Thursday, 27 April 2017

A Bluffers Guide to Watch "Bushing"

Three typical Watch bushes.
Where a steel pivot runs in a brass hole in the movement there will eventually be some enlargement of the hole, often turning it into an oval due to unequal pressure on it.

Occasionally there will be some wear to the pivot as well in which case the pivot will need to be smoothed and reduced in size which will result in it being loose in the hole.

Problems are most common on the fast moving and quite small 4th (Seconds) wheel and on the bottom pivot of the centre wheel which although it moves slowly it has a lot of pressure on it and can't be effectively oiled without taking the movement apart - so it seldom gets done.

To remedy this situation a brass Bush or "Bouchon" is set into the movement.

Part of my collection of Bushes, the two large ones bottom left are for
clocks, those for watches in the top box have an external diameter from
0.8mm to 3mm with a range of hole sizes and depths. These are New
Old Stock, the top set is still available but at £270 the set (120 pieces)
First the original hole has to be opened up to accommodate the size of bush required using progressively larger reamers in the Staking or Jewelling set, this will keep the centre of the hole in the right place.

The bush has to be a friction fit but not so tight that excessive force is required to push it in or the bush could be damaged or the movement distorted.

The reamer on the centre hole of a scrapped bottom plate. The metal is soft and the
tool very sharp so it is just turned by hand with no significant leverage required.
If necessary, which it usually is on the bottom plate, the bush has to be ground down to the same thickness as the plate, on the top plate the shaped end of the bush is left as it looks similar to normal pivot holes and helps with oiling. The bush is then pressed in using the staking set.

My staking set with leaver operation, a different anvil than this would
probably be used when pressing home a bush.

The result will be as efficient as the original.

A re-bushed centre wheel pivot hole that I did recently.

Although more expensive, sometimes a jewel will be used instead of a bush resulting in a higher jewel count than advertised:

This Waltham Traveler has been turned into a 15 jewel watch although I suspect
that was because someone wanted to do it rather than to correct a problem.
On occasion you may find the reverse with a broken jewel replaced with a bush, on the centre wheel this is very unlikely to cause any performance issues - European makers did not like putting jewels on the centre wheel in any case as they add little - except for appearance and marketing - and are susceptible to damage.

Bushes used to replace jewels at the 4th wheel and below will not be as efficient as a jewel and will be more prone to wear in the very long term.

A high grade Keystone Howard Series 5 with the top centre jewel replaced
with a bush. The original jewel was machine set and it was probably thought
that enlarging the hole to replace with a friction fit jewel would have left
the bridge dangerously weak. The brass bush however retains its structural
integrity. There is no noticeable increase in friction and the problem was finding
a mainspring that would not overdrive the movement rather than needing more power.

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