In light of the positive feedback I received from the last post I wanted to share another common issue that comes up with the 861 movement, but really all mechanical watch movements.

When a watch is wound, the barrel arbor turns inside the bearing to allow the mainspring to be wound around it. This is where the watch receives its power.

The below photo shows the barrel arbor sitting in the plate, which is brass. The barrel arbor is made from steel and over time the steel arbor wears away at the brass plate.



When it wears away at the plate we see this happen. The side of the hole has worn away and is no longer an even circle. This scenario isn't the worst that can be seen and sometimes these holes have worn to a very exaggerated oval shape.



The result of this wear can also be seen on the plate itself. You can see that the arrow indicates where the plating has worn away and the ratchet wheel has been rubbing. This is an issue that needs to be rectified.



There are several ways to fix this problem. The first, and the easiest, would be to order a new bridge and install it. This is going to be a costly exercise as the retail price on this is very high.

The best option is to install a bushing in the plate. The advantage of this method is that once it's drilled, it's done and never needs to be done again. When more wear develops, the bushing can simply be pressed out and a new one installed.

To start this process we need to find the exact center of the workpiece so that we can drill accurately and not change the position of the barrel. The barrel needs to be parallel with the mainplate and the bridge so that it runs true and doesn't rub anywhere. If we drill a hole that is off-center, we are going to need a new bridge.



First off we mount the barrel bridge in the faceplate of the lathe. I like to cover the jaws that attach the workpiece with tape as that avoids any scratches. These jaws are polished to avoid marks but you can never be too careful.



It's important to note that we can't always use the part we are drilling as the reference point to line it up to drill. The barrel arbor of the 861 movement does not come all the way through the hole in the bridge, therefore, we can use the original side of the hole as our reference. For a movement where the barrel arbor does come all the way through we need to use a different reference point otherwise, our drilling will be out.

We next use a centering piece on our lathe to center up the hole with the axis that our drill will run on. This is used as a guide only to find rough center. There is not enough accuracy with this method to now start drilling. Perhaps we may get lucky, but I wouldn't advise it!



When we get to this stage we have two methods of finding the exact center. We could employ the wobble stick method, or use a centering microscope. I have opted to use a centering microscope as it's easier and faster. The wobble stick method is incredibly accurate but can take more time. Here is the centering scope mounted.



A photo through the scope. We use the crosshairs to line up the side of the hole and as the work spins the crosshair will stay on our outside line as the piece rotates.



When we first check with the microscope the work is rarely centered so a wooden mallet is used to tap the work true.



Once we have found dead center we can drill. I prefer to use a tungsten carbide drill bit as they have greater rigidity than a HSS drill bit. When using any twist drill there is a tendency for it to wander. The most accurate way to drill something is to use a drill bit to cut a rough hole and then finish the hole with a single point cutter to shave off the last few thousandths to true is up completely. In my opinion, this would be necessary when drilling out something like an escape wheel but for a barrel, I have always found the carbide drill bit to be sufficient.



Once it is drilled we can clean up the hole and insert our new brass bushing ready to receive the barrel.



Our final step is to install the barrel and check to make sure it runs true and is free in the hole. We can see that our gaps are even and parallel with the plates.



I hope you all enjoyed it and it was informative without being too broing!

 

Brilliant, I love these little insights into the complexities of watchmaking. Thanks for taking the time and effort for the write-up.

 

Thank you very much, I truly enjoyed reading. I have always marveled at the skills of watchmakers and their ability to work with such precision in dealing with very small parts.

 

Excellent job, thanks for taking the time to do this.

 

Thanks for such a clear and well illustrated post.

 

Thank you for a very informative post!

I think Archer wrote already before about the wear on the plate from barrel arbor. But it was nice to see the details like using a centering microscope on a lathe.
Also had to google about the "wobble stickmethod", but will have to continue reading tomorrow with fresh head.