The chaser is first forged in blank, for an outside 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.
CHAPTER IV.
CHASERS, ETC.
It is not always an easy task to chase a true thread on a piece of work, and even “the boldest holds his breath for a time,” if he has a nice piece of work all done but the thread, and that in a critical part. It is so easy to make a drunken thread, or one in which the spirals are not true, but diverge or waver in their path around the shaft, that many are made. That they are more common than true threads, is well known to mechanics. To start a thread true is quite easy with an inside chaser; for, strange as it may seem, it is seldom that a drunken thread is made on inside work; only have the bore itself true, and the chaser will run in properly. The case is different when a bolt or shaft is to be cut. With fine threads, the slightest obstruction on the rest will cause the chaser to catch and stop slightly. No matter how slight the stoppage, it is certain to damage the thread. The injury is more perceptible on fine threads than on coarse, for, in the former, if the threads do not fit (as they will not if they are drunken, one crossing the other, when both parts are put together), the drunken thread will not come fair with the other. In coarse threads, however, it will not be so apparent, for, by making the drunken thread smaller, it will have play and accommodate itself to its place. This is not workmanship, it is “make-shift.”
To chase a true thread the rest must be smooth and free from burrs or depressions. Nice workmen keep a special rest, with a hard, polished steel edge, expressly for this purpose.
If the chasers themselves are smoothly finished at the bottom, on an emery wheel, they are all the better. With these precautions, and others noted below, success is certain. When a thread is to be started, take a fine diamond-pointed tool, and hold it on the end of the shaft to be chased. Set the lathe going, and give the tool a quick twist with the wrist, so that a spiral will be traced on the work, like [Fig. 17].
Fig. 17.