The ambition of amateurs, especially, is very commonly centered in
a desire to cut screws in the lathe, and there is good reason for
this, because in the first place there is a difficulty presented
which it is pleasant to overcome, and in the next place, a screw is
of absolute necessity in the greater number of turned works. There
is an apparatus of simple but ingenious construction called a screw
box, which is commonly used by carpenters and others who have not
attained the skill necessary for chasing screws in the lathe, and
which is very convenient even for those who have obtained this power.
A sketch of this is given in Fig. 81. A, shows the tool complete, B
is a view after the top plate has been removed, showing the knife or
cutting tool, the latter being delineated alone at D on a large scale,
C is a section. To make this tool, which is within the power of any
person of average skill, a block of hard wood is first selected, and
drilled with a hole corresponding to the proposed size of the screw
to be cut. If no tap is at hand of the desired diameter and pitch,
this block must be mounted in the lathe, and the thread chased as we
shall presently describe. It is absolutely necessary that the block be
nicely squared up and level on the face. A small place must then be
cut to receive the knife, the edge of which is so constructed as to
form part of the thread cut away to make room for it. It is V shaped
like Fig. D, and very keenly sharpened. The method used to clamp the
knife in position, which is shown in Fig. D, permits the cutter to be
advanced or withdrawn until its position is accurately determined as
above. The top plate of wood is now fitted, and adjusted--the central
hole, _which is not tapped_, but as large as the _outside_ of the
screw-thread to be cut, forming a continuation of that which is tapped
in the lower block. A slot _b_, Fig. A, forms a passage from the
knife, to allow of the escape of the chips. The piece to be cut into a
screw should be shaped like Fig. 82. The part _a_, will be left plain,
_b_, is that on which the thread is to be cut, and must be truly
cylindrical, and of such size as to just enter the hole in the top
plate of the screw box. The part _c_ must pass through the threaded
part of the screw box, not loosely, but just so as not to damage the
threads in the least. The lower part of the central division is sloped
off as seen in the sketch. To cut the thread the screw blank is fixed
in the vice by its head, which, after being turned, should be planed
off at each side. The screw box being then placed upon it the lowest
and smallest part of the blank should just project, as in Fig. 83.
This part is intended to insure the perpendicular position of the
blank in respect of the screw box. The latter is then turned from left
to right until the screw is cut, which ought to come from the tool
clean and smooth. Box wood is especially suitable for this purpose.
This method is, of course, wholly inapplicable to anything but wooden
screw bolts, and for practice the tyro may set to work and make three
or four of the following screw clamps, which are useful to hold pieces
of wood that have been glued together. The tap for screwing nuts and
the jaws of these clamps, is similar to that used for metal, but, the
teeth, or cutting threads, are deeper and more pointed. The jaws of
the clamps shown in Figs. 84, 85, are usually made of beech, which
will take a very fair thread; or of birch, which is still better; and
the screws may be made of the same material, box being too costly and
scarce for such purposes. In making these clamps, there is to be no
thread cut on that part at which the handle of the screws project,
nor is there any thread on this part of the bolts, which pass through
a smooth hole in one jaw and lay hold of the other only. Other forms
will suggest themselves, but the two given will be found serviceable
patterns.
[Illustration: FIG. 81.]
[Illustration: FIG. 83.]
[Illustration: FIG. 82.]
[Illustration: FIG. 84.]
[Illustration: FIG. 85.]
The above method of cutting screws is not of anything like universal
application, nor specially the work of the turner; we shall now,
therefore, speak of cutting them by the chasing tool in the lathe. To
effect this with certainty requires much care and long practice, and
at first the attempt should never be made on a box or ornamental piece
of work, otherwise finished, but on a plain cylindrical bolt, such as
those of the clamps just described. For the inside, or female screw,
the making of chucks will afford endless practice, and a failure in
either of these will be of little importance. The screw tool for male
and female threads is represented in Figs. 86 and 87. It is of steel,
and as each tooth inclines in the direction and with the pitch of the
screw, it cannot be made with a file, but is cut by being held against
a revolving tap (or screw hob, which is of similar form.) There is
certainly a defect in the above common form of screw chaser, and a
slight modification, to be presently described, will be found easier
to use, and, in many respects, easier to make. To cut a thread with
the chasing tool, the top of the rest must be quite level and smooth,
so that the tool may readily slip along it. Suppose an outside thread
to be required on a cylinder of box or other close grained wood. The
rest being firmly fixed so that the upper edge is level with the axis
of the piece, and about half an inch from it, as Fig. 88, the tool is
advanced to touch the work, not in a line with the axis, but so as
to bring the part, _a_, in contact with it first, and the moment the
tool is felt to run along, which it will do as soon as this part of it
indents the wood, the handle is raised a little so that the points of
the teeth come into work. The tool in fact must describe the segment
of a circle, as shown by the dotted line. If this is done cleverly
the tool will not hitch, nor produce a drunken thread, but the latter
will come out clean and sharp. It is, nevertheless, necessary to
practise till the knack of thus chasing a thread is attained, and,
considering that once acquired, the necessity of traversing mandrel or
other expensive (and yet more or less defective) apparatus, no longer
exists, it is evident that the young aspirant should spare neither
time nor patience in becoming an adept in this useful art.
[Illustration: FIG. 86.]
[Illustration: FIG. 87.]
[Illustration: FIG. 88.]
