Battling part barreling

Dear Doc: I cylindrical-traverse grind long, hardened-steel workpieces. Sometimes the resulting parts are barrel-shaped, meaning the OD is larger at the center than at the ends. Why does this happen, and how can I correct it?

The Doc Replies: The most common cause of “barreling” is deflection at the center of the workpiece, where it is less stiff. During grinding, a normal force develops between the wheel and workpiece. The wheel pushes against the workpiece, and the workpiece pushes back with an equal force.

In cylindrical-traverse grinding, the wheel is usually stiffer than the workpiece, so the wheel only deflects a little. Because the workpiece deflects a lot more, especially at its center, the effective DOC is smaller. Let’s say you specify a 25µm DOC. At the ends of the workpiece OD, the actual DOC might be 24µm, while the DOC is 20µm nearer the center and only 10µm at the center. Then, when grinding is complete, the force is removed and the workpiece springs back to its regular position. But, because you took a smaller effective DOC at the center, the result is a barrel shape.

To correct it, you can try beefing up workpiece support by installing a steady rest, but sometimes that’s not possible due to part geometry and the fixturing capabilities of the machine. The other option is to reduce the normal force and, from my experience, that’s usually the best way.

Most companies I visit that are experiencing barreling problems are dressing the wheel dull, usually to improve surface finish. A dull wheel can exert six or more times the normal force—and cause six times the deflection—of a sharp wheel. Therefore, sharp wheels can battle barreling, and the solution is to dress the wheel sharp. Of course, the operator will then complain about a poor surface finish. To overcome that, switch to a wheel with a finer grit size and dress it sharp. Normal forces will decrease, barreling will decrease, and the surface finish will be the same as before.

Other tactics include taking more spark-out passes, dropping the coolant pressure to reduce hydrodynamic push off, or hydroplaning, and slowing the wheel speed to ensure the grits effectively bite the workpiece. Most of my successes in reducing barreling have come from some combination of these tactics.

Dear Doc: I grind nickel-base alloys and use a scrubber nozzle to clean the wheel. However, I’m not sure I’m seeing any benefit. Why?

The Doc Replies: If you suspect a scrubber nozzle isn’t providing any benefit, it’s probably not. The vast majority of scrubber nozzles I see in production environments aren’t doing a darn thing, because they are incorrectly designed. The pressures are too low, the nozzle openings are too large, and the nozzles are aimed incorrectly.

You can assess whether a scrubber nozzle is working by monitoring grinding wheel spindle power. Grind a batch of parts—maybe an hour’s worth—and monitor the spindle power throughout the process. Then, turn off the scrubber nozzle and grind another batch for the same amount of time with the exact same dressing and grinding parameters. If you don’t see a difference in spindle power between the two batches, your scrubber nozzle probably isn’t doing anything. CTE

About the Author: Dr. Jeffrey Badger, independent grinding consultant, www.TheGrindingDoc.com. He’ll be giving his High Intensity Grinding Course May 26-28 in Sheffield, England.