Loading when grinding cermets

Dear Doc: I grind cermets and experience massive loading. Any ideas how to overcome that?

The Doc Replies: Assuming you’re using the right wheel (small grit size, friable grit) and the right parameters (low material-removal rates, low wheel speeds), first match the coolant velocity to the wheel velocity to reduce the rate of loading. Then, stick the wheel frequently to clear loading that does accumulate. If that still doesn’t do the trick, install a small-orifice-area, high-pressure cleaning nozzle. 

Courtesy of J. Badger

Figure 1: When grinding with the wheel face, the aggressiveness is high and the corner breaks down quickly, which is good because the contact area is much larger. 

Dear Doc: I grind with the wheel face and eventually get a taper. Once I get the taper, a fine surface finish is hard to achieve. What is your recommendation?

The Doc Replies: When grinding with the face of the wheel, the aggressiveness is high and the corner breaks down quickly. That’s good, because now the contact area is much larger and you can remove material quicker (Figure 1). However, only the tail end of the fully developed taper cleans the surface finish.

The solution is to dress a taper in the wheel. That way you start with the taper and control its shape. What’s more, you can put a flat in the end to clean the surface finish at the end of the cut. You also eliminate having to scrap parts until the taper breaks in.

This works for conventional abrasives, where you dress the taper with a diamond, and superabrasives, where you “true in” the taper. In both cases, the specific taper and flat you put into the wheel will be much better than a messy taper that naturally develops.

Dear Doc: I grind tungsten-carbide and cermet materials and true the diamond wheel off the machine on a separate truing stand. I true the wheel on the truing station, take it off the truer spindle, remount it on the grinder spindle and start grinding. The wheel salesman said this is asking for trouble, but the wheels are made to such tight tolerances that I think it’s OK. Who’s right?

The Doc Replies: Your salesman is right. Even bores manufactured with tight tolerances have just that—a tolerance. That adds play, which increases runout. Most superabrasive bores are manufactured to an H7 tolerance and sometimes to H6. An H7 tolerance on a 37.5mm bore is -0mm/+0.025mm; an H6 is -0mm/+0.016mm. Even for an H6 tolerance, that’s an extra 0.016mm of runout added to a wheel on a bad day. In addition, the shaft has a tolerance, which can add more runout.

You can grind with that, but the wheel will bang on the high points, causing excessive wheel wear, higher temperatures and higher risk of chatter.

If you want proof with your own eyes, mount the wheel on the truing station and true it. Then measure runout with a dial gage. It should be less than 3μm. Then take the wheel off the spindle and carefully mount it again. On a good day, it’ll be maybe 7μm; on a bad day, it’ll jump to 25μm or even more. That’s too much. 

So follow the golden rule of truing: The wheel and the adaptor always travel together. If your machine doesn’t accommodate an adaptor, you can tap it closer to truth with a rubber mallet or a hammer and piece of wood. A good operator can get it closer to truth, but not as good as keeping the wheel on the adaptor.