The parallel rest.

the machine be fixed so that its cheeks may be, as nearly as can be ascertained, parallel to the axis of the intended cylinder, at the left hand end of which place the tool so that it shall take off a slender shaving. Set the lathe to work, and slowly turn the winch b, till the tool has completely traversed the bar. Repeat the operation, if necessary, till the irregularities in the figure of the bar are removed, and the tool has touched every part of the surface, then, with a pair of callipers or a gauge, examine whether the bar, at both ends, is of equal diameter; if any inequality appear, the rest has not been set parallel to its axis, and it, consequently, is not a cylinder. The matter is rectified by slackening the nuts of the pins hik, and pushing in that side of the rest, which is opposite the thick end of the bar, just half the extent of the error. The nuts being then screwed down, and the tool made to traverse the surface again, the cylinder will be completed.

As it has been supposed that the bar was, in the first instance, by some oversight or other, turned rather conical, the method of making a regular cone with the parallel rest, when occasion requires, needs no explanation. When a flat surface is to be turned, it must be well secured to a chuck, the machine fixed across the bed of the lathe, and the cutting edge of the chisel F precisely on a level with the axis of the mandrel, or some part of the centre will remain unfinished. The tool may be made to cut with so much exactness, that if a rectangular block of cast iron were attempted to be turned flat with it, the edges will not be jagged, when the circle of revolution, extending beyond the shorter diameter of the piece, is not complete.

When the chisel F, in the parallel rest, requires to be moved a little further in, some chuse to alter it by percussion, and keep it only so tight that the blow of a moderately sized hammer will drive it in; others think it better to regulate it by a screw, and provide a frame for the back of it, similar to that at the back of the puppet D, fig. 13, pl. I, in which the screw a acts upon the end of the mandrel.

The section of the cheeks of the parallel rest, to abridge the labour of filing, may be made to resemble that proposed for the cheeks of the foot lathe, fig. 2. pl. I.

Use of the screw-tools.—Traversing mandrel.

Of cutting Screws in the Lathe.

The art of cutting screws in the lathe, constitutes one of the most curious and useful branches of the art of Turning. Accordingly, it generally proves one of the most interesting exercises to the young practitioner, who is further stimulated by the celebrity of those who can cut every description of screw with facility, an attainment commonly considered, among turners, one of the most decisive proofs of skill that can be exhibited.

In proportion as the art of cutting screws has been culti vated, the methods by which the object might be accomplished, have been diversified. We shall notice some of those contrivances which are least expensive, most easily reducible to practice, and most suitable for general use.

If the screw tool, fig. 25, be opposed to a cylinder revolving in a lathe, and at the same time be moved along the rest, with a regular horizontal motion, it will cut a screw on that cylinder, the threads of which will fill the angular spaces between the teeth of the tool. Fig. 25, is an outside screwtool; if the cylinder had been hollow, and intended to be screwed internally, the inside screw-tool, fig. 26, must have been employed, which, when pressed against the side of the cavity, while drawn out horizontally as the cylinder revolved, would have produced the desired effect. There is some difficulty in acquiring the art of cutting screws in this manner, though the process is in very general use among experienced turners. To obviate every disadvantage which attends it, and ensure perfect precision in the operation, was the object of the invention of the traversing mandrel. Of this ingenious contrivance, we shall next, therefore, endeavour to give the reader a description. At the end of the mandrel E, at e, fig. 1, pl. I, there is a screw about two inches long, the thread of which is like that intended to be made. Upon this screw, called the guide, is fitted a piece of wood, the motion of which is entirely prevented by any mode of fastening which may be found convenient. The piece of steel on the headstock C, which falls into the groove of the mandrel, and hinders its horizontal movement, being then withdrawn, and the great wheel turned, the mandrel assumes at once a rotary and rectilinear motion, which is continued till it has gone so far, that the screw e can no longer turn in the piece of wood. If, as soon as this circumstance occurs, or a little sooner, the great wheel be turned the contrary way, the rotary and rectilinear motion of the mandrel immediately takes place again, but in a reversed direction. This

Traversing mandrel.—Traversing chuck.

compound motion of the mandrel, is precisely what is wanted to facilitate the use of the screw-tool, which, while it is going on, only requires to be held steadily upon the rest, against the revolving body, and the screw will be produced. The teeth of the screw-tool must correspond with the screw upon the mandrel, as if made by being held against that screw revolving in a lathe.

It is customary to cut three or four screws, of different threads, one behind another, upon a traversing mandrel, as a single one would be of little use. But as even three or four screws are often insufficient to meet the wants of the artist, and the length of so many together is awkward and inconvenient, it is better to make a concave screw in the end of the mandrel, to which any variety of convex or guide screws may then be alternately attached.—The revolution of the guide screw, without the mandrel, may be prevented by a screw 3, near the end of the latter.

In cutting screws, the proper motion cannot be communicated from a fly wheel to the mandrel by means of the foot acting upon a treadle. If a fly wheel be used, it must be turned backwards and forwards by a winch, through a space proportionate to what the guide screw will allow, so that two persons will be required for the operation. But to cut screws in a foot lathe, the fly of which is unprovided with a winch, and to render one person adequate to the performance, a cord descending from a spring, as in the pole lathe, is coiled round the pulley of the mandrel, and attached to the treadle, the range of which may be suited to the occasion.

