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Y14.5 Feature Modifier Default
2

Y14.5 Feature Modifier Default

Y14.5 Feature Modifier Default

(OP)
At the risk of answering my own question, I am looking for some reassurance that I am making the correct interpretation of the standard.
For tolerance of position, beginning with the 1982 revision of ANSI Y14.5M, it is my understanding that in the absence of a material condition modifier in the feature control frame, the default is S (RFS).  Prior to 1982 I believe this default was M (MMC).  Is it safe then to assume that drawings released prior to 1982 will default to MMC and later drawings will default to RFS in the absence of a modifier?  This seems kind of risky.
Thank you,
smr

RE: Y14.5 Feature Modifier Default

Sadly, you can never be sure.  The only way to be certain is if somewhere on the drawing is a general note that states what ANSI release the drawing is made to.  There were 1970 circa releases of ANSI Y14.5, but many companies did not adapt an ANSI standard until the 1982.  Therefore, many used GD&T type nomemclature, but it was tailored to the company and not strictly to the standard.

In cases like this, you probably have to interpret the drawing as a functional requirement and determine whether it would be better to be MMC or RFS.

--Scott

For some pleasure reading, the Round Table recommends FAQ731-376

RE: Y14.5 Feature Modifier Default

swertel is correct.  You can not always know the implied material condition modifier unless you know the issue year for the Y14.5 standard used to create the drawing.  This should be stated somewhere on the drawing (and is required per all versions of Y14.5) but, many times it is not. But the interpretation problem really only occurs with reading a positional tolerance callout.  

The basic rules for implying a material condition modifer have been covered over the years by what were initially defined as "general rules 2 and 3" in earlier versions of the standard.

"Rule 3" covered all geometric tolerances (with the exception of positional tolerance) and defined that RFS is assumed to apply where no material condition modifier is shown in the feature control frame. This applies to both applicable tolerances and/or datum references.  This has been a consistent interpretation for all versions of the standard to date.

"Rule 2" applies only to positional tolerances and defines differing interpretations depending on the Y14.5 standard year of issue.  The general interpretation rules for positional tolerance apply as follows:

ANSI Y14.5-1973
For positional tolerances only, MMC is assumed to apply where no material condition modifier is shown in the feature control frame. This applies to both applicable tolerances and/or datum references.  

ANSI Y14.5M-1982
For positional tolerances only, no implied material condition exists where the material condition modifier is omitted in the feature control frame. MMC, LMC or RFS must be specified for the tolerance and/or all applicable datum features. Omission of the appropriate modifiers constitutes an incomplete specification.

ASME Y14.5-1994
For all geometric tolerances (including positional tolerances), RFS is assumed to apply where no material condition modifier is shown in the feature control frame. This applies to both applicable tolerances and/or datum references.  

GDT_GUY

RE: Y14.5 Feature Modifier Default

To be safe, I would always check with the design authority of the drawing.  Many companies have internal drawing specifications (which should be called out on the print) but which are rarely distributed with the drawings.  My company has always used implied RFS for all GD&T.  We also have a drawing spec that says exactly which (mostly ISO) standards are used to interpert our drawings.  I'll bet 50% of our suppliers don't have a copy of our drawing spec.

RE: Y14.5 Feature Modifier Default

When we come across an older dwg like you are referring to....if there is a doubt, or the dwg is to the old standards, we will correct the dwg immediately.

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