There are upper and lower bearings for the balance wheel of this clock, which I made from pocket watch center jewels put into settings. The jewels' outside diamter is 2.5 mm and the bore in the jewels is .037". The wire used for the balance shafting is .033", which leaves a side clearance of about .002" when the shaft is running in the jewel. A loose fit, which is great, since the bearings have no weight-bearing role -- they act as guides only.

The jewel settings are made from 3/8" brass rod, using the Taig lathe and Sherline rotary table mounted on the Taig mill. The first operation is to drill a center hole in the 3/8" rod. This hole must be slightly smaller than the 2.5 mm jewel outside diameter, since it will provide the back of a shoulder.

After I drilled the center hole, I unscrewed the chuck from the lathe and screwed it onto the stud on the Sherline rotary table mounted on the Taig mill -- this without removing the brass, to preserve truth. Then the mounting holes were drilled. Again, I first drilled a hole, then rotated the rotarty table a half turn, and drilled the second hole. This way, the holes are uniformly located. The holes are 1/16" in diameter, or the outside diameter of the 0-80 mounting screws.

Then I unscrewed the chuck from the rotary table and remounted it on the lathe. The first operation is to cut a 3/16" spigot on the bottom of the setting. This will slip into 3/16" holes on the horizontal balance plates, the setting to be held by 0-80 mounting screws. The plates are made of 1/8" material, so the spigot is 1/8" deep.

Once the spigot is made, I parted off the setting, and then re-chucked the setting headed in the opposite direction. It's clamped by the 3/16" spigot in a 3/16" WW collet in the Taig lathe.

Next the fun part, which is to enlarge the center hole in the setting to make a bore to hold a jewel. I made the bore with a miniature boring bar, which I made from a piece of 1/16" music wire --hard enough to machine brass -- mounted in a 3/16" brass ferrule. The angles on the bar are ground on by hand, using magnification. The clearance angles on this bar are similar to those on any boring bar.

The Taig lathe has a carriage depth stop -- a highly useful feature. I used it to control the depth of the bore. Using the depth stop, repeated cuts made in the bore will have the same depth.

I made fine cuts, only a couple of thousandths at a time, since the boring tool is small and flexible.

I know of no way to measure such a small bore, other than to trial-fit the jewel in the hole. The jewel is held impaled on a round (smoothing) broach to keep it from disappearing suddenly. Fine cuts are made repeatedly, testing until the jewel just fits in the hole.

I didn't attempt to press-fit the jewel in the bore in the setting. I have a Seitz jewel press, but it has no reamer of the proper size to ream for the 2.5 mm jewel diameter.

So I used the time-tested method of using watchmakers shellac to secure the jewel in its bore. I might also have used Loctite.

Todays' links:

Drilling center hole
Drilling mounting holes
Cutting spigot
Parting off setting
Minibore in toolpost
Jewel on round broach
Making jewel bore
Testing jewel bore
Bearings complete
Setting drawing, GIF