One great difficulty in cutting the screw-threads to the top of a box,
or the inside of its cover, arises from the necessity for stopping
short, and removing the tool instantly as soon as it touches the
shoulder, or the top of the cover. The latter should be made rather
deeper than is necessary, so that there may be a turn or two of screw
to spare. This will give more room for the play and removal of the
inside chasing tool.
The ordinary form of the latter is as shown in Fig. 87, the part under
the plane upper surface (_a_) being either slightly hollowed or flat,
generally the former, from having been cut by a revolving cylindrical
hub.
Now, although this form may be suitable for outside screw tools,
which have to work on cylindrical pieces, it does not appear equally
suitable for inside tools, which are to act on concave work. The
writer of this article has experimented upon many patterns of chasing
tool, and has found it perfectly easy to chase an inside thread with
an ordinary grooved tap, which seldom makes a false cut, or crosses
the threads. From this the idea naturally arose of a convex edged tool
for inside chasing, and a concave one for outside work, as Fig. 89.
[Illustration: FIG. 89.]
[Illustration: FIG. 90.]
Practically, however, the convex edge, Fig. 90, will answer
satisfactorily for an outside cylinder. In order to obtain an
efficient cutting edge from this form, the rounding must be very
slight. The out or inside tool is used with a rolling movement on
the rest as it advances. If for hard wood, a notch cut across in the
line _e, f_, Fig. 90, with a saw-file, will, by making a partially
cutting edge on the convex part, cause the tool to enter more readily
at starting. Tools like the above must be necessarily the work of
the amateur himself. The regular makers have a great objection to
make any tool or machine out of the ordinary routine. Hence the same
patterns are constantly reproduced year after year, until some one
connected with the manufacture invents an improved form, or some one
else of mechanical genius, and possessed of means, registers a new
design. Amateurs are apt to cavil at this system, and in some cases
it no doubt interferes with, and checks improvement in tools and
machines, but the evil is almost a matter of necessity. Tools are made
not singly, one of each pattern, but so many score or hundreds of one
form are forged out, and handed over to the grinding and finishing
department, and it would sadly interfere with the system and order
of the manufacturer to make a single tool or two for individual
purchasers of different pattern to those ordinarily used. If a design
is sent in by a retail dealer who can order a hundred or so at his
own risk, the above objection is obviated, and the new pattern of
tool or machine is at once introduced. If, however, any new design by
an amateur, being submitted to such men as Holtzapffel, Buck, Fenn,
or Whitworth, appears to _them_ good and saleable, they will not
only not object to introduce it, but may possibly give a premium to
the inventor. We have thought it necessary to make these remarks to
obviate the possible disappointment of amateurs in this respect. It
is but natural to suppose that some ingenious device must have long
since arisen to obviate the difficulty of thus cutting screws by hand.
Every turner finds the difficulty, and few perhaps have failed to try
some plan or other to counteract it. There are two methods whereby
this can be done: one by causing the tool to traverse at a given rate
according to the proposed pitch of the screw, the other by giving
similar movement to the mandrel, while the tool remains still. For
such work as screwing the lids of boxes, the traversing mandrel is
commonly used, but for cutting long screws in metal, the tool is fixed
in a slide rest, and the latter is made to traverse, if necessary, the
whole length of the lathe bed, by means of a guide screw driven by
suitable gearing put in motion by the mandrel itself, the speed being
adjusted by a series of cog wheels which can be interchanged to cause
various rates of motion. The latter method belongs to machine lathes,
and will be treated of hereafter in this series.
The author has great pleasure in here introducing a device, not
exactly for _cutting_, but for starting the threads of a single, or
double screw, or, indeed, a quadruple one. It is the invention of a
gentleman whose _nom de plume_ is East Norfolk Amateur, and was by him
kindly communicated to the _English Mechanic_, of June 21, 1867. The
description is here given in his own words:--
"I think the plan to be described will produce to a certainty any
required number of screws and turns to the inch. The screws are
entirely cut with a common comb tool, but started by a revolving
cutter set to the required angle, and applied firmly to the work, on
the T-rest. I call it 'the universal screw guide tool,' contrived
and made by myself, and I believe will prove as useful to others as
it has to me: the drawing will almost explain the tool. The cutter,
A, is 9/16ths of an inch in diameter, turned to a cutting edge, and
finely tempered. The stem, B, in which it revolves, is round, and fits
into the shoe, C, having a graduated collar, D, in front of C, to set
the cutter to the required pitch or angle, the set screw, E, makes it
fast; having turned a piece of rod, of brass, iron, or steel, a little
above the size necessary, and supposing a quadruple screw is to be
cut having ten turns to the inch, there would, of course, be forty
threads when complete; if one of these four can be truly traced, the
comb tool will easily follow by inserting the outside tooth, either
right or left hand, as found convenient, in the line traced, when the
other three will soon appear with perfect accuracy, provided the first
one exactly corresponded to five points of the comb, which is easily
accomplished after a few trials, and if not successful at first,[4]
can be removed by a dead flat file several times, without reducing the
rod too much. When found to exactly fit the five points, the cutter
may be applied with more force to leave a good chase for the comb. The
T-rest requires a smooth surface for the shoe, C, to slide freely on,
and to be set parallel with the work, and the tool held at a right
angle as it proceeds along the rest, or the lines formed would be
of unequal distance. After a little experience it will be found to
work with beautiful accuracy, and for those who have not screw-guide
mandrels, and are not practised hands at flying common screws, it
will be found a great assistance, as it sets to anything. I described
only the quadruple, but the same rule applies to all quick screws;
for a double the chase must correspond to three points, and so on for
any number, that is, one more point of the comb than the number of
screws to be cut, and for a common one the chase must fit the comb
altogether."
