Y14.8-2009, 4.2 Application - ???

[AndrewTT]

Can someone please explain to me what section 4.2 is actually saying? Why, or when, is this note necessary? What is it accomplishing? Why is this note not mentioned or used in Y14.5-2009? This note does appear in Y14.5M-1994 (4.4 Specifying Datum Features in an Order of Precedence).
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"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."
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Maybe comment on this w.r.t. figures (Y14.8) 2-1, 2-2, 2-3, 3-2, 3-23, 4-2, 4-7, or 4-8. This note appears in these figures.

Thank you very much!

 

[AndrewTT]

daily bump.

Does no one have an answer for this? This note is also in Y14.5M-1994, so if you don't have a copy of Y14.8-2009 you can still look up this note.

Thank you.

 

[CheckerHater]

This is simply one of the many ways of reducing redundant FCFs on the drawing. Don't attach any mysterious meaning to it.

14.8 is dealing with the parts where several features being produced "at once" so there is not always necessary to create elaborate system of interrelated cross-referenced datum systems.

Together with notes like "ALL DIM'S ARE BASIC" and symbology like "ALL-OVER THIS SIDE OF PARTING LINE" the note helps to keep your drawing clear.

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

 

[pmarc]

AndrewTT,

If you read the Section 4.2 once again from the beginning you will notice that the role of this note is to relate directly toleranced dimensions (which by default are unrelated to any datums) to a datum reference frame.

Additionally, in that section there is a statement that: "Where directly toleranced dimensions are used to locate or orient features....". In the Foreword to Y14.5-2009, however, it is stated that: "This revision contains paragraphs that give a stronger admonition than in the past that the fully defined drawing should be dimensioned using GD&T with limit dimensioning reserved primarly for the size dimensions for features of size". This could be one of the reasons why the note in question was removed from Y14.5-2009 - locating and orienting features should be done through geometric tolerances, not through toleranced dimensions, regardless of how cleverly they are related to datums.

 

[AndrewTT]

pmarc,

1) If this note appears on a drawing does it have an effect on how you measure a directly toleranced dimension? Do you have to fixture the part (simulate the datums) before you take the measurement? How would that change your measurement? I still don't really see the point of this note. Let's use figure 4-2 as an example. How does this note affect the 34 dimension?

2) Do you think that this note will be removed from the next revision of Y14.8?

3) If I am creating a casting drawing that will bear the wording "PREPARED IN ACCORDANCE WITH Y14.8-2009" does the standard require that I have this note on my drawing?

Thank you!

 

[pmarc]

AndrewTT,

1) Yes, it has effect on how you measure a directly toleranced dimensions. For fig. 4-2, if that figure didn't have the note the drawing specification would simply be ambiguous, as theoretically the dimension could be measured at any angle relative to the datum plane A.

2) I would like to know it, but unfortunately have no idea.

3) No, the note doesn't have to be on your drawing, just like it doesn't appear in each figure in Y14.8-2009. You "just" have to assure somehow that when a directly toleranced dimensions are used, they are specified in unambiguous way (which may not be that easy). But that does not mean I am advocating usage of this note. Quite contrary. As stated in the Foreword to Y14.5-2009, I am much bigger fan of using geometric tolerances wherever possible.

 

[pmarc]

Attached snapshot (taken from 2nd edition of A. Krulikowski's "Fundamentals of GD&T Based on ASME Y14.5M-1994") should also help to understand difference between datum related and unrelated dimensions.

http://files.engineering.com/getfil...-b3fc-4b49-8d66-cd5456d85841&file=capture.PNG