How to Tune Up Your

Starrett #722

Digital Calipers

I picked up a nice Starret #722 caliper at a flea market for what seemed like a good price. It had the nice red case, the depth attachment, and an instruction sheet. Good condition, no dings, dents, or engraving. When I got it home, I was a bit disappointed to discover that it wasn't very accurate. Not off by a huge amount, but about .001" per inch, reading short of the true distance.

I wasn't sure what to do with them, as that's just enough error that I wouldn't use them for measuring parts for CAD drawings, or for machining anything of importance. I considered sending them to Starrett, who is known for excellent customer service, but suspected they wouldn't want to hear about obsolete flea market calipers.

The calipers seemed mechanically ok, and no light was visible through the jaws, but as a last resort I took them apart to be sure there wasn't some piece of grit or grime that was misaligning the parts. What I discovered was not grit or grime, but an electrical connection problem. At the far end of the beam, away from the jaws, is the usual metal strap secured by two slotted screws. On this model, there is some electrically conductive paint on the black scale that insures contact with the underside of the beam. Mine didn't seem to have much paint, and the contact was doubtful at best. I touched it up with some "Silverprint", a silver bearing paint used for fixing PC board traces. You can probably accomplish the same thing with a nickel based paint, or even a small piece of foil. Do this by removing the small metal strap and mating piece on the back side. Improve the contact, then reinstall the strap.

Next, the reader head contacts the beam with two BeCu strips that bear against the area next to the black scale. I took the reader head apart to understand how it works, but you probably shouldn't do that. There's a small rubber "zebra strip" that makes contact with the circuits and tends to fall out; the whole thing is awkward to reassemble. All you need to do is get some contact cleaner, or better yet, contact enhancer, and clean the narrow rails on the front of the beam. Leave just enough of a deposit that the BeCu strips get cleaned when you slide the reader head back and forth. Secret hint- you can get a contact enhancer/lubricant at model railroad shops called "Rail Zip" that works perfectly for this.

After this little electrical tune-up, the calipers were checked again. Much to my surprise, the error was gone! They were now dead on everywhere on the beam. Previously, I believed that whatever accuracy was built into the scale was it. The readout wasn't subject to errors and would either give the right answer, or read out so erratically that the malfunction would be obvious. That doesn't seem to be the case. Small consistant errors are possible, and I'll be checking digital calipers against reference standards much more often!

Alas, after all this I didn't keep them long. The Starrett electronics tend to eat batteries. After going through several batteries, I got rid of them. I now use my trusty Starrett #120 mechanical dial calipers, or sometimes my $19 Harbor Freight calipers.

C. Hoffman