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Some Additional Opinions on the Material Presented in the White Paper



"Issues When Using Existing ASME Standards (Y14.3, Y14.4, Y14.5) for Dimensioning and Tolerancing of Digital Drawings" ( A White Paper ) Version 1.0, by Alex Krulikowski, 21OC98  Read the white paper

By: Lea Irwin
Date: 28-Oct-1998

Introduction
2-D Drawings Based on 3-D Models

A. Additional Issues on Sections of Original Paper:
A1: Specifying Basic Dimensions
A2: User Specified Basic Dimensions
A3: Use of a note to specify basic dimensions
A4: Use of a general note to tolerance basic dimensions
A5: Use of CAD system specified basic dimension for size of a feature of size with only the tolerance shown on the drawing
A6: Dimensioning of 90° angles.
A7: Use of CAD system specified basic dimensions for datum target location, size and orientation

B. Issues when using toleranced dimensions on a solid model.
B1: Use of toleranced dimensions with no nominal specified
B2: Use of max, and min dimensions
B3: Use of a toleranced dimension (which is not a feature of size) with no datum sequence specified
B4: Specifying both a basic and a toleranced dimension to a part feature
B5: Use of overall dimensions when CAD system specified basic dimensions are present
B6: Use of the word “true” when dimensioning a feature not in true projection
B7: Rules for spacing of dimension lines
B8: Extension lines

C. Fundamental Rules
C1: Each surface of the model is at a  basic relationship to every other surface on the model.
C2: CAD system specified basic dimensions applying at assembly levels.
C3: Implied toleranced 90° angles and CAD system specified basic 90° angles.

D. View dependent tolerances
D1: Use of the “between” symbol
D4: Use of the “All around” symbol
D5: Use of “Unilateral” tolerance zones with profile
D6: Use of “Unequal bi-lateral” tolerance zones with profile
D8: Dimensioning to a true profile

D2: Use of the “Straightness” symbol, applied to a surface.
D3: Use of the “Profile of a line” symbol
D4: Use of the “All around” symbol
D5: Use of “Unilateral” tolerance zones with profile
D6: Use of “Unequal bi-lateral” tolerance zones with profile
D7: Bi-directional positional tolerancing rectangular coordinate method
D9: Limited length or area indication
D10: Use of a chain line to specify a partial datum

E. Datum specification issues
E1: Datum feature symbol location
E2: Datum feature symbols on extension lines
E4: Dashed leader lines

E3: Specifying equalizing datum targets.
E5: Datum symbol termination

F. Display issues
F1: The display of cutting planes for sections on a non-orthographic  views
F2: Display of a section cut from a non-orthographic view .
F3: Direction for reading notes
F4: Application of Dimensions
F5: Terminating   leader lines
F6: Text not in plane of screen

Conclusions

Related Topics
 
 
 
 

By: Lea Irwin
Date: 28-Oct-1998
 

Introduction
In his paper, Alex does a good job of identifying many of the issues encountered with the direct application of the ASME standards Y14.3, Y14.4 and Y14.5 on digital drawings. However, I submit that the subject is not really one of the application of these standards to digital drawings, but rather to 3-dimensional models. The term “drawing” has a very specific meaning, one of a series of 2-dimensional views of a product with a specific set of rules as to its construction and interpretation. I believe a distinction needs to be made between this 2-dimensional presentation and the 3-dimensional presentation in order to allow the reader to avoid confusion. Therefore, I consider the 3-D model as an entity distinct from a Drawing. 

Much of my focus deals with the presentation of the symbols and their interpretation based on that presentation. I do not believe the dimensioning and tolerancing concepts defined in the standards are any different in 3-D than they are in 2-D. The question is, how do we communicate the dimensioning and tolerancing information necessary to ensure the manufacture of the desired product? 

The Y14 standards are written assuming a 2-dimensional medium. If that medium is paper or computer, it is still 2-dimensional. We have devised a language based on that 2-dimensional representation of a part. On a drawing, we cannot represent the actual part in 3 dimensions, so we provide a series of 2-dimensional views of the part which are sufficient for the reader to infer the 3-dimensional object which is to be made. With the use of 3-dimensional modelers, we need to devise a means to communicate the same dimensioning and tolerancing design intent, but we now have a 3-dimensional representation of the model with which to work. 

3-Dimensional Presentation of Annotation (e.g. dimensions and geometric tolerancing symbols)
I believe that many of the issues which arise from the application of the drawing standards to 3-dimensional models come from the literal application of the annotation using the same techniques applied on drawings. Although the following statement is an oversimplification, it exemplifies my primary point. In 2-D, we had to connect the annotation to a 2-dimensional representation of a 3-dimensional object. We have well specified rules which a reader employs to infer the 3-dimensional portion of the part to which the annotation pertains. For example, if a GD&T callout is applied to a profile of a surface in a view, the tolerance applies to the surface which goes “into” the paper, the one perpendicular to plane of the view. In 3-D, we can connect the annotation to the actual 3-dimensional object. The reader no longer needs to infer the affected geometry, it can be communicated directly. We need to address the real underlying issue of indicting the controlled geometry in 3-D. The ambiguity which results from utilizing a 2-D presentation technique in 3-D is really a symptom, not a problem in and of itself. 

