§ 76 Arming a Wheel.
A convenient wheel is made out of tin-plate, i.e. mild steel sheet, about one-thirtieth of an inch thick and seven inches in diameter. This wheel must be quite flat and true, as well as round; too much pains cannot be taken in securing these qualities. After the wheel is mounted, it is better to turn it quite true by means of a watch-maker’s “graver”, or other suitable tool. The general design of a rock-cutting machine will be clear from the illustration (Fig. 63).
The wheel being set up correctly, the next step is to arm it with diamond dust. For this purpose it is before all things necessary that real diamond dust should be obtained. The best plan is to procure a bit of “bort” which has been used in a diamond drill, and whose properties have therefore been tested to some extent. This is ground in a diamond mortar — or rather hammered in one — and passed through a sieve having at least 80 threads to the inch. The dust may be conveniently kept in oil.
To arm the wheel, a little dust and oil is taken on the finger, and laid on round the periphery of the wheel. A bit of flint or agate is then held firmly against the edge of the wheel and the latter is rotated two or three times by hand. The rotation must be quite slow — say one turn in half a minute — and the flint must be held firmly and steadily against the wheel. Some operators prefer to hammer the diamond dust into the wheel with a lump of flint, or agate, but there is a risk of deforming the wheel in the process. When a new wheel is set up, it may be necessary to repeat the above process once every half hour or so till the cutting is satisfactory, but when once a wheel is well armed it will work for a long time without further attention.