Pivot Hole Restoration

Once the pivots have been polished and burnished, it’s time to repair the worn out holes that the worn pivots ran in, following decades of service and wear with little or no maintenance.

Select images to enlarge.

1

A tired and worn, but precious and scarce “Quail and Cuckoo” clock from near the beginning of the 20th century will receive new friction bushings to repair worn out pivot holes.

2

A pivot hole from the rear plate of the movement is completely worn out, causing the gear that’s mounted on the associated arbor to mesh poorly with the adjoining gear.

3

A portion of brass from the unworn side of the hole, equal in length to the eroded side of the hole, is filed away. This ensures that the new hole will remain in the correct, original position.

4

The hole is shown after the first filing. While it appears to be asymmetrical, this is an illusion caused by shadows, which must be taken into account when determining when to stop filing.

5

Pressure is applied to the file from different directions to equalize the cut, while inserting the file deeper to approximately round out the hand-filed hole. 

6

Although still in the rough, the re-rounded, re-centered and enlarged hole will enable the D-shaped cross-section of the cutting reamer to rotate more smoothly when the hole is enlarged.

7

A centering tool is affixed to the bushing machine. The point will be lowered into the roughed-out hole, to align the movement plate to the machine, prior to installing the cutting reamer.

8

With the plate captured and held in place by the centering tool, clamps are slid into position and secured, to keep the plate from moving when the cutting reamer is turned.

9

The cutting reamer now takes the place of the centering tool, and is lowered into the hand-filed hole, until the edges of the tapered portion of the reamer contact the sides of the hole.

10

The reamer will be plunged down until the tapered cutting edge has given way to the cylindrical section, forming a true hole with a smooth wall, ready to receive the bushing.

11

As the cutting reamer is gently pushed downward, a wheel on the machine is simultaneously turned to start the cut. As the reamer rotates and deepens, brass is shaved from the hole wall.

12

In order to select the correct friction bushing, the diameter of the restored pivot is measured, as well as the plate thickness. Bushings have three dimensions: I.D., O.D., and Height.

A precision-manufactured brass bushing is selected from an organized assortment.

13

The bushing has two different machined faces, a flat face that will be pressed flush to the plate interior, and a concave face, which will be oriented to plate exterior. The smooth, flat face is designed for minimal friction contact with the pivot shoulder, while the concave face serves as a store of oil. In this view the flat face is depicted, however the bushing will be inverted prior to pressing.

14

A flat-faced pressing tool (inset) now replaces the cutting reamer. The tool is now driven down, firmly pressing the bushing into the plate. The tight, friction fit ensures bushing stability.

15

The restored pivot hole can now accommodate the restored pivot. A slight chamfer is added around the interior hole interior, to facilitate oil flow around the pivot shoulder, into the bearing surfaces. (Interior of movement plate shown).