Smaller Heli-coils and aluminum

- J
- JJ
  
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posted
13 years ago

Sun, Apr 3, 2011 5:06 PM

Has anyone had to specify or repair smaller threaded holes (8-32) in aluminum that have been heli-coiled? How about the difference better regular heli-coil and the "locking" heli-coil that has the circumference slightly deformed triangularly so there is extra gripping by those 3 sides with the fastener. This sounds like a pretty good idea and I wonder if it eliminates the need for thread locking fluid. There isn't a lot of vibration or heavy load (application is an industrial keyboard/control panel)

The manufacturer seems to have specified heli-coil (1 "diameter" long) as part of the original design/construction because of concern about stripping out more due to the fastener being inserted and removed rather than the load being secured.

I'm curious to know if we'd be better off in this application with a stud going into bare aluminum (no heli-coil at all) and probably using an aggressive thread locking fluid. (red) This way there would be practically no insertion/removal in that threaded hole, just the torque/stress from removing the retaining nut. (probably nyloc)

- P
- Phil Kangas
  
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posted
13 years ago

Sun, Apr 3, 2011 5:43 PM

Threads in aluminum don't fare well in repeated removal. Better to use a stud or at least make an insert from an annealed set screw. I've made quite a number of these inserts over the years and they hold up much better than heli-coils. The smallest ones I've made were for 5 x 0.8 mm. phil k.

- A
- anorton
  
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posted
13 years ago

Sun, Apr 3, 2011 5:56 PM

In the semiconductor manufacturing equipment I help design, Heli-coils are used in aluminum not just to protect threads from damage where a screw is frequently removed, but also to prevent generating particles during fabrication. I have not used the locking heli-coil. However, if a screw is not in a environment with a lot of vibration, impact or temperature extremes, then a locking device is really not necessary if the screw is fully-torqued. A fully torqued-screw is stretched just below its yield point, and the friction between the material and the head and threads of the bolt is enough to keep it in place except in extreme conditions (although what often happens in practice is that small screws are over-torqued and large ones a under-torqued). There are some cases where you do not want to fully torque a bolt, so then a locking device is absolutely necessary.

As for protruding studs versus screws; a stud and nut is uglier and there is some risk the exposed threads of the stud can be damaged. On the other hand, studs can be convenient to hang a vertical panel when fastening.