Phantom line gages as datum "features"?

[Sem_D220]

What would be your reaction to a drawing showing a prismatic feature on a part, comparable to those that are found on V-blocks, a phantom line circle tangent to the 2 angled surfaces of the prism, the circle specified with a basic diameter, and a datum feature symbol associated with the phantom line cylinder?

The datum axis derived from the cylinder adjacent to the prism will be used as one of the datums to established a datum reference frame for the part.

The problem I see is that the datum "feature" to which the datum feature symbol needs to be attached and from which the datum axis needs to be derived is not a feature of the part at all, but an external auxiliary component.

Would you be concerned that the scheme is not supported by ASME Y14.5? If it is supported - where and how? If not, what is the closest supported alternative?

 

[pylfrm]

Is there a way of explaining the fixed location of the tolerance zone for the considered hole without dealing with a datum axis and a datum point (as concepts)?

Consider a modified version of ASME Y14.5-2009 Fig. 4-44 without the basic dimension, but with a note that says "REFER TO 3D MODEL FOR DEFINITION OF BASIC GEOMETRY". The model would contain a representation of the part's six surfaces (and nothing else). Also imagine that the datum feature reference of the position tolerance is |A[BSC]| instead of |A|. Hopefully you agree that these changes are not detrimental to the example.

Here's how I would explain it:
[ul]
[li]The simulator for datum feature A is a boundary coincident with the basic geometry of the conical surface.[/li]
[li]The position tolerance zone is a cylinder of diameter 0.2 concentric with the basic geometry of the hole.[/li]
[li]The relationship between the actual part and the basic geometry is constrained by contact between datum feature A and the corresponding simulator.[/li]
[li]The position tolerance is satisfied if the UAME axis of the hole is within the tolerance zone.[/li]
[/ul]

Do you agree that this explanation does not involve the concept of a datum axis, datum point, or datum reference frame?


pylfrm

 

[Sem_D220]

pylfrm, I think I'm finally starting to understand your approach. I'd like to explain it with my own words - please tell me if I'm on the right path.

The drawing and everything related to it includes only the basic geometry with selected features designated as datum features, the required basic geometrical relationships, tolerance requirements and nothing else. It doesn't imply or require the establishment of datums or datum reference frames.

The theoretical aspect of validating drawing specifications includes engaging the actual part with a virtual "fixture" that is constructed of:
1. Envelopes representing the datum feature simulators behaving as prescribed by the material boundary modifiers or lack of them.
2. The theoretical basic geometry constrained to the datum feature simulator envelopes according to the basic relationships imposed by the drawing or cad model.
3. Tolerance zones of all the part features constrained by the basic geometry.

After engaging the part by its' datum features with the virtual "fixture" according to the datum features specified in the feature control frame and their order of precedence the tolerance zones act as a 3D "overlay" for validation of specifications on the actual part.

No datums or datum reference frames involved in any step of the process.
Is that the right way to understand this? Is there anything I'm missing?

 

[pylfrm]

Sem_D220,

That sounds pretty close to my approach. I have some quibbles about the wording of your numbered list (and still prefer the bullet points in my 23 Apr 19 03:57 post), but I fully agree with the other paragraphs.


pylfrm

 

[Sem_D220]

Nice to kbow know I'm on the right track with this. Could you tell what the quibbles regarding the "virtual fixture" be about?

 

[pylfrm]

Could you tell what the quibbles regarding the "virtual fixture" be about?

At the risk of stating the obvious, they are about the differences between the following:

[ul]
[li]The tolerance zones are related to the basic geometry of the toleranced features according to definitions in the standard.[/li]
[li]A drawing or CAD model defines the basic geometry of the toleranced features and datum features, as well as the relationship between them.[/li]
[li]The datum feature simulators are related to the basic geometry of the datum features according to definitions in the standard.[/li]
[/ul]

1. Envelopes representing the datum feature simulators behaving as prescribed by the material boundary modifiers or lack of them.
2. The theoretical basic geometry constrained to the datum feature simulator envelopes according to the basic relationships imposed by the drawing or cad model.
3. Tolerance zones of all the part features constrained by the basic geometry.

pylfrm

 

[Sem_D220]

Then I suppose there are no important differences. Thank you pylfrm. It is great to look at familiar things differently from time to time.