We have a few ways to remove weight from balance screws, and one of the best is undercutting, which removes a clean ring of weight from the flat bottom of the screw head. When done properly, you can’t tell that the screw was modified when it is back in the balance. Unlike screw cutters, then, which gouge dimples out of the screw’s face, undercutters are discrete.
Undercutting is best when there’s a larger amount of material to remove. It is less precise than using balance screw files or escapement files and tends to remove relatively more weight. Undercutting is thus better in the earlier rounds of dynamic poising, and the precision files are better in the later rounds.
As always, it is better than to remove too little weight than too much, so don’t go crazy with the undercutting.
Types of Undercutters
You’ll find two kinds of undercutters for sale. Keep in mind that sellers occasionally call balance screw cutters, which dimple the screw face, as “undercutters.” Don’t be confused—your undercutters should look more or less like these two options.
The first option has a set of posts arranged in a ring. Each post is hollow to allow the screw threads to enter, and the set has a range of sizes. The underside of the screw head thus sits on top of the cutting edges. By inserting a screwdriver into the slot and twisting, you carve a ring of weight from the screw.
The second option is essentially the same. An example, made by Marsh, consists of a stand with 2 removable posts with gravers on each end. Each post can be removed and flipped by loosening the locking screw on the side. This gives 4 different sizes: 2 for smaller wristwatches, and 2 for larger (12, 16, and 18 size) pocket watches.
I find I use this one nearly all the time. It cuts cleanly and feels tight and stable when used.
Before and After
When done right, undercutting removes material without affecting the cylindrical wall of the screw. The key is to choose the right cutter size and to twist with a smooth, consistent pressure.
Here’s an example from a scrap screw. The before picture:
And after some extensive undercutting:
Notice that the screw threads and wall were not affected. Instead, the underside has been hollowed out.
The most common mistake is to pick a graver that is too large. This ends up carving the sides of the screw, leaving an uneven, squashed appearance. When put back in the balance, the screw might not sit flat against the wheel.
When shopping for undercutters, confirm with the seller that the carving tips are okay. The tips commonly break, especially the more delicate posts designed for smaller watches, or get dull and rusty.