Gummy bear stickiness and blobs

Dear Doc: We are cylindrical-OD grinding steel with aluminum-oxide wheels and a cheap, plastic flood-cooling nozzle. We’re getting thermal damage and wondering if better nozzles and higher pressures will eliminate the damage. Thoughts?

The Doc replies: The answer is probably not. In cylindrical grinding, the arc of contact between the wheel and workpiece is very short. That means that the coolant can suck out only a small fraction of heat — say, 10%. If you improve your coolant application (the usual way: V_coolant = V_wheel, aim at contact, and avoid turbulence in the jet), you may get that up to, say, 15%. That improvement is going to suck away 5% more heat, likely not enough to eliminate your thermal damage. Therefore, focus on other things. And the place to start is dressing.

Is there an exception? Yes. If you’re grinding stainless steel (or any material that tends to load the wheel, particularly materials with a lot of chrome), then having good cooling can work wonders. The reason is not that we’re sucking away more heat. The reason is the “quench effect.” Here, good cooling will quench those hot, sticky chips (that want to stick to the wheel), making them cold and less sticky. Imagine a gummy bear coming out of the microwave compared with a gummy bear coming out of the freezer. One sticks, and the other doesn’t. Good cooling can make the chips coming off your workpiece more like frozen gummy bears and less like hot, sticky ones.

Is there a drawback to good cooling? Yes. You’ll get larger hydroplaning forces. And that means greater wheel and workpiece deflection. Is that an issue? It depends. If you’re trying to hold tight tolerances, it could be.

A wise grinder knows when to invest effort in improving cooling — and when to invest energy elsewhere.

Dear Doc: We’re surface grinding hardened steel with an Al₂O₃ wheel and getting loading. Some operators think it’s a big deal and dress away the loading, consuming lots of wheel. Others think it’s not a problem. Is there a way to know?