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School Watch- Balance Staff for ETA 6498

With both the winding stem and pivot gauge examinations out of the way, which I’m pleased to say that I comfortably passed both, we could move on to making balance staffs. A balance staff is riveted to the balance wheel and becomes its arbor. The balance staff also holds the jewelled roller and the hairspring collet. The balance regulates a watch and enables it to keep accurate time, the balance staff is therefore a key component that must be made with great precision.

There are several different parts to a balance staff, all of which are there to serve a purpose. They are shown in the diagram below…
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The balance staff we would be making would be for our yet to be constructed school watch, which will use the ETA 6498 calibre as its base. The 6498 staff is just over 3.5mm in length and 1.6mm in diameter at its widest point, the difficulty of working with something so small would soon become apparent.

Hand Lathe ready for turning…
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The process of making a balance staff starts with parting off a piece of blued steel and turning a point on each end. A small section on one side is reduced in diameter in order to accommodate a carrier that will be needed for turning between centres. The shoulder for the balance seat is marked in the right position and made perfectly flat. The steps that will accommodate the balance wheel and the balance spring collet are turned down next.

The step that accommodates the balance wheel also becomes the rivet that will fix the wheel to the staff, so the shoulder must be undercut in preparation for riveting. A small and extremely sharp graver is used to do this, taking care not too reduce the diameter of the step that will hold the spring collet. Now the pivot can be be turned down to its largest diameter ready for turning the conical pivot and burnishing. The staff can now be turned around ready to complete the other side.

Turning in progress…
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With the staff turned around, the shoulder that holds the roller can be marked and turned down to the right diameter. The diameter has a slight taper to allow the roller be slid on and held by friction. A 45 degree angle is now turned on the reverse of the balance seat to aid in the removal of the roller. The pivot is marked and turned down to its largest diameter. With all of turning on both sides complete apart from the pivots, the pivot on this side can be made, before the staff is turned around again and the same completed on that side.

The pivots on a balance staff are conical in shape to reduce friction and increase strength, both of which are achieved by eliminating the sharp corner of a straight pivot. I had experienced producing conical pivots when turning pivot gauges, they proved to be both harder to turn and trickier to burnish than the equivalent straight pivot. This once again proved to be the case, I successfully turned a number of balance staffs without pivots before destroying them either when turning the pivots or burnishing them.

Ready for pivots…
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The pile of broken balance staffs in my bench continued to get larger by the day, I was stumbling at the same point each time, the pivots. Sometimes I couldn’t turn the pivot to the correct shape but most of the time the pivots were just snapping during burnishing. The pivots were 0.12mm before burnishing and needed to be 0.105mm when completed so it was never going to be easy working with such small sizes. The main problem was the conical shape of the pivot, the pivot could only be supported along the straight section which was halfway along its length. Too much pressure on the burnisher or a small slip, snapped the pivot clean off and it was time to start again!

Some of the casualties…
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After breaking so many, I had decided to turn down a number of blanks and a few balance staffs minus the pivots so that I didn’t have to start from scratch each time I broke the pivots. After two solid weeks of turning, I was still without a balance staff that I could use in my watch but I did have a box full of broken ones and a couple of ‘nearly’ completed ones. Unfortunately, it was time to finish micro mechanics and start preparing for the next exam, so I will continue in my quest to make a completed balance staff when we have some spare time during the rest of the course.

The Process…
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I was feeling slightly disappointed as its the first component that I have so far failed to make. Its one of the most difficult to produce due to its size and need for extremely precise accuracy so I guess I shouldn’t be too downhearted. We will get spare moments from time to time when I will get the chance to try again. I still have a year or so to produce one so I’m sure I will be successful in the end!

How it will look when fitted…
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Posted in The British School of Watchmaking and tagged , , , , .