Another significant difference between interpreting annotation on a drawing and annotation on a 3-D model is that the 3-D  model is free to “move”. On a drawing, the representation of the part is static. In a 3-D modeler, the part model may be manipulated, i.e. moved and rotated. This is a definite benefit as it allows the viewer to study the model in a more natural manner. He/she can manipulate the model in space to gain a better comprehension of its structure. On a drawing, we have to resort to multiple views, sometimes detail or auxiliary views, in order to communicate the 3-dimensional characteristics of the part. It is this ability to move the part from a static viewing direction which causes the literal application of 2-dimensional annotation techniques to become problematical. For example, if GD&T is placed such that it points to an edge of a surface in a particular viewing orientation, when the 3-D model of the part is rotated, the meaning of the annotation becomes ambiguous as it now points to an edge shared by one or more surfaces. We should devise a means for applying annotation to 3-D models which best allows for their interpretation in 3-D and not require the viewer to use static views of the model to interpret the annotation. Although this sounds a bit philosophical, I'll offer it for the reader's consideration. We have actually been limiting ourselves by working in 2-D, albeit out of necessity. We should be able to express our annotation in a more natural fashion, i.e. 3-D, rather than continuing to limit the 3-D world by insisting on the use of more restrictive 2-D annotation techniques. 

2-D Drawings Based on 3-D Models 

The author recognizes the concerns with regard to the generation of drawings and hard copy output from 3-dimensional models. While many companies are moving toward annotating their 3-D models instead of relying on production of 2-D drawings, there is still a need to be able to produce paper based output. However, this need not necessarily imply that the techniques used to annotate a 3-D model be those used on 2-D drawings. If a drawing is produced, then the rules from the Y14 standards still apply.
 

A. Additional Issues on Sections of Original Paper: 

The following comments pertain to the six subsections of Alex's white paper. The following are intended as further consideration of the issues and are not meant to serve as solutions. 

Issues when using measured distances of the model as CAD system specified  basic dimensions of the part
We need to keep in mind that the idea is to communicate the dimensions. This is not intended as an affront to the reader of this paper, who certainly understands that. However, it is easy to forget this point and become embroiled in discussions of “where the dimension comes from”. 

A1: Specifying Basic Dimensions
There may be cases where it is advantageous to explicitly show a basic dimension, in order to properly communicate the design intent of the part, even if the value of the dimension is obtained from the CAD model.
If dimensions displayed on a 3D model are obtained from the model, should they even be allowed to have different values? Do we really need to present dimension values other than those from the model?
A2: User Specified Basic Dimensions
The question posed by Alex is a fair one but I submit this falls into the category addressed by my introductory comment on dimensions. The need is to be able to communicate the dimension value. It is intended as a basic dimension for the purpose of manufacturing or inspection. We need not confuse the intent with the mechanism used to obtain the dimension value, i.e. interrogation of the CAD model.
A3: Use of a note to specify basic dimensions
A4: Use of a general note to tolerance basic dimensions
I would like to add that  issues A3 and A4 are very much related to the ability to analytically process dimensions specified in general notes. We need to be able to enable electronic processing of the digital data models and general notes are problematical.
A5: Use of CAD system specified basic dimension for size of a feature of size with only the tolerance shown on the drawing
I refer back to my introductory comment on dimensions. The CAD system can certainly provide values from the model. If the values are not explicitly displayed, is there really any point of confusion? Is it simply a matter that any explicitly displayed dimension on a CAD model should take precedence, in terms of interpretation, over dimensions interrogated from the model? I am assuming the value will be the same but an explicitly displayed dimension may be toleranced whereas an interrogated dimension probably would not be.
A6: Dimensioning of 90° angles.
No additional comments.
A7: Use of CAD system specified basic dimensions for datum target location, size and orientation
No additional comments.
 
 
 
 

B. Issues when using toleranced dimensions on a solid model. 

B1: Use of toleranced dimensions with no nominal specified
The value used to model the feature is easily obtained from the CAD model. The question is what value should be used to manufacture the part. Is this issue any different on a 3-D model than it is on a drawing?
B2: Use of max, and min dimensions
Is this issue any different on a 3-D model than it is on a drawing?
B3: Use of a toleranced dimension (which is not a feature of size) with no datum sequence specified
Is this issue any different on a 3-D model than it is on a drawing?
B4: Specifying both a basic and a toleranced dimension to a part feature
I refer back to my introductory comment on dimensions. The CAD system can certainly provide values from the model. If the values are not explicitly displayed, is there really any point of confusion? Is it simply a matter that any explicitly displayed dimension on a CAD model should take precedence, in terms of interpretation, over dimensions interrogated from the model? I am assuming the value will be the same but an explicitly displayed dimension may be toleranced whereas an interrogated dimension probably would not be.
B5: Use of overall dimensions when CAD system specified basic dimensions are present
No additional comments.
B6: Use of the word “true” when dimensioning a feature not in true projection
The use of the word “true” is one of the techniques devised to overcome the fact that our presentation medium was a 2-D representation of a 3-D part. In 3-D, all dimensions on the model should reflect the actual measured size. The value of the dimension does not depend on the direction from which the model is being viewed.
B7: Rules for spacing of dimension lines
I concur with Alex's point. The rules for dimension spacing are an artifact of a paper presentation and have their origins in clarity of an image printed on paper.
B8: Extension lines
This is another case where we need a mechanism to indicate the 3-D geometry being dimensioned. Extension lines seem to be one of those tools which are 2-D oriented but are not immediately inapplicable in 3-D. These issues do need to be investigated.
 

