Single Datum Axis from 2 Coaxial Datum Features

[dtmbiz]

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

 

[greenimi]

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…..

 

[greenimi]

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.

 

[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.

 

[AndrewTT]

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

 

[mkcski]

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

 

[greenimi]

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.

 

[pmarc]

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

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).
-----

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

Fully agree.
-----

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.
-----

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/nov2012.htm

 

[greenimi]

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…. 

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

 

[dtmbiz]

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?"

 

[mkcski]

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.

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

 

[pmarc]

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.

 

[mkcski]

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

 

[axym]

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

 

[greenimi]

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.