With respect to the mode of fastening the wood in which the guide screw turns, a word may be expected. Let a stock or horizontal piece, w, be screwed to or cast along with the headstock C; let the end of it be tapped to receive the screw x, which must be taken out previously to fixing the wood upon the guide screw e. When the wood is in its proper place upon the guide, it must hang down over the end of the stock w, and there must be a hole in it just large enough to admit the screw x, by which it can then be made perfectly secure.

The use of the traversing mandrel will probably in a little time give way to that of the traversing chuck, which was invented by Robert Healy, A. B. of Dublin, and a description of it, communicated by him, inserted in the Philosophical Magazine. On the common mandrel A, fig. 1, pl. IV, is screwed the chuck B, to which may be screwed the chucks of the lathe, as R. On the outside of this chuck B, is turned a screw, which is fitted to an inside screw worked in the circular 4. VOL. I.

L

Traversing chuck.

block C, from which block extends an arm D, as long as may be thought fit for the purpose of permitting another arm E, to slide up and down it; a piece of iron should be screwed to the circular block C, of such a length as to be capable of moving in a groove that may be cut in the collar, or adapted to it. This piece of iron should be regular in its shape, and well fitted to the groove; it is intended to prevent the block C, from being turned round, and to allow it only a steady rectilinear motion. The rest, GFO, must not stand as usual parallel to the work, in cutting an outside screw; but at right angles, as when an inside screw is to be cut, in order that the further arm of the rest F, may be joined to the end of the second or intermediate arm E. It is necessary that this second or intermediate arm E, shall be capable of fastening firmly the first arm D, to any part of the rest, GF, as also to have a joint at each end to admit in a horizontal plane its free play. Thus, as the lathe turns to us or from us, the arms must traverse forwards or backwards; which gives a similar motion to the tool H, that is held steadily or fixed with a screw on the further arm F, of the rest; and thus a screw is cut with a tool of a single point. It is unnecessary to mention, that no joggling should arise from the motion of the arms, as that would cause a failure in cutting a perfect screw. If the centre of the rest should be drawn nearer to us, and by that means bring the tool closer to the intermediate arm E, then a screw of a much larger size will be cut; for as the rest, turning within its socket (the thumb-screw for fixing it in the pillar, being in this operation always withdrawn) moves on a centre, the further the tool is moved from this centre, the greater will be the radius of the circle described, and consequently the coarser will be the screw; and, vice versa, the nearer the tool is brought to the centre, the smaller will be the radius of the circle, and thus the screw will be finer. Should the intermediate arm E, be connected with the nearer arm of the rest G, and the tool held on the further one F, then a left-handed screw will be cut, of a thread the distance between the turns of which will vary according to the distance of the point at which the tool is held between the centre and extreme end; for, as the lathe turns to us, the arms receive a forward motion, except the further arm F, of the rest, which receives a backward motion; but when the lathe turns from us, then the further arm receives a forward motion; and as the tool meets the wood, so it cuts a left-handed screw.

It may be apprehended that a piece of wood so far removed from the collar K, might spring in its motion; but this may be obviated by not making use of the traversing chuck B, till the

Traversing chuck.

screw is to be turned; for as the cutting of it is light work, there will be little resistance, and, of course, but little spring; or the traversing screw, B, may be turned on the mandrel A. Another disadvantage would seem to arise from the impossibility of cutting screws when the puppet head is made use of, to prevent the springing of a long piece of wood. But this may be obviated by lengthening the intermediate arm E, to the part where we intend cutting the screw, and thus we have the same screw as that of the traversing one: if a finer or coarser screw should be required, then, by having an arm of the rest to slide in and out, and the intermediate arm to be connected with the centre of the rest, we have just the same power of turning screws as in the former case. A socket S, is represented, the lower part of which slides on the rest, and may be fastened firmly to it by a screw: the upper part, that turns on a pivot, admits the intermediate arm to slide through it, which arm is held stationary in it by a screw.

If the rest were to make a right angle with the piece of wood on which the screw was to be turned, at the commencement of the process, and to become parallel to it when the screw was finished, an approximation would take place from a larger thread to a smaller, or vice versa; but it is impossible for the rest to become parallel to the work, from the connection of the arms. Now let the traversing arm D, lie in the centre of the screw B, on which it plays, and let the rest make a right angle with the wood on which we intend to cut the screw. The rest may traverse thirty degrees on either side of the right angle; which will not cause any sensible approximation in the thread, and will admit a motion sufficiently extensive for turning the common length of screws. But as the method answers for a short screw of a few turns, that is sufficient for every purpose. For, in order to make a long screw, there may be three different ways to do it:

1st, At the commencement, the rest stands at right angles. with the wood on which the screw is to be cut; by its describing an arch of a few degrees, a short screw is cut; then by bringing back the rest to its original angle, the right one, and sliding forward the single pointed tool to the last thread of the screw that was just cut, we proceed to any length by repeating the same process.

2ndly, When one or two threads of a screw are cut, by making use of a common screw tool, the most unskilful hand will be able to continue the screw to any length.

3dly, Should a side tool with many teeth, instead of the single pointed one, be made use of, a screw of any length may be cut, the rest describing its usual arch.