Staking And Bushing Clock Movements

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Staking And Bushing Clock Movements

The term "staking" in clock repair typically refers to using a staking tool for various tasks, but when dealing with worn pivot holes, the proper term is "bushing" or "rebushing." Here's a detailed explanation of the process:

Understanding the Problem

Dust trapped in clock oil acts like a grinding compound, eventually causing the pivot hole to become elongated and allowing wheels to slip out of alignment. The most common problem with a clock movement not running after cleaning and oiling is that pivot holes in the brass side plates have worn larger, usually to an oval shape, because the pressure of the powered gear pushes the next gear away from it.

The Bushing Process Overview

The goal is to bring the pivot hole back to its original size and exact location. This involves removing the worn hole and installing a brass bushing with a fresh, properly-sized hole.

Tools Required

Essential Tools:

• Bushing machine (KWM or Bergeon are the major brands) OR a drill press with minimal runout OR hand tools
• Reamers - Tapered cutting tools to enlarge the worn hole to accept the bushing
• Bushings - Brass sleeves in various sizes (must match your reamer system)
• Centering tool - To locate the exact center of the worn hole
• Drilled stakes - Support blocks with holes for pressing bushings
• Pushers - To press the bushing into the reamed hole
• Broach - To adjust the final size of the hole in the installed bushing
• Chamfering cutter - To bevel the edges of the bushing hole
• Calipers or micrometer for measuring pivots

Step-by-Step Process

1. Disassemble and Inspect

• Take the movement apart and clean out all pivot holes with toothpicks, as dried rubbish doesn't allow you to see all the wear
• Mark worn pivot holes for identification
• Measure the pivot diameter with calipers

2. Select Proper Bushing Measure the diameter of the pivot (for instance 0.67mm), then you would need to use a bushing with a hole 0.60mm diameter. The bushing's outer diameter corresponds to the reamer size you'll use.

3. Prepare the Worn Hole File away a portion of brass from the unworn side of the hole, equal in length to the eroded side, ensuring the new hole will remain in the correct, original position. This step is crucial for maintaining proper gear alignment.

4. Center the Hole Use a centering tool to locate the exact center of the worn hole. This is critical - if your bushing is off center, the wheel and pinion will be too close to or too far from adjacent wheels.

5. Ream the Hole Use a bushing reamer to cut the hole to the correct size, or alternatively use small drills to gradually open the pivot hole, finishing with a drill slightly smaller than the required size.

The holes are tapered and not straight - this reduces friction on the pivot shaft so only the back part of the bushing contacts the pivot, and it allows some gear "shake" for slight misalignment tolerance.

6. Install the Bushing

• Place the clock plate on a drilled stake (a support block with a hole)
• Position the bushing over the reamed hole
• Use a pusher to press the bushing into place until it's flush with the plate
• Ensure the bushing is flush with the plate and centered properly

7. Broach the Bushing Use a broaching tool to adjust the size of the hole in the bushing to fit the pivot perfectly, ensuring the pivot will rotate smoothly. The pivot should spin freely but not have excessive play.

8. Chamfer the Edges Use a chamfering cutter to create a slight bevel on both sides of the bushing hole. Add a slight chamfer around the interior hole to facilitate oil flow around the pivot shoulder into the bearing surfaces.

9. Polish Pivots Before reassembly, ensure all pivots are polished smooth and free from grooves or wear. Worn pivots will quickly damage new bushings.

10. Test Assemble the drive train you've repaired, leaving out the spring, and spin the gears by hand to see if every gear turns freely before main assembly.

Alternative Methods for Hobbyists

Sufficient accuracy can be obtained by installing bushings using common hand tools. Hand reaming is more economical and, with care, can produce excellent results. However, it requires steady hands and practice.

Important Cautions

• Never attempt bushing on an assembled movement
• Check that pivots aren't bent before reassembly
• Use extreme care during assembly to avoid bending pivots
• Proper depthing (gear mesh spacing) is critical - visually inspect gear engagement
• Regularly inspecting and replacing worn bushings extends clock life and maintains accuracy

This is a skill that improves with practice, and many experienced repairers suggest starting on an inexpensive practice clock before working on valuable timepieces.