Making A Balance Staff
By HAROLD C. KELLY
Member National Technical Board
Gravers for Turning Staffs - Three gravers of the styles shown in Figure 1 are needed for staff work. A is for general use, suitable also for square shoulders and the cylindrical portion of cone pivots. B has a rounded point for turning the cone portion of the pivot. C has the point flattened and is used for turning the lower end of the staff prior to cutting it off.
Sharpening the Gravers - Sharpening the gravers does not seem to be given the attention by the average horologist that it should. We have seen workmen trying to cut a square shoulder pivot with a graver having a point like a wire nail. Naturally their work was unsuccessful, yet these workmen did not reflect on the fact that possibly the graver was at fault.
The graver must have a sharp point. Even the best gravers cannot retain a keen edge very long while cutting tempered steel. Therefore keep a sharpening stone handy and make frequent use of it.
Some workmen use an emory or carborundum wheel to grind gravers. This should never be done as the point of the graver is frequently softened and this point is the most important part. Instead we use two stones, a soft Arkansas stone and a hard Arkansas stone. The soft stone is for rapid cutting and the hard stone is for the final finishing. After grinding the face, slide the sides on the stone so as to produce a flat and smooth cutting surface.
Making Pivots for Practice - It is well for the beginner to practice making square shoulder and cone pivots before attempting the making of a staff. The student should secure some pivot wire using the larger pieces. It will be found that pivot wire is excellent for the purpose as the wire is hardened and tempered ready for use.
For cone pivots the usual rule is that the cylindrical portion is twice as long as its diameter. For square shoulder pivots the length is three times the diameter.
The grinding and polishing will be considered when we arrive at that place in our work.
Measuring for the Staff - Now returning to our problem of staff making, the first act is to take the necessary measurements, preferably from the watch for the reason that the old staff may not be correct. The well known Boley gauge serves the purpose very well since it reads both ways, between the calipers and from the end of the foot. See that the balance bridge lies flat with the lower plate. Now remove the cap jewels.
Make certain that the hole jewels are securely pushed in place. For the full length of the staff (A, Figure 2) measure from the side of the lower hole jewel to the side of the upper hole jewel. The height of the seat for the roller table B is found by measuring the distance from the side of the lower hole jewel to the t.op of the lever, adding enough for clearance and the thickness of the roller table.
In like manner the distance for the balance seat C is measured from the side of the lower bole jewel to the top of the pallet bridge and adding for the necessary clearance. The length of the upper end of the staff D is found by subtracting the length of the lower end to balance seat C from the full length A.
The wire is now too bard to turn and we must therefore draw the temper. The wire must be made white in order to blue it. This is done in the lathe by holding a piece of fine emery paper or buff stick against the wire.
The tempering is done by drawing the wire through the flame of an alcohol lamp, or, better still, lay the wire on a curved sheet of copper keeping the wire rolling while being held above the lamp. A full blue color is satisfactory for staffs.
Turning the Staff - Tighten the wire securely in the lathe having extended the wire from the chuck sufficient to include the full length of the staff and about two millimeters additional. With the hand graver turn the upper end; first turning the balance seat to length from the end and almost size, say .05 of a millimeter of the finished diameter. Next turn the collet axis, followed by turning the remaining end nearly to the size of the hole in the roller table.
Turning a Cone Pivot - Turn the cylindrical portion of the pivot almost to size. Next using the graver with the point slightly rounded turn the cone, bringing it down to meet the cylindrical portion of the pivots.
This is followed by cutting the slope between the collet axis and cone of the pivot.
The turning of the upper end of the staff is now completed and we are ready for the pivot polisher and the preparation of the grinding mediums.
The Pivot Polisher - The pivot polisher is a very neat little instrument for grinding and polishing. The superiority of the instrument over any hand method is unquestionable, it does the work in a factory like manner and polishes the pivot most beautifully. It is to be regretted that the pivot polisher is not more generally used.
However the successful use of the pivot polisher depends largely on the proper preparation of the laps and we must therefore digress for a moment and describe the met hod of making the laps and their preparation and uses.
The horologist should make his own laps. The taper chuck shown in Figure 3 is required for the purpose.
Material for the laps may be purchased from material houses or hardware stores. We need laps of cast iron, bell metal and boxwood.
