17: Replacing an impulse jewel

30 01 2016

Escapement jewels may need to be replaced for a variety of reasons. An inexperienced repairer may try to adjust the position of the detent without first blocking the movement and, as an extra precaution, letting down the main and maintaining power. The result is sometimes that the movement “runs away” and jewels get smashed when the detent is released in a panic, instead of quickly getting a finger on to the fourth wheel. Sometimes a sharp mechanical shock will loosen the jewel, for example, when removing the roller from the balance staff; the jewel is secured in its seat with shellac, which is brittle. Injudicious use of alcohol to clean the parts will soften the shellac, which is soluble in alcohol. Sometimes the jewel will just work loose with handling, as happened to me recently when making a trial fitting to a new balance staff I had made.

Replacement is easy, provided one has steady hands and plenty of vision. A loupe may be enough for some people, but I prefer to use a stereo microscope with X 10 magnification, as the working distance is greater and it is easier to get plenty of light where it is needed. Simple ones are quite cheap on the second-hand market. One will also need a low wattage electric soldering iron and a few flakes of shellac, which may be obtained from specialty decorating stores, or possibly from your friendly local French polisher.

First heat up the iron (the business end is copper!) and with a cotton rag wipe off as much residual solder as you can from the tip of the iron. Then anchor the roller with the tip of the iron. Quite soon, you will see residual shellac start to melt, at which point with a pair of fine tweezers feed in a flake of shellac into the jewel seat. A steady pair of hands and two pairs of tweezers are then needed to set the jewel on its end with the acting face aligned with the radial side of the seat (Figure 1). It useful  to pose the parts on a sheet of thin glass, like a microscope slide. This helps to align them correctly and prevents and plastic on the stage of the microscope from melting.

Impulse jewel 001

Figure 1.

The iron is brought into action again until the shellac melts and begins to run, when, with luck, the jewel can be slid into its seat and the shellac allowed to cool. As Figure 2 shows, the business end of a jewel which is simply being put back into its rightful place should be of the correct size , with its working edge concentric with the circumference of the roller. A replacement jewel may be too long, in which case its height will need to be reduced by rubbing the base on a diamond lap. If too short, it simply has to be manoeuvred into the correct place after softening the shellac with the soldering iron.

Impulse jewel 003

Figure 2

The final step is to check that the working face of the jewel has no specks of shellac or dirt on it. Excess shellac can be chipped away with, say, the point of a hypodermic needle and a brief wipe with a rag moistened in alcohol will remove loose chips from the roller.

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