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Strength when threading into Aluminium

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BillTodd:

--- Quote ---Thats 16 threads, you would break the thread rod pulling those threads out of it.
--- End quote ---
Dave's absolutely right;

Your 'allthread' is likely to be the weakest link. The  thread rolling process creates micro fractures as the material stretches
and deforms (longer the thread more chance of failure).

If you want a strong stud use a rod with a cut thread at each end

picclock:
@ DaveJ, BillTodd

Thats why I said I would feel fairly safe with 16 threads. But really the question is about how many threads are actually needed. With similar materials the 6 thread +2 option in the weblink is pretty much a nobrainer, and lots of information exists like that. But when you combine two metals with such differing characteristics and yield strengths the answer is is far less clear cut.

In a moment of (dubious) :Doh: thought I wondered if the ratio of yield strengths might give the correct answer. Typically the yield ratio for naff steel/Aluminium is around 1:3 so the implication is that 18 threads would be the point at which they would start to let go, assuming 6 threads for steel/steel. 

Many thanks for your thoughts

Best Regards

picclock

andyf:
You only need worry about the weaker of the two - aluminium, in this case. I once read a rule of thumb somewhere to the effect that the thread engagement in Al should be one and a half times the major diameter of the thread. I can't cite any authority for that, and suppose that it depends on whether it's pure Al or a stronger alloy.

In your M6 appilication, it would mean 9mm, and thus 9 threads.

Andy

David Jupp:

--- Quote from: BillTodd on April 08, 2013, 10:58:25 AM ---The  thread rolling process creates micro fractures as the material stretches
and deforms (longer the thread more chance of failure).

If you want a strong stud use a rod with a cut thread at each end

--- End quote ---

I have to disagree on that - good quality rolled threads are considerably stronger than cut threads (especially if fatigue is an issue).  That doesn't necessarily go for "cheap 'n' cheerful" hardware store threaded rod.

High Tensile bolts are thread rolled as much for strength as for cost reasons.

vtsteam:
The 6 threads rule seems a little odd, since it would seem to equate 6 threads for a 28 TPI screw with 6 threads for a 14 TPI screw -- the depth of engagement would be double for the coarser screw and the thread lands twice as wide, assuming they were applied to the same diameter screw.

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