Cast Iron and Bell Metal Laps - Laps made of cast iron and bell metal are used principally for grinding. The material may be purchased in rods of various diameters. For most needs of the horologists, rods of about i inch diameter will suffice. Having selected the material desired, saw off a piece about a half inch long and bore a hole in it of the proper size. The hole is reamed out with a reamer of the correct taper and the blank is placed on the taper chuck for turning with the slide rest. Laps of various shapes are needed as shown in Figure 4.
The laps are now turned up. The face and sides must be filed to enable the lap to hold the grinding or polishing medium. Filing, it is admitted, has a tendency to destroy its truth, yet this is necessary in order to do good work. Experience has I shown, however, that it is possible to file a lap many times and still run practically true. File the lap by laying it face down on a Number 4 or 5 file. Holding the lap between the thumb and fingers, slide the lap along l the cutting teeth of the file about an inch or so. The lap should then be turned partly around and another stroke made. This crosses the lines providing a suitable surface for the embedding of the abrasive medium. Next prepare the side of the lap. This is done by drawing the lap on the file as shown by the arrow in Figure 5. Continue in this manner until the entire circumference is filed.
Preparing the Grinding Material - Now that our laps are ready, the preparation of the grinding medium is next in order. We proceed as follows: Apply small quantity of oil stone powder to the first story of our three story polishing block. Add a little watch oil, mix with small knife or spatula until a thin paste is produced.
Place the pivot polisher on the lathe and adjust the lap spindle so that it stands at the same height from the bed as the lathe spindle. Adjust the index at the base of the polisher to 1 degree, so as to give the staff a slight taper toward the end when the grinding takes place. With the belting so fitted that the grinding surface of the lap and the surface of the staff rotate in opposite directions to each other, feed the lap up to the work by means of the hard rubber knob at the rear. Apply thinly, but evenly, the oil stone paste to the lap and grind the balance seat. Continue grinding until the balance just fits the seat. Be sure the under cutting is deep enough so that the front corner of the lap does not touch the staff. If this is the case the balance can be riveted true and flat when the staff is finished. Next, grind the collet axis to size. Now grind that portion of the staff between the cone of the pivot and the collet seat.
Grinding and Polishing a Cone Pivot - Clean the work of the grinding material with a piece of pith previously dipped in Benzole and hold it against the staff. Finish cleaning with a dry piece of pith. For the cone pivot the polisher is set with the spindle at right angles to the pivot with the index set at 0 degrees. It is further adjusted so that the center of the lap stands above the pivot as shown in Figure 6.
This not only reduces the straight portion of the pivot perfectly cylindrical but it also forms the cone, the shape of which can be varied by raising or lowering the spindle of the polisher.
Instead of the caste iron lap we now use a bell metal lap and a grinding medium crocus or Vienna lime. Reduce the pivot frequently trying the jewel until it fits rather closely.
Now remove the bell metal lap and fit in its pace a tin or boxwood lap and polish the pivot using a paste of diamantine or sapphirine and oil prepared on the top story of the polishing block. Continue the polishing until the pivot fits the jewel freely. The slope between collet axis and cone may now be polished.
Turning Hub and Roughing Out Lower End of Staff - Now that the upper end of the staff is finished we next turn with the hand graver a long slope from balance seat to the lower end of the staff. With the pivot polisher set at the necessary angle grind the slope for most of its length using oil stone powder and oil. Polish with bell metal lap, crocus and oil and finally with tin or boxwood lap and oil.
Next cut out roughly the lower part with a graver slightly flattened at the point. Leave the hub a little longer than the finished staff is to be and the roller axis a little larger.
Cut off staff and measure the overall length. The staff, of course, is a little too long. Place the collet axis in a split chuck and stone off the lower end until the staff is worked down to the correct length. Remove staff and re-measure the over-all length between the callipers of the Boley gauge as many times as is required, taking only a little off the end at a time so that the staff will not be made too short. This is positively the most exact met hod of bringing a staff to the correct length. Any other method such as measuring back from a pre-determined amount of extra length and cutting off after the work is cemented up is very likely to result in errors.
The Cement Chuck - The staff having been brought to the correct length we are now ready for the cement chuck. A screw brass lightly screwed in a chuck must have a center turned in it and deep enough to take the staff up to and including the hub. Bore a 0.3 millimeter hole in the bottom of the center to take the pivot. Figure 7 shows the center with ,the staff in place and properly secured in the cement.