It is sometimes desirable to drill a hole in very hard metal. To do this the drill must be made very hard; it must be run at a very slow speed; it must be forced against the work as hard as possible without breaking the point; and it must be provided with an abundant supply of oil. For excessive hardening of a drill, it may be heated to a dull red heat, preferably in a charcoal fire, and quenched in mercury instead of water, in order to make the cooling more rapid. It will also assist in the operation, if the surface of the metal to be drilled is nicked with a cold chisel before work is begun. In some cases turpentine, in place of oil, may be used with beneficial results.

Thin chilled cast iron may be softened by placing a small piece of sulphur on the place where a hole is desired, and then heating slowly to a dull red.

Glass may also be drilled

. There are two methods: one is to use a flat drill moistened with camphor and turpentine; and the other is to use a copper tube with No. 60 emery or carborundum and oil. In the last method, drill half-way through, reverse, and drill to meet, removing the fin at the center with a round file wet with water or turpentine.

Fig. 254. Peening.

Grinding Valves

This is a kind of grinding that is usually done by hand. It consists in fitting a valve and its seat so that they are in metallic contact. In its results, it is the same as scraping. The process is very simple. The valve is coated with oil, and some fine emery sprinkled over it. It is then put on the seat and worked back and forth or revolved. The emery serves to grind off the high surfaces of both valve and seat. After grinding for a time, remove the valve, and wipe both surfaces clean. The metal on each will show where they have been in contact. When these indications appear over the whole of the surface, or in a continuous ring about the seat of a circular valve, the work is completed.