Positional tolerance using floating fastener formula

[ashivu123]

Hi I have riveted assemblies which have rectangular coordinate system tolerance. Now I have to apply GD & T to locate holes. When I used floating fastener formula ending up with .001 or .002 Positional tolerance. Is this possible to inspect? Is there any way to increase positional tolerance?

 

[drawoh]

ashivu123,

To increase your tolerances you either must enlarge your clearance holes, or drill (punch?) after assembly.

--
JHG

 

[KENAT]

Rivets are often match drilled - the close tolerance needed for proper rivet engagement work against making the parts separately and then bringing together.

Carefully designed fixtures or punch tooling etc. can sometimes overcome this.

Alternatively you can separately machine with slightly undersized holes then just open them up by match drilling to size at assembly.

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[powerhound]

I agree with drawoh. This result tells me that the clearance holes are only .001 - .002 larger than the fastener. Increasing the size of the clearance hole will give you more tolerance.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[KENAT]

Not a rivet expert but...

I think many rivets rely not so much on the head and tail clamping the pieces together but by the shank swelling into the hole during installation.

I thought this is why the tolerances on rivet holes are often required to be fairly tight tolerance.

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[powerhound]

Yeah, I guess that's a good point.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[ashivu123]

Thanks for all suggestion. currently assembly are match drill. We want avoid assembly drill operation and drilling corresponding hole in part level.

 

[KENAT]

To get back to your initial question, yes hitting .001 or .002 is achievable in at least some circumstances - however it may well cost you.

Before you go much further seems you need to determine for sure how the rivets you are using work - do they grip on the shaft or just the head & tail & I'm talking nonsense. I assume you got the current hole sizes from a standard or manufacturer data sheet or... so that may tell you.

If that same guide or handbook doesn't answer the question, and you can't find it with a web search then maybe take a look at forum725 and ask there.

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