For cutting external or male threads by hand three classes of tools are employed.
Fig. 292.
The first is the screw plate shown in [Fig. 292]. It consists of a hardened steel plate containing holes of varying diameters and threaded with screw threads of different pitches. These holes are provided with two diametrically opposite notches or slots so as to form cutting edges.
This tool is placed upon the end of the work and slowly rotated while under a hand pressure tending to force it upon the work, the teeth cutting grooves to form the thread and advancing along the bolt at a rate determined by the pitch of the thread.
The screw plate is suitable for the softer metals and upon diameters of 1⁄8 inch and less, in which the cutting duty is light; hence the holes do not so rapidly wear larger.
Fig. 293.