Making a Balance Staff

Using the rollers, hub and balance spring for measurement and a carbide graver, a staff for a 19th century marine chronometer will soon be made.

After cutting a male cone on the end (so the blank can be finished in the turns), I cut to the diameter needed for the balance hub (the largest diameter).  You may be able to see that I use the graver with the ground face to the work.

The hub should be a driving fit on the staff so it will not work loose.  Taper the hub seat so that the hub cannot be pushed more than 1 to 1.5 its height from its final location.

The finished blank.  Part it from the lathe by cutting a male cone on the headstock end.  At this point, because the blank was turned "in one chucking", all surfaces are concentric.  The male cones will be used to finish the blank in "the turns", a dead center lathe.  The advantage of the turns is that the work can be removed any number of times to check the fits; and then returned to the lathe without "losing the center".  I finish all chronometer staffs and staffs for precision watches in the turns.  Depending on the staff design, there are times when I will turn a watch staff completely in the lathe.  However, you need good equipment in order to turn the staff end for end and still have reasonably concentric diameters.  The ONLY way to ensure concentricity is to either use the turns or "turn in one chucking".

The blank is fitted with a driving dog and mounted in the turns.  Note how little of the cone is actually held in the tailstock end.  This allows the pivot (which in this staff will be .15mm) to be cut.  There are various forms of dead center lathes, this is a current production system by Steiner.  It is well thought out and makes its use much more of a pleasure than the traditional tools by Lorch, Boley or the antique patterns.  Plus, accessories can be purchased form the factory.

The driving pin on the pulley drives the dog which is screwed to the staff.  Because the cones do not rotate, you do not have to worry about messing up the attachments with your graver.  In fact, I use the side of the tailstock cone as a guide when cutting the pivots.  Here, you can see I am using the ground face of the graver against the work.  Some use the ground face up; but many find the way shown here gives better control.

The fit of the impulse roller (which requires the larger diameter on this end) is checked.  The roller is upside down.  This allows me to be certain of the tapered fit when I finally assemble the balance.  The oil on the cone collet is from lubricating the male cone of the dead centers.

With the impulse roller in its final location, mark the bottom end of its seat.  The next cuts will step down to the diameter for the unlocking roller.

The fit of the impulse roller (which requires the larger diameter on this end) is checked.  The roller is upside down.  This allows me to be certain of the tapered fit when I finally assemble the balance.

The cone for the pivot is first undercut.  This traps any oil that may migrate out of the jewel.  Then the pivot is cut. while it takes a little practice to cut a pivot of less than .2mm, after some errors you learn the touch.

The fits of the staff length and heights of the various seats are checked.  Here you can see the bottom of the impulse roller seat is slightly above the passing spring of the detent.  This will allow the unlocking roller to seat at the right height, while ensuring the passing spring does not foul on the bottom of the impulse roller.

The staff as removed from the turns.  It has not yet been polished.  You can see the surfaces are smooth and the angles are all sharp.  I hope to put up a page on polishing and finishing in the near future.