How to avoid embedded particles

Dear Doc: When grinding nickel alloys, I occasionally get tiny aluminum-oxide particles embedded in the workpiece. It’s infrequent but unacceptable. What can I do?

The Doc replies: You’re venturing into uncharted territory. This is a problem I hear about occasionally, but I’ve never seen a good solution or good study that explains it — just a lot of half-baked theories. Since it’s infrequent, determining whether process changes had an effect can take months. Having said that, here’s what I tell my customers.

First, look at the possible causes. After diamond dressing, the tips of the Al₂O₃ grits are somewhat fragile and “crumbly.” Immediately upon first contact with the workpiece, the wheel loses 3 to 10 µm (0.0001″ to 0.0004″) off the radius as the tips of the grits break off. This loss matters because you don’t want to finish or spark out with a freshly dressed wheel. Otherwise, you run the risk of these crumbling grit tips embedding in the workpiece. You want to leave at least 10 mm² (0.016 in.²), which is the depth of grind times the length of grind, of material for finish grinding.

This crumbly clean-up period is especially relevant if you’re performing continuous dress. Here, you don’t want to use CD during finishing or sparking out as the wheel will be crumbling off tiny pieces at the end of grind — a bad idea.

Second, examine the type of Al₂O₃. Theoretically, a tougher, or less friable, grit is less prone to break into small pieces and embed itself in the workpiece.