Common datum feature per Y14.5-2018

[ANGIDDT]

1.) Upper case: Would you consider a valid scenario if LMC is used as shown? Copied from Y14.5-2018, but modified to fit my question.
Datum feature C modified at LMC used together with C(RMB)-D(RMB)
Datum feature D modified at LMC used together with C(RMB)-D(RMB)

2.) Lower case: Same as the upper case, but the common datum feature axis is now secondary (instead of primary). The "green star" in FCF in the lower case could be understood as A=primary and C-D common datum feature=secondary.

3.) Related question (additional), but not depicted: how would you use / show the approach per 7-16/page 101, if the two features that make datum feature A have different size? I think 2X cannot be used, correct? Then how would the symbology works in this case? (I am not asking about the case 10-52/ page 232, here datum features ARE entirely different datums, but about how to show the concept of fig 7-16 when the features are different sizes and ARE datum features)

 

[pmarc]

ANGIDDT,

Regarding questions 1 and 2... Semantically, there is nothing wrong with the two approaches.

Regarding question 3... Just take figure 10-52, add separate datum feature symbols (say, C and D) at the extension of the dimension lines for both datum features, and then replace the composite position callout with a single segment 0 at MMC position tolerance with '2X' prefix in front and additional notation like 'APPLIES TO DATUM FEATURES C and D' beneath the feature control frame. Paragraph 10.5.3.4 offers pretty much the same solution but for features that are not datum features and could be labelled with a single auxiliary letter.

 

[greenimi]

Pmarc,

I want to probe it further a little bit: what do you mean by semantically? I sense/”read” that this approach might not be the right one? You might not agree (even if it’s technically correct) or use it yourself?…..

Why am I asking?
Because I remember discussions here on eng.-tips where some people (extremely knowledgeable in this language otherwise) complained about fig 4-20/ 2009:
- Datum feature A is defined RFS on the perpendicularity and it is modified at MMB on the 4 holes position callout (case c). If “A” works as RFS (press fit, etc.) than why the same “A” works as MMB application for the 4 holes. I guess the case shown in the standard is for academic purpose, but still…. I hope you do understand what I am asking….If not, it’s okay, I will rephrase it.

I think the OP’s applies the same approach.

By the way, regarding the OP’s question 1 and 2 I wanted to respond that the shown cases are invalid (based on inconsistencies in using the modifiers and also based on the fact no such of scenarios are shown in any Y14.5 standards), but I did not want to get in trouble, so I gave up. Thank you for indirectly correcting me!
Regarding question 3 and your provided solution (and I remember we talked about this before too): are you envision 2x position Ø0MMC (with note “APPLIES TO DATUM FEATURE C AND D) as being datumless, correct?

Thank you again for your patience and sharing your knowledge.

 

[pmarc]

greenimi,

What I meant by "semantically, there is nothing wrong with the two approaches" is that the dimensioning and tolerancing language used on these two illustrations does not violate any Y14.5 rule. Whether there is nothing wrong with it from the functional point of view is totally different question.

I don't remember people complaining about fig. 4-20 as much as about approach shown in figs. 4-21 (c) & (d). Regardless, I did not consider OP's questions 1 and 2 as having anything to do with datum features C and D referenced at MMB. At least I don't see MMB anywhere on both illustrations.

 

[pmarc]

Oh, and regarding question 3... yes, I meant position with no datum feature references.

 

[greenimi]

pmarc,

I am very sorry to be thickhead, but here are some quote from this discussion:

https://www.eng-tips.com/viewthread.cfm?qid=447050

Figure 4-20(c) mentions maximum material boundary in relation to the MMB version of the callout applied to the pattern of 4 holes. But that version, as well as version (b), still shows RFS version of the callout applied to datum feature A. That was the point of your question, wasn't it?
Figure 4-20(b) could have shown datum feature simulator A at MMB, but that would be very special case and it wouldn't definitely help in grasping the concept of difference between secondary datum feature referenced RMB vs. MMB

pmarc - We're good. My frustration has more to do with the inconsistency within the standard and between versions of the standard, essentially making it a lose-lose vocabulary test. Between the text and the figures the committee throws in so many faults it's tough to keep each straight.

I guess my point is (and that is why initially I wanted to say that shown OP cases are not kosher): a feature either works as an "MMC-feature"/ clearance fit either as an RFS application (press fit). What could be the physical reality that the same (otherwise rigid) feature could work in one place as MMC and in the other place RFS/ RMB. (perpendicularity for A is RFS, but A is shown at MMB in 4-20 (c))

I am 100% sure you could (and you would) explain it in the layman terms so I can understand....

Thank you again pmarc

 

[pmarc]

greenimi,

While I agree that for the cases where the main (only?) functional concern is assembly controlling a secondary datum feature with perpendicularity RFS relative to primary datum plane isn't the optimal choice (as a matter of fact any perpendicularity tolerance of a value T[≠]0 at MMC isn't the optimal choice either), I am hesitant to make a blanket statement that there couldn't be cases where, in addition to just assembly, the requirement is to not allow the datum feature to have the actual perpendicularity error greater than T regardless of its size.