In these, and all similar lathes, the ro-. tation is made backwards and forwards ; And there are some kinds of which such a motion Is advantageous ; but in general it is much preferable that the work should constantly revolve the same way, as shown in the lathe described un der that article, usually known by the name of the foot-lathe. In the regular foot-lathe, work is Very seldom turned between the opposite centres, though this method certainly affords truth and precision. The mandreris here an essential part of the apparatus, 'which is always used. It has been shown that it is supported by a centre on the 'left hand, called the back centre, and by a steel collar in the middle poppet-head ; and that the right hand or nose of the mandrel, terminates in a screw, ei ther convex dr -concave, the latter of which is preferred in the best lathes. The various description of pieces screw ed upon the nose of the inandrel, for holding or carrying work, are called chucks, probably Secapse the work is mostly fastened by being driven, jammed, Or choked into them.
When work is to be turned between centres by the foot-lathe, a eentre-chuck, or steel-piece, carry-Mr' a projecting point, is screwed on the nose of the Man drel ; and as this piece is not harder than blue, and may not always screw home to exactly the same bearing, accurate work men are in the hailir of turning or shav ing the point in its place, so that it shall lie truly 6entered. The opposite centre is afforded by the moveable poppet-head, and ought to be truly in the axis ; and the mandrel is made to carry the work round by an arm and pin, or by any other ready, method of connection.
Work, which is not to be turned be tween centres, is usually fastened to, or Axed in, a block or wooden chuck screw ed on the; mandrel. As it would be al most impossible 'to screw a wooden chuck upon the convex nose of a drel, and take it off as occasion required during the process, without altering the position, it is found much best that the screw of the mandrel should be-hollow, and %brass chuck screwed therein, hav ing its projecting screw to receive the wooden chuck ;' because, by this means, the work may be taken 'off repeatedly, if needftil, without ever separating the brass and the wood ; and the brass and the 'Steel will take 'the same position when screwed together again.
Other.work may be fasten ed to a wooden chuck by cement, or by glue, or by turning a cell in the wood, and driving the work gently and careful ly into it till fixed.
The stronger, the firmer, and the bet ter the workmanship of a lathe, the easi erit will be to perform work with expe dition and truth ; hut a good workman will make true and excellent work with a very indifferent bathe, by taking care to cut so little at a time that the parts of the engine may never be shaken out of their coat M. Metallic lathes, if ever so
strong, have an elastic' tremor, which makes it difficult to cut brass and bell metal as firmly and smoothly as in wood en lathe:;, but the structure of the former admits of greater precision and truth. In a well coitst meted lathe, the hack cen tre, the centre of the collar, and the fore centre, or centre of the moveable poppet head, ought to be in one line, parallel to the bed or sheers. To prove this by trial, set the movable poppet-head as far to the right hand as possible, and screw a stick of wood into tile nose of the man drel : into the middle of the right hand end of the stick, or nearly so, drive a pin or other projecting point, and by gen tle. blows against the stick, cause the point to remain steady in the axis, while the mandrel is turned round. If the cen tre point of the moveable poppet be truly opposite to this revolving point, the three centres are in a line ; and if the same con tinues to be the case when the face of moveable poppet is reversed, it is a proof that the hole in the poppet is bored pa rallel to the bed; and if the same adjust ment continues when the stick is shorten ed, it shows that the bed is straight and parallel to the axis of work. If the col lar and back centre, and the chamfer and point of the mandrel, in a lathe, be truly formed, and set square, the rotation slowly made by hand, when the back centre is rather firmly set up, will be equally stiff in every part, and the wear ing parts when examined, will have the same aspect, slope, and grain, in every part of their surfaces.
The velocity of rotation may be ex tremely swift in wood, slower in brass and bell-metal, still slower in cast iron, and slowest of all in forged iron or steel. The reason for these limits appears to be, that a certain time is requisite for the act of cutting to take place, and that the tool itself, if heated by rotation, will instantly become soft, and cease to cut. Steel and iron require to be kept, wetted. For rough work in wood the guage is a good tool, and after that the chisel, with its edge a little convex, rather than -strait lined. The graver is commonly used for metal; and for strong rough work, the hook tool, which is of excellent' advan tage, even in small work, on account of its extreme steadiness. When steel is to be cut extremely clean, a sharp hard tool may be useful ; but for the most part, in metallic work, even of steel, (if anneal ed,) the hook tool, or graver, need not be harder than purple, or even blue. But to cut steel work or chill cast iron cylin ders at a high temper, the tool must be very hard, the angle of edge obtuse, (say seventy degrees,) and the motion slow.