An excellent bush is one that realigns the depthing of two wheels correctly. The wheel and the pinion pitch circles touch each other as close to their line of centers as possible. The excellent bush is always flush to the inside of the plate, Figure 1, and stands flush, Figure 2, or no more than 10% higher than the front of the plate, Figure 3. Any higher and the likelihood of the bush coming loose in preparation or usage increases markedly, Figure 4. Any lower than the plate and long-term support is compromised, Figure 5. Should the bushing need smoothing on the inside of the plate to bring it flush, the surface must be finished smooth. If it is sanded or filed, it will wick the oil from the shoulder and add drag to its motion, Figure 6. It is usually better to cut a bushing to size on the lathe rather than having to sand it down in place. The bush should have a bright, burnished hole whose edges are smoothly deburred on the back side of the plate and countersunk slightly on the front side, but only enough to allow for better oil retention and/or as close a match to other plate pivot holes, Figure 1.
A faceted pivot hole often results from a dull or worn cutting broach. Cutting broaches need to be reviewed regularly for wear and replaced as needed. These are not lifetime tools but simple cutting devices that wear with use. Smoothing broaches also need touch-up maintenance regularly. As their surfaces become uneven, scratched, or mottled, they must be re-treated (like burnishers—but round) to keep their working surfaces uniform for the best finish. Smoothing broaches, like their relatives the pivot burnishers, should be oiled when used with the same oil types used to lubricate the movement. This reduces the potential for cross-contamination if any oil residue gets through the final cleanup processes.
Expecting a cut but unburnished hole to lap itself smooth is also incorrect. A newly cut hole should be cut and then smooth broached to the exact size needed. A hole cut to size but left unburnished will wear more quickly, as the particles from the ensuing lapping process add to the slurry and cut the new bearing more quickly out of size!
Pivot fit in its hole (sideshake) has several variables to consider as the new hole is sized. The general reference has been a 7º motion in any direction from straight up. This is more of a guideline rather than a hard-and-fast rule. Consider plate thickness, proximity to the main power source, solid- or lantern-pinion engagement, tooth quality, temperature, etc. If the teeth are stubby (or long and thin), extra sideshake can cause poor depthing during performance. With lantern pinions, there is more room for variation. When these types of movements are under load, they can have a subtle flex in the plates that causes the clock to bind—even though when not wound, the sideshakes look fine. When close to the power source, a bit more sideshake may be desirable to ensure the best power flow. If the plates are thick (e.g., Herschede clocks), an allowance must be made for just a bit more sideshake. If the bearings are too tight, it may cause the movement to bind up under load. Ever had a clock or watch leave the shop in the winter and six months later was a come-back? How about the reverse of that problem—summer to winter? For reasons like these, it’s always better to get depthings a bit close and then open them up as needed. A loose hole that, under load, still keeps the depthing on line of centers is still acceptable and quite functional.
One last point, if the plates are flat and the bush work is perfectly flush, the endshakes should be adequate as was the original. However, always check the work, as plate deformity during bushing installation can cause a loss of endshake, and the product can bind up or fail to run properly.