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Hairspring Truing Arbor

From The American Horologist magazine, March 1938

Hairspring Truing Arbor

A true hairspring is one of the important factors in a watch for fine timekeeping. The hairspring must be perfectly fiat, truely circled and well centered. The overcoil must be so shaped that the hairspring develops or breathes out evenly from center. The regulator pins must also be close allowing a minimum of freedom for the hairspring.

The diagram shows construction of hairspring trueing arbor that makes the work of trueing a hairspring simple and accurate. The arbor itself is to be made of soft steel and threaded. It is then hardened and tempered to blue.

The thread can be made from the screwplate, holding the size to .54 mm. This size arbor will take care of hairsprings from 185 to 3/0.


The nut built to specifications, particular care being taken that the cone runs perfectly true when the nut is finished.

The brass disc made to dimensions. Laying a small celluloid disc 13 mm. in diameter in the recess or a porcelain seconds bit. This makes a white background when trueing the spring.

In turning this brass disc it is advisable to bore the hole using a slide rest so the arbor will fit perfectly upright and the disc runs perfectly flat. The nut should also be hardened and blued. 

In using this tool, the hairspring collet is slipped over the threaded part of the arbor and the nut is screwed down to hold the collet. The comb part of the nut will center the collet, holding it in its proper position. The arbor and hairspring is then placed in a trueing caliper and trueing the hairspring this way makes the work easy.

For smaller size hairsprings a similar tool may be made. Instead of having the threaded arbor this part may be slightly tapered so that the collet may be frictioned on. It is important that the shoulder on which the collet rests is perfectly square so the hairspring will take the same position in the watch that it had when on the arbor. The time taken to build this arbor is time well spent as the work will be accomplished much more satisfactorily than the method many watchmakers use.

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Note: Take a look here for examples of these. Making this tool was a common student project.


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