[4] This doubt seems to mar the invention. It is, however, on the
whole a good design.
[Illustration FIG. 90 and 91.]
The above simple apparatus will, it is believed, be of great service
to those who find difficulty in hand chasing of screws; it is,
however, necessary to speak of other methods, and especially of that
so universally used by the turners of "Tunbridge ware"--viz., the
traversing mandrel. This is represented in Fig. 91. A is the poppet;
B, the mandrel, no longer conical at the place where it traverses
the collar, but cylindrical, and passing through _two_ cylindrical
collars. It is prevented from advancing towards the left in its
bearings by the shoulder, K, and in the other direction by a plain
cylindrical collar, or ferrule, C, which slips over the end, and is
secured by a nut, D. The whole is thus ready for use, as an ordinary
mandrel. To cause it to traverse from left to right, as it revolves,
the nut and collar, C, D, are removed, and a ferrule, or guide, F,
which has a screw of the desired pitch cut on its edge, is slipped
on the mandrel over a short feather against a shoulder, where it
is retained by a nut or pin. There are several such guide-ferrules
supplied with the mandrel of different pitches of screws. The nut G
is then removed and the piece E, which is of brass or gun metal 3/8
or 1/2 inch in thickness, with similar screw threads in each of the
hollows is attached. This guide is slipped on at H, and secured by
replacing the nut. The pin, which carries the guide, is frequently
made to slide up and down the face of the poppet, by the action of a
screw, M, working through a brass piece attached to that which carries
the pin. It is thus readily lowered out of gear or drawn up again
to touch the screw ferule. This is better than a pin screwed to the
head of the poppet, and is always adopted in the best lathes. In the
frontispiece is a photograph of this arrangement. This guidepiece acts
like a half nut, and as the mandrel revolves it gears into the ferule
and causes the required traverse. A single point tool, therefore,
held against the work will trace a screw of the same pitch as that of
the guides, and of a length equal to that of the ferule. The above
is not intended for cutting long screws, which would have to be done
in successive short lengths, but for screws of box lids, chucks,
and similar work it is a most excellent contrivance, and peculiarly
adapted for the use of the amateur. The guide threads in the commoner
patterns of these lathes are cut on the mandrel itself, which is
made of greater length than usual, and these several guides are cut
upon the part within the poppet heads, as Fig. 92, which represents
such a lathe as is sometimes used by gasfitters and brass workers in
general.[5] In the latter it will be noticed that there is added a
sustaining screw at the back of the mandrel. This is a good addition,
and indeed almost a necessary one if the lathe is to be used for
ordinary rough work, especially drilling, as it takes off the pressure
which must otherwise come against the shoulder, K, Fig. 91, and it
must be remembered that these lathes are expensive, and, therefore,
ought to be taken care of. The amateur may also be warned against
bad work; none but the first-class makers can turn out a reliable
lathe of this description. The collars and mandrel require _perfect_
fitting, and they must be quite hard, because there is no possibility
of tightening them when worn. They must be kept well oiled, therefore,
when in use, and the oil holes in the top of the poppet should be
fitted with brass covers, to prevent any particles of metal, and
especially emery dust, from working in between collars and mandrel.
Be sure to have one guide screw of the same pitch as that on the nose
of the mandrel, for the purpose of tapping chucks to fit thereon. In
using the traversing mandrel, either the cord should be slackened so
that the pressure of the hand on the pulley may stop the revolution
of the work in a moment, or the flywheel should not be brought into
use, the cord being instead grasped by the left hand, because it
is generally necessary to cut to a shoulder or given point. It is,
however, possible sometimes so to arrange the guides by insertion of
a washer or other expedient as to cause the action to cease of itself
at the required point. In a future chapter, the cutting of long metal
screws will be treated in detail, but before concluding the present
chapter, it may be useful to say a few words concerning the nature of
the screw itself as a mechanical expedient. A screw may be defined
as a continuous inclined plane--or an inclined plane wound round a
cylinder--the pitch being the inclination of the plane, that is, the
ratio of its height to the length of its base. From the mechanical
principle of the inclined plane it follows that the greater number
of threads in a given space the greater is the power of the screw
when used as in a press, or to draw along its nut, as in the slide
rest, in which endlong motion in the screw itself is prevented. It
seems, at first sight, easy to devise a method for cutting screws of
any desired pitch, but this is far from being the case, and when it
becomes necessary to increase the number of threads to fifty, sixty,
eighty, or even more to the inch, with such accuracy that one turn of
the screw shall always produce an _equal_ longitudinal movement of the
nut, the most delicate machinery scarcely suffices for the purpose.
The microscope detects and shows errors even in the best work, and it
is questionable whether a perfect screw of any length _can_ be cut by
machinery, as every imperfection in the latter is communicated to
the work done by it. If the amateur, therefore, requires a screw for
a slide rest, eccentric chuck, microscope, or other delicate piece
of machinery, or philosophical instrument, he had better get it cut
by some practical mechanician, in the possession of the necessary
apparatus.
[5] There should be a collar or shoulder to this mandrel, the same as
at K in the other figure.
[Illustration: FIG. 91.]
[Illustration: FIG. 92.]
