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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

The contents of this paper are solely the opinions of the author. They do not represent any company, organization, or committee position.


Table of Contents

Abstract
Introduction
A. Measured distances of the model as CAD system specified basic dimensions
B. Toleranced dimensions when dimensioning a solid model
C. Y14.5 Fundamental Rules
D. View dependent tolerances
E. Datum specification 
F. Display
G. Conclusion

 

Abstract
Eliminating 2D-drawings has been a goal in industry for many years. It is estimated that by the year 2000, 70% of new designs in the U.S. use a CAD model to communicate a portion of the design requirements. Dimensioning and tolerancing using digital drawings1 based on solid models2 is one of the keys to eliminating 2D-drawings. This paper examines the dimensioning and tolerancing practices described in ASME Y14.5M – 1994 and points out where they may not be directly applicable to dimensioning and tolerancing of digital drawings based on solid models. In these cases revisions to the Y14.5 standard or resolution in other standards (like Y14.41) is needed. It is not the intention of this paper to indicate where each item should be addressed, or what the solution should be in each case. The main purpose of this paper is to bring dimensioning and tolerancing of digital drawing issues to the surface for discussion and resolution.
 

Introduction
The dimensioning and tolerancing of digital drawings based solid models is becoming of increasing interest to firms all around the world. It appears that it will finally break the chains of having to depend on a 2D-drawing for communicating product requirements. Industry has been trying to eliminate 2D-drawings ever since the use of CAD systems became widespread. The topic of dimensioning and tolerancing has been developed over the last 160 years, into a sophisticated, comprehensive language called GD&T.  GD&T although not perfect, does work well for traditional 2D-drawings with orthographic views and with some modifications and additions can be used to tolerance digital drawings based on solid models.

Dimensioning and tolerancing of digital drawings based on solid models creates a number of interesting and sometimes problematic situations. If we want to replace 2D-drawings with digital drawings based on solid models, these issues need to be addressed.  The ASME Y14.5M – 1994 standard is used for dimensioning and tolerancing 2D-drawings in the US and in many other countries around the world. This standard is the premier standard for dimensioning and tolerancing 2D-drawings. It also includes a few practices that apply to digital drawings based on solid models. But to a large extent, the dimensioning and tolerancing is based on specifying the dimensions and tolerances on orthographic views of a drawing. Another ASME standard is in development, Y14.41, it will address many of issues involved with tolerancing of digital drawings based on solid models.

Currently each company using the Y14.5 standard has the ability to, and often does, add or customize the specification of tolerances to suit their local needs. This is legitimized in the Y14.5 standard by paragraph 1.1.4.  When the tolerancing digital drawings based on solid models becomes popular, the CAD software will also check the legality of each tolerance specification. This will be a good news/bad news event. It will be good news because the CAD software will improve the dimensioning and tolerancing of parts and in some cases even be tutorial to the designer. Downstream activities like analysis, manufacturing, and inspection will all benefit from the improved quality of tolerance specifications. The bad news is the software will not recognize a lot of the local tolerancing practices used by many companies. In some cases companies will have great difficulty getting the CAD software to accept their tolerancing information.

The scope of this paper is limited to issues on specifying part tolerances and the views required to accomplish the tolerancing on digital drawings.

Currently there are a variety of methods used by companies are assigning tolerances to solid models. In order to analyze the needs of tolerancing digital drawings based on solid models, a single set of conditions needs to be used. The list of conditions below is the basis for the comments in this paper:
    1. A digital drawing is required.
    2. Measured distances of the model are used as “CAD system specified basic dimensions” and geometric controls are used to tolerance the CAD system specified basic dimensions.
    3. If a toleranced dimension is specified, the part feature(s) involved are no longer CAD system specified basic to each other.
    4. The display of the tolerances must be interpretable by a human without interrogating the solid model  (database).
    5. The dimensions and tolerances shown are associative to the model and can be interpreted for analysis, manufacturing, and inspection.

1Digital drawing – An electronic file that uses a digital representation of the part geometry along with dimensions and tolerances and other annotations to communicate design requirements.

