In lathes that have traversing mandrels to cut screws, the tool itself remains stationary, but as this is obviously a disadvantage in many kinds of work, it is far better to have the tool advance and the mandrel revolve as usual. By this plan much time is saved, a greater range of work is possible with the same gear, and a piece that is chucked, or one that is between the centers, can be cut with equal facility.
Any common lathe can be rigged to do this by putting a shell on the back end of the mandrel, between the pulley and the set screw, and slipping the hub over the shell, with a feather, to keep it from turning. To take a thread from this hub, a round bar must be set parallel with the shears, in easy-working guides. The bar must have an arm at one end, to reach over to the hub, said arm to be fitted with a piece of hard wood, to match the thread on the hub. The other end of the bar has the cutting tool in it; of course, at right angles, so as to run in to the work, and bear on the tool rest. The tool is held in an arm on the bar by a set screw, so that it can be lengthened or shortened.
By this arrangement, a true thread can be rapidly generated on any rod, hollow cylinder, or other kind of work—the pitch depending on the pitch of the hub.
It is necessary to have as many different hubs, varying in pitch, as there are different kinds of work to be done, and, although the thread on the hub is only an inch or half an inch long, perhaps, a screw of any length may be cut on a rod, by simply shifting the cutter on the rest. This same bar is also useful for turning, as with a slide rest, for, by sliding it along gradually, it acts, in a measure, like a fixed tool in a slide rest.
Fig. 20.
From these hints the amateur who takes a lathe in hand for the first time, or is, at best, a neophyte, may learn much to his advantage. Persons of a mechanical turn only need a hint, when the mind springs to the conclusion with surprising rapidity.
The little tool, shown in [Fig. 20], is very handy in many instances, particularly for running under the necks of screws when the thread is cut up to the head. By so making them, the head comes fair down upon its bed, and holds much better.