huangyhg

basic dimensions and first article inspection
does anyone know of a standard, specification, or industry best practice reference that addresses basic dimensions in first article inspections? i've checked as9102 and haven't found any guidence.
the question is whether the basic dimension must be referenced in the fair (first article inspection report), or if the verification/reporting of the locattion of the feature (controlled by the tolerance in the feature control frame) is sufficient? example provided:
drawing req't: hole with true position of .005 rfs, basic dims are 1.000 and 2.000 from the planar xy datums (rfs).
fair: true position reported as ".003 rfs"
i contend that the only contractual requirement is that the true position is reported, since it's value is calculated based on the reference datums and basics, even though general "best practice" would be to report the measurement of the location used to calculate the location of the feature. looking for hard industry standards though.
basic dimensions are theoretical and thus should not be reported.
some companies have reported basic dimensions though. they would report that the basic dimension of 1.000 is basic 1.000, as an example. others have reported them without a tolerance and the results in brackets such as a reference dimension. if a hole was out of position, it would reflect the direction of the actual hole location.
dave d.
only the true position needs to be reported but i have never received an inspection report that did not have the basic dimensions reported as they have to be measured to compute the true position.
basic dimensions themselves should not be reported. the associated geotol is all the information required. but, i think you guys are right about it getting reported.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
i have the same problem i did explained to our customer why i don't need to report those basic dimensions, but they want to see a spec. or any documentations. any luck finding anything?
nope. never found anything -- only a bunch of opinions. so, the way i'm working it is that recording measurements to the basics is best practice. however, until i find a spec or formal industry wide guidence....it aint a requirement.
usually, the requirements are what the customer says they are. in places where i worked w/ qs9000 customers, the inspection and reporting formats were part of customer's plan requirements. usually they wanted measurements included w/ true position calculations.
personally, i would want/expect to see dimensions recorded as a matter of record. the final position is the result of a calculation, but the measurement is the raw data and should be preserved.
it does need to be understood that parts are accepted/rejected based on the true position, not on the measurements alone.
at some cases measuring these basic dimensions would take an extra day or more depends on the part. i have a part here that would prob. take about a week to check those basic dimensions. why would i do that when i have a 3d solid model thats already at nominal. anyway if the costumer ask me again about this issue i will asked him -where does it say i have to record the basic dimensions?- will see how it goes from there.
how can you calculate the true position without measuring the basic dimensions? our cmm machine spits out the values of the basic dims when it prints out the true position calculation. the only exception i know of is when you use a functional (attribute) gage. then it is just go/no go. however, on first piece inspection or ppap submittal i would want actual data to verify the functional gage.
not to point out the obvious, but everything here is opinion. so basically, it's whatever your customer requires at this point. know your customer.....
maybe someone will talk to the as9100 and iso folks for them to provide guidence in their next revision of the fai specs.

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