2Solid Model – In this paper, this term is used as a generic term to refer to part geometry defined in a digital database. Currently companies use solid models, wireframe models, and sheet solids.
 


The issues resulting from using Y14.5 to dimensioning and tolerance digital drawings based on solid models are categorized into six areas: CAD system specified basic dimensions, use of toleranced dimensions, fundamental rules, view dependent tolerances, datum specification issues, and display issues.

The tables shown below are a result of an initial comparison between ASME Y14.5 and the practices used on tolerancing of solid models. Additional items will be found as attention is brought on to this topic.
 

A. Issues when using measured distances of the model as CAD system specified  basic dimensions of the part
Using measured distances of the model as CAD system specified basic dimensions. This is simply the practice of using the model distances as basic dimensions to define part features and then using geometric controls to tolerance the features. Many companies have been using this practice for a number of years now. CAD system specified basic dimensions can save significant design time. However, a number of interesting issues arise.
(Note – When human specified basic dimensions are used, the rules and conventions for display, use and tolerancing are well documented in the ASME Y14.5 standard.)

Currently, the ASME Y14.5 standard does not specifically address these conditions. The list below is a collection of items that  need to be documented on how specify and interpret basic relationships on digital drawings based on solid models.
 
Issue No.
Description
Y14.5 Ref
Issues
Comments
A1
Specifying Basic Dimensions Para 1.1.10 If basic dimensions are automatically specified in the CAD system, respecifying basic dimensions is not necessary and will add confusion for the drawing user. The values may be different between the two basic dimensions.  Should basic dimensions be allowed to be specified when dimensioning solid models? 
A2
User specified basic dimensions Para 1.3.10 hen a basic dimension is shown should it be a reference dimension? 
A3
Use of a note to specify basic dimensions Fig 6-12 Use of a note to specify basic dimensions  Fig 6-12 Y14.5 allows the use of the note “UNTOLERANCED DIMENSIONS ARE BASIC” to denote basic dimensions. Can this practice be used on solid models? 
A4
Use of a general tolerance note to tolerance basic dimensions Para 1.3.9 & 5.2.1.1 Currently, basic dimensions are toleranced by geometric controls. Is it acceptable to use a general note to apply tolerance to a basic dimension?  Rules need to be established on the use of general notes to tolerance basic dimensions. 
A5
Use of CAD system specified  basic for size of a feature of size with only the tolerance shown on the drawing Para 1.3.9 & 5.2.1.1 Currently, basic dimensions are toleranced by geometric controls. Is it acceptable to apply a tolerance to a basic dimension?  This affects the application of Rule #1.
A6
Dimensioning of 90° angles It is impossible to see the difference between a 90° CAD system specified angle and an 89° angle in many cases. Should angles that are near 90 be dimensioned instead of CAD system specified?  Not dimensioning the angles would require the user to interrogate the database to determine each angle. The danger is the drawing user may think an angle is a right angle and not realize the need to check further. Would a user have to check every (near 90°) angle to know what value it is? 
A7
Use of CAD system specified basic dimensions for datum target, location, size, and orientation In solid model dimensioning the dimensions that describe datum targets are CAD system specified. Is it desirable or acceptable to re-specify the basic dimensions as well? Also see issues A1, A2, A3, A4. 

B. Issues when using toleranced dimensions on a solid model.
The practice of using toleranced dimensions to define part features has been around even longer that geometric tolerancing. Most of the standards community would agree on the shortcomings and limited usefulness of toleranced dimensions. Limiting or prohibiting the use of toleranced dimensions has been discussed in National and International standards organizations. Toleranced dimensions, as they are currently defined, present many problems for use with solid models.
 
