Cooling OD burn: General Industry Coverage

Dear Doc: When cylindrical-plunge grinding ODs in hardened bearing steel with an aluminum-oxide wheel, I get thermal damage in the form of a white layer. To battle this burn, I’m going to redo our cooling system to get higher coolant velocities. What do you recommend?

The Doc Replies: My recommendation is this: Don’t bother. If you’re burning with bad cooling, you’ll burn with good cooling. For that OD grinding process, the coolant just doesn’t suck up enough heat to make a huge difference. You’re screwing up something else, and that’s why there’s burn.

There are two exceptions. First, if you’re burning because you’re loading the wheel, improved cooling—getting the coolant velocity close to the wheel velocity and aiming at the wheel-workpiece interface—will slow loading and perhaps delay the onset of burn. But unlike stainless steel and nickel-base alloys, bearing steel isn’t prone to severe loading. So I don’t think that bad cooling is the cause of the burn. The second exception is if you’re grinding with a 30° swivel and getting burn not on the OD but on the shoulder. Here, a dedicated, high-velocity nozzle to cool just the shoulder will reduce burn.

Using a wheel with too large of a grit size and dressing dull likely caused your OD burn. That’s the root cause of most of the burn in cylindrical-OD grinding that I see on shop floors—not bad cooling. Switch to a finer grit size, and dress sharper.

Dear Doc: The old-timer machinists at my shop say I should always dress and grind at the same wheel rpm. Are they right? If so, why?