Fixing holey mainsprings
You need to refinish the hole end of a mainspring because it split out or you had to shorten the spring. First the end has to be softened and right there the trouble can start. It is reasonably hard, too hard to work on with ordinary tools. No problem—heat it up to cherry red and it will be soft. Er, yes, but... A mainspring is very thin, often 10 thou or a little more except in large clocks. Applying a torch, making it bright red and waiting for it to cool down very often does not do.
The problem is that the spring is so thin that it cools far too quickly and it is not difficult to end up with a spring actually harder than originally, one that will resist making a new hole and certainly break when one tries to make that little bend needed right at the end to catch on to the hook inside the drum. The answer is to withdraw the flame slowly but even this is not easy as there is no colour to be a guide, the spring steel is just black if it is not red.
One very helpful way is to move the flame from side to side, starting by hardly moving the flame off the metal and immediately returning it and then off the other side. Each traverse should be made a little longer always at the same distance from the metal until these quick movements are maybe two inches long. and almost as if one is brushing the metal with the flame.
Now the flame can be moved gradually further away as it continues its brushing action. Within a minute the flame is switched off and quite quickly the metal is cold enough to handle. Now a bend with the pliers will produce a bend and not a break. The metal has been tempered properly or if not properly at least well enough. If it hasn’t taken a good temper just do it right away again. It doesn’t take long and one
soon gets it right.
Ted Wale
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