15 Replacing a locking jewel

27 07 2015

A friend recently sent me the detritus from an accident which had been brought about by not blocking the movement and letting down the main and maintaining springs while adjusting the escapement. Because of this, the movement train had “run away”, rotating at great speed. Fortunately, this did not run the escape wheel off its pivots, but the escape wheel, on re-engaging the locking jewel, broke it off, and the detent flexed so violently that the spring broke off close to the foot of the detent. The moral of the story is that you should always block the movement securely before working on the escapement and, as an additional safety measure against forgetfulness, you should let down the springs.

Only the passing spring was salvageable and I was provided with an intact detent with a broken jewel, together with a new jewel, to assemble. This is not a difficult task though it is a tedious one. It is important to have some means of holding the detent securely and I used a small vice to grip it by the base of the horn, where the passing spring is normally attached by a screw. The old jewel was secured by means of shellac and a brass wedge, though the wedge is mainly there to hold the jewel in the right place while shellac is applied. I removed the jewel by applying a soldering iron to the pipe until the shellac started to bubble and then pushing from below with a needle. Figure 1 shows the parts separated.

Figure 1: Broken jewel removed.

Figure 1: Broken jewel removed.

The old shellac then has to be removed from the pipe by means of a drill, 0.8 mm in diameter in this case. The old wedge was a piece of brass wire with a flat filed part way up and was oversize for the new jewel, so after a bit of experimentation, I used a piece of 28 gauge copper wire and increased its thickness by squeezing it between the jaws of a small pair of pliers so that it would hold the jewel in place. This is quicker and more satisfactory than trying to reduce the thickness of a piece of thicker wire with a file. Figure 2 shows the result of using the pliers and is reminiscent of an old bodge of increasing a shaft’s diameter by knurling it to make it an interference fit in a hole.

Figure 2: Copper wire wedge.

Figure 2: 28 G Copper wire wedge.

With the jewel in place, the wire is inserted from below until its end can be seen in the top of the pipe, when it is cut off flush with the bottom of the pipe. There is enough friction to hold the jewel in place, but not so much that the angle of its working face cannot be adjusted to between 8 and 12 degrees of draw. Figure 3 is a drawing showing this for a German “Einheits-Chronometer” (Standard chronometer), from which the Soviet MX6 chronometer was copied and it shows a draw of 12 degrees. The draw is needed to help lock the escape wheel tooth. Without it, the instrument is very liable to trip and let go a tooth  in response to minor shocks.

Figure 3: Escapement drawing.

Figure 3: Escapement drawing.

When you are happy with the angle, the soldering iron can be reapplied and flakes of shellac fed into the pipe to hold the jewel securely in position (Figure 4) Once the shellac has cooled, the wedge can be filed off flush with the bottom of the pipe and any excess shellac chipped off with a needle. I find an 24 gauge hypodermic needle excellent for this purpose, as it has a very sharp bevel.

Figure 5: Securing jewel with shellac.

Figure 4: Securing jewel with shellac.

Re-attaching the passing spring can be tiresome, as the screw is so tiny, though readers who are used to watch repairing will have no difficulty. For others, it is helpful to secure the detent with a blob of Blu Tack or similar to the corner of a wood block to that you can approach the hole with tweezers and screwdriver held at the correct angle (Figure 6). It is helpful too to spread a white cloth like a well-washed handkerchief underneath the work area so that you can see the screw when it is dropped, and the cloth seems to discourage it from bouncing very far. You will of course already be wearing a white apron clipped to the underside of the work bench to catch the screw if it wanders further afield.

Figure 6: Work-aid for re-attaching passing spring.

Figure 5: Work-aid for re-attaching passing spring.

Figure 3 is copied from “Das Deutsche Einheits-Chronometer”, (ISBN 978-3-86852-597-7) a magnificent book full of close-up colour photos and drawings of the instrument from which about 50,000 Soviet chronometers were derived. It is well worth owning even if you cannot read German.

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