When a wheel is to be cut of large size, or of substance exceeding that of clock-wheel work, the above method is not suitable. The wheel is then generally laid flat, and the cogs are shaped by a slotting machine, the chisel of which has a vertical motion. The lathe is then no longer used; as a separate machine is more convenient and economical.
A most serviceable addition to a lathe, especially an amateur's foot lathe, is the circular saw, with guides for cutting parallel, taper, or mitred work. Great rapidity of work is thus combined with perfect accuracy. A five-inch lathe will of course take a saw nearly ten inches diameter, but it is not advisable to attach one of quite this size, for the larger the saw the greater is the leverage against which the turner has to contend, and the friction caused by a deep cut in stuff of two inches diameter is quite sufficient to make the labour considerable. When such work is necessary, it must be very gently brought to bear upon the saw, and the flywheel of the lathe should be heavy. The cord should also pass from the latter to the slowest division of the pulley. If the workman, amateur or professional, desires a lesson in practical mechanics, he has nothing to do but turn a piece of ash six inches in diameter, with the lathe-cord extending from the flywheel to the smallest part of the pulley, the diameter of which is about half that of the object to be turned. This will teach him what hard work is. Then let him try the job with the cord, from the smallest part of the flywheel rim, to the largest diameter of pulley. The change to a slower motion and greater power will not be disagreeable. It must be remembered that a circular saw of six inches diameter will not penetrate three-inch stuff, owing to the boss or nut by which it is attached to its spindle. The above size will not make good work of stuff exceeding two inches in thickness, and even less thickness would be preferable. As to the size of saw, indeed, that is most suitable to a five or six-inch foot lathe, much depends upon the proposed work, and still more upon the weight and size of the flywheel. As a general rule it is better to err upon the size of smallness. The service to which this tool is commonly put is but light; sawing narrow strips of mahogany, such as used at the angles of bird-cages, cutting strips or segments of ivory (for which let the saw be kept wet) sawing out mitred or dovetailed joints, and similar work is within the compass of a five or six inch saw, and it is better not to exceed this. The teeth should be tolerably fine for hardwood and ivory, and coarser for deal and soft woods. Smaller saws of hard steel, and made of thick plate, are used for metal.
Figs. 183, 184.
Fig. 184, 185.
Fig. 184A.