Datum line that is not a straight axis 3

[Burunduk]

The two most recent versions ('09 and '18) of ASME Y14.5 define a 'datum' as:

datum: a theoretically exact point, axis, line, plane, or combination thereof derived from the true geometric counterpart.

With "line" mentioned as a separate kind.
In past discussions such as thread1103-452649 no clear conclusion was reached about what can be the intent behind the addition of "line" to the definition in the '09 edition of the standard.

Please see the following diagram.
Do you think, based on the Y14.5 standard, that the shown curved line could be a valid datum corresponding specifically to the "line" type (and only) in the above definition? If not, why?

 

[greenimi]

What about derived median line DML?
Is DML a "line"? Or is more like a curve or more like a spine?
Maybe it is well defined and it is interpretable per the Y14.5 and/or per math standard Y14.5.1, but could create same questions as you pointed above

 

[3DDave]

Yes. Because if you are the target audience and have already decided to create your own definition, I'll understand what you intended.

No, for anyone else, because the alternative organization for which CMMs are also programmed, only support a straight line. There is no support in Y14.5 for what you propose. There is, however a surface interpretation that does not require any other datum than is already exemplified, so your suggestion would not be noticed / does not matter. It's just a complex feature, so datum plane, datum axis, and datum point to align to the DRF.

 

[jassco]

Hi, Burunduk:

Both "Axis" and "Line" are lines in math. The difference is construction of them. "Axis" is a special case for features with axis while "Line" is generic and for everything else. "Axis" is a straight per definition of current standard.

I don't see a point in debating "Axis" vs "Line".

Best regards,

Alex

 

[Burunduk]

Thanks, the three of you, for the responses so far. Definitely looking forward to more opinions on whether the curved line is a valid 'datum' per Y14.5, even if not exemplified.


Here is another question about the above diagram, not directly related to the original context:
Would you classify datum feature A as a "complex feature" (as 3DDave did) - which constrains all 6 DOF? Or do you think a secondary datum reference had to be used to fully define the location of the holes relative to a datum reference frame?

 

[jassco]

Hi, Burunduk:

You are not supposed to specify profile of surface to datum feature A (a torus) with reference to datum A. That is like a dog traces its own tail. Or, you are trying to push a boat while you are in the boat.

Best regards,

Alex

 

[Burunduk]

Alex,
I did not suggest that controlling datum feature A relative to itself makes sense - didn't even think of it.
My second question was - does datum feature A belong in the category of "complex feature" that constrains 6 DOF as shown in fig. 4-3 in Y14.5 '09 or 7-3 in '18:

I'm also not sure what point were you making in your response to the first question. It's clear that an axis is also a line. The purpose of the first question was not "debating Axis vs Line", it was to find out whether a line that does not meet the convention of what an "axis" is (in the example, because it is curved), can be a valid datum per the quoted datum definition. Do you think the shown line can or cannot be considered a 'datum'? Thank you.

 

[jassco]

Hi, Burunduk:

These objects are datums:

1. Plane XY;

2. Plane YZ;

3. Plane ZX.

The line you drew is not a datum, not even a feature.

Best regards,

Alex

 

[Burunduk]

These objects are datums:

1. Plane XY;

2. Plane YZ;

3. Plane ZX.

The following objects are also datums:

1. The center point related to a spherical datum feature.

2. The axis related to a cylindrical datum feature.

3. The axis and the apex point related to a conical datum feature.

The line you drew is not a datum, not even a feature.

The 3 datums I mentioned above are also not features. Nevertheless they are datums. Planes, including the ones you mentioned are not features either. They are derived from true geometric counterparts of (datum) features.

 

[drawoh]

Burunduk,

How do you propose I pick up your datum with a fixture?

I think that is the real test.

Datum targets?

--
JHG

 

[Burunduk]

drawoh,
A fixture is not mandatory.
The part including the datum feature is scanned by a CMM or an optical measurement machine and a cloud of points that includes the scanned datum feature (or "extracted" datum feature as they call it in the ISO standards) is generated. Then the software uses the "extracted" feature for calculation/simulation of an envelope that constitutes the "datum feature simulator" and represents the "true geometric counterpart", a virtual fixture if you wish - from which the datum is derived.

