When a surface flatness has been specified, the DMP flatness is inherently also controlled (ASME)?

[Kedu]

Do you find these statements ambiguous or incorrect?

When a surface flatness has been specified, the DMP flatness is inherently also controlled, but the converse is not true. In other words, when a DMP Flatness is specified, the surface does not have to be flat at all, although it must now be symmetrically out of flatness with its opposing side of the feature for the derived median plane to remain flat within the specified DMP flatness tolerance zone.


I am questioning the validity of those statements "When a surface flatness has been specified, the DMP flatness is inherently also controlled."
If one surface is controlled within (lets say an example) 0.2 tolerance (for a block defined with plus-minus dimension, 20±2) should I understand that the DMPF should be also smaller than 0.2?
Or both sides of the block shall have their flatness smaller than 0.2 and then ( and only then) the DMPF is controlled within 0.2?

What is your understanding?

 

[jassco]

Hi, Kedu:

It makes sense to me although "When a surface flatness has been specified" is incomplete. DMP is derived from a feature with TWO flat parallel surfaces rather than one.

Best regards,

Alex

 

[Kedu]

It makes sense to me although "When a surface flatness has been specified" is incomplete. DMP is derived from a feature with TWO flat parallel surfaces rather than one.

So, if only one surface flatness is specified (not two surface flatnesses) is the statement above make sense or does not anymore?

I mean the original statement: "When a surface flatness has been specified, the DMP flatness is inherently also controlled, but the converse is not true. In other words, when a DMP Flatness is specified, the surface does not have to be flat at all, although it must now be symmetrically out of flatness with its opposing side of the feature for the derived median plane to remain flat within the specified DMP flatness tolerance zone."
is true or not?

 

[jassco]

It makes sense. Imagine you have a block with wavy surfaces. If top and bottom surfaces are mirrored, then flatness of the DMP is "ZERO". Flatness of surface(s) control DMP flatness, but not the other way around.

Best regards,

Alex

 

[greenimi]

Guys,

Now I am very confused, which granted, does not take too long.....
If one surface flatness is limited to 0.2 (to keep the OP's original value) and the opposite surface flatness is limited to 0.3 (for the sake of conversation) then how much would be the DMPF (derived median plane flatness) limited to?
Can we even calculate this DMPF value?

What about if both surface flatness are controlled by the SAME flatness tolerance (pretend 0.2) should I understand that the DMPF is also limited to 0.2?
Can someone clarity my doubts?

 

[jassco]

DMP flat will be 0.25 max. which is average of 0.2 and 0.3.

Best regards,

Alex

 

[greenimi]

Based on what you concluded that the "average" is the right answer?
Is the ASME supporting your assessment?

 

[jassco]

Based on Math.

 

[greenimi]

Based on Math Standard Y14.5.1?

 

[greenimi]

I am still not convinced that the answers provided are the correct ones. Average approach seems fishy at best IMHO.

 

[Burunduk]

greenimi,
I agree with Alex that the average would give the right answer.
Suppose you have a rectangular part in which the top surface is allowed to be out of flat within 3 mm and the bottom is allowed 2 mm.

When just the top becomes convex within 3 mm the median point under the highest point would offset by half of that - 1.5 mm, from its nominal location. The effect of one of the surfaces curving within a certain amount is half of that amount for the median plane.

If both were curved (top convex, bottom concave) by 2 mm the median point would move up by exactly 2 mm too. The effect of both curving the same amount is that same amount for the median plane.

If the top curves within 3 mm and the bottom within 2 mm, the median point between the extremes would curve within 2 mm and an additional 0.5 mm as affected by the top curving more than the bottom by an additional 1 mm.

So the average calculation would give the right answer, but I would describe it with a different mathematical expression that might be more intuitive and give the same result - it's the sum of the common amount of variation and half of the difference between the individual variations of both horizontal surfaces.

 

[greenimi]

Burunduk,

Thank you for your answer. I appreciate it. Looks like I have a lot to learn.

Just a quick follow up question (and I don't want to put words in your mouth)

Do you agree or disagree with the OP's initial statement? Is this statement correct 100% of the time?

" When a surface flatness has been specified, the DMP flatness is inherently also controlled, but the converse is not true. In other words, when a DMP Flatness is specified, the surface does not have to be flat at all, although it must now be symmetrically out of flatness with its opposing side of the feature for the derived median plane to remain flat within the specified DMP flatness tolerance zone."

