Stub ACME Thread Form Limits

[Sircrashalot]

By defination the stub ACME thread form has an included 29 degree angle. Does anyone have any experience in rounding slightly the upper crests of the thread to break the resulting shart corner?

How much rounding can you do and still conform to the standard?

Have male thread in F51 running in a female thread in 4130 (much softer). Concern is that the sharp edges of the male threads will bite into the female if there is any misalignment. Suggestion is to round the threads slightly to break the sharp corner, but QA has concerns that this will deviate from the standard thread form and will bear certification issues.

Thanks,

Andy

 

[KENAT]

I'd get a copy of the thread form standard and take a look.

I haven't looked at it for several years (and when I did it was in the UK version) so can't remember off the top of my head. However I'd expect their to be limits defined, if not explicitly then implicitly.

What we did do on ACME threads to stop problems from misalignment is remove the partial lead in thread until a full thread form was achieved. This was on a male thread, I can't remember what the mating female thread had as we didnt' design that part, on a large locking ring.

 

[metalonis]

You definitely have a legitimate concern. We've experienced a lot of problems with ACME threads not having the corners sufficiently broken. ASME/ANSI B1.5 shows an "optional" 45° chamfer of 0.0945·P max. I recommend you exercise that option.

 

[btrueblood]

Metalonis, the chamfer is only allowed for the ACME standard thread form, not for the Stub ACME form, at least according to Machinery's Handbook. That said, we all know that good shop practice would be to break that edge, the question is how much is allowable. Kenat is right, get the spec. and read it. In lieu of any specific limits noted there, I would use a chamfer whose max. dimension is reduced by half of what is allowed for the full ACME thread, to avoid losing too much contact area on the shorter stub thread faces.

 

[israelkk]

FED-STD-H28/13A Title: SCREW-THREAD STANDARDS FOR FEDERAL SERVICES, SECTION 13, STUB ACME THREADS

Free at

http://assist.daps.dla.mil/quicksearch/

 

[mrainey]

The information I found when researching my thread data calculator shows max chamfer values of .0945P for General Purpose Acme, but the chamfer is optional. Centralizing Acme have a mandatory chamfer - min of .0707P, max of .0945P.

The values refer to the actual flat width.

I didn't see anything in MH about chamfers on Stub Acme.


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