Fig. 1328.
After the chaser is cut on the hob, its edges, as at c, and the corner, as at d, in [Fig. 1328], should be rounded off, so that they may not catch in any burr which the heel of the hand tools may leave on the surface of the hand rest.
Fig. 1329.
For roughing out the threads on wrought iron or steel the top face should be hollowed out, as shown in [Fig. 1328], which will enable the chaser to cut very freely. For use on cast iron the top face should be straight, as shown in [Fig. 1328] at a, while for use on soft metal, as brass, the top face must be ground off, as shown in [Fig. 1329].
Fig. 1330.