Truing calipers are used to help make a wheel turn true. Occasionally you find a bent wheel in a watch. The truing caliper will help you see how far out of true it is, locate the low (or high) spot and make the correction to get it turning true again.
There are two main types of calipers. The set at the top are “lyre” style calipers and the two on the bottom are “figure 8” calipers. The lyre calipers are specifically designed for balance wheels. The guage which comes out from the center can be used to see the wobble in the wheel and also to make the correction. Generally the other calipers are used for wheels with larger pivots. Some of them are jeweled and others are simply bushed. The nice thing about the lyre calipers is that it is easy to make and fit different tips for them. I made a set of tips with a large conical cup on the tip for center wheels because the center wheel pivot is usually larger than any of these calipers.
The Rolex calipers in the middle are held closed with hand tension and there is a set screw to set the maximum closed dimension so that the wheel has just enough endshake to spin freely without moving up and down. The bottom calipers are set with the thumb screw in the middle to a specific size and no tension needs to be applied with the hand. This would be useful if you needed both hands to perform an operation on the wheel. I can’t think why you would need to have both hands free.
To see the lyre calipers in action you can visit my post entitled How does this happen? Truing calipers are one of those tools (like poising tools) that you can pick up inexpensively on eBay or at a used tool shop. There seem to be plenty of them and they usually seem to be in good condition.
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