Machine Design

Safety Files: The Case of the Exploding Grinder

A worker needed to grind the OD of a part, so he set up a grinder borrowed from another company on the bed of the shop’s lathe. He’d borrowed the grinder before, frequently enough that his grinding wheel was still on the tool’s arbor from the last use. Everything looked like it was assembled properly.

When he started the grinder, the guard over the power transmission rattled. After tightening the guard’s fasteners, he started the grinder again. The rattling disappeared, so he was just about to begin his task when the wheel exploded, injuring him.

The potential for grinding wheels to fail catastrophically is well known. For that reason, grinding-machine manufacturers, ANSI B7.1, and OSHA regulations all require the use of guards during grinder operation. But the company to which the machine belonged didn’t include the guard when loaning out the grinder. It also forgot to send along the owner’s manual, which would have indicated the need for a guard.

The worker should also have removed the grinding wheel from the arbor for inspection before using it. In addition, the power transmission driving the grinding wheel was actually misassembled so the pulleys were reversed. Instead of reducing the rpms supplied by the motor, the transmission multiplied them, spinning the wheel above its upper limit.

Workers shouldn’t store grinding wheels on arbors because it is tempting to start work without inspecting them or checking whether they are the right wheel for the job. Anyone loaning a machine should be sure to include all guards, tools, accessories, and manuals that came with the machine. And manufacturers should add interlocks and error-proofing so that their machines can’t be run in potentially catastrophic ways.

Edited by Jessica Shapiro

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