Fig. 2012.
Fig. 2013.
Thus in [Fig. 2013] it is seen that the corner c of the emery wheel is closer than corner d, and being at the back of the wheel and out of sight it is apt to touch at c unless a thin emery wheel be used.
In a left-hand spiral, [Fig. 2012], it is the corner d that is apt to touch the next tooth, the liability obviously being greatest in cutters of large diameter.
The emery wheel should be of a grade of not less than 60 or more than 70. If it is too coarse it leaves a rough edge, which may, however, be smoothed with an oilstone slip. If the wheel is too fine it is apt to burn the cutter, or in other words, to soften the cutting edge, which may be known by a fine blue burr that may be seen on the front face of the tooth, the metal along this line being softened.
The diameter of the wheel may be larger for small cutters than for large ones, since the teeth of small cutters clear the wheel better. The larger the wheel the less the curvature on the top of the tooth.