The second class consists of a stock and dies such as shown in [Fig. 293]. For each stock there are provided a set of dies having different diameters and pitches of thread.
In this class of tool the dies are opened out and placed upon the bolt. The set screw is tightened up, forcing the dies to their cut, and the stock is slowly rotated and a traverse taken down the work.
In some cases the dies are then again forced to the work by the set screw, and a cut taken by winding the stocks up the bolt, the operation being continued until the thread is fully developed and cut to the required diameter. In other cases the cut is carried down the bolt, only the dies being wound back to the top of the bolt after each cut is carried down. The difference between these two operations will be shown presently.
The thread in dies which take successive cuts to form a thread may be left full clear through the die, and will thus cut a full thread close up to the head collar or shoulder of the work. It is usual, however, to chamfer off the half threads at the ends of the dies, because if left of their full height they are apt to break off when in use. It is sometimes the practice, however, to chamfer off the first two threads on one side of the dies, leaving the teeth on the other side full, and to use the chamfered as the leading side in all cases in which the thread on the work does not require to be cut up to a shoulder, but turning the dies over with the full threaded teeth as the leading ones when the thread does require to be carried up to a head or shoulder on the work.
Fig. 294
Fig. 295