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Single Datum Axis from 2 Coaxial Datum Features

Single Datum Axis from 2 Coaxial Datum Features

(OP)

Is it possible to identify 2 coaxial features of size (cylinders) @MMC for both to, establish a single Datum axis ? (e.g. A-B)

RE: Single Datum Axis from 2 Coaxial Datum Features

Yes, it is possible. See figs. 4-24 and 7-59 in Y14.5-2009.

This approach can be also used for coaxial cylinders which are not of the same size. Assign one of them A, the other one B, and the reference to the common datum axis will then be A(M)-B(M).

RE: Single Datum Axis from 2 Coaxial Datum Features

(OP)
pmarc

I am specifically asking about the "A-B" concept. Fig 4-25; where Datum feature simulator ("simulator" spelled wrong in 2009 figure)
I probably wasn't clear enough.

I do not believe MMC is valid in this scenario.

Thank you pmarc

RE: Single Datum Axis from 2 Coaxial Datum Features

Yes, it is valid only if you are controlling the considered feature with position or profile. If you are controlling the considered feature using runout then you cannot use the MMB modifier.

EDIT: It is definitely not valid in figure 4-25 since the considered feature is controlled by runout.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

RE: Single Datum Axis from 2 Coaxial Datum Features

(OP)
powerhound


For A@MMB-B@MMB (position control) there would be a gage built with hole sizes equivalent to each features MMC ?

If the feature is smaller than MMC then there would be datum displacement (wobble within the gage) ?

And from that, how is a "single axis" defined? The axis is from the features' coaxial holes @MMB ?


RE: Single Datum Axis from 2 Coaxial Datum Features

dtmbiz,

I agree with pmarc and powerhound. Referencing two coaxial datum features at MMB is possible. The A(M)-B(M) case is not illustrated in a figure, but I don't see a problem with it. This would not be possible for Fig. 4-25, but only because of the special rule for runout tolerances that the datum features must be referenced RMB only.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Single Datum Axis from 2 Coaxial Datum Features

(OP)

Thank you to all responders

RE: Single Datum Axis from 2 Coaxial Datum Features

Sounds like you got the answer you needed but since you asked me a question right there at the end, I'll answer it as best I can.

Quote (dtmbiz)

And from that, how is a "single axis" defined? The axis is from the features' coaxial holes @MMB ?

The datum axis would be the axis of the aligned datum simulators.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

RE: Single Datum Axis from 2 Coaxial Datum Features

dtmbiz,

As powerhound said, the datum axis is the axis of the two datum simulators (in other words, the datum axis is defined in the gage). Having said that, the part can wobble (rotate and translate) within the gage. This is one of the tricky things about datum features referenced at MMB - from the part's point of view, there are many different possible datum axes (one for each different "wobble configuration" of the part in the gage). There isn't one unique datum axis. Imagine the part as stationary, and the gage (with its datum axis) wobbling relative to it within the clearance. That's the situation.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Single Datum Axis from 2 Coaxial Datum Features

Quote (pmarc)

This approach can be also used for coaxial cylinders which are not of the same size. Assign one of them A, the other one B, and the reference to the common datum axis will then be A(M)-B(M).

pmarc, Evan, John,

Can we even assign one of them as A and being controlled/positioned/ located to A(M)-B(M), the other one B and being controlled/positioned/located to A(M) -B(M) and the reference to the common datum axis will then be A(M)-B(M). Am I correct or some caveats are needed?

RE: Single Datum Axis from 2 Coaxial Datum Features

(OP)

Axym
You stated exactly my concern. With "many axis" possible, it doesn't seem to fit my definition of "a single axis".
Yes each axis is single, however there isn't a "single repeatable axis" as I see it.

pmarc
There are many concerns I have with this dimension scheme. For what ever reason, I do not like the looks of it.
One thing I definitely do not like its that both A & B are a length of .038 mean value.

I do not have access to mating components.

see attached

RE: Single Datum Axis from 2 Coaxial Datum Features

dtmbiz,

In my first post I did not suggest to control datum features A and B with position to A(M)-B(M). Position with no datum feature references would be fine (as shown in the standard). I meant that the toleranced feature should be controlled to A(M)-B(M).

If your concern is that datum features A and B are short, then the question should be asked if from design standpoint they are good candidates for datum features at all. To me, even though they are short indeed, their separation looks to be sufficient to make them feasible primary datum features.