C. Fundamental Rules 

C1: Each surface of the model is at a  basic relationship to every other surface on the model.
Are we getting too restricted by the literal interpretation that, because a CAD model can provide a measured value (i.e. a basic dimension) for any surface on the model, that each dimension should be considered as having been explicitly specified?
C2: CAD system specified basic dimensions applying at assembly levels.
Is it sufficient to control the visibility of the part dimensions in the context of an assembly?
C3: Implied toleranced 90° angles and CAD system specified basic 90° angles.
No additional comments.
 
 

D. View dependent tolerances 

D1: Use of the “between” symbol
D4: Use of the “All around” symbol
D5: Use of “Unilateral” tolerance zones with profile
D6: Use of “Unequal bi-lateral” tolerance zones with profile
D8: Dimensioning to a true profile
If we do not restrict ourselves to the use of 2-D techniques for indicating the controlled geometry, many of these issues are easily solved. The issue is that we need to directly indicate the 3-D geometry now that it is available. Should we propagate a 2-D mechanism into 3-D? 

D2: Use of the “Straightness” symbol, applied to a surface.
D3: Use of the “Profile of a line” symbol
D4: Use of the “All around” symbol
D5: Use of “Unilateral” tolerance zones with profile
D6: Use of “Unequal bi-lateral” tolerance zones with profile
D7: Bi-directional positional tolerancing rectangular coordinate method
D9: Limited length or area indication
D10: Use of a chain line to specify a partial datum
These issues are good examples of a dependency on the 2-D nature of the drawing presentation. An equivalent mechanism should be derived for 3-D representation which communicates the design intent while being as robust as possible in a 3-D environment where the part can be dynamically manipulated.
 
 

E. Datum specification issues 

E1: Datum feature symbol location
E2: Datum feature symbols on extension lines
E4: Dashed leader lines
If we do not restrict ourselves to the use of 2-D techniques for indicating the controlled geometry, many of these issues are easily solved. The issue is that we need to directly indicate the 3-D geometry now that it is available. Should we propagate a 2-D mechanism into 3-D? 

E3: Specifying equalizing datum targets.
No additional comments.
E5: Datum symbol termination
No additional comments.
 
 

F. Display issues 

F1: The display of cutting planes for sections on a non-orthographic  views
Just a small addition to Alex's points. We need to be careful as we devise presentation techniques for 3-D that they do not add unnecessary clutter to the model. As all annotation may now be presented on the model at the same time, the annotation techniques should not add clutter or obscure the geometry. For example, planes will be more obtrusive than lines.
F2: Display of a section cut from a non-orthographic view .
No additional comments.
F3: Direction for reading notes
No additional comments.
F4: Application of Dimensions
I concur with Alex's point. The rules for dimension spacing are an artifact of a paper presentation and have their origins in clarity of an image printed on paper with ink.
F5: Terminating   leader lines
No additional comments.
F6: Text not in plane of screen
Another example of a dependency on the 2-D nature of the drawing presentation. An equivalent mechanism should be derived for 3-D representation which communicates the design intent while being as robust as possible in a 3-D environment where the part can be dynamically manipulated.
 

Conclusions 

I concur with Alex's assessment that the display issues are a central question in this discussion. I would counter however, that the use of the phrase “human interpretable” to mean “interpret using literally the same techniques and rules applied to a 2-D presentation” is too restrictive and somewhat misleading. It seems that humans can interpret what is seen on the 3-D display even if the model can be dynamically manipulated. 

Related Topics 

It is no longer sufficient to have the annotation “just look right” to a person. We need to explicitly identify and capture in the CAD data model, the portions of the part to which the annotation pertains. This is essential to enable the integration of computer based design and manufacturing packages. However, as this is along the lines of a solution, I have not included it in the previous material. 

Another subject is that of dynamically querying the 3-D model to obtain information about the controlled geometry. I feel this is an extremely powerful tool and one which should be leveraged to its fullest extent in 3-D modelers. However, as this is something of a solution instead of an issue, I have purposefully avoided the topic. The technology is rapidly becoming available which makes this viable, even in applications outside of the CAD system. Also, because this tool cannot be leveraged in a paper presentation of a part model, it is not sufficient in and of itself. It needs to be combined with adequate visual interpretation of the annotation.
 
 

Issues when using ASME standards for solid models  L. Irwin
 

 



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