Maximum possible contact and a single solution

[greenimi]

Per ASME Y14.5-2018
7.12.4 Pattern of Features of Size RMB
When RMB is applicable in a feature control frame to common datum features of size used to establish a single datum, the true geometric counterpart of each feature shall be fixed in location relative to one another. The true geometric counterparts shall expand or contract simultaneously from their worst-case material boundary to their LMB until the true geometric counterparts make maximum possible contact with the extremities of the datum feature(s). When irregularities on the feature(s) may allow the part to be unstable, a single solution shall be defined to constrain the part.
Questions:
1.) What means maximum possible contact?
2.) What means "single solution" default for unstable/rocking datum features (that replaces the former default of a candidate datum set) ?

 

[mfgenggear]

Another ad hoc answer to a gap in the standard.

The section and the figure are created for the single case of two nominally coaxial features and fail for any other case of such references. It's a single case and a likely ad hoc solution that doesn't apply to most any other RFS "pattern."

Clearly, if the features are of nearly perfect form and identical size there is no problem. I think that is obvious enough to not be worth using as a basis for a conclusion of any kind.

" It is all matter of technique, which is always imperfect and the Y14.5 standard is not expected to cover."

Gaps in Y14.5 cannot be excused for technique being imperfect. The standard is intended to communicate the limits of imperfect part variation and perfecting the basis of measurement, not ignoring imperfections in measurement.

I mean that I don't mean any malice.
And that's the problem. To quantify your post here. ASME does its best to explain every circumstance. It's literally impossible to
Try to cover ever billion circumstances.
But to say it's the best we have. And it is what it is it is humanly impossible to make it perfect.
I think we can acknowledge that and deal with it.
No mater how hard it can be.
I think the best way to learn from it is to break it down section by section
Then define its meaning.
My thoughts to much wording to me , becomes to confusing. And probably represent most guys on floor. Because that's where I started from.

I am a big believer in presentations , graphical in nature. It just makes more sense. I remember when an other posted geotolerancing hand book
It was a very concise in explaining GD&T.
Don't remember the exact name so correct me if incorrect.

 

[Burunduk]

The standard does have gaps but this one is not exactly one of them. The only problem here is some unnecessary and partial details (true geometric counterparts contracting or expanding simultaneously, in 7.12.4) that should not have been mentioned. Once they were they are a cause of confusion and distraction. The way to avoid falling into that trap is to know how to separate the wheat from the chaff. The intent is simultaneous constraints with equal ability for the pattern members to effect, and that intent does get communicated.

 

[mfgenggear]

The standard does have gaps but this one is not exactly one of them. The only problem here is some unnecessary and partial details (true geometric counterparts contracting or expanding simultaneously, in 7.12.4) that should not have been mentioned. Once they were they are a cause of confusion and distraction. The way to avoid falling into that trap is to know how to separate the wheat from the chaff. The intent is simultaneous constraints with equal ability for the pattern members to effect, and that intent does get communicated.

Yes very well said, continuous improvement is real. Thank you for your contribution.

 

[greenimi]

The standard does have gaps but this one is not exactly one of them. The only problem here is some unnecessary and partial details (true geometric counterparts contracting or expanding simultaneously, in 7.12.4) that should not have been mentioned. Once they were they are a cause of confusion and distraction. The way to avoid falling into that trap is to know how to separate the wheat from the chaff. The intent is simultaneous constraints with equal ability for the pattern members to effect, and that intent does get communicated.

So, if "The true geometric counterparts shall expand or contract simultaneously" should NOT have mentioned in 7.12.4, then how the end user would have understood the DEFAULT condition as being "simulaneously" (simultaneity) and not being the "maximum possible contact" ?

Other figures RMB within 2018 make reference to "make maximum contact" (7-34) hence I think consistency should play a key role in those figures.
Therefore, should I understand that the "maximum contact" is the main driver for a single feature at RMB, but when a pattern of features is used RMB then the main driver become "simultaneity" (and the maximum contact becomes a secondary concern)?

By the way, I've never thought about "different rate" to achieve this "simultaneous" condition......you might be right. I am still trying to get my arms around the physical realities of such concept.

 

[Burunduk]

greenimi,
My opinion is that both the simultaneity of application of constraints and the "maximum possible contact" are required characteristics of a pattern of features of size (or "common datum features" of size) at RMB. These two things should not be in conflict (no reason for them to be). "Maximum possible" contact is related to the fact that the true geometric counterparts are basically constrained to each other, so they can't make the fullest contact with the actual features like an Unrelated AME does, but they must make the "maximum possible" (contact, under these conditions). I think there are already several guidelines offered in this thread for practically workable "between the lines" interpretations of the Y14.5 wording.

 

[mfgenggear]

The standard does have gaps but this one is not exactly one of them. The only problem here is some unnecessary and partial details (true geometric counterparts contracting or expanding simultaneously, in 7.12.4) that should not have been mentioned. Once they were they are a cause of confusion and distraction. The way to avoid falling into that trap is to know how to separate the wheat from the chaff. The intent is simultaneous constraints with equal ability for the pattern members to effect, and that intent does get communicated.

I believe what the issue is , the cart before the horse. Unless an Engineer under stands the concept of different types of assemblies and what the intent is it will be more interpretable.
As the video I posted earlier the management of tolerancing. What goals are, and what the intent.
For example a bearing on an axel. Where is the press fit, and where is the slip fit. And what type of bearing.

What type of fit between with a bolted assembly.
What is precision fir required on shaft rotating 10k rpm. What is the out of balance required.
and how do I apply GD&T.

 

[mfgenggear]

GD&T is only part of the game plan on drawing.
What is material required.
What heat treat required and what hardness.
What is the application .
What is the precision
Which GD&T is more applicable .
GD&T is only required to inspect and record for Quality Assurance, and the part is to the drawing. But as said earlier, the Engineer must be experience to apply it correctly.

 

[mfgenggear]

I am changing hats , So as a precision mechanical inspector. The Engineer or Engineers have applied requirements. Gd&T.
The tolerancing applied is not for debate.
My position is to know what means and how to measure it. I have philosophy can I setup and measure at least to methods. Of the results are
Marginal. I will measure it differently. To verify my results. CMM or on the surface plate. I can do both. My job is to make sure it is in compliance.
If it passes is great . If not write up an discrepancy report and for forward it to engineering. And wait for the Engineer to disposition.

 

[mfgenggear]

In my young days I had the privilege to be in an apprenticeship. That allowed me to work and learn different departments. Shop, inspection, engineering, tool design, old style drafting board.
Met lab, NDT level 2,
My point is not to boast but to state that every person get a good under standing of all these areas. As well as get the education which is so important. It will all fall in place. apprenticeship will very much help in one's career. It has done me well.

 

[greenimi]

In my young days I had the privilege to be in an apprenticeship. That allowed me to work and learn different departments. Shop, inspection, engineering, tool design, old style drafting board.
Met lab, NDT level 2,
My point is not to boast but to state that every person get a good under standing of all these areas. As well as get the education which is so important. It will all fall in place. apprenticeship will very much help in one's career. It has done me well.

I agree with you. With only one caveat: We have to understand what the theory -read "the standard"- requires (and what does not require) - before we can apply such of said theory in any sort of practical way. We are debating/ clarifying here mostly our individual understandings of the theory. Burunduk, looks like, has a very good grasp on the thoery so we are finding a common denominator ............