basic dimensions and first article inspection 1

[kelroy]

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.

 

[dingy2]

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.

http://www.qmsi.ca

 

[dgallup]

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.

 

[fcsuper]

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
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[fazekas]

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?

 

[kelroy]

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.

 

[TheTick]

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.

 

[fazekas]

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.

 

[dgallup]

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.

 

[kelroy]

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.

 

[KENAT]

http://www.draftingzone.com/guides/

If you have a membership, ours lapsed.

KENAT, probably the least qualified checker you'll ever meet...

 

[fcsuper]

Kelroy,

That's actually not so true. The contract/PO is what determines the requirements. If the matter was not addressed in the contract specifically and the standards (ASME or ISO) are called out on the drawing, then the standards become part of the contractual agreement. The standards do very clearly establish the basic as just that:

1.3.9. Basic dimensions are "the basis from which permissible variations are established by <em>other</em> tolerances on other dimensions, in notes, or in feature control frames."

The true value is determined by the GeoTol. It is the GeoTol that gets measured against, not the nominal. I used QPlus and other places for out-of-house first article measurements. They've never reported on a basic...only on the GeoTol itself.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[MintJulep]

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"

That doesn't tell you EXACTLY where the feature on the FAI part is does it? It only tells you that the feature was located somewhere within a zone that was smaller than necessary.

So that report is no more useful than a report that simply states "conforming".

 

[PSE]

MintJulep,

At the risk of deviating from the intent of this thread, I would disagree with your last statement. I find variables data significantly more useful than attribute data. With variables data you can begin looking at process capability, trending and potentially control. Something not easily achieved with attributes only. In this instance it indicates that the feature fell within a smaller zone than the specification ie. "conforming". It was not perfect but it was not at the tolerance limit either. If I had seen it at .005 (still conforming), I would still be a bit concerned.

I realize on the basis of First Article, it may not mean as much. However the variables data from a FAI part can help establish the initial baseline from which to watch the process evolve.

Regards,

 

[HGMorgan]

Hello,
A FAIR only needs the True Position result to show if it's in tolerance.
The CMS report that the FAIR was created off of will show the Basic Dim values.

Consider a product with 100 holes all normal to contour:
1. The CMS report will show the Diameters, XYZ location nominals, actuals, devations and out of tolerance conditions.
2. The FAIR will call-out: 100 ea .250 Dia Holes .010 True Position Dia ck up to .0078 TP Dia.

A FAIR has to show all the "toleranced" dims or features and address them. BASIC DIM's have no tolerance. The need for BASIC DIM values is addressed by the CMS report, which has to be saved by the Inpection Organization for a minimum of 2 years, and a maximum as determined by contract.

 

[MintJulep]

PSE,

A report of one datapoint does nothing for SPC. Otherwise I agree with you.

Kind of makes you question the true value of a First Article Inspection. At the end of an FAI all you really know is that the vendor was able to make one good part.

 

[MintJulep]

I would also note, that - dependent upon manufacturing process - knowing only the zone that the feature lies in, and not the actual position, or trends in each direction, may not help you solve the specific process problem.

 

[fcsuper]

MintJulep,

BASIC dims have no tolerances by themselves. They are nominals. Measuring their location directly violates the design intent and provides no useful information within the context of their design intent. A direct measurement of a BASIC dim does nothing to say if the feature is in tolerance. If one feels it is necessary to measure directly, they need to invoke some other method of tolerancing instead of BASIC.

If GeoTols are needed and do not provide enough information when measured, then the callout is either incorrect or incomplete; or the measurement methods are incorrect or incomplete. If GeoTols capture the design intent, then you will get enough information from them in ways that are much more powerful and useful than direct measurement can provide.

When someone believes they need a direct measurement of a BASIC dim, they may be thinking in linear terms that may not be the actual design intent.


Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[TheTick]

The problem with not having dimensions on FAI is that if there is a feature out of tolerance, there is no data for making adjustments. As useful data goes, a simple position error does nearly nothing to help with process control.

The PC-DMIS reports I see always have the measurements included with the resultant position.