The weight which drives this clock is suspended at its back. So that it doesn't swing and do damage if bumped, the weight has an axial hole surrounding a rod which is fastened at its bottom to the wooden base of the clock and at its top to a post on the back of the weight bridge. As the clock runs, the weight slides down the rod. The rod also does double duty as an additional support at the back of the clock. It is made of 3/32" music wire.
In order for this arrangement to work, the winding drum must "surround" the rod, having a groove at its center to allow the rod to pass by. The weight is suspended equally by two separate cords, one on each side of the rod, which wind up on the drum on each side of the rod. The cords wind in opposite directions on the drum.
The drum is made of 1/2" brass, with two small disks soldered at each end to inhibit the cords from falling off. The drum is a straight-forward machining job. I cut the central groove with a 60-degree thread-cutting tool.
The main arbor is made from 3/16" music wire. This material is great for clock arbors, since it's already sufficiently hard, but not so hard that it can't be cut with a sharp lathe tool or a file.
The winding square on the end of the arbor is made with a file. While I do have a milling machine, this is one of those jobs which is easier, faster, and of equal quality to a milling job.
In the lathe, I cut a V-shaped groove near the end of the arbor. This forms the limit of the winding square. Then, I clamp the arbor in the vice and file some of one facet. Then I rotate the arbor 90 degrees and file some of the next facet, and so on, until there are facets on all four sides -- the vice acts as an indexing device.
You judge how much you've filed by keeping an eye on the "lands" -- the uncut round areas -- on the edges of the facets. I don't try to cut all of one facet at a time -- keep rotating the arbor in the vice. I keep the lands the same size and parallel-sided. If you're careful, you can cut the arbor perfectly square withing a few thou, and do it in fifteen minutes.
The part of the arbor beyond the V-groove is protected from file damage by a sleeve of brass tubing (or washers) which comes up to the center of the groove.
Then I cut a center in the front end of the arbor with a center drill, and cut a point on the other end. These will prove useful when the arbor is later placed in the depthing tool when the arbors are planted between the plates.
The arbor must slip through the 3/16" opening in the ball bearing. The music wire I had for this job proved to be a bit tight -- it was about .001" oversize. It's fairly easy to file this small amount of material from the arbor, but it's hard to know how much material has been removed. So I extended the arbor some from the chuck of the Taig lathe, filed it a bit until the bearing just slipped over the rod. Then I extended the arbor some more and filed a bit more, sliding the bearing down as I worked, repeating until the entire arbor slipped nicely through the hole in the bearing. When the rod extended quite a bit from the lathe chuck, I supported the end with a live center in the tailstock. The results were quite good -- I then polished the rod with abrasive paper.
The central hole in the drum is drilled under 3/16", then reamed to 3/16" minus .001" with a "one-under" reamer, available commonly at tool houses. The drum is then heated with a propane torch, and the 3/16" arbor is tapped into the drum. When the brass cools, the drum shrinks and grips the arbor.
There are two holes drilled through the drum which will hold the ends of the weight cords. These holes are drilled at an angle through the drum, so that the holes will miss the hard music wire arbor inside the drum, and because the cables will be bent at lesser angle where they pass through the drum.
It's difficult to drill slanted holes in a round rod. So first, I mounted the arbor in a vice on the Taig mill, and put a small drill in the mill chuck. This I used to make a visual check at the end of the arbor, to be sure that the drill would miss the hard arbor. Then I used a 3/32" end mill to make a small recess in the rod -- end mills aren't bothered by starting at a slant. The recesses are cut just deep enough to serve to contain the knot at the end of the weight cord.
Then I center drilled the hole at the bottom of the recess and drilled a .040" hole through for the cable. The process is repeated to make the second hole on the other side of the groove in the drum.
The weight slide rod is mounted to a post which extends out the back of the weight bridge which I made last posting. The post is made from 5/16" brass, with a 3/32" hole drilled transversely. The end of the post is drilled and threaded for a 2-56 set screw, which holds the rod. The inner end of the rod is made with a spigot, which passes through a hole in the weight bridge in the top "ear" of the bridge. The spigot is riveted into the hole.
Today's links:
Main arbor grooved Filing square Thinning arbor (note bearing gauge) Arbor with drum Milling drum recesses Drum finished Post with rod Back of clock