I'm repairing an old musical instrument and need to replace 4 small torsion springs. Measuring one of the unbroken springs gives: wire diameter 0.5mm, 2 1/4 turns, inside dimeter 3.5mm, one arm about 22mm with 2 small bends in it, the other arm is about 11mm with 1 bend. I'm planning to take a naive approach and simply wind some piano wire around a 3.2mm mandrel, add the bends and then bake it at 250C for an hour or so. Will this work? How do I hold the start end against the mandrel?
Winding a spring from piano wire of the appropriate diameter should do the trick, it is unlikely that you will need to heat treat it. Do not try to b e too economical with the initial wire length. Use a long enough piece to get a good hold on it at each end and try to keep some tension on it as you wind it. There are a number of designs of spring winders marketed to the instrument repair trade. While not worth buying for 4 springs, looking at the instruction pages for them might give some clues for your application, try having a look at Votaw Tools springwinder which has a video and manual for their tool:
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In the UK you can buy spare parts for many instruments from Dawkes Music http://www.dawk es.co.uk/index.php. They list springs and spring wire under repair materia ls and formed springs under spare parts. Their site has good pictures of t he spare parts so if your particular brand/model isn't listed have a look a t other makes to see whether there is something that looks similar that mig ht be adapted. I have always found them to be very efficient and helpful.
need other items such as reeds, lubricants or cleaning brushes if you deci de to place an order with them.
I just have a hole in my mandrels, but that wouldn't work for the arms. What about just tapping a small hole in the side of the mandrel for a screw? Might make a washer shaped to capture the wire if needed.
Springs are tempered to a higher temperature than cutting tools in order to avoid the brittleness that comes with the cutting tools' hardness. There's a
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