The ordinary methods of cutting screws have already been described. Where great accuracy is not necessary, the threads may be chased by hand. A chaser, or chasing tool, differs from the ordinary thread-cutting tool, in that it has a number of cutting points instead of but one. When a chaser is operated by a power feed, it is customary to have a shaft revolve at the same rate or at an even multiple of the rate of the lathe spindle. This shaft carries a master thread into winch a section of a nut drops. The handle connected with the nut carries the chasing tool. When the nut is in contact, the tool is cutting. At the end of the cut, the tool is lifted out, and with it the nut disengages with the thread.

Hand-chasing requires a great deal of skill in order that a good piece of work may be done. The chasing tool has a number of points, as shown in Fig. 167. The work must be run rapidly in the lathe. The tool is held in both hands, and is supported on a rest similar to that shown for the hand-turning tools in Fig. 90. The first left-hand tooth of the chaser is brought lightly against the right-hand edge of the work. The handle is given a quick twist from left to right, throwing the teeth in the opposite direction. It is well, after the first twist, to stop the lathe and examine the work. If the operation has been properly performed, the second tooth will be found to have entered the groove made by the first. A short length of thread will have been cut out, the pitch being the same as that of the chaser. If this is correct, the lathe may again be started and the chaser applied as before. On the second trial the thread may be run to its full length. The finishing of the thread is done by merely repeating the operation. A fine cut is taken with each application of the chaser for the whole length of the thread, until the full depth has been cut. In doing this work, the rear or right-hand side of the chaser should be pressed more firmly against the piece being cut than the front, because the threads with which that portion of the tool is engaged are more deeply cut than at the front. In addition to cutting, these teeth also guide those in front. The reason for running the lathe at a high rate of speed, is that the movement of the chaser is less likely to be checked or thrown aside by seams or inequalities in the density of the metal than it would be if the lathe were to run slowly. Inside threading may be done by means of the inside chaser shown in Fig. 168.

Fig. 167. Hand Chaser Cutting Outside Thread.

Fig. 168. Hand Chaser Cutting Inside Thread.

Fig. 169. Drill Held by Drill Chuck.