STOCK, Rule#1 & geometric tolerances

[Burunduk]

5.8.2 Form Control Does Not Apply (Exceptions to Rule #1) The control of geometric form prescribed by limits of size does not apply to the following:
(a) items identified as stock, such as bars, sheets, tubing, structural shapes, and other items produced to established industry or government standards that
prescribe limits for straightness, flatness, and other geometric characteristics. Unless geometric tolerances are specified on the drawing of a part made from these items, standards for these items govern the surfaces that remain in the as-furnished condition on the finished part.

It seems like starting from "Unless geometric tolerances..." (bolded) there is an exception to the exception?
Should I understand that if the drawing has a dimension labeled "STOCK" and there are geometric tolerance anywhere on the same drawing, the tolerances from the standard for the stock item do not apply?
Or maybe my interpretation of the text is pedantic, and the intent was that only if a geometric tolerance is applied to the feature which remains "as-furnished" from the stock item, then the tolerances from the stock standard do not apply to that feature?
Then, if the tolerances from the industry/government standard do not apply, does this mean that the said feature must be fully toleranced, and rule #1 applies to it? If so, does it even make sense to indicate it as "STOCK"? If someone understands this, any clarifications will be much appreciated.

 

[CheckerHater]

There is no exception from exception.

You can identify part as "STOCK" and accept it being "produced to established industry or government standards that prescribe limits for straightness, flatness, and other geometric characteristics"

Or you can apply geometric tolerances (or envelope modifier) and demand part to be "straitened out" beyond regular stock tolerances.


"For every expert there is an equal and opposite expert"
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[ctopher]

It seems to me that there is no tol to stock material. But, if there is a surface on the finished part that is the stock material surface (not machined), it holds to the dwg tolerances stated. A confusing note, one that I would never use.

Chris, CSWP
SolidWorks
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[3DDave]

There is often tolerance on stock metals - it is contained in the material specification, which is where "standards for these items govern" comes in.

However, those tolerances are often dependent on the original mill size/width of the material being produced so if it is then sheared to a smaller size the buyer will have no idea. It is atypical to specify the original width so the designer is left with no firm idea of the tolerance the stock material was accepted as being within.

 

[Burunduk]

All,
As we know from the fundamental rules section, dimensions identified as STOCK do not require a specification of a tolerance (on a drawing of a part where features remain without modification from the stock item). It is so because the tolerances required for complete definition of the part are in the documents that control the stock item variations.

The question is this - when a drawing, where STOCK dimensions are specified, but there are also toleranced features unique to the product, contains any geometric tolerances, does this mean according to 5.8.2(a) that the stock feature is no longer controlled by the industry/government standard for the stock item?
Again;
"Unless geometric tolerances are specified the drawing of a part made from these items..."
If I take the "unless" condition literally, I would say the answer is "yes", the "STOCK" feature in that case is uncontrolled without the regular drawing tolerances. But then, if it has all the necessary tolerances shown, why label it "STOCK"?

 

[3DDave]

Clearly a wordsmith issue. Again, the committee manages to try to clarify a constraint by making a bad rule.

They were so close and screwed it up so badly.

Thanks Burunduk for the keen identification of the flaw.

"Unless geometric tolerances are specified on the
drawing of a part made from these items, standards for
these items govern the surfaces that remain in the as-furnished
condition on the finished part."

Could have been:

Unless geometric tolerances are specified for the
particular feature(s) derived from these items, standards for
these items govern the feature(s) that remain in the as-furnished
condition on the finished part.

 

[Burunduk]

Unless geometric tolerances are specified for the particular feature(s) derived from these items, standards for
these items govern the feature(s) that remain in the as-furnished condition on the finished part.

To me, this suggestion makes much more sense than the original text, and I think this is what the intent could have been.
Still, even with the altered wording, the reasoning behind this conditioning is not totally clear, especially as a blanket statement.

For example, suppose that from whatever reason someone decides to apply a tolerance of position regardless of feature size to a feature that remains unmodified from a steel bar, and the tolerance is specified relative to a machined datum feature (which is a requirement that can be a bit counter-intuitive to the machinist but still achievable and absolutely inspectable). The standards that define the allowable variations for the stock item still control the size and form of that stock feature. Why and how would anything else be the case as long as the diameter of that feature is indicated as STOCK?

 

[3DDave]

The main purpose is to exclude users thinking titleblock tolerances apply to control the envelope or size variation and that flatness and/or straightness, et al, are controlled by some other document. Look, for example, at the Aluminum Association tolerances for extruded shapes.

It's up to the maker to decide if a more restrictive tolerance is to be used to reject stock material.

 

[Burunduk]

The main purpose is to exclude users thinking titleblock tolerances apply to control the envelope or size variation and that flatness and/or straightness, et al, are controlled by some other document.

The purpose makes sense, but the result achieved seems to be the opposite of the purpose.
They made the condition that the industry standards apply unless a geometric tolerance is specified.
So if one specifies a tolerance of position at RFS to a stock diameter, the stock standard no longer applies to it. This leads to the interpretation that title block tolerances do.

 

[3DDave]

The situation is that no one else reads the standard with legal intensity. I also would not expect the special cases you create to be seen on user drawings. It's applied to stock plate thicknesses or rod diameters and the like to tell inspection to not inspect those features to the drawing tolerances; hopefully the stock was inspected for conformance to the standards when it was purchased and hasn't been damaged.