Issue No.  Description  Y14.5 Ref Issues  Comments 
B1
Use of toleranced dimensions with no nominal specified. Para 1.3.2.8 On toleranced dimensions where the nominal is not stated, it results in confusion on the value to model the feature.  Rules and conventions need to be established on the use of toleranced dimensions on solid models. Perhaps all toleranced dimensions on solid models should have a nominal stated? (Some believe that when using a model, the model size should be defined as the nominal.)
B2
Use of max, and min dimensions  Para 2.5 When a max or min dimension is specified, no nominal exists. The value the model is made to is not defined clearly. Rules and conventions need to be established on the use of max and min dimensions on solid models. Perhaps a max dimension could be presented as 2`+0 / -2`? 
B3
 Use of a toleranced dimension (which is not a feature of size) with no datum sequence specified? A toleranced dimension has no datum sequence specified, therefore assumptions must be made on how to interpret the dimension for analysis or measurement. If a toleranced dimension is to be used for analysis or inspection, a datum sequence is needed. Rules and conventions need to be established for the use and interpretation of toleranced dimensions on solid models. Perhaps it is time to limit the use of toleranced dimensions. 
B4
Specifying both a basic and a toleranced dimension to a part feature  Fig 4-33 Y14.5 allows the use of both a basic and a toleranced dimension shown to a part surface. With solid models, a CAD system specified basic dimension already exists to every part surface and when a toleranced dimension is added it overrides the CAD system specified basic dimension.  Rules and conventions need to be established for the use of toleranced dimensions and CAD system specified basic dimensions
B5
Use of overall dimensions when CAD system specified basic dimensions are present Para 1.7.7 
Fig 1-17, Fig 1-18
 In Y14.5 when an overall dimension is specified, one of the sub-dimensions must be a reference dimension. With solid models, the sub dimensions would often be CAD system specified basic dimensions. How can a CAD system specified basic dimension be designated as a reference dimension?  Rules and conventions need to be established for the use of overall toleranced dimensions and CAD system specified basic dimensions 
B6
Use of the word “true” when dimensioning a feature not in true projection  Para 1.8.2.3 In Y14.5 it is required to use the word “true” when dimensioning a part feature that is not in the plane of the paper. In a solid model drawing many of the dimensions are shown in views where the feature is not in true projection. This requirement to use “true” may need to be relaxed for solid models. 
B7
Rules for spacing of dimension lines Para 1.7.1 When dimensioning a non-orthographic view should the rules for spacing of dimension lines shown in Y14.5 fig 1-6 be used or should new rules be created? 
B8
Extension lines Para 1.7.2 Extension lines applied to a solid model do not always communicate the design intent, should new rules for extension lines be written? Perhaps extension planes should be used? 

C. Fundamental Rules
In paragraph 1.4  of Y14.5 there are 14 fundamental rules. The rules are written to a large extent to cover 2D drawings. When applying the rules to solid models, several issues need to be addressed.
 
Issue No.
Description Y14.5 Ref  Issues Comment
C1
Each surface of the model is at a  basic relationship to every other surface on the model.  Para 1.4 Fundamental rule (a) Is it necessary to specify a tolerance for all the basic relationships? Currently, Y14.5 requires each dimension to have a tolerance.  Does each CAD system specified basic dimension need a tolerance? Perhaps each feature should require a tolerance not each dimension.
C2
CAD system specified basic dimensions applying at assembly levels.  Para 1.4 Fundamental rule (n) A measured model distance as a basic dimension appears at each level the model appears. Does a CAD system specified basic a dimension  apply at all levels where the model appears automatically? In certain cases the intent may be to have the CAD system specified basic dimension from the detail apply at the assembly. How do you invoke certain CAD system specified basic dimensions and disallow the rest.  Since a measured model distance as a basic dimension appears at each level the model appears, there needs to be additional rules describing the levels where a CAD system specified basic dimension does apply. 
C3
Implied toleranced 90° angles and CAD system specified basic 90° angles. Para 1.4 Fundamental rules (i) and (j) Does the implied 90° angles only apply to orthographic views? 
Is there a need for 90° implied toleranced angles?
The rules for basic and toleranced 90° angles need to be revised.