RE: Single Datum Axis from 2 Coaxial Datum Features

For me the shown drawing looks good (correct method to express a very common true design requirements within the definitions and rules on the Y14.5) except a small "mistake": NO diametic symbol needed for the perpendicularity callout on the datum feature C.

RE: Single Datum Axis from 2 Coaxial Datum Features

(OP)

pmarc
understand and agree

greenimi
FYI
Agreed it is technically correct.
I do not like the MMC due to thin wall tube with tolerance stack
I am not sure how to factor in the datum displacement in this scenario, considering that
there is more than likely "wobble" built into it. This wobble can potentially and probably
will be different at both ends of the part.

BTW
This is not my dimensioning scheme

RE: Single Datum Axis from 2 Coaxial Datum Features

I do not agree that the drawing is technically correct. The chapter on Runout is clear that "Runout tolerance may be applied to a datum feature(s) and related to the datum axis derived from that datum feature(s)" (section 9.5.5 - 2009). I can't find anything in the chapter on Position that allows this same concept. Said another way, The ∅.3125 features cannot reference themselves in their FCFs.

RE: Single Datum Axis from 2 Coaxial Datum Features

I agree with AndrewTT - runout is different than position in this area. Although I understand the concepts with datums with runout; from a practical mfg and QA standpoint, good luck with establishing the datum axis from the entire surfaces and then checking the FIM runout.

Certified Sr. GD&T Professional

RE: Single Datum Axis from 2 Coaxial Datum Features


AndrewTT and mkcski,
You might not agree that the shown drawing is technically correct (except the Ø symbol), but that does not mean is not.

Number#1:
pmarc, Evan and John are the best around here. Read some other threads and convince yourself.
To successful disagree with pmarc or Evan (to name just a few) is EXTREMELY hard and painful. You “almost” have no chance to win.

Number#2:
If the concept is not shown in the standard that does not mean is not correct or legal.

Number#3:
Quote:” The ∅.3125 features cannot reference themselves in their FCFs.”

A classic common misunderstanding in the industry = is that both datum features A and B are referencing a location back to themselves. However, that is not the case. The datum reference frame is simply specifying that the “Datum Axis” a single axis, is to be established using both datum features A and B together at the same time.

RE: Single Datum Axis from 2 Coaxial Datum Features

I did not say that the shown concept is FUNCTIONAL,--I have no clue how is part works-- (however, as shown, is most likely reflecting the design intent, if the two “rather small” surfaces works together to align/orient the part in assembly), but for sure is legal.

“The figure illustrate the most practical functional requirement where both datum features A and B will be used to together to hold the part in an assembly. The two datum features are still allowed some misalignment in both location and orientation. The amount of misalignment permitted is often specified by bearing manufacturers and other design guidelines.
Because of such said above common misunderstanding, the intended coaxial control of the datum features to themselves as well as the considered features controlled with total runout/ position is often specified relying on a FUNDAMENTAL APPLICATION of the GD&T language, but lacks the advanced understanding of what the permissible variation may actually be.”


Cite / quote from Applied Geometrics Advanced GD&T book, author Mark Foster.

RE: Single Datum Axis from 2 Coaxial Datum Features

greenimi:

Yes, disagreeing with the "masters" is not recommended. But to me, part of the learning process is to reflect your understanding back "at" the master and let him "correct" any misunderstanding so as to obtain more clarity. What really matters is the tact used to respond to the master. Challenging or arrogant will get you "slapped" (and detention hahaha).

Quote (greenimi)

A classic common misunderstanding in the industry = is that both datum features A and B are referencing a location back to themselves. However, that is not the case. The datum reference frame is simply specifying that the “Datum Axis” a single axis, is to be established using both datum features A and B together at the same time.
I have a challenge - find an example in Section 7 (or any other material) where a feature is positioned to itself (like runout allows)

I mentioned "practical" in my earlier post. So, considering fig 4-25 in the lower "means this" frame - how do you inspect the runout on the surface of the datum feature with a dial-indicator (or other device) when the datum simulators that establish the datum axis are covering it up?

Certified Sr. GD&T Professional

RE: Single Datum Axis from 2 Coaxial Datum Features

Greenimmi - Only one of of the people that you mentioned commented after the drawing was added to this thread (pmarc). I'm not sure that one can take the support they (pmarc, powerhound, axym) gave for the OP's original question and assume that it carries over into support for the drawing that was shown later in the thread.

