Pass Along DM
April 08
Grinding on the Mill
by George Bulliss
t one company that I worked for in the distant past, we had a requirement for a 6" diameter bore through a piece of H-13, 6" thick and hardened to 46-48RC. The tolerance on the bore was plus or minus .0005". Sending out to a jig grinder or purchasing the appropriately sized boring head was rejected as too expensive and too time consuming.
Our solution for the finishing of the bore was to mount a grinding wheel, 2-1/2" diameter X 3/8", to a piece of 1" diameter CRS with a 3/8-16 screw and washer. We mounted our new arbor in a tool holder and loaded it into our vertical CNC. We then dressed the grinding wheel against a diamond dresser that was clamped to the mill table. Running at approximately 3500 RPM and 10 IPM we were able to take .001" depths of cut. The hole turned out well within tolerance and had a great finish also.
After the first dressing, we checked the OD of the grinding wheel and added a few thousandths to our tool offset for any runout the arbor may have had. Take a light cut first and then measure the work to arrive at the exact tool offset to use. If you set a work offset for the location of the dresser, you can keep track of how much is removed during re-dressing and adjust your tool offsets accordingly.
This grinding wheel was brought out for service in many other tasks. Deep, straight-walled features that would be impossible to cut without end mill deflection were a breeze with the grinding wheel. Just make sure to cut close to the finished dimension before switching to the grinding wheel.
A few words of advice: The spindle speed that we settled on was quite a bit lower than what we originally thought would be optimum and probably was the result of our arbor being out of balance. Use coolant, both for cutting and dressing. On some steels it was best to relieve the wheel so that it was cutting with less than the full 3/8" of contact. Dress the wheel often.
Of course, this isn’t the quickest way to remove metal, but it works wonders in the right situation.
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