Usually, the center arbor (the arbor which holds the minute hand) runs through the front plate and is supported on the other end by a pivot on the back plate. In this clock, the center-line of the clock is occupied by the balance-wheel suspension spring. For that reason, the back pivot runs in a bridge rather than the back plate, which leaves room behind it for the spring to descend. Thus, the arbor is much shorter.

The bridge is made from 3/16" brass, with two posts made in the same way as those made previously. Except that the posts are screwed onto the front plate with two 2-56 brass flat-head screws. I decided to rivet the posts to the bridge, instead of making screws for them.

I drilled the holes in the bridge in the mill, clamping the bridge in the mill vice. First, I drilled the center-hole, then drilled post holes at equal distances on either side of the center hole, insuring equality by using the gradations on the dials on the mill's long axis to measure the offsets.

Once the holes in the bridge are made, I transferred the hole locations to the front plate of the clock. There is a dot in the Deltacad template still glued to the front plate of the clock which indicates the location of the center arbor. At this position, I drilled a hole which is the same size as the pivot hole in the bridge, then used the drill to "pin" the bridge to the front plate. Then I used a transfer punch to transfer the locations of the two post holes in the bridge to the front plate. This way, all the holes line up. Once the holes are drilled in the front plate, the center hole can be enlarged to arbor size.

I cut tangs on the posts which are a close fit in the post holes in the bridge, then rivetted them over. To do this, I set the bottoms of the posts on a bench block and used small taps of a light hammer to rivet them in place. Once this is done, the excess can be filed away or ground off with a belt sander. There are only slight traces of the hole remaining in the back of the bridge. The key to successful riveting is that the work must be a close fit, and the rivetted material must not extend more than about .020" above the surface.

The center arbor is made from 5/64" music wire. I cut the pivot on the end with a very sharp graver in the old watchmakers lathe. The graver is made from 1/8" high-speed steel, which is hard enough to cut the hard music wire, but it must be sharpened often. A dull tool will burnish the hard carbon steel of the music wire, making it hard to cut.

I made the 20-pin center pinion in the same way as I made the maintaining ratchet pinion; the pinion is spool-shaped, with the 20 holes in the bottom of the pinion drilled with a tiny drill, using a needle-center drill to mark the hole locations, as I did with the ratchet pinion. The center hole is drilled a bit under the 5/64" which will be the size of the center arbor.

The pinion is tap-fit on the arbor. First, I tapered the center-hole in the pinion with a clockmakers 5-sided broach. These broaches are nominally tapered at about .001 for each 1/4" of the broach, which is about right for a press fit.

I also tapered the end of the center arbor by making light cuts with a sharp graver, finishing with a fine file. When the tapered arbor fits into the tapered hole in the arbor with only about an 1/8" "to go", its time to tap the end of the arbor with light taps with a small brass-headed hammer. This seats the pinion in the arbor. The end of the pinion is supported on a bench block during the process.

The pins in the center pinions are made from .023" needles. Needles are quite hard, and the finish on them is probably better than anything I could do in my shop. They make ideal pinion pins. The needles are inserted one at a time, the ends being ground off with a Dremel cutoff tool mounted in the Dremel tool. They're quite a snug fit in the holes in the pinions, but I used Loctite on each pin to be sure.

Then I smoothed the ends of the pins on the arbor. Once the pins are all in place, I put the arbor with its pinion in the watchmakers lathe and rotated it slowly, grinding the ends of the needls with a Dremel cut-off tool in the Dremel tool. I finished the job with 250-grit emery cloth, then Scotchbrite and then steel wool. The look is quite neat.

The pinion has a spigot on it, which holds the center wheel. The wheel is mounted on the spigot with Loctite, then it is hollowed out with a "serpents-tongue bit" of my own manufacture. The bit can make both right-hand and left-hand cuts, yet doesn't chatter because of the V cut in its center.

Hollowing out the wheel makes it much lighter and more attractive. It has the same effect as spoking the wheel, but it's much easier to do.

Today's links:

Bridge and posts
Bridge complete
Drilling front plate
Bridge on front plate
Perfect pinion
Cutting with serpent-tongue bit
Serpents tongue bit
Center arbor finished
Center arbor finished 2
Center arbor and bridge