Watchmaking: Crown Wheel Seat Repair

Wanted to share a small repair that I completed recently, just to expose people to the sort of issues that are seen on vintage watches. What made me think of this are the claims of the specialized teams of watchmakers that will be assembling the new Cal. 321 Speedmasters. As I’ve said, that would be a dream job assembling these from new parts, rather than dealing with an old worn out movement. Now this isn’t a Speedmaster, but it shows the kinds of problems that old worn out movements can present.

The movement in question is a vintage Bulova 7AP, and when it arrived it had a lot of wear on it. I ended up buying several old donor movements, and my customer even ended up finding a couple and sending them to me, but as often happen, they all had many of the same problems. So the problem I’m solving here (one of many on this movement) is related to the crown wheel. The crown wheel turns when you wind the watch, and is driven by the winding pinion. The crown wheel in turns drives the ratchet wheel, and that winds the mainspring.

In this watch, the area that the crown wheel sits on at the barrel bridge is worn:



You will see a red arrow that points to a worn spot on the bridge. This spot is allowing the crown wheel to tip under load, and it is tipping so much that the teeth of the crown wheel are not staying engaged with the teeth of the winding pinion. The watch would get maybe 75% to full wind, and the teeth would slip. This damages the teeth of the winding pinion - rounds them off as you can see here:



And the underside of the crown wheel teeth - damaged on the left, and good on the right:



I’ve taken this video to show how much the crown wheel can move before the repairs:


Of course the wheel needs to be able to move, and in the first part of the video I am lifting it up, which is fine. It’s the tipping of the wheel when I press down on it with my tweezers that is the biggest issue.

So what I’m doing in this repair is I’m leveling out the area where the boss is worn, because part of the problem is that this area is uneven. So I want to turn it on the lathe, but holding it presents a bit of a problem because I’m turning something that is not in the geometric center of the part - it’s offset. I could use a face plate style chuck, but the movement in question is quite small, and I don’t like getting close to those as I’m working with a loupe. To show what that looks like, I grabbed this photo off the net (I do have one but this shows it with work clamped in so you can see how it functions):



So how to hold onto this relatively small parts? I considered a number of ways - one was mounting the bridge to a separate plate, and then clamping that plate in my faceplate style chuck, but that would require drilling a tapping holes in the plate, sourcing taps, etc. If I was repairing a bunch of these, creating a fixture is certainly the way to do it.

But for a one off I had another idea...it's a bit of a long road to get there so I'll post this in a few parts.

I started by sharpening the gravers - first up are the high speed steel gravers, and I use a Crocker graver sharpener to mount them and keep the angles right. Many people sharpen their gravers by hand, but personally I like having a consistent angle between steps and also between gravers, so I use the jig. I start with some oil on the India stone, using the rough side to lean up the surface:



Note that there are a few things in this photo that should be noted. First, everything is on a heavy sheet of glass on my bench (welding glass that I can’t really remember where I got it from), and this provides a very nice smooth surface for the jig to ride on, that I know will be flat. Second, you will note that the India stone is on top of a piece of abrasive paper - this helps keep the stone in place, but it’s there more for spacing than anything - it will become evident in a minute why it’s there.

After roughing in the graver cutting surface with the darker side of the stone, I flip it over to the fine side:



Once I’ve done the fine side, you can maybe now see why I had the paper there:



Since the angle that the cutting surface is polished to is partly determined by the height of the stack of items on the glass, I put the paper under in the first two steps to make sure the height of all the items is the same for this step. Terrible iPhone photo of the cutting surface - I didn’t have a better camera with me and tried to shoot this through my spare 4X loupe. Trust me, the surface is mirror like, which is what you want.



For carbide gravers, I use a diamond lap, and this time instead of oil for lubricant, it’s just water - I prepare the graver by using the rough side of this diamond lap, then the fine side, and it’s good to go:



So now that the gravers are good, I had to find some material for this project. That proved to be more difficult than I had anticipated, but eventually I found some 25 mm diameter brass bar stock:



Step 1 is to cut off a small piece, since the watchmaker’s lathe can’t swing a long bar:



Now to the lathe - I’ve selected my 3 jaw chuck for this work:



Work piece is running true, and I’m ready to start making chips!



Unlike an engine lathe, with a watchmaker’s lathe of this type the graver is hand held, so this is more like turning on a wood lathe than on an engine lathe:



Need to remove a lot of stock...yes there are other ways of constructing the item I’m making here that would have saved stock, but I wanted this to be all one piece when it was done:



Not too concerned about dimensions at this stage, so just hogging off material the fastest way possible:



And after a while...it’s getting there:



Test fit of the collet I’m going to use to hold this:



Now that it’s close, I can focus on making the surface finish better, so here is the end of part 1:



Okay time for a break...part 2 to come later.

