Roll-formed ACME Screw - should there be cracks on run-out?

[usma82]

We recently outsourced an acme screw with the following specs:

Stainless 303S per ASTM A582
THD.-1-5 NA-2G
Major Dia.- 1.000
Pitch Dia.- .8920
Minor Dia.- .7800
Single left/right hand ACME Thread
Pitch - .200
Thread Thickness at Pitch Line - .100
Thread angle - 29 Degrees

During the first article inspection we noticed 2-3 hairline fractures on the run-out thread of the screws, but after to the last complete thread. The fractures are perpindicular to the thread run, at the crown. These occurances were only on the run-out threads.

When we asked the supplier about these we were told that it was due to a number of conditions - initial hardness of the stainless steel and work hardening during forming. They also said that since the die is not "filled by material" during run-out the incomplete thread at the end of the forming process may have the surface fractures.


From what I've read this seems like a reasonable explanation, however it also seems that it should be possible to prevent this from happening - slower feed possibly?

Can anyone tell me what my expectations should be. The only previous experience I have is with cut threads and this was not a problem on the same part. Thanks

 

[Goahead]

May I ask a question? Are you talking of thread rolling? Stainless 303 is a free machining material, to be used for machined items. Why should you use a free machining material if you are not going to use its specialty?
Could you switch to 302 or 304?
If I misundertood your exposition I apologize, but I do not like cracks, either hairline or otherwise.

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[usma82]

That is correct, roll-formed threads. The initial run of screws we outsourced were machine cut, however the threads were rough and matched poorly to the mating threads. Our supplier offered the alternative of roll-forming to produce a smoother fit-up.

Other than the cracks on the run-out thread the screws work excellent. I too am suspicious of cracks, however the screw is for adjustment and not load bearing. The run-out threads are never in contact with the mechanism, so the likelihood of a thread failure affecting function are moot.

We do not have the option of using 302 or 304, 303 was specified by the customer.

Would an alternative material have avoided this problem?


Thanks for the reply.

 

[Goahead]

Thank you for the details. I would assume that a different material would have a better chance to behave properly.
But to answer your question one should determine by metallurgical investigation if the harline cracks border with the sulfur based inclusions that provide the free machinability to material 303.

http://www.welding-advisers.com/

 

[EdStainless]

Follow Goaheads advise and see if the sulfides in the 303 are, in fact, the cause of the cracking. I suspect that the answer is yes.
What really concerns me is that there may be cracks in the rest of the part, but they have been pushed closed so you can't see them. You need to lightly grind and then acid etch some samples to see if there are cracks anyhwere else.
You could have samples checked with Eddy Current or other NDT.

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Corrosion never sleeps, but it can be managed.

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