The wheel will usually determine the correct RPM. For cylindrical tool post grinding, there are basically 3 speed ranges within which the grinding wheel performs best. These speeds are 5,500 to 6,500 SFM, 8,500 SFM, and 12,000 SFM. Because of the maximum spindle speeds, most tool post grinding is done around 6,500 SFM.
The maximum safe operating speed of the wheel should be clearly marked on the wheel blotter. Always check to make sure the spindle speed of the grinder does not exceed the maximum safe speed of the wheel. All Themac tool post grinders are supplied with a spindle speed chart on the motor nameplate or on the belt guard.
To find the wheel RPM, the following formula should be used:
3.82 x SFM
Example: What RPM should be used with a 6" wheel at 6,500 SFM?
3.82 x 6,500
Just as the work speed can be manipulated to compensate for variations in wheel grade, so can the wheel speed. If the wheel appears too hard, the speed should be decreased thereby slowing it to the work speed ratio. This increases the cutting force per grain, and the wheel will act softer.
Similarly, increasing the wheel speed will cause a too soft wheel to act harder by raising the work speed to wheel speed ratio, resulting in less cutting force per grain. Never increase the wheel speed above its rated maximum safe speed.