The second of our two micro mechanics exams was to produce a pivot gauge. We once again had a time limit of 8 hours to complete it, turned between centres using a hand lathe. The task would be a lot harder than the winding stems because this time we would be using blued steel, which is much harder material to turn. The hardness of the blued steel blunts gravers extremely quickly, it punishes a small slip of the graver by breaking off the tip or an edge. This can be very frustrating at times!
There are four parts to a pivot gauge, the handle, the hub, the cone and finally the pivot. The process of making a pivot gauge starts with a piece of 1.5mm blued steel bar, from which a piece the length of the pivot gauge must be parted off using a graver. A point is turned on each end and the piece of bar is placed in between centres ready to turn the pivot gauge.
The handle is the first part to turn down, it must have an evenly turned finish as no polishing is allowed. The shoulder meeting the hub must also be flat and have a clean corner. To achieve a good finish, a few hundredths of a millimetre are left to take a few final small cuts that leave a nice consistent turned finish. Lots of practice is required to get a good standard of finish.
Next up, the width of the hub is marked to create a shoulder on the cone side, but the blued steel of the hub must remain unmarked. The hub is left slightly oversize while the cone is turned down so a clean and flat shoulder can be created at the end. The cone must be turned down to set a diameter at each end, obtaining perfect conicity, which is measured using the flat gauge that we made previously. The cone must also have a nice turned finish to match the handle, this is made even harder due to the fact that you are trying to turn a perfect cone at the same time.
The final part of the pivot gauge is to turn the pivot ready for burnishing, the pivot can range in size from 0.07mm to 0.23mm and be straight or conical in its shape. Straight pivots are turned down using a normal graver but conical pivots are turned down using a modified graver with a rounded tip. Conical pivots are used in watches for increased strength and reduced friction.
The pivot is left a few hundredths oversize ready for burnishing, straight pivots are burnished using the flat side of the burnisher and conical pivots are burnished using the curved side of the burnisher. Although the pivot gauge examination would be assessing straight pivots, conical pivots prepared us for the balance staffs we would start making in the next part of the course.
The first few pivot gauges were made in just over a day and each subsequent pivot gauge was produced in less than 8hrs. The week before the exam, I could comfortably produce a pivot gauge in around 4hrs that was to a good standard. I felt confident in my ability to produce a pivot gauge within 8hrs, that would be more than good enough to pass the examination.
Overall pivot gauges seemed to be technically easier to produce than winding stems but with very few parts to be assessed, it was crucial that each part was produced to a high standard. Each part had its own difficulty, the turned finish on the handle and cone, the burnishing on the pivot and making sure not to mark the blue on the hub. It was a challenging experience but with weeks of practice I soon became a master.