Re-bushing a barrel is such a crucial aspect of watch servicing, but it is seldom done. Anytime a watch doesn’t have a jeweled barrel or center wheel, there will be wear.
A barrel and center wheel have steel arbors or pivots. When they use a jeweled bearing the jewel is the harder of the two materials. When they just sit in the brass mainplate and bridge the steel is the harder of the materials.
Due to this fact, the plates wear before the pivots and arbors.
When this happens the plates must be drilled out and new brass bushings need to be installed.
The arrows below point to the plates that have worn holes. That is where we will drill and install the bushings.
Below is the lower barrel hole in the mainplate. You can see where the rose gold plating has rubbed off – that shows that the hole has become sloppy and a bushing needs to be installed.
Then we see the upper bridge. It also hasn’t wear on the plate and a new bushing needs installing.
Here is a close up of the hole. You can see that they wear has ‘cut’ into the plate. That is what gives it the sloppiness. If left unchecked, the barrel will move up and down and run on the plates causing the amplitude to drop.
Before we start drilling, dead center needs to be found. We do this with the tailstock which is equipped with a rod that has a pointed end. This helps us to roughly center the work. The workpiece is placed in the faceplate to allow for off-center drilling of the hole in question. The clamps are tightened, but not completely to allow for adjustment.
We then use the wobble stick to find dead center. The wobble stick is really any pointed stick that is sharpened to a point on both ends. It is best to make it from blued pivot steel, but brass also works. It is placed on the tool rest like so with one pointed end entering the hole in the plate that we are trying to center. The workpiece is slowly rotated. As it is rotated the stick will move up and down indicating that the work is not true.
We use a ruler at the other end to measure how far out of true the piece is. When there is little to no play left, the work is centered.
If the workpiece needs to be adjusted, we use a wooden mallet to gently tap true.
Once the workpiece is centered, the clamps can be tightened and the truth is checked once more. We are now ready for drilling. We use a tungsten steel drill bit slightly smaller than the diameter of the hole we desire. If looking for a 1.2mm hole, we use a 1.1mm bit.
Once drilled, the hole needs to be removed to size with hand reamers. First, we use a cutting broach to get to a close size, then we use a smoothing broach to get to the correct size.
Once the size is correct we can fit the new brass bushing. Generally, the inner hole for the pivot or arbor will need to be reamed to size as well. We use the same method with a cutting broach and then smoothing broach.
Once the desired size has been found we use a countersink cutter to make a chamfer in the hole.
Here is the new bushing fitted in the bridge with the barrel in place.
Here is the bushing in the mainplate (or in this case, the lower bridge).
This is the last step we need to check. We also need to check that the barrel is parallel to the bridges. If we didn’t center the piece properly, this would be out and the barrel would sit skew.
Now we can assemble the rest of the movement.