What is this tool?

[dsmith]

Thanks for your help

[Mark]

Truing Calipers. 
 

for testing the true of a balance wheel

[Neverenoughwatches]

11 hours ago, dsmith said:

Thanks for your help

There are 2 different end nibs. The nibs with the holes that allow the pivots of a balance staff to pass through safely. This end is for straightening out a balance wheel (truing), the staff stays on the wheel. If you look closely inside the nibs at this end you will see a very small convex shape for the cone of a staff to fit into. This transfers the pressure to a meatier part of the staff and prevents the pivots from breaking off while manipulating the balance wheel to straightness with your fingers. Unfortunately on this one the index and carrier arm are missing, this is what gives you the visuals to straighten out, the 2 fastening screws are still there though so you should be able to manufacture the parts needed to complete the tool. The other set of nibs have no holes, this side of the calipers is for poising the balance wheel (finding an unbalance in the wheel ie. it's heavy spots). Good calipers will have jewelled nibs here with a lightly loaded sprung pin behind the jewel for the pivot to press up against. Cheaper ones will have just a shallow recess in the nib to hold the pivots, be careful not to bend the pivots when bringing the nibs together on the cheaper version.

[JohnR725]

5 hours ago, Neverenoughwatches said:

Unfortunately on this one the index and carrier arm are missing,

I found some pictures so you can see the missing arm and other styles that have different arm arrangements. Usually the arm stays with the tool because it's held in place with the screw.

In one of the pictures there's something on the left-hand side it's a bit fuzzy though. At the end of it is a slot that slot goes over the rim of the balance wheel allowing you to straighten out your balance wheel and true things. Typically that gets separated from the caliper and is never seen again. Which is why When you typically find the truing calipers you will not find that because nobody associates that with the truing caliper.

[dsmith]

Many thanks, guys!