Vintage WorkshopServices for Brough Superior motorcycles and their contemporaries
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Vintage Workshop Services for Brough Superior motorcycles and their contemporaries |
A tricky repair to a SMITHS chronometric speedo head
| Recently, I bought a Smiths speedo head on ebay for my project. Or
rather, one to go on my MX 80, which is still bearing an instrument with a
wrong calibration constant, reading about 8% high.
I just love these little mechanical marvels. And I had, so far, not come across one that needed more than cleaning and oiling and some cosmetic attention. |
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| Unfortunately this one proved to be a lemon. A peep into the bulb holder hole told me there was a problem with the balance wheel. Opening the case took me the best part of an hour, as the bezel was horribly tight, and there was a dent in it, which went right through into the thread on the shell. With the movement on the table, the problem was obvious: |
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| Now that would not have been so bad, but either somebody had tried to
put it back by poking into the bulb holder aperture with a screwdriver, or
the whole unit has been dropped hard. Whatever the reason, the balance staff was bent on one end and broken on the other. |
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| I took the balance to pieces and made a drawing. Mind you, these are millimetres... A pretty small bit to manufacture, but just imagine the dimensions of the corresponding component of a wristwatch!! |
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| I did not fancy turning such a small bit on my lathe. And besides, I
would like the pivots to be hardened.
So I decided to grind it from a 1.5mm drill shank on my lovely little Deckel S0 tool grinder. This is a pic from grinding one of the pivots... |
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| ... and here, the other one and the tapered portion.
I found the diameter of the taper needs to be accurate to 0.01mm, otherwise the pressed-on balance wheel and the escapement bit will not go on or it will be too loose. |
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| This is the finished balance staff and the bits to go on it. |
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| An this is the assembled balance unit. I just wonder if the very hard HSS material will be too brittle. Experience will tell..... |
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| I put everything back together and the movement went just fine!
Altogether I spent a lot of time, but I learnt a few lessons. And I got into some websites dealing with watch and clock repair, and I can only say I have a deep respect for these people! see e.g. http://www.abbeyclock.com/
or |
- sorry pic missing |
Any kind of feedback to
is
appreciated
(sorry, this is not a clickable 'mailto:' hyperlink. If you want
to write me, please type my address in your mailer. )