In the "Manual Bergeron" is an ingenious contrivance, by an amateur,
which is worth notice, although unsuited for any work where extreme
accuracy of pitch is required in the screw. The following, Fig. 93,
is a description:--The mandrel is made to traverse in its bearings,
as before detailed in this series, but instead of its motion being
governed by guide hubs, it is dependent on the action of a pair of
differential pulleys, B. A bent lever, C, is pivoted at E to the
face of the poppet, having a bit of hardened steel fitted to work in
a semicircular groove in the mandrel itself, and so arranged that
on raising the tail or long arm of the lever the mandrel is thrust
forward from left to right, while a reverse action of this lever
causes a similar movement in the opposite direction. The movement of
the lever is thus regulated:--At the extreme end of the long arm is a
pulley and hook, as shown in the drawing; the double, or differential
pulley, is fixed to the end of the mandrel, and from the smaller
part depends a cord which passes thence through the pulley on the
lever, and is wound round the larger one on the mandrel when its end
is secured. On the hook is hung a weight. It will be evident, on an
inspection of the drawing, that on putting the lathe in motion the
cord on the differential pulley will coil itself round the largest
part of the same, and will draw up the end of the lever with a speed
proportionate to the difference of diameter between the larger and
smaller parts of the double pulley. The short end of the lever will
at the same time with similar proportionate motion move the mandrel
and work, and cause the fixed tool to cut a spiral or screw thread on
the latter--a good deal of ingenuity is displayed in the above, and it
has the advantage of being easily fitted up, but it is evident that
some alterations and additions would be required to adapt it to any
other use but that specified. A contrivance similar to Fig. 94, may
in some cases be a sufficient makeshift, when a more perfect one is
not at hand. A screw is here cut on the outside of the chuck, and a
kind of double tool is used, the tracer which is in contact with the
guide thread being adjustable as to its length, and the cutting tool
having a sidelong adjustment as well. The rest being placed between
the connecting bar of the tool and the work, the former will be held
with sufficient steadiness to enable the workman to traverse the
whole easily by hand. The use of this tool is of course limited, but
the plan is simple and fairly effective. The only really serviceable
plan is the slide rest, to be hereafter described. But one other plan
is here added, which is called "Healey's chuck." The description and
sketch are from Holtzapffel's work, in which it was however copied
from an older treatise. The author, it must be understood, has never
seen the contrivance himself, and there is a fault in its principle
of construction which must militate against its use except in a very
limited degree. Since, however, Holtzapffel has considered it worthy
of a place in his work, it is at any rate well to introduce it to the
reader, especially as its defects will not be of great importance
in tracing screws of half a dozen threads or so. The apparatus is
represented in Fig. 95. in plan. C is the chuck which carries the work
to be screwed, and T is the tool which lies upon R, R, the lathe rest,
that is placed at right angles to the bearers and is always free to
move in its socket S, as on a centre, because the binding screw is
either loosened or removed. On the outside of the chuck C is cut a
coarse guide screw which we will suppose to be right handed. The nut
N, N, which fits the screw of the chuck is extended into a long arm,
and the latter communicates with the lathe rest by the connecting rod
C, C. As the lathe revolves backwards and forwards, the arm, N (which
is retained horizontally by a guide pin, G), traverses to and fro,
as regards the chuck and work, and causes the lathe rest R, R, to
oscillate in its socket S. The distance S, T being half S, R, a right
hand screw of half the coarseness of the guide will be cut, or the
tool being nearer to and on the other side of the centre, S, as in the
dotted position T, a finer and left hand screw will be cut. The rod C,
C, may be attached indifferently to any part of N, N, but the smallest
change of the relation of S, T to S, R would mar the correspondence
of screws cut at different periods, and therefore T and R should be
united by a swiveljoint capable of being fixed at any part of the
lathe rest R, R, which is omitted in Mr. Healey's perspective drawing
of the apparatus.
[Illustration: FIG. 93.]
[Illustration: FIG. 94.]
[Illustration: FIG. 95.]
This is one of the least perfect modes of originating screws, it
should, therefore, be only applied to such as are very short, as,
owing to the variation in the angular relation of the parts the motion
given to the tool is not strictly constant nor equable. When in the
midway position the several parts should lie exactly at right angles
to each other in order as far as possible to avoid the error. The
inequality of the threads is imperceptible in a short screw. A little
modification of the screw-chuck of Healey would result in a more
correct and serviceable arrangement.
The disadvantage, for instance, of being obliged to set the T of
the rest across the face of the work is apparent at once, and it is
difficult to understand how such an arrangement could be made to
answer when the work might be of a length to require the support of
the back poppet. The following plan, Fig. 96, would obviate this and
the other disadvantages, and make a more efficient apparatus. B is the
chuck with screw chased on the outside, A the nut travelling upon the
same. To this is attached a bar H, which passes through the bar K,
to which it is clamped by the binding screw visible at H. Two short
pillars, F, F, are screwed into the bed of the lathe, in which there
might be more than one hole for each to permit the pillars to be fixed
at different distances from the line of centre. Through slots in these
passes the square bar of polished iron or steel E, supporting the
traversing rest socket, D, of which two other views are given at Figs.
97 and 98. It will be evident on inspecting the drawing that as the
nut A travels to and fro, it carries with it in a line parallel to
the lathe bed the rest socket and T. To enable the workman to steady
the tool, the latter should rest against two short pins fixed in the
top surface of the rest in holes made for the purpose; with the aid
of these the tool will be made to traverse the work with great ease
and regularity. In chasing a right-handed screw, the tool would have
to lie on the left side of the pins, and the latter would insure its
traversing with the rest. A longer T (or half T) being turned in its
socket to stand across the face of the work will enable an inside
thread to be cut with the ordinary tool either of one or more points.
