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School me on 100* countersinks?

no704

Aluminum
Joined
Sep 14, 2017
Location
Tempe, AZ
I’m working on a project that we would like to use some 10-32 and 8-32 flat head machine screws to attach hardware to. Seems like the 100* taper is more common for riveting applications. However we wish to use screws. I have been able to locate screws with the 100* counter sink in plated steel that are 60kpsi rated. Would like to be using alloy screws rated at 120kpsi but am only finding these in the standard 82* counter sink, making the counter sink too deep even with back cut screws.

Any ideas? Thanks.
 
If the countersink is too deep that makes me think your sheet is very thin, in which case the pullout strength is probably a much larger concern than screw strength.

An 82 degree countersink sunk "too deep" so the screw is flush should still have about the same pullout strength due to the same shear area (but may be slightly lower due to reduced cross section of the annulus that could tear).

I've never seen a 120kpsi rivet.
 
Thanks everyone, that’s what I was afraid of. Guess this one might have to go into throw it back at the engineer pile.
 
SS sheet .060” is being held in shear.

If the rivnuts are Al, you might not see any ultimate strength improvement with 120ksi+ fasteners, although drive socket life will be better. But with .200 thickness for the flatheads, that should be thick enough with regular 82 degree #8 and 10 screws to fit without issue - why are you looking for 100 degree fasteners? Is there a Cbore on the bottom of the .200 to clear the heads of the rivnuts?

If these are going into a product exposed to moisture or other corrosive environments, consider using antiseize on all the threads, there's nothing more frustrating than a spinning rivnut due to the screw locking into the nut thread.
 
Riv nuts we have in mind are steel. Attempting to secure a .060 SS plate in shear to a .200 aluminum extrusion.
 
Can you countersink the aluminum piece and then pull the stainless down with the screw? There won’t be a lot of thread left in the .2 thick aluminum so you may have to pre-form the stainless (an air hammer works well)
 








 
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