish them by hand. We may
here also state the necessity of knowing when the slides of the rest
are at right angles to each other, without which no work can be turned
correctly. It is necessary to ascertain this by help of a small steel
square. Once fixed truly, it is only necessary to make a mark on the
quadrant (which should be marked in true degrees), by which the same
position can be found again at any time. We have spoken here of the
quadrant--we mean the arc, or arcs with slot--allowing the circular
movement of the compound slide rest. The chief use of these is to
enable the workman to turn true cones instead of cylinders, which
latter will only be produced when one slide is parallel to the lathe
bed. Such is the common use of the slide rest; and it will be evident
from these few remarks that there is an infinity of work, not only
produced with ease by its aid, but which cannot, even with expenditure
of time and labour, be produced without it; hence we advise this to be
the chief ambition of the tyro after he has mastered the difficulties
of ordinary hand turning (and not before). The cost of a fair one
for 5-in. lathe will be £5, or thereabout. At Munro's, £7 or £8; at
Holtzapffel's, £10 or £12. Both the latter are of course perfect.
We have directed to tap the chuck where it is to be screwed to
the mandrel with a set of three taps, or to cut it with stock and
dies if an outside thread is required. In both cases more true and
satisfactory work may be produced by the chasing tool. We speak of
the latter as used by hand; an account of cutting the threads by
help of the slide rest we reserve for the present. Mount the work
in any convenient way, either driving it into a wooden chuck, or by
clamping it to the face plate if you have one. Now in this way you
have advantages. In the first place you need not have a drill the
exact size, though it is convenient to have such a one, and also a
cylinder bit. You can drill and enlarge the hole by the slide rest
tools, and you can also with the slide rest ensure the perpendicular
position of the hole with respect to the end. Thus it is sure to fit
up close and snug to the shoulder of the mandrel. When bored thus
it will be in position for chasing. It is not difficult to chase a
thread in brass, as it does not chip away like wood, but cuts clean
and sharp. Follow the directions already given and you will succeed in
a few turns in getting the tool to run. Then let it have its own way;
hold it lightly, but steadily, and do not force it either to cut too
deeply, or to advance too quickly; it will run along of itself after
the faintest thread is cut or scratched, and the lathe can be worked
by means of the treadle all the time as soon as you have attained the
knack of dropping the chaser into its place at the commencement of its
cut, and suddenly withdrawing it when it has reached the bottom of the
hole. A chuck thus turned and screwed entirely in the lathe is sure
to prove a good fit, and there can be no better practice than to cut
screws in all your brass and boxwood chucks in this way. A very good
chuck to hold flat plates of brass was invented by a Mr. Wilcox, of
Bishop's Stortford, some years ago--an amateur of rare ingenuity and
mechanical knowledge, and who made all his own apparatus for plain and
eccentric turning. The chuck, Fig. 143, as described by him in some
unpublished manuscript, is made of boxwood, or may be of metal. It
is a plain disc or surface chuck with three slots A B C. and a steel
centre. This last must penetrate rather deeply into the wood of the
chuck, but is only kept up so as to project from the surface by a
spiral spring below it. Hence, when pressure is made upon its point by
the application of the object to be turned, the pin recedes into the
body of the chuck, suffering the work to lie flat on its surface. In
the three slots are three screws, with nuts at the back of the chuck
the screws pass through two pieces of brass, forming a pair of jaws,
one of which is shown separately.
[Illustration: FIG: 143.]
Fig. 143 shows the chuck complete. Suppose we have to face up a round
disc in its central part, or to perform any surface work in which the
jaws will not be in the way of the tool, the centre of the plate is
marked at the back of it, and this mark laid on the central pin. The
work can be clamped down by the three jaws, and the necessary work may
be done. Now so far the chuck is but a simple affair, and the receding
pin does not show itself to such great advantage; we will therefore
suppose a plate is to be drilled at several spots; let these be marked
at the back by a centre punch. It is now only necessary to bring these
marks in turn upon the central point, and clamp the plate in position.
Bring the point of the drill against the work and keep it up to cut by
the back poppet screw. In the same way eccentric work or any operation
may be done to such a plate, with the certainty that the point to be
thus worked upon is precisely central with the axis of the pulley,
or mandrel. Many similar applications of this chuck will present
themselves to the reader upon due consideration. We present the chuck
in this place because we have had occasion to speak of drilling as
connected with the slide rest, and there are many pieces of work that
could not otherwise be conveniently held in the desired position.[10]
[10] The MS. book containing the above was kindly lent to the author
of the present work by Mr. Hoblyn, of Bishop's Stortford, who is the
inventor of a new form of slide rest, which will be introduced in a
later page. The chuck in question is, however, commonly attached to
the watchmakers' lathes of the present day, and therefore may not, as
the writer supposed, be actually the invention of the late Mr. Wilcox.