The last exercise to complete before we started making the first winding stems was to perfect the art of filing squares. We had to make a set of 5 squares with the last of the set being just 0.60mm in diameter with a tolerance of +/-0.03mm. This would be the start of very small tolerances that are crucial in making watch parts.
We used the headstock on the Horia lathe that we were able to lock in four positions and then turn to create an evenly sided square. A roller fixed to the lathe bed was used to support the file and allow the forward motion to file the square. A section of steel bar was turned down slightly larger than the required size of the square and a steel bush fitted over the original diameter of the bar. This would help the file to create a flat shoulder, then an even amount was filed off each of the four sides to create a perfect square.
We started with larger squares as they were more sturdy and less likely to bend. Once we had a perfect square of the required size we could move on to the next smaller and more challenging square. In practice this was extremely difficult once the squares became smaller as too much pressure caused the bar to bend. If the bar wasn’t straightened perfectly the square would file unevenly and measure smaller at one end than the other. Then you would have to start all over again!
Once the right amount of pressure on the file was mastered and the file kept flat, it became possible to repeat the process again and again with the same success nine times out of ten, although I still wouldn’t describe the process as easy. With a full set of squares completed I could look forward to the next challenge.
Once again, I must thank Mr Alatairy for the excellent photos!