Of course, in this modification of Healey's chuck, the screw cut will
be of the same pitch as that of the chuck itself; but as the latter
may be of boxwood, there would be no difficulty in having from three
to six with the most generally useful pitches of screws, as the arm
may be screwed into the near side of the nut, and, therefore, it,
and all the other parts of the apparatus, would answer for the whole
set of chucks. If the work to be screwed is merely a box cover, or
some such work where great length of screw is not required, it is
evident that to rig up this kind of contrivance, or Healey's, or
indeed anything of similar elaboration, would appear like summoning
a gang of navvies to remove a mole-hill. Hence, if a traversing
mandrel cannot be obtained, by far the simplest plan is the chasing
tool used by hand; it is therefore well worth while to get into the
knack of using this tool. To give confidence (which is essential to
success, the least nervousness generally proving fatal to such work)
the part on which the screw is to be cut may be left larger than will
finally be necessary. The screw is then commenced, and if a failure
takes place it is again levelled, but if, as is more probable, the
attempt is successful, the chisel and chase are alternately used, the
cuts of the latter not being obliterated by the former but always
left sufficiently deep to form a guide until the desired object is
satisfactorily accomplished.
[Illustration: FIG. 96.]
[Illustration: FIG. 97.]
[Illustration: FIG. 98.]
Having treated of screw cutting so far as we are able without
trespassing on that section of the present series which is to be
devoted to machine work with slide rest and change wheels, we shall
enter on the matter of spirals, or Elizabethan twists, the method of
making which was long kept a secret by the trade. These twists are
essentially screws of very extended pitch, and generally rounded
threads. The latter sometimes embrace the central cylinder or core,
and sometimes are detached so as to assume a more open form like a
corkscrew, and in the latter case two, three, or more threads can be
cut, so that the spirals appear to intertwine. These spirals, too,
are frequently ornamented with the aid of the eccentric chuck, and
thus the work becomes fit for the adornment of the drawing-rooms
of the highest in the land. No turned work can in short exceed in
beauty these delicate and elaborate specimens of the turner's art.
To commence with a single twist of one thread. A cylinder of the
requisite length must first be turned. The number of turns the
thread or cord of the spiral is to make in a given space must next
be determined. We will suppose the cylinder one foot long, exclusive
of any mouldings or tenons at the ends, and that the spiral is to
make three turns round it, that is, one turn in four inches of its
length. Divide the cylinder into three equal parts by the lines B, C,
Fig. 99. Next rule equidistant lines D, E, F, along the cylinder in
the direction of its length; in the present case let there be _four_
such lines. The three divisions first made must be subdivided each
into four (always the number of the longitudinal lines). The angles
of the parallelograms thus formed must then be connected diagonally
as shown in the figure, which diagonals being continued, will be
found to describe a spiral line. A second similarly constructed
spiral determines the thickness of the thread or cord of the twist.
These spirals may now be cut with a tenon saw, and all the material
outside the cord carefully removed with the gouge, so as to form
a semicircular hollow between the threads. This cannot be done by
putting the treadle and flywheel in motion, but the work itself must
be grasped with the left hand, while the right holds the gouge upon
the rest and guides its edge. After the work has thus been roughed
out, it must be finished by rasps and files, or by a kind of plane
with a semicircular cutting edge. While the latter is being used, the
flywheel must be brought into action, so that the work may be made to
revolve with sufficient rapidity to ensure a clean and smooth cut. A
hollow plane may be used to round the cord of the twist, and the whole
finished with glass paper and polished.
[Illustration FIG. 99.]
The "kind of plane" named was designed by a regular workman for his
own use, and was made thus and grasped in the fist, or rather hollow
of the hand. The iron was like _c_, sharp at one edge, the block of
wood being slightly bevelled off in front of the cutting edge. The
workman, who made scores of these twisted works for the trade, of all
dimensions, would run this along the hollows, while the lathe was
in motion, with great speed and accuracy. He marked the spirals as
described, and then grasping the work in the left hand, and the gouge
in the right, turning the work round with the former (the cord thrown
off the wheel), he cut out the wood boldly in large pieces with little
apparent care, and perfect ease. Then came the plane described above.
Fig. 100 shows a simple spiral thus made. If the piece to be turned
is dark it is not easy thus to mark out the divisions. In that case
the following method will answer equally well. It is the plan used
and contrived by the writer,[6] and specially handy when a number of
similar twists are to be cut, as in ornamented pieces of furniture.
A, Fig. 101, is a straight edge of hard wood, through which, at any
given angle (regulated by the number of turns the cord is desired to
make in a given length of cylinder), a knife edge is fixed. If this is
held as in the figure, and the blade is pressed down upon the cylinder
to be cut, the lathe being put in motion, a very correct spiral will
be traced, which can be at once deepened by a tenon saw as before.
