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Floating fastener formula

Floating fastener formula

Floating fastener formula

The floating fastener formula from Appendix B of ASME Y14.5-2009 gives only:
This is the simplest case where one part has the same hole size and feature-relating positional tolerance as the other. But what is the general case where the hole sizes and feature-relating positional tolerances are NOT the same?

H. Bruce Jackson
ElectroMechanical Product Development
UMD 1984
UCF 1993

RE: Floating fastener formula

Worse - it only applies when the datum reference frames of the two parts are perfectly aligned, which is the actual general case.

I suppose it's H1= F + T1 and H2 = F + T2, but then that assumes the fastener is the same diameter for each portion instead of having a step, so H1 = F1 + T1 and H2 = F2 + T2.

Other than that there's an infinite range of ways to shift tolerance for location from one hole to the other with the projected fixed fastener formula being the limiting case.

RE: Floating fastener formula

I don't even think those formulas needs to be remembered. As long as the VC virtual conditions are the same everything can be shifted accordingly/per the design intent.

RE: Floating fastener formula

If H1=F+T1 and H2=F+T2 then does it follow that:

H. Bruce Jackson
ElectroMechanical Product Development
UMD 1984
UCF 1993

RE: Floating fastener formula

Yes - so there's one equation with up to four unknowns. Not exactly useful and only applicable for a fastener with a constant diameter for parts where the datum reference frames are perfectly aligned. For some reason the Y14.5 Committee overlooks assembly tolerance.

RE: Floating fastener formula


I do not bother with the formulae. This can easily be modelled from first principals.

In the floating fastener case, you have a fastener with a maximum diameter, positioned exactly at nominal. Your holes must not intrude into that diameter. There are multiple solutions to this problem.

In the fixed fastener case. your keep-out is the maximum diameter of your fastener, plus its positional tolerance. Again, there are multiple ways to not intrude into the space.


RE: Floating fastener formula

Keep reading that appendix. There are more general cases with differing tolerances further into it.

Jason C. Wells

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