Hybrids, temperatures and chatter: General Industry Coverage

Dear Doc: What’s your take on hybrid bond wheels when grinding carbide?

The Doc replies: I’m a fan, especially if you have a lot of material to remove quickly, such as in flute grinding of endmills. Keep in mind that when you try them out, you may need to increase your grit penetration depth (compared with resin-bonded wheels). How do you do that? In surface grinding, you increase the depth of cut, increase the feed rate or decrease the wheel speed. In cylindrical plunge grinding, you increase the plunge speed, increase the workpiece rpm or decrease the wheel speed. In cylindrical traverse grinding, you increase the DOC, increase the workpiece rpm (and probably simultaneously increase the traverse velocity so as not to change the overlap ratio) or decrease the wheel speed. Or you do some combination of these. I’ve found that if you use hybrid bond wheels, you can go longer without having to stick. Or if you’re really skilled, perhaps you never have to stick — or almost never.

Dear Doc: I attended your three-day class and learned that speeding up workpiece rpm reduces grinding temperatures in cylindrical plunge grinding. Is this also true for cylindrical traverse?

The Doc replies: No, absolutely not. In fact, it’s just the opposite: In cylindrical traverse grinding, increasing workpiece rpm increases workpiece temperature. How can that be? It’s because cylindrical plunge and cylindrical traverse are two different animals.

In cylindrical plunge, when you increase your workpiece rpm, you simultaneously decrease your effective DOC. (Effective DOC in mm = plunge speed in mm/min. divided by workpiece rpm.) That moves you from the slower, deeper regime to the faster, shallower regime. In cylindrical grinding, grinding faster and shallower reduces workpiece temperature.

In cylindrical traverse grinding, the effective depth is fixed. It doesn’t change as you increase workpiece rpm. Increasing your workpiece rpm simply increases your workpiece surface velocity (at the same DOC). That means larger temperatures in the (now smaller) cutting width of the wheel. In other words, you’re asking less of your wheel (a narrowed width) to do more of the work. And that increases temperatures.