REBUSHING PIVOT HOLES IN MECHANICAL MOVEMENTS

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The most common problem with a clock movement not running after it has been cleaned and oiled is that some of the pivot holes in the brass side plates have worn larger.  (Usually due to movement not being serviced and oiled regularly.)

In most cases the hole has worn to an oval shape, because the pressure of the powered gear pushes the next gear away from it.  In simple terms, all you need to do is bring the pivot hole back to its original size and exact location.  The first move is to identify which pivot holes are worn.  I mark their position with a circle using black permanenet texta.  It is best to take the movement apart and clean out all pivot holes with toothpicks,as the dried up rubbish in some pivot holes does not allow you to see all the wear.  If it is one only pivot hole to repair, you can sometimes part the side plates just in that location, take out the gear wheel and then rebush.

POLISHING PIVOTS

The running of a clock movement requires as least amount of friction through the drive trains as possible.  So, you will notice that the pivots diminish in diameter the further they are from the driving force.  To help reduce the friction, pivots need to be polished to a shiny smooth finish as stated in previous written guide on clock cleaning and repair part 2.  Now measure the diameter of the pivot with calipers (say for instance it is 0.67mm dia.) therefore you would need to use a B04 bush that has a hole 0.60mm dia x 3.00mm high.  To open up a pivot hole in the side plate to fit the bush, a clock repairer would have a 1.97mm dia. bushing reamer to cut this hole.  But with costs of around $30 - $35 each cutter, unless you are doing repairs professionally, they can become expensive.  You would need around seven sizes plus a handle.  An alternative method would be to use a small set of drills to gradually open up the pivot hole, finishing in this case with a 1.9mm drill.  Make sure the hole is located in its original position, sometimes the oil sink can give you a good indication of its position.  Finally the hole needs to be opened another .07mm larger.  For this you use a cutting broach to ease out the hole so that the bergeon bush would just sit in the top of the hole from the outside of the brass plate and be tapped in with a small hammer.  If you think it is not a tight fit use Loctite around the outside.  The oil well on the bergeon bush is on the outside of side plate and the inside surface of the bush should be level with the inside of the side plate.

The reason you will need cutting broaches is you will now need to ream out the pivot hole so that the pivot will go into and turn freely with gear shaft sitting square to the side plate.  Most of the bergeon bushes I have are the longest available.  You may be fitting a 3mm long bush into a 1.5mm thick side plate so what sticks out on the outside needs to be cut down witha drill until it is level with the surrounding side plate, but still remains an oil sink.  The angle of an oil sink is 90 degrees inclusive so it is best to regrind a  drill which has an angle of around 120 - 90 degrees and keep this solely for cutting the oil sinks.           * It is safer to hold the drill in a pin chuck and turn by hand rather than a power drill which may grab the brass and go right through.* 

It is good practice to assemble the drive train you have repaired, leaving out the spring and spin the gears by hand to see if every gear turns freely before main assembly.  You should already have checked that the pivots are not bent and use extreme care when assembling that you do not bend any pivots, as the clock will not run and the fault not detected until you pull apart once again.

In our shop on eBay we sell bergeon bushes, sets of cutting broaches and flat emery buffs for polishing pivots. 

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