 

[cr7]

Someone with enough knowledge and access to the latest GOM Inspect (now called Zeiss Inspect), should try to create this kind of datum and report back, I think it should work.

 

[pmarc]

Burunduk,

Is the part in your example a bent cylinder or an extruded shape? If the latter, then I don't understand why the question is about the line as indicated. If the former, then the datum feauture A is a portion of torus that ends up being the same type of datum feature as cone. Table 7-3 in Y14.5-2018 explains datums for this type of datum feature and none of them is curved line.

 

[CheckerHater]

Burunduk,
I've never seen Profile of a Surface being interpreted as axis / centroid or any other kind of line. Profile of a Line on the other hand... (wink, wink)

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

 

[Burunduk]

pmarc,
The shape of datum feature A is a nominally bent cylinder.
If this is not an example that can justify the mention of a line (alongside axis) in the definition, do you believe there could be another example? Or is the mention redundant since "axis" covers all possible cases that could ever be attributed to "line"?
I agree that this torus-shaped datum feature has something in common with a cone - 5 degrees of freedom are constrained and one rotational degree of freedom remains.
In my example, it is manifested by the fact that holes accurately spaced and centered to the spine of the torus-shaped TGC could end up anywhere along that spine and pass as conforming. That would not be the case with a "complex" datum feature.

 

[Burunduk]

I've never seen Profile of a Surface being interpreted as axis / centroid or any other kind of line.
Profile of a Line on the other hand... (wink, wink)

It's not clear what you mean here. The topic is the datum derived from the datum feature's TGC interpreted as a line. Not a profile tolerance interpreted as a line. The profile tolerance controls the surface of the feature but doesn't dictate how the datum is obtained or what type of datum it is.

 

[pmarc]

If this is not an example that can justify the mention of a line (alongside axis) in the definition, do you believe there could be another example? Or is the mention redundant since "axis" covers all possible cases that could ever be attributed to "line"?

I believe it is bad that "axis" and "line" are both mentioned in the Y14.5 definition of datum. Only one of them should be listed and if it was my choice, I would choose "line" (as more generic term than axis) and more specifically I would call it "straight line" (as there can be curved lines as well).

I cannot think of a practical example where anything besides a "point" or "straight line" or "plane" or combination thereof would be needed as the definition of a datum derived from the datum feature. Whether this is or isn't a convenient/straightforward/user-friendly description of datums in the datum theory, as has been already discussed a few times in this forum, is a different story.

 

[Burunduk]

I would choose "line" (as more generic term than axis) and more specifically I would call it "straight line" (as there can be curved lines as well).

That would make the Y14.5 datum terminology very similar to the ISO descriptions, but I guess it is OK in this particular context (I just hope "situation features" and other unnecessary stuff is avoided).

Here's another observation which I wonder what you think about:
Usually one can relocate the datum reference frame from any default location to any arbitrary place which may be found convenient, as long as the relationship between the DRF axes and the true geometric counterparts that interact with the datum features is theoretically exact and constant (do you disagree?). I think the default for a torus-shaped primary datum feature would be to establish the DRF origin coincident with the axis of the TGC of the feature (that axis would be directed into/out the page, located a radius away to the left of the part). In that case it's clear that a rotational DOF about that axis remains. However if for convenience I translate the DRF to where it is currently shown in my figure (to bring it closer to the part) it is no longer clear which DOF remain free and which are constrained. Relative to the currently shown CSYS, the tangential transformation looks like a translation in a non-constant direction, and it doesn't look like any rotational DOF is unconstrained. It means that at least in this particular case the DRF has a meaningful placement that shouldn't be overridden. What would you say?

 

[Kedu]

Pmarc,
I would like to ask, if you don't mind to use this discussion, what is "situation feature" in ISO? Burunduk mentioned it above and I read it a bit in ISO5459 with no chance for me to understand it or even see a good application of such term.
If you guys prefer I can open a new discussion here or on GD&T and GPS World on linkedin.
Just let me know if my inquiry is related with Burunduk's original post.

 

[Burunduk]

Kedu,
Situation Features are elements of the types plane, straight line, point or helix derived from the "associated feature" - which is the ISO term for true geometric counterparts. ISO 5459 defines that location and orientation of toleranced features is established from the Situation Features. A Datum consists of one or more Situation Features.