The thickness of the cord being determined it only remains to place
this tool again upon the work so that the second line shall be traced
at the required distance from the first. A second, or any number of
cords, may be thus traced in succession parallel to each other by this
simple method. By a slight modification of this instrument, which
allows the knife to be clamped at any desired angle with the straight
edge, the inclination of the cords, and, consequently, the pitch of
the spiral can be varied at pleasure, and a second blade can be added
to trace the second line, determining the thickness of the cord with
one movement of the tool along the cylinder. The edge of the knife may
be across the rest, the piece of wood just overlapping the T on the
side next the workman, if the blade is long enough to reach across the
work when thus held. The tool will be steadier and perhaps more easy
of management in this position. One hand should then lightly press the
back of the knife, while the other retains the wood against the rest
as a straight edge. East Norfolk Amateur's design of a screw guide is
on the same principle, the only difference being the substitution of a
revolving instead of a fixed knife edge. In using either it requires
some care to allow the tool free traverse, as the least check would
spoil the thread.
[6] A similar plan is noticed in Holtzapffel's mechanical
manipulation, which the writer had not seen. It is satisfactory to
him to find the method thus authorised.
[Illustration: FIG. 100.]
[Illustration: FIG. 101.]
The next class of spirals is that in which no central core exists,
but the coils stand separate and distinct, two or more rising from
the same base. The coils are sometimes flat, sometimes rounded, and
still more frequently, in the best work, exquisitely, and (as a
casual spectator would say) _impossibly_ carved. The process is as
follows:--Turn a cylinder of ivory or hard wood, forming at the end
any required mouldings as a base and capital. Determine the number of
coils and the pitch, and by one of the previous methods mark out the
same. The cylinder is now to be bored out from end to end, leaving
sufficient for the thickness of the required cords. This bore may
with advantage be slightly larger at one end than at the other, so
that a mandrel of wood may be fitted into it, to be afterwards easily
withdrawn. This will certainly be necessary if the ivory cords are
to be of light substance, as they require support to enable them to
bear the action of the tool. After the cylinder has been bored as
above directed, let a mandrel of common wood be inserted, and the
lines, marked as shown, be cut quite through. The intermediate parts
between the intended threads must then be removed carefully (with a
round rasp, if of ivory) with any convenient tool if of wood. The
cords must then in a similar way be rounded or otherwise moulded, and
afterwards the common and now damaged mandrel removed. In the case of
ivory the piece of work will not only be strengthened by the insertion
of a polished mandrel of ebony, but its appearance will be improved.
Sometimes, however, it may be preferable to line it with red velvet or
silk, or it may be left entirely open. The further ornamentation of
the cords, depending on the eccentric chuck or eccentric cutters, will
not be described in the present paper. These open spirals are worth a
vast amount of patience and trouble, which their elegant appearance
when finished will amply repay.
The reader is not to suppose that this method of cutting spirals by
rasp and file is the only or best method, especially when ivory is
the substance operated upon. Further on will be described various
modes of accomplishing the same ends by self-acting machinery, and
by the spiral apparatus designed for use with the ordinary lathe,
but all these need the slide rest, whereas it is quite possible in
the foregoing manner to make spirals by hand tools alone, which for
correctness and finish may vie with those which may have been worked
with more elaborate and costly apparatus.
Before quitting the subject of plain hand turning in wood, a few
more words may be necessary in respect of certain details of lathe
manipulation, foremost among which comes Chucking work. This is often
carelessly done, especially by amateurs, who, in consequence, are
frequently annoyed by the shifting of the material under the influence
of the cutting tool. If this is hard and valuable--as are many of
the best woods for ornamental turning--the fork or prong chuck will
not enter sufficiently to sustain the piece, and at the same time
the stuff is too valuable to allow of the waste incurred in screwing
it direct to the mandrel, or inserting it sufficiently far into a
brass cup chuck. In such cases the best plan is to screw a piece of
common wood upon the mandrel, face it truly, and cut a few shallow
concentric circles upon the end thus levelled, both for the purpose of
a guide to centrality, and also to give a hold to the glue by which
the more precious material is to be attached to it. For this purpose,
both chuck and work are to be well warmed, and the glue--boiling
hot--brushed upon the parts to be united. The latter are then to be
rubbed together a few seconds, and when the piece to be turned runs
truly, the back poppet with boring flange attached--if the right hand
end of the piece is level, otherwise, the point allowed to remain--is
to be brought up and screwed as a clamp against it till dry. This
process requires time, but is well worth the trouble, as the material
will be securely held, and can be safely operated on. None but those
who have had to contend personally with a tyro's difficulties, and
have, in consequence, seen the work shift in the chuck when nearly
completed, can truly appreciate the advantages of efficient chucking.
In the case detailed, there is absolutely no waste of material, no
possibility of the work becoming loose or out of truth; and the
ornamentation by eccentric cutters, drills, and so forth, can be
proceeded with, and carried out with that confidence which never fails
to promote good workmanship. Even with the above arrangement the back
poppet should be used while the excrescences are turned down, and
retained as long as the gouge has to be used in bringing the design
into an approximation to its intended form. This should be removed,
however, before taking the final cut, as the work will generally seem
to drop a little when the support is taken away, in consequence of
the mandrel, which has been forced against the back centre, returning
to its place in the collar.
Pieces of six or seven inches in length, and of one or two inches
diameter, requiring to be hollowed out, may frequently be turned by
reversing the usual method and boring out the interior, previous to
shaping the outside. A case for pencils, for instance, or a bodkin
case, may be thus worked:--Mount in the square hole chuck, an American
screw auger, sets of which are now to be had beautifully finished and
polished. The kind meant has a scooping kind of edge above the screw,
Fig. 102, and cuts cleanly and rapidly. The piece of wood--soft wood
alone is meant--is brought against the tool, being grasped by the
left hand, while the back centre, with flange, is steadily advanced
with the right hand against the opposite end. This auger will run
straight through several inches without requiring to be withdrawn, as
the borings pass freely along the polished threads of the instrument
and escape. If necessary, however, it can be readily withdrawn by
reversing the action of the lathe, and replaced without difficulty.
