Custom wheels and a CNC safety issue

May 1, 2008 by

For the mass market, auto aftermarket custom wheels are priced around $200 each.   For the high-end luxury market (think Escalade and up), a set can cost up to $8,000 (tires included!)  Have you ever wondered how they were manufactured?   While some wheels may be cast aluminum, they typically are manufactured one at a time on a CNC (computer numerically controlled) horizontal boring machine.   The machine starts with a solid block and trims, carves, rotates, flips, carves some more, turns and cuts using multiple rotating cutting bits, operating on up to 5 different axes.   Frequently the cutting edges are sprayed with cutting oil to lubricate and cool the machined surfaces.

The swarf (or turnings and shavings) can be exceedingly sharp, creating a major handling safety issue.   To contain the hazards of rotating blocks, spinning bits, flying shavings, noise, and splashing cutting oil, operations are performed in a closed compartment within the machine.  Typically, the operator walks into the compartment, mounts the blank into the chuck, adjusts or replaces the tool bits and then exits the compartment, closing the small doorway behind him.   Safety interlocks prevent operation of the machinery when anyone is in harm’s way.  At least that is how it is supposed to work.GEI was called when a man was caught inside the machining compartment of such a machine, which proceeded to begin operations with him still inside. Was there a defect in the machine’s safety system? Our expert examined and photographed the machine in question.   A manager described what occurred in the incident in question.

He reported that the injured employee was operating the machine in his normal manner.   In this operation he would carry a wheel blank into the compartment and mount the piece in the horizontal chuck.   Ordinarily, after tightening the chuck on the part to be machined, he would exit the compartment, close the door to the cabinet behind him, (which would disengage the safety interlocks), and then he would push the start button which then would begin the computer controlled machining process.

This time, however, he dropped the chuck wrench, which was used to tighten the wheel blank in the chuck, and he bent down to retrieve it.  Also, when he first entered the compartment, he did not use the lock-out key in the doorway as directed by the large warning sign posted on the machine, right next to the doorway.

The manager then reported that the injured man’s supervisor (who also happened to be his brother) walked by the machine.   He saw that the machine was standing idle and the part was mounted and ready to be machined.   He closed the door and pushed the start button to begin the machining operation.   Immediately, the arm on which the wheel blank and chuck were mounted inside the cabinet swung to the machining position and moved all the way back to the end of the cabinet, where the employee was trying to retrieve the chuck wrench.   The employee’s foot was then trapped in the mechanism.

The manager reported that the time for the arm to rotate and pull back was almost instantaneous, and the employee would have had no time to react and move to a safe position before the mechanism would move to where he was located.

There were several safety mechanisms on the machine, which were intended to prevent this type of accident:  On the control panel there was an interlock switch, which was operated with a key.  This key was properly set to the “normal” position so the machine would not operate if the door was left open.   Further, the door itself had a lock-out stop, which, when used properly by rotating the stop out and removing the key, prevented the sliding door from closing entirely and therefore prevented the machine from starting when the start button was pushed.  If the employee had taken the key with him when he entered the machine compartment workspace, the machine would not have been capable of being started and a visual means from the outside of the machine would have been available to indicate that somebody was inside the compartment.

Our expert’s conclusion was that the accident in question was completely due to human error on the part of the employee and his supervisor.   There was no defect in the machine in question.   A lock-out/tag-out device was available and ready to be used when the employee entered the machinery compartment.   The safety mechanism was operative and was not bypassed or defeated in any way.  Had he followed the lock-out/tag-out protocols and used the lock-out/tag-out key, the incident would not have happened.   His supervisor also failed to check and see if anybody was in the compartment when he assumed control of the machine. The operator was apparently absent from his workstation, and the supervisor closed the door and hit the start button.  The supervisor failed to verify that the machine was safe to operate.