Machine Design

Warnings, Guards, and Safety Analysis Could Have Prevented Router Injury

A worker cutting plywood with a router noticed that the roller clamp holding the plywood to the router table had loosened again. While the router was still running, he applied a wrench to the adjustment bolt. As he did so, his hand slipped. It hit the router’s spinning bit directly below the bolt, injuring him.

The worker’s employer was a job shop with a contract to a larger company. The contracting company had designed the router, table, and roller clamps, and supplied them to the job shop cutting plywood for trailer floors. The design included a handheld router that was bolted upside-down to the bottom of the table. A frame over the table supported roller clamps that held the plywood down to contact the router bit.

The clamp-adjustment bolts had a habit of working themselves loose after about 5 min of operation. To tighten them, workers had to apply a crescent wrench to the bolts, located about 7 in. above the router bit.

If a board was already loaded in the table when adjustment was needed, operators would tighten the clamps while the machine was still running. If no board was loaded, they would unplug the unit from the wall before making the adjustment.

Disconnecting the power was the only way to stop the router bit from spinning; there was no emergency kill switch or on-off-control on the router. In addition, the router’s bit was not guarded.

Investigators found that the contracting company had designed and supplied the router system without conducting any safety analyses, writing a user manual, or warning operators via printed labels on the system.

Turning off the router by whatever means necessary would be a logical precursor to adjusting the clamps. However, because operators had to tighten the bolts every 5 min, and because there was not a readily accessible power-off mechanism, it was reasonably foreseeable that they would tighten the bolts with the router running. In fact, on-the-fly adjustments were standard practice at the contracting company as well as its job shop.

The worker could have prevented his own injury by unplugging the router before adjusting the clamp. However, his employer and the contracting company had a responsibility to make sure operators had clear instructions and warnings about using the equipment.

If the machine’s designers had conducted a safety analysis, they would have seen the need for an accessible power-off mechanism and proper guards against both intended and reasonably foreseeable uses. After the accident, the contracting company added a metal guard over the router bit.

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

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.