Symmetric Parts and Inspection of Features

[GravyBagel]

I'm working through something funny. I've got a long extruded rectangular (slightly off square) prism that is symmetric in X/Y/Z. I primarily care about the rectangular ends being perpendicular to the long sides and parallel to each other as these are measurement locations. I utilized two of the long flat sides as datum features and listed perpendicularity on each end.

First, is this correct?
Second, it seems that because of the symmetry, it's impossible to tell which face is the datum face.
Does this mean that the geometric call outs must be supplied in all possible orientations?
Am I choosing my datum features incorrectly because they are ambiguous?
How can I proceed forward?

Thanks!

 

[ctopher]

It's projected. The left view, face toward you, is datum A. Bottom surface is B.

Chris, CSWP
SolidWorks
ctophers home

 

[3DDave]

It is a poor practice to have the part datum features be indistinguishable. Typically this is managed by adding a feature or features to ensure correct identification. For example on one end of the part, adding a chamfer where datum feature A intersects an end face will provide a unique orientation.

In the 2018 version of ASME Y14.5, paragraph 7.8:

"Therefore, in the case of symmetrical parts or parts with identical features, physical identification of the datum feature on the part may be necessary."

 

[GravyBagel]

This is a test specimen. It is intended to be symmetric to produce good test results. Adding features is adverse to that goal. I suppose a superficial mark (paint/ink) would satisfy without violating the intent of the part.

Thank you.

@CTopher, I think you misinterpreted the question. A could be the front or the back, B could be the top or the bottom. I could have both correct, A correct and B reversed, B correct and A reversed, or both reversed. They would look essentially the same in all circumstances, but under some conditions, may fail inspection, while under others, would pass.

 

[ctopher]

The face that you see is Datum A. It's projected from the right view. Regardless if you want to call it front/top/back/whatever.

Chris, CSWP
SolidWorks
ctophers home

 

[Burunduk]

GravyBagel,
It looks by the right view that the part section is square. It also makes the height and width interchangeable, and the order between A and B datum features can't be maintained repeatably. If you prefer not to add any features or marks, and you don't really care about the datum precedence order and about which side is datum feature A (near or far in the left view), and which side is datum feature B (bottom or top in both views), then consider using width type feature of size datums. The width could be datum feature A and the height could be datum feature B, and they could be referenced for perpendicularity as A-B in a single compartment of each feature control frame. This would solve the ambiguity and also communicate that datum precedence doesn't matter. You may also need to control the parallelism* within each pair of opposed surfaces, but this would be required anyway if you wish to prevent the situation that you described in your own words as:

They would look essentially the same in all circumstances, but under some conditions, may fail inspection, while under others, would pass.

*Note that if your company follows ASME Y14.5 and Rule#1, the width and height tolerances applied to the dimensions may prevent the surfaces from being too out-of parallel.