The Mariner's Chronometer

A friend from Norway recently asked my advice about a chronometer which had been damaged in transit to him. It seems likely that one or both balance staff pivots have been broken. If so, he will need to access and remove both end stones and hole stones, to remove any bits of the pivots and to check the hole stones for cracks and chips.

The upper stones are easy. He just needs to block the movement and remove the balance cock. However, to access the lower stones, he will have to dismantle the chronometer and to do this the main and maintaining power springs MUST be let down. As someone who pulls apart chronometers and clocks from time to time I have made a proper let down tool that will fit winding squares of various sizes, but it is scarcely worthwhile to make such a tool for one job.

I have viewed a video on the internet which shows a repairer using the chronometer key itself to let down the spring while holding the movement between his knees. He does so successfully, neither bruising his fingers on the key as the spring runs away from him nor dropping the chronometer on the floor. The key can be used with less risk of discomfort and disaster, at the expense of a little woodwork. Figure 1 shows a sketch of a simple key holder. I have not given dimensions, but the octagon is about 40 mm across the flats.

Figure 1: Key holder sketch.

Start with a piece of wood about 100 mm long and 40 mm square, and cut a slot about 30 mm deep in it, the same width as that of your key. For the Soviet MX6 chronometer, this is 3 mm, while for most others it is 2 mm.Then drill a hole across the slot. For the MX6, which has a 4 mm hole in it, you can rest the key on a flat and use it as a jig to position the hole for a cross screw. It does not need to be tapped for a screw as a screw passed through the wood and the hole in the key will suffice to keep the key in place. For others, you will need to tap the hole in the wood for a suitable screw to hold the key firmly in place, though if you drill the hole a little larger than tapping size, the screw will usually make its own thread if the wood is fairly soft. This won’t work with hardwoods like mahogany, oak and teak.

Now plane the corners off the square piece of wood  to make a regular octagon. It helps to mark out the octagon, but it does not have to be exact as long as it looks correct. To finish, sand off sharp corners, and stain and varnish the wood. In my stock of stains, I found an old can of stain  labelled “Nordic teak”,  which I thought might be just right for my new Norwegian friend. but when I considered the place of origin of teak, I thought again… Figure 2 shows the finished object with an MX6 key in place.

Figure 2: Finished let down tool.