Several years ago, while I was working for a small manufacturer of video coordinate measuring machines, I was sent on a marketing and service trip to Asia. The Singapore leg of the trip was all trade shows and schmoozing, but I knew on the Taiwan leg I was going to have to visit a customer who had been complaining of an intermittent problem.

The customer's unit -- our top-of-the-line air bearing, linear motor, fully mechanically decoupled video cmm -- had a recurring issue where the X and Y readouts would start jumping around. Only a few ten-thousandths of an inch, but it wasn't holding perfectly still like a stable servo system should. I poked, prodded, and tested the machine for about two hours, during which time the intermittent readout jumping occurred three different times, each time for 10 or 15 minutes. It started, then stopped, with no apparent change to the state of the machine or stability of the servo drives.

The unit worked the way it was supposed to, whether the readout was jumping or not. Yet I couldn't get the jumping to stop. I was just about to give up and admit that I couldn't identify the problem when it started again. This time I noticed a faint sound. Previously, the sound blended into the ambient whoosh of the cleanroom filtration system. The faint sound started at the same time the readout started jumping.

I waited for the readout to stop jumping. Sure enough, the sound stopped at the same time. I asked the technician whether he knew what the sound was. He motioned for me to follow him. In the next room, he pointed to a machine that trimmed the edges of soft contact lenses by die-punching them. Our unit was installed in a cleanroom with removable tiled-over metal lattice sub-floor, the kind that makes routing cables convenient. It had effectively been functioning as a seismograph whenever the die cutter turned on. The readout marked time with every punch cut.

This entry was submitted by Dan Whitlock and edited by Rob Spiegel.

Dan Whitlock is a mechanical engineer in Healdsburg, Calif.

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