Fig. 677.
It is essential that the work revolve in a direction opposite to that of the emery wheel, for the following reasons. In [Fig. 677] let a represent a reamer and b a segment of an emery wheel. Now suppose a and b to revolve in the direction that would exist if one drove the other from frictional contact of the circumferential surfaces, then the pressure of the cut would cause the reamer a to spring vertically and a wedging action between the reamer and wheel would take place, the reamer vibrating back and forth under varying degrees of this wedging; as a result the surface of a would show waves and would be neither round nor smooth.
Fig. 678.
In the absence of a proper grinding lathe, an ordinary lathe is sometimes improvised for grinding purposes, by attaching to the slide rest a simple frame and emery wheel arbor with pulley attached as in [Fig. 678], in which a is the emery wheel, c the pulley for driving the arbor, and b the frame, d being a lug for a bolt hole to hold the frame to the lathe rest.
In some cases the work may remain stationary and the emery wheel only rotate. Thus, suppose it was required to grind the necessary clearance to relieve the cutting edge c of the reamer, then a could be rotated until c stood in the required position with relation to b, and the revolving emery wheel may either be traversed along, or the work may traverse past the wheel, according to the design of the grinding lathe, but in either case a remains stationary during each cut traverse; after each successive traverse a may be rotated sufficiently to give a cut for the next traverse.
