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?

 

[greenimi]

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?

Burunduk, jasson, 3DDave,

So, if I am probing this further should I understand that the DMPF is limited to:
Average: (0.2+4)/2 = 2.1
4 is coming from 20±2 (size tolerance=4)--initial post OP requirements
0.2= actual requirement imposed by the OP

Then, if DMPF is 2.1 and the upper feature flatness is 0.2 how come the initial statements holds true?
Initial statement “ "When a surface flatness has been specified, the DMP flatness is inherently also controlled.".

I am sure I am missing something, but I have no idea what the book’s intention has been.
Or maybe if the flatness for one surface (let’s say upper surface) is limited to 0.2, then THE OTHER feature’s flatness (the opposite feature) could NOT be the maximum flatness driven by the size tolerance (4)? Then what COULD be (its maximum flatness)? Can we even calculate it, theoretically?

Anything you guys can do to shoot some lights on this issue?

 

[3DDave]

Apparently operated by Mark Foster, Applied Geometrics is still in business and should be able to clarify what they meant.

 

[greenimi]

3DDave,
Well that's up to OP's discretion to do just that.
My scope is to gain more knowledge (maybe even different opinions, I realize that) from this forum to improve myself in this area of expertise.

 

[3DDave]

Well greenimi, what do you think about it?

You said "I am sure I am missing something, but I have no idea what the book’s intention has been." which can only be answered by the source.

 

[greenimi]

I always assume that I am the one which is at fault and missing information and knowledge. Therefore, I take what the author said in the book as true and trying to understand it first.
Again, Applied Geometrics book is correct, now the ball is in my yard to understand it

 

[greenimi]

Just to rephrase, I am questioning the OP's initial statement against my assessment: which one is more plausible to be incorrect?

When a surface flatness has been specified, the DMP flatness is inherently also controlled,

VERSUS

So, if I am probing this further should I understand that the DMPF is limited to:
Average: (0.2+4)/2 = 2.1
4 is coming from 20±2 (size tolerance=4)--initial post OP requirements
0.2= actual requirement imposed by the OP

Then, if DMPF is 2.1 and the upper feature flatness is 0.2 how come the initial statements holds true?
Initial statement “ "When a surface flatness has been specified, the DMP flatness is inherently also controlled.".

I am sure I am missing something, but I have no idea what the book’s intention has been.
Or maybe if the flatness for one surface (let’s say upper surface) is limited to 0.2, then THE OTHER feature’s flatness (the opposite feature) could NOT be the maximum flatness driven by the size tolerance (4)? Then what COULD be (its maximum flatness)? Can we even calculate it, theoretically?

 

[Burunduk]

Greenimi,
First off, your calculation is correct, the Max. out-of-flatness of the Derived Median Plane would be 2.1 as I showed in the sketch below (assume 10+/-2 instead of the original 20+/-2, changing the value helped me to do a more compact sketch).

The only problem with AGI's verbiage ("When a surface flatness has been specified, the DMP flatness is inherently also controlled") is that they didn't communicate properly that the indirect DMP Flatness control results from the flatness tolerances specified (or derived from Rule#1) applicable to both surfaces of the width FOS and not just one surface. But the intent is clear from the context so I forgive AGI for that.

 

[3DDave]

I'd recognize that font and the old Pro/E background color anywhere.

That median curve looks like a circular arc, which it can only be if both arcs are the same radius. Since it's just showing 3 point sets it's fine.

 

[Burunduk]

Yes, because it's just a quick sketch the yellow line representing the median plane is a circular arc through 3 mefian points. It's not an accurate representation which would be a curve through multiple median points. The general idea remains the same, though.

 

[3DDave]

No, no. I realize it was a hasty effort. It's fine. Had there been straight line segments the median would have been more obvious. I just happen to notice poor geometric constructions.

A more generalized approach would have been two splines at the outer material limits and then have the software create the median so that a variety of solutions would be seen, a task perfect for a parametric modeler.

 

[SeasonLee]

Some one may be interested in watching this video

When to Use DMP Flatness

 

[3DDave]

It has 590 views and a presenter who suggests avoiding using it.