Fig. 2040.
It is obvious that if a piece of work, as a in [Fig. 2040], be held steadily upon the rest r, its end will be ground to the curvature of the emery wheel w, and that if it be required to grind the surface flat the piece must be raised and lowered as denoted by the dotted lines, the amount of this motion being determined by the thickness of the piece.
Fig. 2041.
Furthermore, if the piece of work be of a less width than the thickness of the wheel, as in the top view in [Fig. 2041], the work a will wear a groove on the wheel, and its side edges will therefore become rounded off unless it be given sufficient motion in the direction of d and e to cause it to traverse across the full width of the wheel face, and as this motion would require to be simultaneous with the vertical motion explained with reference to [Fig. 2040], it is not practicable to grind true level surfaces upon the perimeter of the wheel. As the sides of the wheel are flat, however, it is self-suggestive to apply the work to the side faces. But in this case, also, that part of the wheel surface that performs grinding duty will gradually wear away, leaving a shoulder or projecting surface upon the wheel.
Fig. 2042.
Suppose, for example, that in [Fig. 2042] the duty has been confined to that part of the wheel face from a to the perimeter and the wheel would wear as shown, the result being the same whether the width or distance from the shoulder a to the perimeter of the wheel represents the width of the work held steadily against the wheel or the traverse of a narrower piece of work.