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)
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Single Datum Axis from 2 Coaxial Datum Features
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RE: Single Datum Axis from 2 Coaxial Datum Features
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
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
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
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
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
Thank you to all responders
RE: Single Datum Axis from 2 Coaxial Datum Features
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
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
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
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
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
RE: Single Datum Axis from 2 Coaxial Datum Features
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
RE: Single Datum Axis from 2 Coaxial Datum Features
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
“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
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).
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
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.
RE: Single Datum Axis from 2 Coaxial Datum Features
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
Or any other material:
Applied Geometrics Advanced GD&T book, author Mark Foster.
RE: Single Datum Axis from 2 Coaxial Datum Features
RE: Single Datum Axis from 2 Coaxial Datum Features
RE: Single Datum Axis from 2 Coaxial Datum Features
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
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
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).
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Fully agree.
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Again, fully agree.
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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
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….
You have such a track record….. don’t worry about the two threads you were less than “perfect”. We (I) don’t count those
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
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
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.
e
Certified Sr. GD&T Professional
RE: Single Datum Axis from 2 Coaxial Datum Features
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
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
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
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.