Avoiding high wheel wear: Turning Performance

Dear Doc: I use electroplated CBN wheels to grind cobalt-chrome alloys. I get a lot of wheel wear for the first few parts, creating part-dimension headaches, before the grinding operation settles into a steady-state regime. Also, during this high-wear period, surface finish is atrocious. Is there any way to avoid this period?

The Doc Replies: Yes. Purchase a conditioned electroplated wheel. Unconditioned electroplated wheels, because they are not dressed, have rogue grits sticking up in just a few points around the circumference. These rogue grits wear quickly, but until they do, just a few do the cutting and the space between them is huge. Therefore, the chip thickness is massive and the surface finish is poor. Conditioned wheels are already “broken in,” so you don’t have to endure the break-in period.

Dear Doc: I plunge-roll dress a form onto a wheel with a diamond roll. I like to dress at a 0.4-mm/min. plunge speed, whereas my colleagues say I should dress faster, at 4 mm/min. Who’s right?

The Doc Replies: Avoid thinking about plunge speed—just about any speed will work, depending on the wheel speed. Instead, focus on dressing depth, or the distance the diamond roll plunges in one revolution of the wheel, measured in mm/rev. or ipr.

To calculate dressing depth, divide the plunge speed in mm/min. by the wheel’s rotational speed in rpm. This provides a result in mm/rev. Then, multiply that by 1,000 to convert the result to µm/rev.

Typical values for dressing an Al2O3 wheel are 0.1 to 1.0 µm/rev. or 4 to 40 µin./rev. Small values produce a duller wheel, more grinding power and a finer surface finish. Large values provide a sharper wheel, less grinding power and a rougher surface finish.