Quote (pmarc)

In my first post I did not suggest to control datum features A and B with position to A(M)-B(M). Position with no datum feature references would be fine (as shown in the standard). I meant that the toleranced feature should be controlled to A(M)-B(M).

I don't want to put words into anyone's mouth, but I read that to agree with what I stated. Hopefully pmarc can come back and clarify his position.

I am open to the possibility that I am dead wrong. ponder But like mkcski pointed out, there does not seem to be any direct support for this in the standard.

RE: Single Datum Axis from 2 Coaxial Datum Features

AndrewTT,

Let me go back and ask you what means in your opinion “technically correct”

You used the words ---quote: “I do not agree that the drawing is technically correct” .

Could you please define what means that “technically correct”? Means legal? Functional? Inspeactable? Manufacturable?

I, personally, used the words “drawing looks good”. Go back and check. Just semantics…..

RE: Single Datum Axis from 2 Coaxial Datum Features

Quote (mkcski)

I have a challenge - find an example in Section 7 (or any other material) where a feature is positioned to itself (like runout allows
mkcski,


Or any other material:

Applied Geometrics Advanced GD&T book, author Mark Foster.

RE: Single Datum Axis from 2 Coaxial Datum Features

What I meant by " not technically correct" was that the drawing in question (IMO) did not comply with the ASME Y14.5-2009 standard.

RE: Single Datum Axis from 2 Coaxial Datum Features

Greenimi - can you post the figure(s) that goes along with the text that you pulled from Mark Foster's book?

RE: Single Datum Axis from 2 Coaxial Datum Features

greenimi:

I do not have this book in my library. I will have to get a copy. In the mean time ,could you please post a page or two that shows a feature positioned to itself?

Certified Sr. GD&T Professional

RE: Single Datum Axis from 2 Coaxial Datum Features

Quote (AndrewTT)

What I meant by " not technically correct" was that the drawing in question (IMO) did not comply with the ASME Y14.5-2009 standard

Could you, please, explain why do you think the drawing in question did not comply with 2009? What paragraph is violating or not abiding to?
Just because you don't see an example (in the standard) I don't think is good enough.

RE: Single Datum Axis from 2 Coaxial Datum Features

Here are my thoughts about some of the things that have been said since I last replied (in random order):

Quote (greenimi)

Number#1:
pmarc, Evan and John are the best around here. Read some other threads and convince yourself.
To successful disagree with pmarc or Evan (to name just a few) is EXTREMELY hard and painful. You “almost” have no chance to win.

I am speaking for myself here, but I am pretty sure that Evan and John will agree with me. The fact that someone won 99 discussions does not automatically mean that she or he will win discussion #100. So I would be very careful with using this kind of argument in any discussion (as a matter of fact I remember at least two threads recently in which I was proven that I was not correct and I admitted that).
-----

Quote (greenimi)

Number#2:
If the concept is not shown in the standard that does not mean is not correct or legal.

Fully agree.
-----

Quote (greenimi)

Number#3:
Quote:” The ∅.3125 features cannot reference themselves in their FCFs.”

A classic common misunderstanding in the industry = is that both datum features A and B are referencing a location back to themselves. However, that is not the case. The datum reference frame is simply specifying that the “Datum Axis” a single axis, is to be established using both datum features A and B together at the same time.

Again, fully agree.
-----

Quote (AndrewTT)

I don't want to put words into anyone's mouth, but I read that to agree with what I stated. Hopefully pmarc can come back and clarify his position.

Not exactly, Andrew. I rather wanted to clarify the intent of my initial reply in this thread. Having said that, I don't think that referencing to A(M)-B(M) in position callouts for both datum features would be illegal. It is not shown in the standard and I would not probably use it (if this was my drawing), but that does not mean that the concept is flawed. In other words, no rule from the standard (other than lack of explicit statement and figure) is in conflict with the concept.

You may want to take a look at the link below. It is from Jim Meadows' Neswletter that can be found on his website. Scroll down a little bit to Subject: "Total Runout Question".
http://www.geotolmeadows.com/newsletters/2012/nov2...

RE: Single Datum Axis from 2 Coaxial Datum Features

Quote (pmarc)

The fact that someone won 99 discussions does not automatically mean that she or he will win discussion #100. So I would be very careful with using this kind of argument in any discussion (as a matter of fact I remember at least two threads recently in which I was proven that I was not correct and I admitted that).

