The Making Of A Balance Staff
By Emanuel Seibel
In making a Balance Staff we must know how to get the measurements in case no sample is available and never take it for granted that the one in the watch is correct. Which is a very good rule to follow in anything to be replaced in a movement, especially Mainsprings. The greater majority of the men at the bench do not know how to go about selecting correct material, muchless know how to measure a movement for a correct Staff.
We will now consider a relatively simple way of getting at the correct measurement for a Staff for a particular movement.
In our illustration, we show the essential parts in getting these measurements. We have taken a type that is used in practically everything from 16s down. For 18s the system of measuring is exactly the same except you use the top plate in place of the pallet bridge in our illustration.
1. is the lower or main plate 2. is the balance cock 3. is the pallet bridge Now, the first essential is to know the overall (a) length that your staff must be.
This is gotten by removing both cap jewels and being positive that both hole jewels are positively seated in the bottoms of their respective sinks. Use a slide or a spring millimeter gauge and measure the distance from the face of one balance joint to the face of the other jewel.
This is the direct method. You can accomplish the same by measuring over the cap jewels on either and deducting the thickness of both cap jewels from the first measurement.
Next, we must find the correct distance from the end of the upper pivot to the seat for the balance or the top of the hub (b). For this we will use the slide gauge and get the distance from the top of the balance cock to the top of the pallet bridge, and subtract the thickness of the upper cap jewel; but this would let the balance rest on the pallet bridge, so we must deduct .01 or .02 mm from this last figure so as to give balance wheel the proper clearance over the pallet bridge (e).
We still have 2 more preliminary measurements to find before we can begin to cut our staff for we only know how long the upper ball (b) of the staff is and the overall (a).
We must now find the height of the hub so as to get clearance between the under side of the roller table and the top of the pallet fork. We can get this by measuring with the slide gauge from the top of the pallet bridge to the top of the pallet fork.
From this measurement, deduct clearance between roller and fork and add the clearance allowed between balance and pallet bridge (e).
This is the total measurement of hub and roller. Then deduct the thickness of the roller and the result is the height of the hub (d). The remainder of the overall length is the roller shoulder and the lower pivot (c).
In the ordinary 16 and 12 size movement, we can allow a slight variation in the height of the hub, but in thin models and ultra thin movements, the dimensions must be exact and no mistake about it.
Now, as to the length of the hairspring shoulder, it should be as high as the hairspring collet and its diameter should be about .01 larger than the inside diameter of the hole through the collet. This can be gotten by slipping the collet on a burnishing broach and gauging the broach directly under the collet against the broach. The collet must spread almost imperceptibly in going onto its shoulder. Of course, the balance shoulder is made to fit opening in the balance arm without any side shake and just about .01 or .02 mm at the most higher than the thickness of balance arm.
The pivots. in any watch should be the same size but especially in a fine movement. They should not be any longer than necessary to reach through the hole jewel and touch cap jewel without binding or riding on the cone in the edge of the hole.
Pivots should be as large as the jewels will stand and still allow positive freedom. The endshake should be just sufficient to guarantee positive freedom.