Countersunk Blind Rivet Flushness Against Surface 3

[bmnovak]

Hi there!

I am working on an aluminum shelter that involves about 55,000 various rivets. The particular rivets I have a question on are NAS1921 rivets, 100 degree flush head countersunk blind rivets. We use various size and grips lengths along with an internal installation spec, heavily referencing MIL-STD-403C but it doesn't specify a 'flushness' requirement or guideline. I also referenced the Huck Unimatic Blind Rivet catalog with no such luck either.

Our internal document does state "The countersunk rivet head shall be flush with the riveted surface within +0.005 - 0.008 inches." However being engineers there is debate whether that ranges from -.008 to +.005 inches or from +.005 to +.008 inches and I can't trace back where these dimensions originated from.

Does anybody know what are the limits of how flush the manufactured head should be against the skin after the rivet is installed or are there any? A referenced document would be great too!

Thanks for any help.

 

[KENAT]

Your internal spec implies that with the riveted surface acting as the 'datum' you can be .005 above it or .008 below it. Who is suggesting +.005 to +.008, are they experienced at applying or interpreting tolerances?

Posting guidelines faq731-376

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(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[racookpe1978]

Seems like an awefully (literally!) small tolerance for a riveted surface in a mill sheet metal process. Is that not a rocket/supersonic jet/aero surface specification? DO you really, really, really need that small a tolerance?

 

[bmnovak]

Kenat, one of the lead engineers is claiming the wording (within +0.005 - 0.008 inches) is ambiguous and could insinuate the tolerance is + .005 to + .008. The existing design is 40 years old so we are up to interpreting these tolerances and specs.

Racook, these are missile command specifications and this unit is part of a missile command system however this is a mobile-land unit. I don't have a choice in the matter for the small tolerances as they are set in stone and are near impossible to change, however with my experience on this unit these tolerances are necessary.

 

[KENAT]

The fact you have your own spec implies that will take precedent over any industry standard or vendor recommendation etc.

So unless you can find a standard that matches your spec and just clarifies ++ or +- then it won't be much help.

Posting guidelines faq731-376

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(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[bmnovak]

Kenat,

Yes I realize that fact of precedence, hence this post to attempt to trace back those values if they exist somewhere. Just simply having other referencing information would give me leverage to updating our own specifications if...more likely when, an issue occurs.

 

[hydtools]

That MIL-STD specifies the countersink diameter tolerance. You have no means to tolerance the 'flushness' other than that resulting from installing a spec rivet into the countersink created with the mil spec dimensions. That old flushness tolerance would be double-dimensioning or require a post-installation process.

Ted

 

[MintJulep]

How are you going to inspect that 55,000 times?

 

[1gibson]

So, that engineer is arguing that one should reject a perfectly flush rivet? That makes perfect sense!

 

[hendersdc]

Star for 1gibson for the reality check of what that tolerance would mean in the real world!

 

[bmnovak]

Hydtools - Correct. That's what my quality group is not understanding, the fact we can't possibly reference two contradicting standards.

MintJulep - Fortunately we only spot check the rivets but there are many we check.

1gibson & hendersdc - Lol