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Y14.8 4.2 APPLICATION

Y14.8 4.2 APPLICATION

(OP)
4.2 APPLICATION

Orientation and location controls require one or more datum reference frames established by datum targets or datum features (see Fig. 4-1).

Dimensions may be directly toleranced or indirectly toleranced in the case of basic dimensions and geometric tolerancing. Where directly toleranced dimensions are used to locate or orient features, and it is necessary to relate the dimensions to a datum reference frame, the desired order of precedence shall be indicated by a note such as

UNLESS OTHERWISE SPECIFIED, DIMENSIONS ARE RELATED TO DATUM A (PRIMARY), DATUM B (SECONDARY), AND DATUM C (TERTIARY)

This note is not to be used in lieu of indicating datum feature references in a feature control frame for geometric tolerancing applications.

-----------------------------------------------------
I understand that the note does not allow me to leave off the datum references in a FCF. I have no idea what the note does for me though. Can someone shed some light on what the intent of this note is? Can anyone provide a drawing that utilizes this note and explain what it accomplishes?

Thank you!!

RE: Y14.8 4.2 APPLICATION

It pertains to dimensions that have nothing to do with a feature control frame. Think about length, width, and height: Without that note, those three dimensions would be measured independently; thus the corner angles could be anything -- what nominally should be a square might be a rhombus or parallelogram and it would still pass.

With that note, though, the three dimensions would have to be measured while fixtured on that datum reference frame, thus the corner angles (orientation) would be maintained within a tolerance.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Y14.8 4.2 APPLICATION

(OP)
Thank you Belanger. That is what I thought it meant but couldn't be sure. Always nice to have the answer form a certified senior level professional.

Why is this note not mentioned in Y14.5? Or is it and I just missed it? Isn't this pertinent for non-castings as well?

RE: Y14.8 4.2 APPLICATION

That was a good question. Some questions are more valuable than the answers.

So, could we extend this practice into non-molded parts?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: Y14.8 4.2 APPLICATION

Quote (AndrewTT)

Why is this note not mentioned in Y14.5? Or is it and I just missed it? Isn't this pertinent for non-castings as well?

That depends on which version of Y14.5 you are talking about.

If Y14.5M-1994, it is specifically stated (although using very small font wink) in para. 4.4.

If Y14.5-2009, it is indeed not there. I guess (but it is just a guess) it is mainly because of the general approach the committee took, which essentially boils down to a statement that: "directly toleranced dimensions are primarly resereved for controlling size of features". Any other characteristics, like orientation or location, should be controlled through geometric tolerances. Whether they did or did not follow this approach throughout the entire Y14.5 document is another story.

RE: Y14.8 4.2 APPLICATION

(OP)
So what is your opinion on using this note in a drawing prepared in accordance with ASME Y14.5 - 2009?

What is your opinion on using this note in a drawing prepared in accordance with ASME Y14.5 - 2009 and ASME Y14.8 - 2009?

RE: Y14.8 4.2 APPLICATION

Personally, I do not like this type of note, but in the same time I do not want to say it should not be used at all.

Also, by no means, I am not trying to say its usage is illegal - paragraph 1.7 of Y14.8-2009 is pretty clear, in my opinion, that in case of any conflict between the text of Y14.8 and any standard referenced in that paragraph, Y14.8 takes precedence.

But, if you for example take a look at the top picture of fig. 2-1 in Y14.8, I am having hard time to understand why, instead of using profile of surface callouts referencing to A|B|C, dimensions like 71.6, 35.6, 2X 50.8, 9.3, 17.8, 22.9, 15.2, which are classic examples of non-feature of size dimensions, have been +/- toleranced.

RE: Y14.8 4.2 APPLICATION

(OP)
Yes, Figures 3-23 and 3-24 are more of what I would expect from a casting drawing. Profile of a surface (all over) seems perfect for a lot of casting applications.

I think they only used that note in figure 2-1 because they were demonstrating the separate views method of drawings and wanted to keep all other things simplified. They had not address profile of a surface yet in this standard.

Interesting that the note appears in figure 3-23. Again I think they just didn't want to clutter this figure with a FCF that had nothing to do with Profile.

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