Simulating a datum center plane 2

[semiond]

I need help in understanding this issue regarding simulating a center plane from a datum feature related to an external width dimension: let's say a width dimention is defined as a primary datum feature. When using a vise-like physical datum feature simulator with two almost parallel faces that close on the part, unless the tangent planes on both sides of the datum feature are perfectly parallel (and in the real world they're not), one of the vise faces will act similar to a primary datum plane - touching on 3 high points, and the opposite face will touch on only one point, similary to a tetriary datum plane. Now, depending on which side of the datum feature will make the more stable contact with the simulator, we might get a different separation width between the vise faces, and therefore the simulated datum plane will also be different. For example, if the measurement set up has the vise faces oriented horizontally, the side of the datum feature facing down will orient the part in the fixture, and if you flip the part upside down for a repeated measurement you might get different results on whatever control called out that datum. Now, I understand that there is only one "actual mating envelope" to the datum feature per ASME and only one of the sides facing down will produce the "minimum separation" condition per fig. 4-13, But that means that you have to mount the part twice in the fixtute and re-check your results, and I somehow doubt that this is the recommended practice... on the other hand, if the vise is oriented vertically, we will have no control over which side we stabilize better in the simulator - which is even worse. Everyone's insight will be much appreciated... Thank you!

 

[3DDave]

semiond -

Datums are theoretical and not functional by definition
the functional datum will be a center plane

Which is it?

 

[semiond]

I knew you will be nitpicking on that wording once i went over the text after pressing submit You have to appreciate how i didn't edit the post though, following your advice about not "looking unreliable".

Ok, here is an explanation: in the first sentence the word "functional" is used with a direct meaning (making contact in assembly, submitted to forces, etc.) In the last sentence, the same word was used with an indirect meaning (a theoretical datum that is derived from a functional datum feature is - indirectly "functional")

Don't tell me you didn't get the main message though. I'm sure you heard the same concept countless times from people with much better phrasing skills than mine.

 

[CheckerHater]

If you are able to change the drawing, you could assign the width as datum feature and then control each of its sides with paralellism to the datum center plane.

Although it would look weird, it would not be datum self-referencing.

How exactly this is not self-referencing?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[pmarc]

semiond,
Theoretically, datum features simulator should contact entire datum feature, unless otherwise specified. In reality that is not often the case (refer to theory vs. reality dillemma).

Although the callouts I proposed are definitely not the most inspection-friendly, there are ways to verify them. One idea is to use a special device, as you mentioned. Another one could be CMM where both opposed surfaces would be probed and then based on the collected points cloud the CMM software would do the derivation of the datum center plane and evaluation of both paralellism requirements.

Functionally, the center plane doesn't contact anything so it is not a functional interface.

I may be mistaken here, but as far as I know there is no single axis in the world that would ever contact anything, yet the hole/pin-like features are "quite" common datum features. Why is it different in case of center planes?

 

[pmarc]

CH,
This is not self-referencing because each parallelism callout proposed by me requires inspection of a single face relative to the center plane derived from two faces.

If my proposal was to assign the width as datum feature (say C) and then associate parallelism tolerance with the width dimension and use C as datum reference in that callout, then that would be self-referecing.

 

[semiond]

CheckerHater,
I think that the reason that it would not be self-referencing, is because the datum is derived from the unrelated mating envelope, which is influenced only by the high points of the flat surfaces. The parallelism to datum control measures the entire surfaces (or sufficient sections of them), thus giving a clear enough evaluation regarding the orientation of the features to the datum, and indirectly - to each other.

 

[CheckerHater]

If my proposal was to [highlight #FCE94F]assign the width as datum feature[/highlight] (say C) and then [highlight #8AE234]associate parallelism tolerance with the width dimension and use C as datum reference[/highlight] in that callout, then that would be self-referecing.

Isn't it exactly what you propose?

you could [highlight #FCE94F]assign the width as datum feature[/highlight] and then [highlight #8AE234]control each of its sides with paralellism to the datum center plane[/highlight].

If you use [highlight #FCE94F]width[/highlight] as datum feature, your datum is [highlight #8AE234]center plane[/highlight], isn't it?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[pmarc]

The two are not the same, CH.

 

[greenimi]

The width is the centerplane (datum), but the controlled feature is the face (each single face would have its own parallelism) because the parallelism is NOT associated with the width dimension.
Is my understanding correct?

 

[CheckerHater]

pmarc,
I really want to believe you... as soon as you provide the reliable reference where datum feature is a width and datum is not the center plane.

greenimi,
how parallelism is associated with plane and NOT associated with width if PLANE IS DERIVED FROM WIDTH?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[semiond]

pmarc,
Thank you for the explanation about measurement. I'm usually not comfortable defining callouts that are too hard to inspect without CMM. But seems like a special inspection device might be an option. I imagine something involving plates with slots through which a probe could go.

 

[greenimi]

I really want to believe you... as soon as you provide the reliable reference where datum feature is a width and datum is not the center plane.

I would say the datum is the centerplane, but the controlled features are the two individual faces that make/ construct the width.

 

[CheckerHater]

greenimi,
Could you make a sketch with GD&T symbology applied?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[greenimi]

CH,
pmarc said :"you could assign the width as datum feature and then control each of its sides with paralellism to the datum center plane."
Datum feature is the width:
Datum plane is the centerplane of the width
Controlled feature(s): each individual faces/surfaces:
GDT: parallelism of each individual features within some tolerance to C ( datum plane)

I think I understand how this works however I haven't used it neither found to be shown in the standard

 

[pmarc]

CH,

Hope this helps:

https://files.engineering.com/getfi...f3ca071d4968&file=datum_width_parallelism.JPG

 

[CheckerHater]

greenimi,
If you look at my post from 19:28 and pmarc's post from 19:33 you will see that somehow it is not so.

I am still waiting for the sketch / reference.

So far I only have one solution when datum is plane not derived from width:

Make your choice.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[CheckerHater]

pmarc,
I got your illustration.
I really don't think creators of the standard have seen it this way.
Have you seen this dimensioning scheme not even referenced, but at least discussed somewhere else (beside this thread )

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[pmarc]

Yes, I have seen it used and discussed. Does it matter?

 

[CheckerHater]

Yes, because creating datum from feature being controlled is still no-no in my book.

I would rather assign A to one side, B to another side and control wrt A-B (it's not like I really like it). At least compound datum is not single datum.

But, whatever. Let's hope everybody else understands it the way you do.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[semiond]

CheckerHater, how about what i wrote earlier - about AME (orgin of the datum) versus the entire actual surfaces (being controlled)? The AME and the surfaces are related - but not the same thing.