Thanks for your message. I didn’t explain it clearly when I wrote this article.

You’re right if we were looking to make a perfect square from the bar, but we’re looking to make a square for use in horology.

This means that we don’t want sharp edges, but rather curved ones. So each of the four corners of the square is actually the surface of the original bar.

Sharp edges on squares are not used in watches. One of the reasons a winding stem cannot have a perfect square is that the “square” hole in the sliding pinion will always have rounded inside corners due to it being impossible to drill a perfect square – so if the winding stem’s corners were sharp they would jam inside it.

I hope that makes it clearer.

Colin

The largest square that would fit inside a circle 1.2mm diameter is just under 0.85mm square, so where does the extra 0.03mm (0.015mm on each side) to take it up to 0.88mm come from?

It seems a small amount, but it is three times greater than the 0.005mm (5 microns) you allowed on each side for lapping.