A Manual of the Hand Lathe / Comprising Concise Directions for Working Metals of All Kinds, Ivory, Bone and Precious Woods

by Egbert P. Watson

Some part of this will correspond with the pitch of the thread to be cut, and there is less liability of making it drunken. By a little practice, one is able to hit the pitch of the chaser exactly in making a start.

“There is no trouble, after you once know how.” We have chased quantities of small screws, with forty-eight threads to the inch, and not a sixteenth of one inch in diameter. If the chaser once hesitates on such screws, they are spoiled. For heavy threads—seven and eight to the inch, which is about as hard work as any one wants to do,—it is the custom of some turners to use a tool with only two teeth, and some use only a sharp-edged cutter, like [Fig. 18], to deepen the thread, the chaser being used afterward, to rectify the job. There is danger with this tool, unless it is used by an expert, of digging out the thread, so that the last end of it will be worse than the first.

Fig. 18.

Another tool, used in chasing heavy threads, is a doctor. This consists in having a fac-simile of the thread to be cut on the back of the chaser, and in applying a short set screw behind, so that, as the iron is cut away, the chaser may be followed up behind. [Fig. 19] is the doctor, but the follower opposite the chaser is too narrow, and should be made nearly half a circle to avoid slipping; with this exception it is all right.

These tools, and the screws made by them, are all inferior to those made by lathes with traversing mandrels; that is, a mandrel which slides in and out of the head stock, as in a Holtzapffel lathe.

Fig. 19.

This lathe has a series of hubs, unlike the one shown previously, slipped over the back end of the lathe spindle (furthest from the workman) and a fixed nut on the head-stock, which, being put in communication with the hub on the mandrel, drives the same in and out, according to the direction the cone-pulleys are turned. Of course, with such an attachment as this, there is no danger of making drunken threads, for the hubs which start the threads, are cut with a train of gears in an engine lathe, so that it is impossible for them to be incorrect. Moreover, a square thread, or a V-shaped thread, can be made with them, which is not the case with common chasers.