Machine Design

Lathe operator killed when guarding system fails

A lathe operator was fatally injured when the bar stock he was machining came loose and struck him. The bar was guided and guarded by a pneumatic bar feeder which had not been properly secured.

The CNC turning lathe the worker had been using to machine a 7/8 th-in.-diameter metal bar was running at about 2,500 rpm at the time of the incident. Air pressure in the bar feeder was around 25 psi.

To load the machine, the worker slid the workpiece into the steel feeder tube of the bar feeder. A locking system and a support held the tube in place. The locking system consisted of two C-shaped sections that secured with four bolts to apply clamping friction to the tube. The friction kept the tube in place axially with respect to the lathe so workers weren’t exposed to the rotating workpiece. The support had a slot which let the tube slide between loading and machining positions. Technicians could activate a locking lever to hold the tube in machining position radially with respect to the lathe.

To load a new workpiece, the worker had to unbolt all four bolts on the locking clamp, release the locking lever on the support, slide back the tube, load the bar, then rebolt and properly tighten the locking-clamp bolts and reposition the locking lever. This happened multiple times during a single shift.

In this accident, improperly tightened lockingclamp bolts let the tube slide back as the machine vibrated, and the unsupported section of the rotating workpiece bent at an angle to the machine tool. The bent rod hit the worker on the head, cutting his scalp to the bone and fracturing his skull. He died 12 hours later at a local hospital.

Because the feeder system was both guiding and guarding the workpiece, the locking mechanism should not have relied on friction alone to hold the tube in place. Workers should have been able to more easily lock and unlock it to prevent errors and negligence. And it should have included interlocks that prevented the lathe from operating if the bar feeder was not secured. The bar feeder also needed warnings or instructions posted on it and in its manual.

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].

© 2011 Penton Media, Inc.

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