The piece thus bored may then be mounted in the lathe and finished on
the outside. To do this satisfactorily, an arrangement is requisite
by which the centrality of bore is insured, else in the process of
cutting the external surface, the material will, in all probability,
be cut through in one part, while in another it will remain of
considerable thickness. If the piece is bored quite through, so as to
become a tube, Fig. 103 will be satisfactory, as the cones preserve
centrality, whilst the use of the carrier will prevent the necessity
of screwing the cones up so tightly as to endanger splitting the wood.
This is the best way to chuck small cylinders and brass tubes. The
more obtuse the angle of the cones the better.
[Illustration: FIG. 102.]
[Illustration: FIG. 103.]
[Illustration: FIG. 104.]
In this method the bottom of the case must be turned as a plug and
glued into its place. If the bottom of the case is left solid, an
arrangement like Fig. 104 will answer well. The plug chuck, A, must
not be at all conical, and the part that enters the work must be at
least an inch long. If this is attended to, and the face of the work
and of the chuck is square, the tube will be truly centred, only
requiring the back poppet to take off the strain upon A, when the
tool is applied. If A is chalked, there will be no slipping, provided
it has been accurately fitted. Observe, nevertheless, that as a
general rule, hollow work should be placed _inside_ and not _upon_ a
chuck, unless you have to work upon the _whole_ exterior surface. By
this plan, there will be no likelihood of splitting the object, an
undesirable consummation which not unfrequently takes place when the
contrary method is pursued.
Thin discs of wood or brass are most conveniently turned upon a
face-plate, to which they can be attached by turners' cement, already
described. If, however, one surface only has to be worked, and the
plate is not of less thickness than 1/8th of an inch, it may be
mounted on a flat chuck with small projecting points, the back poppet
being used to keep it firmly against the face of the chuck. Even a
plate of brass may be thus turned if placed first of all against the
chuck and gently tapped so as to mark the position of the points, and
then drilled to suit them. Bread-platters are thus easily chucked,
first of all _face downwards_, and then reversed with the bottom
against the points, so that in the latter position, the chisel or
broad may be applied to the face and the marks removed. The larger
designs on these platters are carved by hand after their removal from
the lathe, and the small figures forming the ground, which often
appear round the main design, are made by figured punches.
_Chucking Egg Shells_:--The method of doing this so as to enable the
turner to cut the shell evenly in two parts, is given by Holtzapffel,
in his "Mechanical Manipulation," and has been copied elsewhere. It
is ingenious and effective. The object is simply to obtain a pair of
delicate vases, to be edged with ivory, and mounted on a pedestal, as
a curiosity. The following account is from the pen of the inventor,
Mr. G. D. Kittoe, as communicated to Mr. Holtzapffel:--"In the
accompanying figure--Fig. 105--is represented the nose of a lathe,
with an egg chucked ready for cutting." Fig. 106 is the chuck used
first "to prepare the egg, to be mounted in the above way. The latter
is generally termed a spring chuck, and is made by rolling stout paper
with glue upon a metal or hardwood cylinder, the surface of which
has been greased to prevent the paper sticking to it, and upon which
it must remain until perfectly dry, when it may be removed and cut or
turned in the lathe as occasion may require." [N.B. Nothing is said
in the above account of the evident necessity of fixing the paper
cylinder to a wooden block, in which a screw must be cut to mount it
on the nose of the lathe.] "This sort of chuck is very light--easily
made and well adapted for the brittle material it is intended to hold.
Before fixing the egg in it, the inner surface should be rubbed with
some adhesive substance (common diachylon answers exceedingly well);
when this is done the egg should be carefully placed in the chuck,
the lathe being slowly kept in motion by one hand whilst with the
other the operator must adjust its position until he observes that
it runs perfectly true, then, with a sharp pointed tool he must mark
the centre and drill a hole sufficiently large for the wire in the
chuck, Fig. 105, to pass freely through. When this is done the egg
must be reversed, and the same operation repeated on the opposite end,
its contents must then be removed by blowing carefully through it.
It is now ready for cutting, for which purpose it must be fixed in
the chuck, Fig. 105. A is a chuck of box or hard wood having a recess
turned in it at _a, b_, into which is fitted a piece of cork as a soft
substance for the egg to rest against. B is a small cup of wood with
a piece of cork fitted into it serving the same purpose as that in
A. A piece of brass, _d_, is to be firmly screwed into the chuck A,
and into this a steel wire screwed on the outer end, on which a small
brass nut _e_ is fitted to work freely in a recess in the piece B.
When the egg is threaded on the wire through the holes previously made
in it, this nut is to be gradually tightened up until it presses the
cup B against the egg sufficiently to hold it steady and firm enough
to resist the action of a finely-pointed graver used to cut it. The
tool requires to be held very lightly, as a little undue violence
would crush the shell. Neither should the latter be pinched unduly
tight in the chuck, as otherwise when the point of the tool divides
the shell the two parts might spring together, and be destroyed by
the pressure. It requires some delicacy of hand to attach the rings
to the edges of the shell to constitute the fitting. The foot and top
ornaments are fixed by very fine ivory screws, the heads of which are
inserted within the shell."
