Checking the balance
To check the balance for truth in the flat mount the balance in truing calipers. To check flat swing blade over the rim of the balance and while slowly turning the balance observe the line of light between the balance rim and the blade, if there is any deviation in the thickness of the light line you will need to adjust the balance. Adjust the balance by bending it with your fingers and a balance truing tool which is a lever with notches cut in it’s ends that you can slide over the balance rim or arm giving you leverage and an accurate way to manipulate the balance.
To check the balance for truth in the round follow the instructions above but slide the blade to the side of the balance rim instead of over the top. Adjust the balance in the same manner as described above.
See the illustrations below
see an animation of checking the balance for truth in the flat using the calipers
see an animation of checking the balance for truth in the round using the calipers
Poising the balance
To check the balance for poise means you check to see if it is heavier on one side than the other. The poise of the balance is very important, you will not be able to time the movement to positions other than dial up or dial down if the balance is out of poise. To poise the balance you must first make sure the balance pivots are straight, polished and clean, Now make sure the poising tool is clean and level then set the balance on the poising tool so that only the pivots are resting in the blades of the tool, next take you hand away and let the balance roll freely, when it stops take note of the place on the balance that is farthest down and rotate the balance so that point is on top, then let go of the balance again and if it stops with the place you noted in the same spot as before the balance is probably heavier at that point and needs to be adjusted however it is always a good idea to check this several times until you are absolutely sure before you proceed. To poise the balance you need to take weight away from the heavy point on the balance, you can do this in several different ways but the method I use and find quite satisfactory is to remove weight from the back of the screw where it can’t be seen. I you a screw undercutting tool which only removes material from the inner portion of the back of the screw and leaves the rest of the screw intact, this leaves no marks to be seen from the outside and the balance is left looking untouched. There are other tools that can be used that take material away from the head of the screw, I know of two tools that do this, one of which takes the material away from the outer edge by chamfering it and the other bores a small hole in the center of the head of the screw. One method I do not recommend is filling the heads of the screws down, this method leaves the balance looking hacked and is in my opinion the sign of poor workmanship. Solid balances without screws can be poised by drilling them, usually on the underside of the rim.
When poising the balance remove only very small amounts of material at a time or you will be chasing it around the balance for ever. And remember every bit of weight you remove from the balance will directly affect the rate. Before you start removing weight check to see if any of the screws are loose or missing, check the balance arms to be sure they are not bent, make sure the balance is absolutely clean, check the pivots to be sure they are not bent and also check to see if someone has put a timing washer on only one side of the wheel, any of these things are easy to repair by themselves but if you don’t see them in the beginning you will just be compounding the problem. You can also add weight by using timing washers but as with taking weight away adding weight also affects the rate. If you suspect that one of the screws is not right for the balance, try finding the proper one, even the lightest timing washer will add a surprising amount of weight. Poising a balance is a fairly simple operation when the proper care is taken.