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Using drawing tolerances as part of justification

Using drawing tolerances as part of justification

(OP)
Hello,
Discussion at work about using drawing tolerances for justification. I'd like to hear your thoughts / pointers to guidance material.

Hypothetical Examples:
An L-section extrusion had gouge damage on the bulb which has been blended out.

1) The dimension of the bulb is less than the drawing minimum tolerance. All the other dimensions have been measured and are WITHIN tolerance. The section structural properties (Fcc, I, A, I/z, etc) for the actual part are acceptable compared with undamaged section AT MINIMUM tolerance.

2) As above, except other dimensions have been measured and are BETWEEN NOMINAL AND MINIMUM tolerance. The section structural properties for the actual part are still acceptable by comparison.

3) The MoS from the stress report is 0.00, based on nominal dimensions.

Cheers,
Tim

RE: Using drawing tolerances as part of justification

For our stress analysis if the thickness of a part is below about 0.050" we have to analyze the part with a reduced thickness that considers a number of factors but is essentially about half the machining tolerance. So a part that is 0.040" +/-0.010 is stressed for 0.035". Above 0.050" the nominal thickness is used in the analysis.

Essentially the assumption here is that it is unlikely that a part will be machined with all dimensions at minimum tolerance and if that did happen you have your ultimate factor of safety to cover that scenario. This is somewhat analogous to B-Basis material properties, it is possible that any one part may have a tensile strength less than the B-Basis properties and that possibility is covered by the ultimate factor of safety.

Thus comparing actual section properties with section proprieties based on minimum drawing tolerances to show a damaged part as OK is not acceptable as you are purposefully and knowingly degrading the factor of safety compared to the analyzed condition and I would say that neither of scenario (1) or (2) above are acceptable considering a M.S=0.00

Comparing actual section properties to nominal (or as analyzed) section properties is an acceptable approach to show a part good by comparison.

RE: Using drawing tolerances as part of justification

It depends on how conservative you want to be. Some (most?) OEMs use nominal dimensions, some will (no doubt) use mininium tolerance, some (like Stezza's) use something inbetween. Some might even consider the wear/corrosion tolerance.

RE: Using drawing tolerances as part of justification

TMcK....

IF this is for a new design/analysis then the organization will have established analytical standards... and will [usually] be validated by FEM and by testing.

IF this issue is for a unique damage-REPAIR then the extent of material loss from NOMINAL will have to be adjudicated based on revised geometry. In general... conservative, in all aspects, is better... unless there is a significant MS [in the original analysis and verified by testing]... when 'shortcuts/simplifications' can be taken.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

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