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.

In your case, if the wheel is running at 3,600 rpm and you're plunging at 0.4 mm/min., that's 0.11 µm/rev. (1,000 × 0.4 ÷ 3,600), a timid dress. Dress more aggressively to sharpen the wheel. If the surface finish goes out of spec, stick with the aggressive dress but switch to a finer-grit wheel.

Dear Doc: I took a high-pressure coolant system from a lathe and installed it on a grinder. It worked wonders for turning, but doesn't seem effective for grinding. Why?

The Doc Replies: Extremely high-pressure coolant might be fine for turning, but it is not good for grinding. You want a pressure that provides the right coolant exit velocity—one that matches wheel velocity. The basic calculation when applying a water-based fluid is pressure in bar equals (wheel velocity in m/sec.)2 divided by 200, or pressure in psi equals (wheel velocity in sfm)2 divided by 535,000.

For example, for a wheel speed of 40 m/sec. (8,000 sfm), you need a nozzle pressure of about 8 bar (120 psi). Anything more than that is not only wasted, it's detrimental to cooling performance.

Cleaning nozzles, on the other hand, are a different story. Here, you want a pressure of 50 to 150 bar, depending on the wheel bond.