My grinding wheel is mounted on my radial arm saw. That means I can slide my mandrel holding jig under the grinding wheel. It also means I can lower the grinding wheel incrementally. I ground all three flats to just a bit less depth than I wanted so the last "cuts" could be very light. This was to insure that each of the final cuts would not be affected to any degree by part of the grinding wheel wearing away during heavier cutting on previous flats. The hex nut on the mandrel made a handy guide for rotating the mandrel 120 degrees between the three flats. To grind I simply slid the mandrel and its supports laterally toward the grinding wheel.