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?

The Doc replies: There are different types of loading. Some are terribly detrimental to grinding operations, and some don’t affect much at all.

Is your wheel simply discolored? Perhaps with some swarf becoming lodged in the pores of the wheel? This kind of loading typically has little effect on a grinding operation. I’ve seen baby blue wheels turn gray and grind just fine. I’ve seen pretty pink wheels turn dark gray and grind just fine. Of course, if it’s excessive, that’s another story. But some discoloration and swarf in the pores is not going to cause huge problems.

“Blob loading” is where the trouble lies. I’ve seen pretty pink grinding wheels with blobs cause lots of problems.

Look at the wheel surface with a magnifying glass. Do you see blobs of compacted swarf at the surface? That’s what’s going to affect your grinding operation. Or do you see swarf smeared over flat portions of the grit? That’s also a problem and more difficult to see than the blobs.

The best way to determine if loading is affecting your grinding is to monitor spindle power. After dressing, if power gradually increases as you continue to grind (for the same speeds and feeds), then there’s a good chance that wheel blob loading is affecting the grinding operation. (Grit dulling is another possibility that can cause power to increase.)

But discoloration? That doesn’t necessarily mean that loading is affecting the process.