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Threads passed inspection but wont assemble.

Threads passed inspection but wont assemble.

(OP)
Hi All,

I have a 1.625-18 UNEF Internal thread. We have changed supplier and we are now struggling to assemble the nuts onto the corresponding shaft (still same supplier). Some nuts are ok but the majority start to bind after a few turns or need excessive force to turn. The Threads are CNC turned. The supplier simply tells me they are checked OK with a thread gauge.

Any ideas? Clearly by changing supplier my suspicion is firmly on them, but rather than point fingers i'd like to go back with some educated questions.

My understanding of thread gauges is limited, but i understand your really only checking a few things, primarily pitch diameter.

Thanks,

R.



RE: Threads passed inspection but wont assemble.

Shaft and Nuts are the same supplier?
If you have the tooling, check the threads, of both pieces. Check the roots of both for a clean cut.
Ask the supplier if "he" agrees the nuts and shaft threads should engage.
Send both back to "him" for his assessment.

RE: Threads passed inspection but wont assemble.

(OP)
Shaft and Nut is different supplier.
I like the idea of turning the question around and asking them if they think it should engage.
Annoyingly the shaft end (from the other supplier) is ITAR restricted so I cant send them for evaluation.

RE: Threads passed inspection but wont assemble.

What class of thread 2 or 3 or...

Could be work looking at the tips & roots of the thread.

How long are the nuts - standard thread tolerances only work up to a certain limit in length of mating threads.

We've had trouble with 80 & 100 pitch class 3 threads where buying the nut & bold separately tended to result in a lot of them binding. In many cases we went to buying as a matched set.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Threads passed inspection but wont assemble.

(OP)

Quote:

What class of thread 2 or 3 or...

Could be work looking at the tips & roots of the thread.

How long are the nuts - standard thread tolerances only work up to a certain limit in length of mating threads.

We've had trouble with 80 & 100 pitch class 3 threads where buying the nut & bold separately tended to result in a lot of them binding. In many cases we went to buying as a matched set.]

Class 2.
What would I be looking for?
I haven't got the drawing but about 1/ 1.5 inches long.
Your last point is interesting. Never had to buy matched nuts and bolts before, I've always assumed if parts had the same threads they were interchangeable, clearly not.

RE: Threads passed inspection but wont assemble.

Another thought is material, are these stainless steel or other material prone to galling?

WRT what to look at on the roots and crests, essentially dimensional anomalies especially on the roots.

In our situation with matched sets there were 2 issues.

First class 3 doesn't give much room for error being a line fit worst case.

Secondly these are adjustment screws and rather than 'nuts' they were mating to threaded bosses significantly longer/more threads than typical nut.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Threads passed inspection but wont assemble.

The symptoms suggest a pitch error in one or both parts.
Ordinary thread gages are short, so wouldn't detect such an error.
Find a way to measure the actual thread pitch.
Look for a roundoff error in the CNC program that determines the cut pitch.



Mike Halloran
Pembroke Pines, FL, USA

RE: Threads passed inspection but wont assemble.

Since you have two suppliers, they will most likely say "their" threads are perfect, it must be the other guys threads.
That will go on forever if you let it.
Most expedient method to resolve it is to check the threads yourself (your company). If you do not have the capability you need to find someone that does. If you can only check one side (male of female), check that to establish you are certain it is "good or not good". Send the other half back to the supplier and be ready to defend the "good" half.

RE: Threads passed inspection but wont assemble.

The reason for having a matched set of master thread gauges is to ensure vendors machining different parts of the mating thread connection will produce components with the proper fit. The vendor machining the external thread should make it fit with the internal master thread gauge. And the vendor machining the internal thread should make it fit with the external master thread gauge.

RE: Threads passed inspection but wont assemble.

What I've seen before is that what a company calls a "gage" is something they made themselves.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

RE: Threads passed inspection but wont assemble.

R1chJC,

Maybe you need to inspect the incoming items with something better than a ring and a plug gage. Try something along the line of what is offered at this company. Link

John H. Dunten, CD
Certified Drafter

RE: Threads passed inspection but wont assemble.

As a minimum, you should specify class 3 for your 1.6250-18 UNEF thread joint. And then include a requirement for each mating part to conform to a common set of master thread gages.

You also noted that the engaged thread length could be up to 1.5", or almost 27 pitches. You might consider reducing this engaged length to less than half that amount.

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