For some reason the poising tool is one of those watchmaker tools that you can find anywhere. There always seems to be one at every flea market, in every used tool store, and of course in every watchmaker’s shop. In fact, there are two in my shop. One is my personal poising tool, the other belongs to the store. A quick search one eBay yields 8 poising tools in good condition with bids starting as low as $6.99. Why can’t I find an escapement meter or heater at that price?

A well poised balance wheel is an essential element for a watch to tell accurate time. A poised balance wheel is one whose center of gravity exists at the center of the balance staff. If the center of gravity is not in the center of the balance wheel, gravity will affect the balance wheel differently when the watch is in different positions resulting in a larger variance of rates between the different positions. (See timing variations.)

There are two ways to “poise” a balance wheel. One is to remove the hairspring and statically poise the balance wheel. The second is to dynamically poise the balance wheel while it is installed in the watch based on timing results. When dynamically poising it is important to be sure that there are no other problems with the watch or you may end up with a balance wheel like the one I wrote about on Friday. There is a good article about this in the latest issue of the Watch & Jewelry Review.

The poising tool is used for static poising. First the poising tool has to be perfectly level and the ruby jaws need to be clean and static free. The balance with hairspring removed is placed on the jaws (like in the picture). The wheel is lightly rotated. If it turns freely and comes to a rest it has a heavy spot. If it can be stopped at any position around it’s circumference then it is properly poised. If it has a heavy spot a small amount of metal is removed from the underside of the rim of the balance wheel (or from the shoulder of the screws if it is screwed). The process is repeated until the balance is poised. If you remove too much material than you end up making more holes in the balance wheel and eventually you end up with swiss cheese so it is best to go slow.