Battling grinding burn

Dear Doc: I battle burn and long cycle times when grinding ODs in bearing. The wheel manufacturer has specified a series of wheel changes to solve the problem, but it means buying a lot of new wheels. What’s your take?

The Doc Replies: Without even knowing what wheel you’re using, I can say with 99 percent confidence that the wheel you have is just fine for the job. Instead, consider the grinding parameters: material-removal rates, grit-penetration depth, rpm ratios when grinding, and plunge depths and rpm ratios when plunge-roll dressing. There is also the classic danger of slowing the workpiece rpm when burn appears. Establish the right parameters and just about any standard wheel will do the job.

I frequently visit companies with the sole purpose of eliminating burn. A few weeks before each visit, an engineer emails about what new wheels he should order in preparation for my visit. I reply that whatever he’s using will be fine. He thinks the key to eliminating burn is finding the right wheel. It almost never is. The one exception is when using a grit that’s too large for the job, then dressing it dull to achieve the specified surface finish—in that case, you need a new wheel with a finer grit. But even then, it’s not about grit type or wheel grade—just grit size.

The vast majority of burn is caused by bad dressing and grinding parameters, not bad wheels.

Dear Doc: I pinch-peel grind to put forms on tungsten-carbide shafts. Wear on the roughing wheel is a killer. Some people say numerous small cuts are better at reducing wheel wear, while others claim the opposite. Some say to slow the workpiece rpm, while others stress the opposite. What’s the story?

The Doc Replies: The geometries in pinch-peel grinding can get a little strange and, therefore, may seem counterintuitive. Unlike cylindrical-traverse grinding, where all the grinding action occurs on the bottom of the wheel, all the grinding action in pinch-peel roughing occurs on thin section on the front of the wheel, which results in significant wheel wear.

Here are some strategies to reduce wheel wear:

1. Don’t take multiple cuts at a small depth and fast feed rate; take fewer, large-depth, slow-feed cuts.

2. Slow the workpiece rpm.

3. Increase the wheel speed.

4. Use a high-concentration, hard-grade wheel.

The downside is this will increase grinding temperatures. But if the issue is holding form, they’ll help with that provided temperatures don’t get high enough to soften the resin to and cause enormous wheel wear.

Dear Doc: I’m grinding tight-tolerance forms with electroplated CBN wheels. It seems I’m sending them for plating prematurely. What will extend wheel life?

The Doc Replies: Try a finer-grit wheel. This may seem counterintuitive, as larger grits tend to give longer wheel lives. But electroplated wheels are different animals. They exhibit three wear periods: break-in, where wear is large and rapid; steady state, where wear is less and life is longer; and “crash,” where everything rapidly goes bad.

You may be tossing wheels when they’re still in the first period. Radial wear is roughly proportional to the grit diameter during break-in, when fine-grit wheels reduce radial wheel wear.

Another—and probably better—alternative is to buy a “conditioned,” or already broken-in, wheel. It might provide four or five times the wheel life by avoiding the break-in period. CTE