The clock is driven with a weight which hangs at the back of the clock, suspended from cable wrapped around a drum on the back of the main arbor. The outer end of the arbor with its drum is supported by a bridge which stands away from the main plates of the movement. This to give clearance between the weight and the plates as well as between the weight and the balance wheel as it descends.
The drum which contains the cable and supports the weight is 1/2" in diameter, but the weight must be centered in the clock dome, so the clock's main arbor is offset from the center of the clock by 1/4". This permits the cable to reel off the drum in the center of the clock.
I made the bridge from 3/16" brass. I fastened it to the back main plate by two screws which run through two pillars. The bridge also contains a small ball-bearing, which supports the weighted end of the main arbor.
I sawed the bridge from a Deltacad template, as ususal, but before sawing it out of the mother brass, I made three holes in it. Two are for mounting it to the back plate of the clock and are center-punched and drilled in the usual way. The third holds the ball bearing, and is 1/2" in diameter. This hole must be precisely located, so I thought it better to bore the hole in the lathe, rather than to drill it. Drilled holes -- especially larger ones -- tend to wander when you're drilling them.
To bore the hole, I first screwed a piece of 3/16" brass to the Taig face plate. This faceplate has slots for mounting screws, so the brass was drilled for the screws in places which will later be cut away as waste. Then I mounted the faceplate in the lathe and used a center drill to make a center mark in the brass.
Then I removed the faceplate from the lathe without disturbing the brass. There is a center mark for the 1/2" hole on the Deltacad template. I sprayed the template with adhesive and positioned the template on the brass, using the template center mark and the center divot cut in the brass for alignment.
Before the adhesive dried, I returned the faceplate to the lathe, and fine tuned the placement of the template on the brass with the lathe running slowly. You can center such things so that there is no wiggle left in the dot indicating the center of the 1/2" hole.
After the adhesive dried, I first drilled a 1/4" pilot hole in the bridge. Then I used a boring tool to bore the 1/2" hole to size. I was able to achieve a light friction fit between the bearing and the bridge. Had there been a little extra, I would have used Loctite to hold the bearing. The bearing cannot be crowded, or it will turn rough -- a big no-no in a clock.
I remove the shields from ball-bearings I use in clocks. They add friction, and they also make it impossible to ultrasonically clean the clock at a later time. I clean out the grease, and relube the bearing with clock oil. This bearing will not be turning at 20,000 rpm under a heavy load, but it must turn smoothly.
I mounted the bridge to the main back plate by locating the dots which are present on the paper Deltacad template which is still glued to the back plate. I drilled one mounting hole in the back plate, tapped it, and screwed the bridge onto the plate on before drilling and tapping the second mounting hole. I used a transfer punch to center-punch the second hole.
I also used a 1/2" transfer punch to center-punch the center of the hole through the back plate for the main arbor through the 1/2" bearing hole in the bridge. This hole is drilled large enough for the 3/16" main arbor to pass through.
The screws are 6-32 screws, which I made by hand and heat-blued. The pillars are similar to other pillars I've made for this clock, except they are drilled completely through for the screws. I threaded the screws in the 9x20 lathe, using a hand die-holder supported by the tailstock. This method ensures that the threads on the screws will be straight. I turned the lathe spindle by hand.
The top of the bridge has another "ear" which will later be used to hold another post.
Today's links:
Mother brass on faceplate
Template centered on brass
Bridge with bearing
Drilling back plate
Threading screws
Screws and posts
Bridge, post and screws
Bridge mounted