D. View dependent tolerances
When dimensioning and tolerancing digital drawings, non-orthographic views are often used. The use of non-orthographic views requires additional documentation of rules and conventions for display of dimensions and tolerances. (Note – When orthographic views are used the rules and conventions for display of dimensions and tolerances are shown in the ASME standards.)
 
Issue No. Description Y14.5 Ref Issues Comments
D1
Use of the “between” symbol Para 3.3.11 Which surface does the profile control apply to, the front or the top? Should the use of the “between” symbol be restricted to orthographic views?
D2
Use of the “Straightness” symbol, applied to a surface.  Para 6.4.1.5 and fig 6-6 In which direction does the tolerance zone apply?  Should the use of the “Straightness” symbol, applied to a surface, should be restricted to orthographic views or should ASME define new rules for its use and interpretation 
D3
Use of the “Profile of a line” symbol  Para 6.5.3 and fig 6-18 In which direction does the tolerance zone apply? Should the use of the “profile of a line” symbol should be restricted to orthographic views or should ASME define new rules for use and interpretation 
D4
Use of the “All around” symbol Para 6.5.1 and fig 6-19 In which direction does the all around symbol apply? Should the use of the “all around” symbol should be restricted to orthographic views or should ASME define new rules for its use and interpretation?
D5
Use of “Unilateral” tolerance zones with profile  Para 6.5.3 and fig 6-11 It is difficult to determine which surface is toleranced.The  “Unilateral” tolerance zone specification can become obscured when the model is rotated. Perhaps a new modifier or revised rules would eliminate the need for view dependency or should unilateral profile be limited to orthographic views?
D6
Use of “Unequal bi-lateral” tolerance zones with profile Para 6.5.1 and para 6.5.3 Is a CAD system specified basic dimension OK for the offset? When the model is rotated, the specification can become difficult (or incorrect) to interpret. Perhaps a new modifier or revised rules would eliminate the need for view dependency or should the use of  “Unequal bi-lateral” tolerance zones be restricted to orthographic view 
D7
Bi-directional positional tolerancing rectangular coordinate method Para 5.9.1 and fig 5-41 What determines the direction of the tolerance zones? Should the use bi-directional positional tolerancing be limited to orthographic views or should ASME define new rules for its use and interpretation? 
D8
Dimensioning to a true profile Para 6.5.1 A profile control must be applied to a true profile. In a non-orthographic view a true profile doesn’t exist. Should we limit the use of profile to an orthographic view or should ASME define new rules for the use of profile?
D9
Limited length or area indication Para 1.7.3 and fig 1-11 The chain line to could be out of site when the model is rotated. How far away from the view can the line be shown? 
D10
Use of a chain line to specify a partial datum Para 4.5.10 and fig 4-23 Does the chain line apply to the top or side of the part?. How far away from the view can the line be shown? When the view is rotated, the chain line may be hidden behind the view. 

E. Datum specification issues
When specifying datums on digital drawings, in many cases, the current Y14.5 conventions apply. However, there are a few areas where the Y14.5 conventions do not directly apply. In these cases new conventions need to be established.
 
Issue No.
Description Y14.5 Ref Issues Comments
E1
Datum feature symbol location Can the datum feature symbol be shown on the edge between two surfaces to designate one of the surfaces as a datum feature?
E2
Datum feature symbols on extension lines When the datum feature symbol is shown on extension lines in a non-orthographic view  it is not clear that the entire surface is the datum feature.
E3
Specifying equalizing datum targets. Para 4.6.6 
fig 4-38
The absence of a basic dimension denotes the ability for the targets to adjust to center the part. In solid model dimensioning, datum targets are located by basic dimensions that are CAD system specified. This creates difficulty when specifying an equalizing datum target.  Perhaps a new symbol for movable datum targets should be considered along with revising the current convention on the absence of a basic dimension indicating a movable target. 
E4
Dashed leader lines Para 4.6.1 As the model is rotated datum targets that appear on the far side of the part can move to the near side of the part.can If dashed leader lines are used, they become incorrect Eliminate the use of dashed leader lines 
E5
Datum symbol termination Para 1.7.4 When directed to a surface, should the datum feature symbol triangle be attached to the surface or should a leader terminating with a dot be used?

