All-or-nothin' burn

Dear Doc: I use water-based coolants when creep-feed grinding hardened steel. I get burn, but it doesn’t appear gradually or come and go. I get no burn and then—wham!—the workpiece is burned to death, along with all subsequent workpieces until I dress the wheel again. Why is the burn not gradual?

The Doc Replies: It sounds like you’re getting what’s called “burn-out,” or “film boiling.”

When creep-feed grinding, the coolant absorbs a lot of heat. Water is an effective heat conductor, and if you get an adequate supply of coolant within the pores of the wheel, the coolant will absorb about 70 percent of the heat. Some of that coolant evaporates as it sucks up the grinding heat. That’s called “nucleate” boiling.

But if too much evaporation occurs, the result is a film of steam at the wheel/workpiece interface. Steam has poor thermal conductivity and, therefore, absorbs only a small amount of heat, which causes film boiling. This means more heat enters the workpiece, producing high temperatures and burn. The high temperatures cause the wheel to become dull and loaded, and the burn continues until you dress the wheel.

The solution to keeping coolant burn-out at bay involves three steps. First, make sure a lot of coolant enters the pores of the wheel, which is best achieved with a high coolant-exit velocity and by aiming the coolant nozzle directly at the wheel/workpiece interface. Second, keep the amount of generated heat down by sharply dressing the wheel. Third, grind aggressively enough that the grits dig into the workpiece.

Dear Doc: I sell grinding wheels. Frequently, a wheel that works magnificently for one customer fails miserably for a different customer with a similar application. Why?

The Doc Replies: Every combination of wheel, grinding machine, coolant type and workpiece material has a “sweet spot,” and the right aggressiveness number (also called chip load, chip thickness or grit-penetration depth) works well only for a specific combination. This means the grits penetrate just deep enough to form a chip, but not too deep to cause excessive wheel wear or too shallow to cause excessive rubbing and heat generation. Over time, machine operators learn how to tweak a machine’s speeds and feeds to find this sweet spot.

Let’s say Mike Machinist has run a wheel for 2 years when Joe Wheel Salesman arrives with a new wheel. The wheel worked wonders at a previous customer with a similar application, so Joe says, “Mike, try this wheel. You’re going to love it.”

Mike tries the new wheel, but runs it at the grinding parameters for the old wheel—the best ones! After all, he’s been tweaking them for 2 years.

What’s the result? The new wheel fails miserably. Why? It’s not a low-quality wheel, but the new wheel has a different sweet-spot aggressiveness number and Mike didn’t run it at that setting.

To do a fair test, Mike needs to spend 2 years tweaking the new wheel to find the sweet spot, which is a bit extreme and impractical. But Mike should at least take a few hours to see how it performs when it’s near its own sweet spot. Otherwise, it’s not a fair comparison, and the gains the new wheel might have provided are never realized.

That’s why so many test wheels fail; they’re run under the wrong conditions.

The next question, of course, is how to cut to the chase and quickly find the sweet spot. I’ll discuss that in my next column, scheduled for the August issue. CTE

About the Author: Dr. Jeffrey Badger is an independent grinding consultant, visiting companies and helping them improve their grinding operations. His Web site is www.TheGrindingDoc.com.