A BALANCE staff which has been riveted to the wheel should not be punched out in the staking tool. Even the method of turning off the riveted part, followed by punching in the staking tool, is not to be recommended. For, in spite of the removal of the riveted portion, the remaining stock inside the hole is still tapered in an unfavorable direction, and cannot be punched out without further spreading of the hole. Horologists often wonder why a standard balance staff cannot be found to fit a standard wheel. It is because balance staffs have been repeatedly punched out of the wheel. The hole has, thus been enlarged to the extent that a regular stock staff will no longer fit it.
Proper equipment for making a balance staff should include such measuring instruments as micrometers and end gages (see Suggestion Department in this issue), a pivot polisher or wig-wag and a good supply of hand gravers. A special pivoted slide gage is also very useful.
Before any work on the staff proper is begun one should make a sketch of the staff, and fill in all the measurements as taken from the sample, at their designated places. If the old sample has been lost measurements may be taken direct from the watch in the following manner.
A micrometer is used for determining the length of the staff. The thickness of the watch is measured with the balance bridge screwed to the main plate and both endstones in place. From the total measurement the thicknesses of both endstones, which are measured separately, are now subtracted, giving the length of the staff.
The height of the roller table may be measured with the special slide gage shown in Figure 1. In a similar manner the location of the balance can be obtained. The dimensions of the roller table hole and that of the hairspring collet are obtained by measuring them with a round tapered broach. For the sizes of the pivots the jewels are measured with a jewel gage, allowing a small amount for freedom, a half of one hundredth of a millimeter being considered a close fit.
The same method of turning the staff for finishing with the pivot polisher is used for the wig-wag. The upper portion of the staff is turned to shape first. It is to be noted that all sharp corners are left undercut. An especially deep undercut is made for riveting the balance. Each cylindrical or conical portion is left about .02 of a millimeter larger in diameter.
A steel or cast iron lap is recommended for grinding. Diamantine should be used on bell metal and tin laps. A set of laps suitable for all occasions is shown in Figure 2.
All cylindrical and conical sections or the staff are ground and polished with the spindle of the pivot polisher set parallel to the bed of the lathe. Such set-up is shown in Figure 3. The conical pivots of the staff, however, are finished with the pivot polisher spindle set at right angles to the bed of the lathe. The length and shape of the cones are determined largely by the amount which the lap is set above or below center, as shown in Figure 4.
The pivots should be made with the parallel section long enough to project through the jewel a sufficient amount.
The ends of the pivots arc burnished with a steel burnisher which is moistened with oil. Contrary to instructions found in some publications that the ends be made absolutely flat a slightly rounded pivot end is the correct form. After completely finishing the upper part of the staff it is cut off nearly to length. The shape of the staff before it is cut off from the rod is shown in Figure 5. The other end is then stoned off to exact size while frequently checking with the micrometer.
For finishing the lower half the staff is held in a chuck by the collet hub. The various shoulders are now turned to the measurements indicated in the sketch (Figure 6) and are ground and polished in the same manner as described above.
The tapered portion on which the roller is fitted is best finished with a lap having a corresponding taper. If one desires to finish this taper by setting the spindle of the pivot polisher at an angle the flat front of the lap should then be made conical. Failure to observe this rule will result in a cupped and scratched roller seat.
Particular attention should be given to the following points. The balance must fit the staff with no side play or it will not be riveted on true. The amount projecting through the arm should be no more than actually necessary for riveting. The rollers must go on to the staff freely so that when placed on the staff with a tweezer the space between the roller and its position when being pressed on in the staking tool will be no greater than its thickness.
A tightly fitting roller may either split or bend the staff when being driven on.
We are all familiar with the term WigWag, describing a method of pivot polishing used in some factories. A small device which may be used by hand can easily be made by anyone desiring to do so. No special skill or equipment is needed.
In connection with this wig-wag it is necessary to prepare a taper with an eccentric pin, which allows adjustment to different sizes of pivots as shown in Figure 7.
In Figure 8 is a wig-wag made from a section of a tube. Two blocks with cross holes provide means for fastening the handle.
For square pivots the wig-wag is held as squarely as possible to the bed of the lathe. For conical pivots the corner of the wig-wag is slightly rounded and the tool is held at angle which corresponds closest to the shape of the cone. Figures 9 and 10 show the wig-wag in actual use. The fact that this device can be used on the most delicate pivots even when they cannot be made to run true makes it a desirable accessory even to the workman who owns a regular pivot polisher. The same grinding material which is used on the pivot polisher may be used on the wigwag.