In an engine, or power lathe, all screws are cut by trains of gears, as mechanics well know, but in the hand lathe, which was the first machine, screws, both male and female, must be made by chasers or hubs, both inside and outside. The chaser itself must be made first, however, and that is done by a simple tool called "a hub."

Fig. 13.

The chaser is first forged in blank, for an outFig. 14.

Fig. 15.

side chaser, as in Fig. 14, and as in Fig. 15 for an inside tool. It is then filed up, and held against a hub, shown in Fig., 13, running in the lathe.

This rapidly cuts away the chaser blank, and forms the teeth in it perfectly. The lines across it are spiral grooves, cut completely round from one side to the other, so that the hub cuts the blank like any other tool. Fig. 16 represents the chaser.

Fig. 16.