Do you find these statements ambiguous or incorrect?

When a surface flatness has been specified, the DMP flatness is inherently also controlled, but the converse is not true. In other words, when a DMP Flatness is specified, the surface does not have to be flat at all, although it must now be symmetrically out of flatness with its opposing side of the feature for the derived median plane to remain flat within the specified DMP flatness tolerance zone.


I am questioning the validity of those statements "When a surface flatness has been specified, the DMP flatness is inherently also controlled."

 

[Burunduk]

Greenimi,
This paragraph makes sense to me, and I agree with it. Wonder where it was quoted from? Kedu (OP) didn’t say. Whatever it is, it could be a good source to learn more from.

 

[Kedu]

Burunduk, jassco, greenimi and all,

Wonder where it was quoted from? Kedu (OP) didn’t say. Whatever it is, it could be a good source to learn more from.

My source is Applied Geometrics training material for 1994 standard (ASME Y14.5M-1994), but I think the year is irrelevant for my question.
I know the name has been changed (as explained in the picture), but I don't think the concept changed at all so my inquiry is applicable to 2009 or even to 2018 versions of the standard.
2009 and 2018 is now called derived median plane flatness DMPF

So adapting the picture's text (again for 1994) to the 2009 or 2018 standard, the verbiage would become as I wrote in my initial post

When a surface flatness has been specified, the DMP flatness is inherently also controlled, but the converse is not true. In other words, when a DMP Flatness is specified, the surface does not have to be flat at all, although it must now be symmetrically out of flatness with its opposing side of the feature for the derived median plane to remain flat within the specified DMP flatness tolerance zone.

Do you find any problems with the statement above (from my initial post)? Any discrepancies on how would YOU translate the AGI verbiage from 1994 to 2009 or 2018?
Is the statement true? Is it arguable? Is it debatable or controversial?
That’s I am looking for ……

 

[3DDave]

It should have been called the Derived Median Surface as the chances it will be a plane are zero. A plane has zero thickness while a surface can be seen to occupy a gap between two planes and therefore has dimensionality.

 

[Garland23]

But a surface is something that can be physically touched, so that term doesn't seem to work either.

 

[3DDave]

A surface is not exclusively something that can be touched. Mathematically defined surfaces cannot be touched.

 

[Kedu]

Gentlemen,

I would appreciate if we can focus on my original question and leave the "unnecessary" distraction alone.

Burunduk,
Did I answer your question?
Would you mind now, to provide your pertinent opinion about my initial inquiry?

Thanks

 

[3DDave]

Kedu,

What is your expectation for answers? You did not include the illustration that is the introductory part of the statement, the illustration the statement is explaining. "Notice in the illustrations that follow ..." is information withheld from the original question and remains withheld.

It makes no particular claim about the mathematics of the result, so there's nothing to "understand".

jassco already gave a sufficient explanation.

Do you find any problems with the statement above (from my initial post)?
No. Because there is not enough to determine if there is a problem.

Any discrepancies on how would YOU translate the AGI verbiage from 1994 to 2009 or 2018?
I would not do any translation because the connection between surface flatness and median flatness is not useful.

Is the statement true?
It doesn't matter because there's no demonstrated value in linking them.

Is it arguable?
Argue away. What is your understanding of the problem?

Is it debatable or controversial?
Everything can be debated or considered controversial, but unless there is a reason to understand some outcome of debate there's no purpose to doing so.

Find a mechanism or application where knowing the link between these two factors is critical to the operation of a machine, then it's worth investigating.

Worth noting is that the median "plane" has no interaction with anything. It is a placeholder.

 

[Burunduk]

Kedu,
Thank you for mentioning the source of the statement, But I already provided my "pertinent opinion" about your initial inquiry.

As I said above:
"This paragraph makes sense to me, and I agree with it."
This was not before I explained how the described indirect control of DMPF works in the case of flatness tolerances applied to the two surfaces of the width feature of size.

On why this doesn't work the other way around - I think the original statement is pretty self-explanatory:
"...when a DMP Flatness is specified, the surface does not have to be flat at all, although it must now be symmetrically out of flatness with its opposing side of the feature for the derived median plane to remain flat within the specified DMP flatness tolerance zone."