Metal work

Bushing is both a relatively simple but also a highly precise process for repairing worn pivot holes on clock plates.

As noted in our last episode, when clock wheel pivots wear the brass pivot holes of the clock plate off of their manufactured round, the resulting channel dug into the brass can create enough friction to slow and ultimately damage or stop a clock. 

The pivot hole must be restored to the proper diameter and exact original location of the originally manufactured pivot hole.

Once again here is our worn pivot hole. The pivot dug into the brass to the right so the left side of that hole is effectively as original. You can see that the left side looks well centered to its original design but the right side is creeping to the edge of the cupped area that surrounds the pivot hole. That cup is an oil sink that facilitates oiling the pivot hole.

Worn pivot holes are extremely common in older clocks and can go unnoticed for decades until the clock slows or stops. Because it is so common there are a wealth of resources for bushing and it requires a few very specific tools. 

First and foremost it requires a bushing (yes the piece of brass itself and the work to repair the plate use the same word) which is a manufactured piece of brass or bronze with very specific dimensions of its bore (the inner diameter / future pivot hole), outer diameter and height. And there is the bushing I'll use in the photo above.

Bushings come in hundreds of sizes but there is a reasonable selection that can be used and lightly altered to work with many different clocks.

Here's a sampling of my small collection of bushings. 

To bush you cut the brass plate with a special cutter, a "reamer", re-centering the hole to its original location but now, of course, its diameter is wider. Reamers are designed to cut a hole to highly specific standard outer diameters that will match the standard sized outer diameters of the bushings (known as sizes I, II, II, IV and V but also 1, 2, 3, 4 and 5 are used interchangeably). Actually the reamer cuts a hole less 0.02mm smaller than the outer diameter of the bushing. Thus the bushing can be fitted/wedged into the plate very snugly.

The diameter of my pivot at the end of the wheel is almost exactly 0.8mm and the plate is 2mm thick. 

So I will use a number 11 bushing and a II / 2 reamer.

The hole must be cut so that the bushing hole will sit absolutely perpendicular to the surface of the plate. You actually cut the non-worn side of the pivot hole and even it out to the worn site to make a larger, round, correctly-centered hole. The cutting of the hole is called broaching. A lot of guys use machines to do this but I've learned that many of the skilled NAWCC crew do it by hand/eye. 

Thusly...

 

Once cut, the bushing is seated over the hole...

And then pressed into the plate. I read about the use of parallel pliers. 

Here's the bushing in its new home. That's the rear view and here's the front.

Note that the bushing has an oil sink built in that matches the original pivot hole oil sink. And yes for those of you who are paying close attention the new pivot hole is 0.7mm and I need a bit more than 0.8mm (probably 0.9mm) for the pivot to insert and rotate with enough play.

So the pivot hole on the bushing needs to be widened by about 0.1-0.2mm with a special file. Essentially the hole is just shaved open a bit larger.

This diagram from the NAWCC best summarizes how the worn pivot hole needs to be re-center to the cross mark, broaching it with the reamer, fitted with a bushing and then the new pivot hole is broached to properly fit the bushing.

Our new pivot is tested and ready to go.

 

You can see how that oil sink built into the bushing aligns to the original oil sink. 

Fast forward through rebuilding of the movement and here is the repaired plate in place with its wheel pivot happily sitting in place and working well.