[Illustration: FIG. 105.]
[Illustration: FIG. 106.]
Box wood is decidedly the best material for ordinary chucks, as it
takes a screw almost as well as brass, is pleasant to work, holds
the material firmly, and is of good appearance, which last is not
unimportant to those who possess good lathes, and like to see
everything in decent order about the workshop, and it is certain
that a disorderly workman will commonly produce slovenly work. This
wood, however, though tolerably plentiful, is sufficiently costly to
be worth preserving, and by a little management chucks may be made
to answer for a longer period than might be at first supposed. A
chuck, for instance, too large to hold the work, may be plugged with
a worn-out chuck of smaller bore, or with wood of inferior character,
to save the necessity for hollowing out a new piece of box wood. The
latter material, moreover, excellent as it is, may be replaced by
other kinds of wood, provided the latter will bear a good screw.
_Beech_, if dry, will answer very well for the purpose.
_Pear_ is tough and screws well.
_Apple_ is little inferior.
_Ilex_ or _evergreen oak_ is sufficiently hard and tough and will be
found quite satisfactory. Elder of large size is good, and screws well.
_Sycamore_ screws well, but is not always equally tough.
All hollow or cup chucks should be furnished with rings of iron or
brass to prevent splitting. About six sizes of rings will suffice
for a great number of chucks. Bergeron, speaking of the barrel stave
chucks already alluded to, prefers the encircling rings plain and not
screwed. He gives the following reason:--"If a piece of work entered
in such a chuck does not run quite truly, a tap on the ring in the
proper place will, by closing the sawgates more in that part, rectify
the error, whereas with a screwed ring this is impossible." There is
reason in this, but at the same time it would be easy to unscrew the
ring a turn or so, give a light tap to the work, test its position
by putting the lathe in motion, and when true fix it securely by
screwing up the ring. There is, however, one precaution to be taken
in making these useful chucks--namely, to cut the staves of equal
width, else they will not yield equally to pressure, and the work
will not be so readily centred truly. A grip chuck of inexpensive
make (one additional pattern of which is introduced from a design by
contributors to the _English Mechanic_) should always be provided. A
rough block of ivory for instance may be seized in its jaws, and the
exterior useless part cut off by a parting tool as a ring, leaving the
nicely rounded material ready for chucking. Ivory nuts or _corosos_
which are peculiarly awkwardly shaped for mounting in the lathe, may
also be thus seized, and one portion faced up and rounded so as to
allow of being fixed on a face chuck by glue or cement, or fitted into
a cup chuck. Rough pieces, too, thus mounted may be faced up, bored
and tapped to fit the mandrel as chucks, and a thousand similar works
may thus be handled. The simple grip chuck in question is important
as having the very useful addition of a centre point which the writer
would, if he did not abominate and eschew puns, direct attention to,
as the chief "point" of interest--"I call it," says the inventor,
a "Universal Self-Centering Grip Chuck." The drawings 1, 2, and 3,
almost speak for themselves, to practical turners.
[Illustration: FIG.(drawing) 2.]
[Illustration: FIG.(drawing) 1.]
[Illustration: FIG.(drawing) 3.]
1, is an elevation of one of the grips. 2, a section through centre
of chuck. 3, a side section of ditto. The body of the chuck is made
of cast iron, to screw on to the mandrel; and the grips, 1, are moved
simultaneously by a right and left-handed screw acting in a circular
groove. The jaws of the grip are serrated and tempered, the same
as in ordinary vices. In the centre of the grips, when closed, a
three-sixteenth hole is bored true to the centre of the lathe. Behind
this there is a true centre point screwed into the body of the chuck,
as marked at _a_. The above hole and this centre point are to be
particularly attended to, as on their truism depends the correctness
of your work.
If I want to turn a solid cylinder I make the usual centre at each
end; put one on the above centre point and the other on the back
centre of lathe, and then screw up the grips tight; but if the work is
short you need not apply the above centre point or the back centre,
as the grips are alone sufficient. The hole in the jaws of the grips
admit of any kind of drill or other tool being put into them without
using any centres, and the grips will admit anything up to two inches.
In fact, I do almost every sort of thing with this chuck, and I think
amateurs, if not others, will find it a most valuable and handy
contrivance.
Fig. 108, A and B, represents a modified form--a chuck already spoken
of and recommended for ordinary plain turning, in which the work
is supported at both ends. The present form is to a great extent
self-centering and will hold the work also without the saw-cuts
otherwise needed, the sharp edges of the double fork entering the work
with the pressure caused by the back centre. The chuck is useful not
only for ordinary work, but for re-mounting pieces centrally, which
it may have been necessary to remove when partly finished, and to
return to the lathe for completion. A still further addition to this
chuck of a steel point sliding through the centre, as in the section,
Fig. 109, makes it a very complete and serviceable apparatus, as by
this means it is easy to reverse the work without destroying its
centrality. The point is intended, as in the chuck of Wilcox, to slide
back stiffly (being if necessary kept up by a spiral spring as shown),
as it is only intended as a guide to assist in mounting the piece. If
the mandrel is not bored the chuck must be long enough to receive the
pointed wire within its substance. This will be found in every way a
most serviceable chuck. It may be of iron or brass, or even of wood,
if a round plate of brass is mounted on its face, to which the holding
pieces can be soldered or brazed.
[Illustration: FIG. 108.]
[Illustration: FIG. 109.]