Dial Caliper Autopsy

Copyright 2000 by Frank J. Hoose, Jr  Home
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Caliper_y.jpg (45981 bytes)

My Chinese-made $20 dial caliper recently jammed and would not move. I tried blowing compressed air through it and flooding the gears with light oil to flush out any metal chips that might be caught in the gears but had no success.

Since it was useless to me in its non-working state, and because I'm always curious about how things work, I decided to take it apart and see if I could fix it. It became evident that it would be necessary to remove the dial cover and pointer to get to the innards so I pried the pointer off using a pair of little levers I formed out of thin brass.

Here is the crystal, spacer ring and pointer:
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With the dial removed, a machined brass plate is visible:
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On the back side of the dial are two pinions that ride along the rack on the caliper frame.

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After removing the brass cover on the back side of the dial, an internal gear train is visible:
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Here is one of the two internal gear assemblies. It consists of a gear and a plate connected by an anti-backlash spring mechanism. The gear is approximately 3/8" diameter. I photographed it through a 10x microscope.
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Here's the other gear. It has a spring wrapped around the shaft and extending in a loop above the surface of the gear.
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This is the middle gear that sits between the two larger gears. In operation, the dial pointer is mounted on the narrow shaft.
Middle_gear.jpg (12396 bytes)

I was surprised at the complexity of the internal mechanism. I would guess the original inventor of the dial caliper started out with a simpler design and it evolved to this more complex form as improvements were made to increase accuracy.

There must be close to 100 parts in the caliper overall. I'm amazed that the Chinese can make this device, package it in a plastic box, ship it to U.S. and still sell it for under $20. I guess the workers didn't take home much pay. Considering the price, though, I think the craftsmanship is pretty impressive.

I found no obvious reason why the caliper was jamming, so I reassembled it. I had no parts left over, which is always a good omen ;-). Unfortunately, it still jams and I have pretty much run out of ideas for fixing it. At $20 to replace it, it's not worth spending any more time on, but I hate to give up. After a while it becomes a matter of pride more than practicality...