Datum targets and their relationship

[Andera]

ASME Y14.5-2009 or 2018

Is there any requirements for datum targets to be related (orientated or located) to its higher precedence datums?
Searching for applicable and relevant figures in the 2009 and 2018 standards, I cannot find a single figure where datum targets are used AND the relationship between primary and secondary are shown with geometrical relationships (callouts).

 

[pmarc]

Assuming the committee deliberately decided to leave a perpendicularity callout wrt A on datum feature B off of the fig. 4-53, I guess the reason might be that this common way of defining the orientation relationship between primary planar and secondary cylindrical datum features would lead to non-repeatable setup of the part against the datum target simulators A1-A3 and create a need to modify the figure to address the issue.

 

[greenimi]

...and create a need to modify the figure to address the issue.

pmarc,

I would agree with your statement up to the quote above.
What do you mean by THE NEED to modify the figure?

I guess the reason might be that this common way of defining the orientation relationship between primary planar and secondary cylindrical datum features would lead to non-repeatable setup of the part against the datum target simulators

What is the difference between what you described above and the candidate datum set / rocking datum feature-default per 1994/ 2009?
Wouldn't be the same concept? I would say it would.
So, I still don't think the reason you described above is good enough[/u] or at least in my humble opinion, the reason does not hold water.
But for sure you know how the committee works.

Therefore, I don't disagree with you pmarc, I am just trying to get more details, if possible.

 

[pmarc]

greenimi,
Just to clarify, I didn't describe the rocking datum feature scenario. I was thinking about a situation when there is no control over how the part should be located relative to the set of datum target simulators A1-A3 for the measurement of the perpendicularity tolerance wrt to A and the relationship may be completely different for the measurement of the geometric characteristics referencing other DRFs like A|B or A|B|C.

 

[chez311]

pmarc,

Seems like that could be solved by adding the note at the end of Y14.5-2009 para 4.24.14 and shown in Y14.5-2018 fig 9-16, right?

Regardless, do you believe that the application of datum targets establishes any kind of implied or indirect control on the location/orientation of their associated datum features?

 

[pmarc]

chez311,

Yes, seems like the note was designed for that purpose, although personally I have always struggled with the wording it uses, and the lack of pictorial example in the standard did not help either.

No, I don't think that the application of datum targets establishes a control between datum features. This should be established by tolerances.

 

[greenimi]

pmarc,

At least you are consistent with what you said 2 years ago... in the above referenced thread.
"This isn't really a question about datum targets. Regardless if datum feature B is referenced RMB or MMB and regardless if secondary datum axis is derived from datum targets or from the entire feature, it shall have a perpendicularity control relative to A, othwerwise the feature won't be fully defined."

At least to me your today's quote "No, I don't think that the application of datum targets establishes a control between datum features. This should be established by tolerances." is in aligmnent with your general idea of having to have an estblished a relationship between datum features regardless if datum targets are used or not.

Is my understanding correct?

Even, if you said yes for the question (Is my understanding correct?) the lack of pictures is still puzzling, at least to me.

 

[pmarc]

greenimi,

My answer to your question is yes.

 

[chez311]

greenimi,

In light of this - do you still believe some ambiguity exists about any implied/indirect controls? I don't see anything presented here which contradicts my initial statements.

 

[greenimi]

In light of this - do you still believe some ambiguity exists about any implied/indirect controls? I don't see anything presented here which contradicts my initial statements.

I would like to thank you for your contribution in this discussion. I have no further questions (at least not for now).
I had to re-read your statements mentioned above and I think I do understand what was your stance in the begining. Thank you chez311

 

[greenimi]

pmarc,

I forgot to say THANK YOU to you too.
Based on your input and with your help, I understood better the Y14.5 current statements (regardless if there is room for improvements or everything is crystal clear)

 

[pmarc]

No problem, greenimi.

 

[chez311]

greenimi,

Of course, always a pleasure discussing the finer points of GD&T with you. And pmarc as well of course!