Figs. 3 and 4.
The foregoing passage must, however, be amplified and modified, but this I will do further on, for you will understand the reasons better if I insert it after what I had written further with regard to the cutting of glass.
How to Sharpen the Wheel Cutter.—The right way to do this is difficult to describe in writing. You must, first of all, grind down the "shoulders" of the tool, through which the pivot of the wheel
goes, for they are made so large that the wheel cannot reach the stone (fig. 6), and must be reduced (fig. 7). Then, after first oiling the pivot so that the wheel
may run easily, you must hold the tool as shown in fig. 8, and rub it swiftly up and down the stone. The angle at which the wheel should rest on the stone is shown in fig. 9. You will see that the angle at which the wheel meets the stone is a little blunter than the angle of the side of the wheel itself. You do not want to make the tool too sharp, otherwise you will risk breaking down the edge, when the wheel will cease to be truly circular, and when that occurs it is absolutely useless. The same thing will happen if the wheel is checked in its revolution while sharpening, and therefore the pivot must be kept oiled both for cutting and sharpening.
Fig. 5.
It is a curious fact to notice that the tool, be it wheel or diamond, that is too sharp is not, in practice, found to make so good a cut as one that is less sharp; it scratches the glass and throws up a line of splinters.
