John B Shadle, CMC
A lot of us fix cuckoo clocks, and since some cuckoo clocks play music, we fix music movements, too.
Usually, the most worn part of a music movement is the little governor that controls the speed, shown in Figure 1. Until a few years ago, replacing the governor was easy, since parts houses carried exact replacements. But these days, the selection of replacements is more limited, and we have to make do with "one size fits all" replacements. Unfortunately, "one size fits all" means "fits few", so you wind up spending a lot of time modifying and fitting.
So instead replacing the governor, why not fix it? I believe that repairing instead of replacing can be time- and cost-effective, and you'll wind up with a governor that is actually better than the original. While you still may want to replace some governors, my method will give you an option you didn't have before
The worst wear is found in the pivot holes of the two little gear arbors. Pivot holes are worn egg-shaped, and because the pivots are short, they're also undercut. I've attempted to re-bush these pivot holes, without success.
Instead of bushing pivot holes, my method involves broaching the worn holes round and re-pivoting the arbors with new, oversize pivots. I've not found a way to repair the fan arbor pivots, but it's usually not necessary.
Here's how to do it. First remove the two little gear arbors. Do this by bending back the tabs that have the pivot holes in them. It's best to bend the tabs up from the base, as shown in Figure 2. Use care, bending the tabs just enough to permit removal of the little arbors. The result appears in Figure 3
Second, bend the tabs back to their original positions, without mounting the arbors between them. Put a broach or a needle through the holes to check the alignment of the holes, as shown in Figure 4. They should be aligned as they were before bending, and the space between them should be right for the arbors. Don't worry about being exact at this point -- you can make final adjustments later.
Next, use a cutting broach to ream out the pivot holes. Broach to remove eccentricity and undercutting. Broach through both holes to align the holes, as shown in Figure 5.
Now, we'll re-pivot the arbors. We'll use pivot material that has a larger diameter than the original pivot diameter, is much harder, and will make a better pivot than the originals. I use needles, which are hard, cheap, highly polished and come with a convenient taper.
First, re-pivot the short arbor, which consists of a wheel and pinion combination. You can chuck it by grasping the pinion in a WW collet, or mount it on a piece of tubing held in a lathe chuck, as shown in Figure 6.
If you use tubing, first square the end of it with a tool bit, then clamp the pinion/wheel to the end of the tubing with super glue or watchmaker's shellac. You can hold the pinion/wheel centered while cementing by using a 1/8" brass cone center in the tail stock to support the pivot. Run the lathe to ensure that the wheel is running true before applying cement. The brass center is home-made, and appears in Figure 6.
Next, remove the exposed pivot with a graver or a file. Then chuck the finest center drill you have in the tailstock and make a little center in the end of the pinion as shown in Figure 7. Drill a hole completely through the pinion, as shown in Figure 8. Drilling will also remove the pivot on the backside of the pinion.
Drill with quality American-made drills -- don't expect to drill small holes with cheap imports. Center drills should be size 5/0 or 4/0, with a tip that's only .010" or .015" in diameter. These can be bought from any machine tool supply house.
The size of the hole you drill is determined by the size of the holes in the brass governor tabs achieved earlier and the size of your pivot material. If you use a .035" needle for pivot material, drill a .035" hole through the pinion.
(Clearances between the new pivot and the holes in the tabs should be a bit loose, as they were in the original governor when it was new. If you have doubts about what clearances should be, examine a new governor and duplicate the clearances.)
You can use a press fit, or you can use Loctite to secure the pinion to the needle pivot. Just remember that the pivot material will need to be slipped through the pinion as well as the pivot holes during re-assembly, as shown in Figure 9.
Remove the pinion/wheel from the brass in the lathe by heating it gently with a micro torch or alcohol lamp flame. Use acetone to remove super glue residue.
When re-pivoting the long arbor, use a steady rest. To re-pivot the end with the pinion, I clamp a clock bushing with an internal diameter the size of the pinion in the steady rest, and run the pinion in the bushing, as shown in Figure 10. (Use a drop of oil in the bushing.) Center and drill as in before, but of course, without drilling clear through. Just drill deep enough to support a replacement pivot. To re-pivot the opposite end of the arbor, support it with a steady rest as well, as shown in Figure 11.
When the parts are all finished, put them together with Loctite. Slip the needle through the pivot holes and the pinion of the short arbor. It is held in place with Loctite. The pivots of the long arbor are first slipped through the pivot holes and then into the holes drilled in the ends of the arbor. They, too, are held in with Loctite. The result appears in Figure 12.
Dab away excess Loctite with a bit of tissue. Since necessarily, some Loctite will get into the bearings, put a drop of oil in the bearings and turn them a few times while the Loctite sets to keep the bearing from being cemented. When the Loctite is set, trim off the ends of the needles with a Dremel cut-off wheel. You may need to bend the brass tabs a bit to adjust depthing between the two arbors as well as arbor end-shake.
Finally, clean the rebuilt governor in your ultrasonic to remove all cuttings. Then oil all pivots, of course, and also the worm gear. The governor fly should spin easily, and should be driven smoothly by the gear train.
If you’re like me, you probably have a box full of old governors to practice on. You'll find that many of these can be re-used when properly repaired.