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6-32 440C Screws, All Thread, etc.

Repeated pneumatic hammering causes it to break.

Sounds like a fatigue problem rather than a yield strength problem. Are you controlling the assembly torque to ensure that there's a high enough preload to keep the joint tight and prevent bending motion?

Are the faying surfaces flat and smooth so that the load in the stud is purely axial?

I did find a company overseas that boasts a grub screw that is CNC milled in-house out of "hardened stainless steel."

Bad idea. Rolled threads have better fatigue resistance than machined threads.

The failure is not in the threads, but at the connection point between the two objects. It snaps off.

It could also be that the screw is being exposed to shear at the parting line between the 2 parts. That would be a recipe for fatigue. If the preload or joint friction can't be increased enough to prevent shear it might help to eliminate threads from the parting line by using a stud like this:
stud.png

I wonder if just counterboring the faying surfaces a little might help to prevent thread shearing and bending. A tight-fitting hollow pin could maybe be put into the recess formed by the 2 counterbores to keep the shear load off the threads. The joint you're describing is a little unusual.
 
I wonder if just counterboring the faying surfaces a little might help to prevent thread shearing and bending. A tight-fitting hollow pin could maybe be put into the recess formed by the 2 counterbores to keep the shear load off the threads. The joint you're describing is a little unusual.
Yes, to both of those suggestions. The other evening, I was thinking about turning a recess on one face and a matching spigot on the mating face. Taking shear loads across threads is terrible engineering practice. Do almost anything to avoid it.
 
I suspect if you got steel grub screws or SHCS from Mcmaster they will be quality made in USA, Holokrome or equal. These are way stronger than stainless steel.
 
Evening gentlemen,

I just wanted to drop back by with an update and thank everyone for the help. After leaving here, I finally managed to get a response from Holo-Krome. Well, actually as you guys had said early on, the original emails went unanswered, but once I called, things started moving and I was put in touch with one of their engineers. I think we're in good shape now.
 








 
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