Questions Y14.5-2009

[aniiben]

Q1:
If in fig 4-24, the size of the tolerance zone of datum feature A is changed from zero at MMC to 0.5 RFS (for example), would the new scheme make sense ? I am asking from the fact that now datum feature A is modified at MMC in the position callout for the 8mm hole and RFS for its own positional tolerance. What I am trying to ask is if there is any issue for such a combination where a feature is called at RFS (datum feature A) but in another FCF is called at MMC?

Q2:
Fig 4-42: What is the significance of the tertiary datum feature C in the FRTZF? Otherwise stated, what degrees of freedom C in the lower segment would stop? Would the meaning of the callout be changed if C (again tertiary) is removed?

Q3:
Fig 4-45: If a basic dimension is added from datum B to the apex of the cone, can the datum feature B be modified at MMB in the positional callout for the hole? In other words, I am trying to establish a mutual location relationship to the primary datum feature A and then modify the planar surface B at MMB? Would this approach be valid?

Q4:
The paragraph 4.22 states: ".....(b) the degree(s) of freedom to be constrained by each datum feature referenced in the feature control frame shall be explicitly stated by placing the designated degree of freedom to be constrained in lowercase
letter(s) [X, Y, Z, u, v, or w] in brackets following each datum feature reference and any applicable modifier(s)."

Why the text specifies that the degrees of freedom shall be shown in the lowercase letters, but actually shown as a CAPITAL letters [X, Y, Z]? Is it just a mistake? As far as I understand the axes shall be specified in the CAPITAL letters and not the degrees of freedom. The underlined text is talking about the degrees of freedom -translation and rotation --, correct?

 

[greenimi]

I will try to have some input on Q4 and I am sure the experts on this forum will chime in on the other questions.

4.21 states:
”Where a datum reference frame has been properly established and it is considered necessary to illustrate the axes of a datum reference frame on the drawing, the axes or center planes may be labeled to determine the translational degrees of freedom X, Y, and Z.”

Now, I am self admitting my English is not the best and I am not so sure the sentence cited above means that the axes are X, Y, Z (capital letters) or the translational DOF’s are X,Y,Z (capital letters). I am inclined to say the former.
Anyone with better understanding than mine?

 

[AM Engineer]

Q1

Datum feature controlled with datumless positioning at RFS dia 0.5 as per your inquiry. My answer is that there will no virtual condition boundary. I dont think we can modify this feature at MMB in FCF of dia 8 hole.

 

[greenimi]

Regarding Q1, I found these two discussions that might be helpful

https://www.eng-tips.com/viewthread.cfm?qid=317540

https://www.eng-tips.com/viewthread.cfm?qid=350055

However, I am not sure what would be the correct answer on your Q1 question.

On Q2: I would be tented to say "C" as tertiary in FRTZF is redundant. Anyone else?

Q3: Not valid, but I realize my statement will be subject to scrutiny. Customized DRF with MMB?? Not sure if it is valid. Moreover "B" is a planar surface, hence modified at MMB looks fishy......

 

[axym]

Q1: I don't think that there are any rules or limitations on the boundary condition(s) that datum features can be referenced at in relation to the material condition that the datum feature's tolerance has. A datum feature can be referenced RFS in one FCF and referenced at MMC in another FCF on the same drawing.

Q2: This is a very interesting question, that brings up some subtle issues. I would say that the meaning of the callout would change if the C reference was removed. The C reference helps to constrain the actual part relative to the A and B datum targets. If the C reference was not there, the actual part could translate relative to the A and B datum target simulators and the contact points (and therefore the DRF) would change. Once the ABC datum reference frame is established, then the lower segment tolerance zones can translate relative to it.

Q3: I'm not sure exactly what you mean by "establish a mutual location relationship to the primary datum feature A". I don't think that adding a basic dimension would change anything in the position tolerance. A basic relationship between the apex and B is already defined, if the model geometry is considered basic (no directly toleranced dimensions are specified on the drawing). I would also say that datum feature B could be referenced MMB, but I'm not sure what the purpose of that would be.

Q4: I believe that capitalizing the letters X,Y,Z in 4.22 was a typo - they should be lower case. This was corrected in Y14.5-2018.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[aniiben]

Thank you Evan for your point by point reply.
May I ask a few quick follow up questions:
Regarding Q1: if the features that build datum feature A primary have different sizes is it mandatory to modify the applicable position at MMC and how the true position callout would look like (since 2X preceding the geometric tolerance is no longer be used)?

Ref Q2: I've always been under the impression that C tertiary in the FRTZF is constraining rotation only and since A primary took 2 rotations and B secondary stopped 1, there is nothing left for C tertiary. Why this concept does not apply to the FCF's with DATUM TARGETS? How can I think of datum targets to make more sense?

