Workshop Hints
Time keeping - Positional Error
By Alan Burtoft FBHI
To get the best performance from a wrist watch it is important to reduce positional error . Attempts to overcome this has resulted in the development of the Tourbillon in high quality time pieces. and the Breguet over coil to reduce pinning point error and the imbalance of the balance spring as it coils and uncoils. These are only two of many innovations to reduce positional error to a minimum.
The majority of everyday good quality work will be the traditional jewelled lever escapement. Without going into great depth and theory, here are practical tips to help towards that goal when servicing.
One should not assume that a balance is poised especially if a new balance staff has been fitted at any time.
A quick check on the timing machine will reveal a positional error, but not necessarily a poising error. This will be more evident as the amplitude rises or falls, a poise error cancels out at approx. 210 Deg.
Assuming the escapement has been checked see lever escapement to make sure any error is not caused by a badly adjusted escapement.
To verify a Poising error.
Release the mainspring and place the watch on the timing machine, wind until the balance amplitude is just less than 180 Deg. and, from the pendant down position, rotate in stages of 45 Deg. and record the result at each stage.
If there is a significant gain in one position that decreases to a loss in the opposite position, then there is a poising error . The heavy point will be at the bottom where the greatest gain is recorded.
Check that the error is not caused by the balance spring as it develops unevenly from the centre.
Remove the balance and spin slowly between callipers and observe the centre coils. These should give the impression of a smooth even spiral. The spring should also develop flat from the centre..
Make a small mark on the balance rim adjacent to the stud for accurate re assembly and remove the balance spring. Check for bent pivots and true between callipers.
Using a poising tool , verify the heavy point. and remove metal under the rim with a drill, or the centre of the head of a screw.
Take care before removing weight from the balance as this will make the watch gain overall. If more than a small adjustment is necessary, add weight (timing washer) to the light point if appropriate. Equal weight should be added to the opposite side, .to maintain overall weight .
Replace the balance spring and check for flatness and centering. Re fit balance assembly
With the movement flat on the bench (in a holder) the outer coil can be adjusted with the top shock setting removed so you can see there is no side pressure on the balance staff at rest. This will also reduce positional error.
Curb pin error.
The balance spring should beat evenly and come to rest between the curb pins of the regulator. These pins should be close together so that the spring movement is only just visible but not gripping the spring. Curb pins set too wide will cause a loss at low amplitudes
Re test on timing machine
Hope the above helps you achieve improved results.
Alan Burtoft FBHI.
