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Machine Design

Miter Saw Design, Poor Safety Training Lead to Student’s Severed Thumb

A junior-high shop-class student started up a compound miter saw to cut an angled corner for a wooden jewelry box. As he pulled the saw down with one hand, the blade grabbed the wood he was cutting and pulled his other hand in, severing his thumb.

The saw blade works with a miter box that lets the user adjust the cut angle relative to the fixed back fence between –45 and 45°. A gap about 4-in. wide in the back fence accommodates the sweep of angles. The saw blade has an integral guard that covers the blade except where it touches the workpiece. Apart from the workpiece, the guarded saw, and the miter box, the student didn’t have any additional accessories for making the cut.

Although the shop teacher had explained the saw’s operation to the class, he had not posted an operators’ manual or instruction sheet near the machine. He was helping other students and had his back turned to the saw at the time of the accident.

The saw’s safety materials and the teacher’s instructions indicated the workpiece should be secured firmly against the saw’s fence, but neither explained that clamping the piece was safer than holding it with a free hand. The saw didn’t have integral clamps, and the teacher did not place any clamps near the saw in his classroom.

An interview with the student after the accident revealed that he had not been aware that clamping the workpiece to the fence was safer than holding it, or that the saw blade tends to catch and kick up smaller workpieces.

Investigators felt that the classroom setup created an unsafe condition for the inexperienced students using the saw. Although each student had to pass a safety quiz about each piece of equipment in the shop, students were not required to demonstrate their proficiency in front of the teacher before being allowed to use the equipment solo.

In addition, the gap in the saw’s back fence necessitated a large space between where the workpiece was secured (whether held by hand or clamp) and where the saw blade touched the wood. This long moment arm made it more likely that the saw’s power would violently pivot the wood.

The saw’s manufacturer publishes separate instructions on building supplemental fences that accommodate specific cutting angles while permitting clamping closer to the cut. However, no such fence was available in the wood shop. Current compound-miter-saw models have a back fence the operator can move to minimize the gap. If adjusted properly, such a setup would have prevented the accident. MD

This month’s safety violation comes from the files of Lanny Berke, a registered professional engineer and Certified Safety Professional involved in forensic engineering since 1972. Got a safety violation to share? Send your images and explanations to [email protected].

Edited by Jessica Shapiro

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