Ref Q3: I read here on the forum that the only way to legally modify a planar datum feature at MMB is if (and only if) this planar datum feature has a location relationship to the primary, so consequently, in my mind, adding a basic dimension between B and apex made perfect sense. But if you say that the basic dimension is already there by default then I would ask what about the customized DRF? The translation along the cone's axis should be stopped by B and not by the cone itself, correct?

 

[3DDave]

Q1 - referring to a datum feature in a way that it was not initially constrained is a potential problem for understanding the condition. See previous discussion where I make clear that Figure 4-16 (c) is clearly evaluated incorrectly in the standard.

Q2 - I agree with Evan. It's a clumsy method that is necessary to somehow distinguish relaxation of control that normally works OK with non-target features; when applied to targets rather than part features the only thing indicating where the part is relative to the DRF is when all targets needed are mentioned. The standard writers could have better used a separate indicator that the part is to use the upper segment DRF without change, eliminating confusion about reinterpreting the DRF contents with possible confusion about redundancy.

Now I'll have to look up to see if there is wording that precludes the part from being rocked between the upper and lower segment evaluation.

Q3 - I dislike that diagram. Show a case where the part is located in every direction by the cone except axially. Contrived example is terrible. Adding a basic dimension makes it just as terrible in a different way.

 

[axym]

Q2: I would say that the underlying issue is how the following statement from section 7.5.1 of Y14.5-2009 is interpreted:

"(2) If datums are specified in a lower segment, they govern the rotation of the FRTZF relative to the datums and within the boundaries established and governed by the PLTZF."

Specifically, exactly how do the datums (this should actually say "datum features") specified in the lower segment govern the rotation of the FRTZF? I have found that there are two main interpretations:
a) The datum features in the lower segment only control rotational degrees of freedom (and thus do not constrain translational degrees of freedom). With this interpretation, datum feature C in Fig. 4-42 would have no effect because there are no more rotational DOF's left to constrain.
b) The FRTZF is constrained in rotation relative to the DRF, but not constrained in translation. In other words, the FRTZF is allowed to translate relative to the DRF. The datum features in the lower segment constrain any applicable rotational and translational degrees of freedom, just the same as they would in the upper segment. With this interpretation, datum feature C in Fig. 4-42 would constrain one translational DOF.

Unfortunately, there are indications in Y14.5 that support both of these interpretations. But after extensive study I have found that interpretation b) works, and interpretation a) does not work. Specifically, interpretation a) leads to problems in Figs. 7-38 and 7-39, where there are multiple composite FCF's. It breaks down completely with Figs. 7-42 and 7-43 (and 4-42) where the same datum features are referenced in both segments.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

For what is worth:
Q4: 2009 applicable paragraphs are full of disagreements capital letters versus lower case letters (AXES versus translational degrees of freedom respectively). I guess this is the highest area of mistakes per square inches (or square mm) from the entire standard.
Q2: Regarding the Chevy hood issue with composite, the new version updated the GDT to multiple single segments. I haven’t read and more important understand the new (2018) standard so, the issue presented by Evan in the above post (interpretation a.) versus interpretation b.) could still be present and could cause issues on the equivalent figures mentioned by Evan.

 

[axym]

Q1: If the two features that make up the primary datum feature are different sizes, then I think that the A-B notation would need to be used. It wouldn't be necessary to reference them at MMB - there is an example in Y14.5-2009 where they are referenced RMB (Fig. 4-25). I don't think that the "smallest pair of coaxial circumscribed cylinders" is uniquely defined, but Y14.5 allows it.

Q3: Questions on customized datum reference frames are difficult to answer, because Y14.5 does not really explain the details of how they work. I'm still not sure of the intent of the question - what would be the reason for referencing datum feature B at MMB instead of RMB? I don't think I see what the intended result is.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

The datum features in the lower segment constrain any applicable rotational and translational degrees of freedom, just the same as they would in the upper segment

Evan,

Would you mind commenting a little bit on the above quote? I am not quite following it.
Does not fit my understanding specially when you say "translational degrees of freedom".

You said in previous discussions that you spent extensive amount of time understanding composite tolerancing. Can we know what YOU've found?
I thought I knew how composite works, but looks like that might not be the case. Thank you Evan.

 

[greenimi]

Evan,

Q1: If the two features that make up the primary datum feature are different sizes, then I think that the A-B notation would need to be used

May I squeeze one more question: why do you think 2X before the FCF position to some tolerance at MMC and this callout to become a datum feature, does not make sense ? (and A-B, multiple primary datum feature would be needed) for features that have different sizes?

So the sizes (different) to be shown separately and then 2X before the position within some tolerance (or even zero) at MMC.