Repairing a mainspring

This is in an old camera actually. There's a spring that is 'charged' by
winding the camera and flips up the viewfinder mirror when taking a
photo. I seems that the loop end has sraightened out. The spring fits
inside an open-ended barrel with a small pin on one side around which
the loop must wrap. Except there's no more loop left.
I'd like to re-bend the end of the spring into a new loop*. But what's
the proper procedure for re-tempering the steel once bent? Keep in mind
that this is rather small part (about 1/8 inch wide) and I din't have
any jewelers brazing equipment. My torch will get the whole thing
glowing red in a fraction of a second. The gas stove might be better.
*The spring isn't timing critical. And there's enough of it so I could
cut a quarter of an inch off and start over if necessary (if I screw
Reply to
Paul Hovnanian P.E.
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Springs are not usually so hard they can't be bent cold at least once. That's often how they're made in the first place, with music wire anyway.
Reply to
Don Foreman
I've found with flat spring stock if I heat it dull cherry red and let cool slowly I can bend the loop if it is not too tight a bend - and bending it when cherry red almost always works. Then heat and quench to harden/set the shape - then draw the temper - a hot oven for 10-20 minutes, then quench works reasonably well for recoil starter springs.
From an old text book:
Spring Tempering.-The method employed in hardening a spring in oil is as follows : First, heat to a cherry red as in hardening in water ; cool all over in oil; hold over the fire until the oil upon the surface blazes. This is called "flashing." Cool again in oil. This "flashing" is done three times before the process is complete. Another method of hardening a spring employs water instead of oil. Pass the spring over the fire or through a flame until it is hard enough to make a pine stick show sparks (???????) ; then cool in water and a spring "temper" results.
Reply to
as with anything for the first time, you need to run a trial with another piece of iron wire the same dia to work out the best way of heasting just the end of it. Do try your gas stove, or any small gas flame ,barbeque? or an oil lamp or even a candle. That is surprisingly hot for a say 1/2mm thick wire. then when you have worked out your technique on the scrap wire only then do it on the real piece. Old jewellers/silversmiths trick. Ted Dorset UK.
Reply to
Ted Frater
Nor wire. Flat ribbon. But I get the idea.
I also figure I'll try to treat just the loop end rather than risk buggering up the entire spring. I should be able to heat sink the majority of the spring by holding it in a pair of vice grips and shielding it from the burner with aluminum foil.
The object is to 'set' the new loop raher than maintain its spring constant. As long as I don't screw up the rest of the spring.
Reply to
Paul Hovnanian P.E.
For this sort of thing I go to clock and watch repair books. Very similar to repairing a mainspring on a pocket watch. They used to embed the main portion in a chunk of potato for protection and just use an alcohol flame with a blowpipe to heat the end they were interested in. If it were thin enough, they could dispense with the blowpipe.
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