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The Fixed and Floating Fastener Formulas Are Not Just for Fasteners (#144)

(In accordance with Y14.5M-1994 standard)

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Nearly all assemblies fall into the categories of either fixed or floating features. A floating feature assembly might be a shaft floating within two bushings. Where a part floats between multiple features, the floating fastener formula will usually apply. The ASME Y14.5M-1994 standard gives the floating fastener formula as H = F + T where:

For floating type assemblies, just substitute the internal feature’s smallest size (MMC) for H, the external feature’s largest size (MMC) for F and the allowable positional tolerance (T) may be calculated.

When a screw or dowel pin is assembled, it becomes "fixed" to the mating part. Therefore, the fixed fastener formulas work whenever features are "fixed" on mating parts. These assemblies include bosses fitting into holes, tabs fitting into slots, etc. The ASME Y14.5M-1994 standard gives the floating fastener formula as H = F + T1 + T2 where:

For fixed type assemblies, just substitute the internal feature’s smallest size (MMC) for H, the external feature’s largest size (MMC) for F and the allowable positional tolerance (T1 + T2) may be calculated. T1 and T2 need not be equal and may be distributed to reflect process capability of the processes being used. For the assembly shown below. 

The two tabs fitting into the two slots is a fixed fastener assembly. MMC of the two tabs is 10.1. MMC of the two slots is also 10.1.
Therefore H = F + T1 + T2 or 10.1 = 10.1 + 0 + 0.

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Back to Tips Tip added Jan 2002