Making a plug for the broken end of an extension spring

The same neighbor with the broken recip saw has a broken 7" angle
grinder that he'd like me to fix for him.
It's the switch on the grinder that's broken - the return spring on the
trigger has a broken end (it's an extension spring with hook ends).
I've checked the usual sources & can't find this one, but I have a good
repair, I think.
The repair is to make a plug with threads to match the coiled spring
profile. The plug would just thread into the broken end & have the
needed hook. The spring has a very high spring rate & extends very
little, so there would be little chance of the plug escaping during
As far as turning the threads on the plug, how closely do I have to
match the profile? Can I just use a standard 60 degree thread (with the
same pitch, of course), or do I have to use a form tool to get a
semicircular thread?
This will a small plug: the ID will be 3/32 & the pitch 24 tpi.
Anybody ever done this?
BTW - the spring is 3/16 OD, 1-9/16 OAL, .044 wire, preloaded - if
someone has a REALLY good source for springs (i.e., much better than McM-C)
Reply to
Bob Engelhardt
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I made the plug - it went off without a hitch & works a treat, as they say.
I made 60 degree threads, kind of. The profile is:
The minor diameter is just the ID of the spring and the major diameter is the ID + .73 * D. Where D is the diameter of the spring wire and the .73 comes from some geometry & trig.
In this case, making the plug had a huge advantage over reforming a coil into a new hook. After the plug was turned I didn't cut it off from the stock it was made from. This left that stock as a handle to manipulate the spring as I reassembled the switch. Which involved putting about 15 lbs tension on the spring.
Thanks for the comments, Bob
Reply to
Bob Engelhardt

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