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# Grinding Wheel Speed - Tool Post Grinders

The grinding wheel will usually determine the correct rpm. For cylindrical tool post grinding, there are basically three speed ranges within which grinding wheels perform best – 5,500 to 6,500 sfm, 8,500 sfm and 12,000 sfm. Because of the maximum safe wheel speed of most vitrified grinding wheels and maximum spindle speeds, most tool post grinding is done around 6,500 sfm.

The maximum safe operating speed for a grinding wheel should be clearly marked on the grinding wheel. Always check to make sure the spindle speed of the grinder does not exceed the maximum safe speed of the grinding wheel. All Dumore tool post grinders include a spindle speed chart on the motor nameplate or belt guard.

Use the following formula to determine the grinding wheel speed (rpm):

RPM  =           3.82 x sfm
Wheel Diameter (inch)

Example: What rpm should be used for a 6” diameter grinding wheel at 6,500 sfm?

RPM  =    3.82 x 6,500 (sfm)    =  4,318
6 (in)

Just as the workpiece speed can be manipulated to compensate for variations in wheel grade, so can the grinding wheel speed. If the wheel appears too “hard”, the wheel speed can be decreased thereby slowing it to the work speed ratio. This increases the cutting force per grain, causing the wheel to act “softer”.

Similarly, increasing the grinding wheel speed will cause a too “soft” wheel to act “harder” by raising the work speed to wheel speed ratio and resulting in less cutting force per grain. Never increase the grinding wheel speed above its rated maximum safe speed.