Fig. 120 (above). Center Punch. Fig. 121 (below).
Exercise No. 20—Hand-Punch.
Hand-punches are made of various sizes of stock, ⅝ in., ¾ in. and ⅞ in., and are used for hot punching. [Figure 122] shows the size of stock for a punch that will be useful in the school shop, and [Figure 123] shows the completed punch. It is made in the same manner as described for the center-punch. This punch must not be tempered. For punching square holes the punch is drawn square, and the ends of all hand-punches are made smaller than the hole to be punched.
Fig. 122. Stock for Punch. Fig. 123. Completed Punch.
High speed steels, due to their hardness and durability, retain their edge when cutting at extremely high speeds.
It has only been of recent years that high speed steels came into use. Before this time self-hardening steels were made by Jessop and Mushet which were in general use. They were tempered by heating to a dark red and left to cool in the air. The high speed steels of today are heated to 2,000° or 2,200° Fahr., or a white heat bordering on a welding heat.
The chemical composition of these new steels are only known by their makers. However, it is said that they contain carbon, tungsten, chromium, manganese and other elements.
The great advantage in using high speed steel, is that a machine can be run three times as fast as one using carbon steel, without destroying the edge of the tool. The output is therefore greater. Of course, in order to force this steel to do a great amount of work the machine tools should be constructed to stand heavy strains. All kinds of tools are now being made from high speed steel.