Cheers, Al

Great post Archer, I look forward to seeing how your repair pans out.

Really interesting post and I appreciate greatly the work a good watchmaker can do.

Okay here is part 2...

Time to turn down the larger OD to clean it up, and then cut a groove for parting off the work piece:



The jeweler's saw comes in handy for cutting off the work:





After this, the work has been flipped around and mounted in the lathe collet. I need the front of this part to be completely flat, so how do I ensure that as I work on it? If I had a lathe with a cross slide and tool holder, this would be easy, as that precision is built into the machine tool on something like an engine lathe. But cutting freehand like this, it's not straightforward at all. Essentially I have to cut this surface flat by hand, so it requires experience to keep the graver presented to the work in a way that gives you the best chance of accomplishing that. The graver is small compared to the large work surface, so it's harder to do than it may appear.

So how do I even check this? Well I could use a precision square (I do have one) but I don’t want to remove the work from the collet to check it all the time, so how else can this be accomplished? Traditional methods of measuring flatness visually would involve a bluing and a precision flat plate, but again this would require removing the work from the collet. Fortunately, the same concept to highlight the high points can be done using just a file:



I stop the lathe and file across the face of the work lightly, rotating the work as I do this until I’ve filed across the whole surface, and then take a look:



You can see that the filed areas are around the outside of the face of the work, but the middle hasn’t been touched, so it’s concave. I keep working away at the face, and keep checking it:



And here when the file marks are all the way across the face, it’s good:



Next I cut grooves in the face of the work:



So what is this thing that I've made? I'm sure the watchmakers here figured it out a long time ago, but this is known as a wax chuck. Contrary to the name, I won't be using wax, but shellac - the same shellac that is used in movements for things like holding pallet fork jewels and roller jewels in place.

I mount my tailstock on the lathe:



I heat the chuck and put some shellac flake on it, and spread it around:



I have some old movements I used for spares (all had this same wear on the bridge) to use for test purposes. So I mounted a spare bridge on a spare main plate, heated the chuck, placed the work on the shellac, and used the tailstock center to hold it in place until it cooled:



Now I can turn the work centered on the spot at the location I want:



That’s it for Part 2...

Okay here is the last part...

The next step is to make sure I have the tools needed to turn that small area, and I took one of my smaller HSS gravers, and ground it down - not quite there yet but this shows the original size on the right, and the ground down one on the left:



Now ready:



After performing some test cuts on the spare bridge, I get the bridge I want to fix - spare on the right and the original bridge from the watch on the left:



The main plate was left on the chuck, so all I had to do was unscrew the test bridge from the plate, and screw the good one in place:



Making the cut - took all of about 30 seconds:





Here you can see that the raised boss that the crown wheel rides on has now been turned flat again:



I’ve now removed the bridge from the plate on the lathe, and taken it back to the bench. It’s been mounted and here’s a video showing the movement now:


You can see that it is still freely moving, but doesn’t tip as it did before. So the wheel cannot tip enough to allow the teeth of the crown wheel and winding pinion to disengage from each other.

So this problem is now solved. However that’s not the only issue this watch has - the original main plate for this was is badly worn, and as you may be able to see here the center hole is worn oval, and this has allowed the center wheel to tip so much that the cannon pinion teeth have carved away a bunch of material at the red arrow in the middle (enlarged view bottom left):



So back to the parts bin to find a good plate. I also looked at many train wheels, etc. to find a set that were in good shape:



And here a final test to make sure it could be fully wound without slippage between the winding pinion and crown wheel teeth:


So I hope you enjoyed seeing how one small problem on a vintage watch is resolved. Now the real work getting the movement performing properly can be started.

Cheers, Al

Excellent writeup and pictures.

If you don't already follow him, I find "This Old Tony" on YouTube a fun watch...

Your point about the set-up taking longer than the actual job is certainly relevant, as it is often the case. It certainly was here, but I went into this knowing that the wax chuck would be a handy thing to have for other jobs. Yes, I could have bought one, but where is the fun in that? 😀

Having said that, I am losing my shirt on this particular service/repair, so I feel that there may be a Sherline in my future sometime soon...that would have made this job soooo much quicker.

Cheers, Al

Time to sell advertising space 😉

A machinist lathe would have been a great help indeed. What is the advantage of a watchmaker's lathe? I know parts can be mounted on one with the precision collet but the handheld gravers seem difficult to master. Why aren't gravers mounted like those found on a machinist lathe?

Sorry, you lost me...

To help pay for the lost time.

Such photos!

They suck one right in to see the parts in intricate detail. Makes him feel like "the man on the scene."

Thanks!

Great post and as always simply outstanding photos. Thanks for sharing.