In dynamic poising, we add weight to the balance wheel’s lightest spot or remove weight from the wheel’s heaviest spot. What tools work well for removing weight?
The amount of weight to remove can be tiny, almost imperceptible, for fine adjustment, so we need tools that are precise and easily handled. And ideally we would remove weight without visually marring the balance’s appearance. What are our options?
Balance Screw Cutters
I don’t recommend this tool, but you see them everywhere.
Balance screw cutters come in sets for different watch sizes. They look like screwdrivers, but the tip has a sharp graver protected by a brass collar. The collar is spring loaded, so when you place it over a balance screw and push, the graver contacts the screw’s face. Pushing and twisting the cutter removes weight by carving a circular dimple out of the screw.
This tool is a poor choice for a few reasons. First, it’s hard to have fine control with a screw cutter. These mostly come in handy if you need to remove a lot of weight.
Second, balance screw cutters seriously hurt the look of the watch. Here’s what a scrap Hamilton 730 balance looks like after a bit of attention from this tool. The difference between the dimpled and flat-faced screws is too obvious and salient.
Third, you can weaken a screw by deeply dimpling it. The slot can become fragile, making it hard to remove the screw remove safely.
And finally, a selling point of this tool is that you can supposedly remove weight from a screw without removing the balance from the watch. But in practice, you can easily snap a balance staff by grinding a screw cutter against it.
Balance Screw Undercutters
Balance screw undercutters are a much better option. You remove the screw and place it in the cutter. The cutter is hollow, to allow the threads to enter, and it has graving edges. If we press on the screw and twist it, just as if we’re trying to screw it into the cutter, the graving edges will remove material from underneath the screw. The balance’s appearance isn’t affected, but a lot of weight can still be removed if necessary.
These are easy to use: we dig into the details of screw undercutting here.
When buying a screw undercutter, be sure all the tips work. It’s common for the tips for the smallest screws—the ones you’ll use for wristwatches—to be blunt or broken.
Balance Screw & Escapement Files
Balance screw and escapement files are my go-to tool for fine adjustment.
This balance screw file has 2 working edges. The fine edge is serrated, giving it a saw-like cutting effect for creating and enlarging a screw slot. You can insert this edge into a screw slot and remove weight without noticeably changing the screw’s appearance.
The flat face of the tool is intended to file the sides of the screw slot, so it has a fine filing surface. For very fine adjustment, you can also lightly sand the face of a screw with this surface.
Escapement files are small, fine files that are ideal for fine adjustment of screws. The image shows two Vallorbe files. The rectangular one is an “equaling” file—it is rough on all 4 faces—and has a cut of 4 (fine). The other has a “barrette” form—it is rough on only the wide, flat face—and has a cut of 8 (super fine).
These small files are well suited to working in tight spots. They are fine enough to remove small amounts of weight without leaving scuff and scratch marks on the screw’s face.