The contrate wheel drives the escape wheel arbor. Since the escape wheel arbor is vertical while the rest of the train gears are on horizontal arbors, the wheel must be a contrate wheel.

The wheel has 60 teeth and it's made from 3/16" brass, which is thick enough to permit hollowing it out into a rim and back. It is 1.085" in diameter.

I made the wheel using a modified Taig blank spindle arbor. (The Taig lathe and mill have many such nice programmable accessories.) The bottom of the arbor has internal 3/4" x 16tpi threads which screw directly onto the Taig lathe spindle nose. It also fits on a stud which screws into the center of the Sherline rotary table. You can thus put a part on the arbor, mount it on the lathe, machine it and then transfer it to the mill for further machining without removing the part from the arbor. This preserves the concentric truth of the part.

To mount the contrate blank on the Taig arbor, I cut a 3/16" spigot on the end of it, which fits snuggly in a hole in the contrate wheel blank. Normally, I'd secure the wheel blank onto the arbor with a screw, but such a screw would stick up high, where it could interfere with the fly cutter when the gear is being cut. So I used Super Glue to glue it to the arbor. This is sufficiently strong to permit machining the blank. The glue bond is broken with heat when the wheel is finished.

I made the hollow in the contrate wheel with a specially-ground lathe bit which I call a quarter-round bit, since it's shaped like the wooden molding with the same name. The bit must have a rounded bottom to do cutting inside a small circle, as shown here . The bottom of a normally-ground lathe bit would rub on the inside of the wheel rim.

The quarter-round bit is ground by hand from a 1/8" high-speed steel bit blank. First, I grind a clearance angle on the front of the bit blank, which leaves the cutting edge as a point, as shown here. This is the same way you would prepare a normal bit. Then, the bit is ground round on its left side. The result is shown here.

An 1/8" bit is used because it can fit into tighter places than than the usual 1/4" bits native to the Taig or Sherline. Since the Taig accepts 1/4" bits in its tool holder, the 1/8" bit must be raised up 1/8" for the point to be on lathe center. Another 1/8" bit works perfectly for this, set under the quarter-rounder in the tool holder.

I hollowed the contrate blank until the rim is 1/16" thick and the back of the wheel is also 1/16" thick. This is thicker than it will be when finished. The extra thickness is for added strength during tooth cutting.

The arbor with its cut blank is screwed onto the rotary table, and the fly cutter is gradually lowered until the depth of cut is right, as described previously. As before, the inside and outside limits of the mill cross feed is marked to prevent running the flycutter into the opposite side of the contrate wheel blank. The wheel is indexed 6 degrees for each cut.

When the teeth are cut, the arbor is moved back to the lathe, and the rim and back are thinned to 1/32" with the quarter-round bit. The wheel is polished with Scotch Brite and steel wool.

I've also made another 60-tooth wheel of ordinary sort which will be used as the clock's center wheel. The wheel is made as previously described for the 80- and 120-toothed wheels.

Todays' links:

Blank arbor bottom
Spigot on arbor
Untoothed contrate and rotary table stud.
Hollowing contrate blank
Cutting contrate teeth
Contrate on arbor
Contrate finished
Center and contrate, two 60-tooth wheels