Gummy bear stickiness and blobs

Author Jeffrey A. Badger, Ph.D.
Published
May 10,2022 - 05:00pm

Related Glossary Terms

  • coolant

    coolant

    Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.

  • cylindrical grinding

    cylindrical grinding

    Grinding operation in which the workpiece is rotated around a fixed axis while the grinding wheel is fed into the outside surface in controlled relation to the axis of rotation. The workpiece is usually cylindrical, but it may be tapered or curvilinear in profile. See centerless grinding; grinding.

  • dressing

    dressing

    Removal of undesirable materials from “loaded” grinding wheels using a single- or multi-point diamond or other tool. The process also exposes unused, sharp abrasive points. See loading; truing.

  • flat ( screw flat)

    flat ( screw flat)

    Flat surface machined into the shank of a cutting tool for enhanced holding of the tool.

  • grinding

    grinding

    Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.

  • surface grinding

    surface grinding

    Machining of a flat, angled or contoured surface by passing a workpiece beneath a grinding wheel in a plane parallel to the grinding wheel spindle. See grinding.

  • swarf

    swarf

    Metal fines and grinding wheel particles generated during grinding.

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: Vcoolant = Vwheel, 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.