Well pmarc, correct me if I am wrong in my approach, but I tend to side with someone who has 99% (as per your provided numbers) chances to win….bigsmile

You have such a track record….. don’t worry about the two threads you were less than “perfect”. We (I) don’t count thosebigsmile

Anyway, I learn A LOT, but A LOT from the discussions in which you have an input.
Thank you again

RE: Single Datum Axis from 2 Coaxial Datum Features

(OP)

AndrewTT, mkcski,

I agreed (politely conceded; with reluctance) to the "technically" correct; as there is no text in the standard that says it is not permitted. Just because it doesn't say "no"; doesn't mean it is a good callout though. I see problems with it in the real world setting. (not just as model in the virtual world)
However
My argument and marked error that the MMC should not be applied to A-B to the customer (prior to EngTip posting)
is exactly based on what your comments express .. particularly yours mkcski, for the same reasons.

My position has caused a lot of negative scrutiny and some comments from their management that what marked as error (for their further consideration) was absolutely wrong.

My position was based on practicality... "how will you do that customer?"

RE: Single Datum Axis from 2 Coaxial Datum Features

greenimi: Thanks for the drawing. I will have to order the book at $125... ouch

pmarc: Thanks for the J Meadows reference. My issues are exactly expressed in the very last paragraph - see qoute. The GDT on the drawing may communicate design intent but inspecting the part to confirm it meets the drawing is challenging.

Quote (pmarc)

Some feel uncomfortable in saying each datum feature diameter is controlled to their compound axis, in that if we were to “chuck-up” on both, they would be obscured and not measurable. The way around that is to not “chuck-up” on both, but rather probe them with a CMM and analyze the collected data, or put both in Vee-blocks which leaves them exposed to measure, or center drill the part and put it between centers, then measure every diameter to see how far off they are from each other. If everything is measured from the same axis, whether that axis is the proper datum axis or not, then every diameter is related to each other to within the sum of their tolerances to that common axis. In other words, if the part measures within the tolerance, it is good. If not, another measurement tactic can be used.
e

Certified Sr. GD&T Professional

RE: Single Datum Axis from 2 Coaxial Datum Features

mkcski,
No one said that GDT on the drawing must always be easy to inspect. At least in this case there are some decent alternatives - not only in the inspection phase, but also prior to drawing issuance.

RE: Single Datum Axis from 2 Coaxial Datum Features

pmarc:

I was not implying it needed to be easy. And yes, a "smart" organization would get QA input before the design (GDT, drawings) is released. Several times in my organization QA was not involved up front. They "confronted" Engineering with the planned inspection program (dollars and worse, contract schedule delays) only to find that the GDT could be adjusted making the inspection more reasonable, while maintaining functionally.

Certified Sr. GD&T Professional

RE: Single Datum Axis from 2 Coaxial Datum Features

greenimi,

Here's how I approach following others' opinions:

1. Listen to and consider everyone's opinion, no matter who they are.

2. Scrutinize and question everyone's opinion, no matter who they are.

This has served me well at ASME meetings, and on forums like this. I've learned from people at various experience levels. I remember you in particular bringing certain details up in posts on this forum, that I wouldn't have thought of myself and led to a deeper understanding. I appreciate that you would consider my opinion as likely to be correct.

As you may have noticed, I often begin posts with the phrase "I agree with pmarc". It's not because I just assume that he is right, even though he almost always is (the key word being "almost"). I've learned a lot from him.

Regarding the discussion in this thread, I agree that referencing A(M)-B(M) in position tolerance for features A and B is allowable but not optimal. Y14.5 doesn't show an example like this, but I don't see it as any worse than the "self-referencing" A-B callouts in the Y14.5's runout section.

I've attached a crudely edited version of dtmbiz's drawing, with alternative tolerances for the datum OD's that I believe are functionally equivalent to the A(M)-B(M) strategy that was specified originally.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Single Datum Axis from 2 Coaxial Datum Features

Quote (Evan)


1. Listen to and consider everyone's opinion, no matter who they are.

2. Scrutinize and question everyone's opinion, no matter who they are.


Thank you Evan,

Do doubt that I have to improve my listening skills. I am working on it.

Thank you also for the equivalent datum schemes provided. I am learning new tips and trics each day and improving my GD&T expertise.



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