F. Display issues
Dimensioning and tolerancing of digital drawings is complicated by the use of non-orthographic part views. The trend seems to be to tolerance a non-orthographic view. Problems begin to occur when the rules for tolerancing orthographic views are being used on non-orthographic views. The list below demonstrates a number of issues that need to be resolved when using Y14.5 to tolerance digital drawings.
 
Issue No. Description Y14.5 Ref Issues Comments
F1
The display of cutting planes for sections on a non-orthographic views 
Y14.3 
Para 3.2.1
On orthographic views cutting planes are shown by using a cutting plane line which represents the edge view of a cutting plane. On non-orthographic views, a convention for the direction the cutting plane cuts through the part needs to be established. Perhaps a full plane should be shown instead of the edge view of the cutting plane as currently used in orthographic views.
F2
Display of a section cut from a non-orthographic view .
Can a section be cut from a non-orthographic view be displayed as an orthographic view? 
F3
Direction for reading notes 
Para 1.7.5.1, Y14.4 para 3.7.2
The direction for reading notes is from the bottom of the drawing. Do we apply this convention to digital drawings or are new conventions to be used?
F4
Application of Dimensions
Para 1.7
Y14.4 Para 2.7
In orthographic views there are rules for dimensioning which cover the following. 
- Spacing of dimension lines 
- Alignment of dimension lines 
- Leader lines. 
When tolerancing non-orthographic views, these rules may not be applicable.
Rules are needed for which dimensions used on non-orthographic views. 
F5
Terminating leader lines
Para 1.7.4
When a leader terminates on a surface, is it permissible to use a leader terminating with an arrow, or must a dot be used? Rules are needed for terminating a leader line on a surface of a non-orthographic view.
F6
Text not in plane of screen
Y14.4 
Para 3.7
In many CAD systems the tolerances and notes are placed in the plane of the work coordinate system. When the model is rotated, the tolerances and notes also rotate. The tolerances and notes can become unreadable or their meaning can change. Rules and conventions for the readability and legality of rotated tolerances need to be established 

G. Conclusions

Y14.5 works well for 2D drawings and a large portion of the concepts can be carried over into digital drawings. This paper points out many of the areas where revisions or additions to Y14.5 are needed. If these items are not addressed, companies will go in various directions and digital drawings will take much longer to become the primary drawing type used in industry. The items discussed in this paper should be documented in a national standard or each software supplier may end up with their own version of how these items are addressed.

Display issues are the largest problem in moving from tolerancing 2D-drawings to tolerancing digital drawings.  It is here where the decision will be made on whether tolerancing digital drawings will be a natural step in the evolution of tolerancing or if a new system of tolerancing is being born. The decision can be reduced to the following questions. Do we want to dimension and tolerance solid models, or do we want to make digital drawings using solid models. Do we want to build on the existing rules used on 2D-drawings or do we want to create a new dimensioning and tolerancing system, which is non-human interpretable?
 
 
Display Method
Amount of current ASME standards that could be used.
Comments 
The display is interpretable by a human without interrogating the database.
75-90%
  - May require the use of stored model views for tolerancing. 
- Easier transition for the workforce 
- Can work with electronic data or printout 
- More difficult for software manufacturers 
The database contains a portion of the information needed to interpret the display.
20-40%
- Porcupine views or charted tolerances would exist
- Difficult transition for the workforce 
- May need to make human interpretable drawings as well 
- CAD system dependent until translators are perfected 
- Easier for software manufacturers 

I believe that industry would be best served by a system that would allow as much of the current language of dimensioning and tolerancing as possible to be used and to preserve the ability for human interpretation of digital drawings.

2005 Update
The ASME Y14.41-2003 Standard on Digital Product Definition Data Practices defines the exceptions and additional requirements to existing ASME standards for using product definition data or drawings in 3D digital format. The new standard advances the capabilities of Y14.5, Dimensioning and Tolerancing, the widely used standard pertaining to 2D engineering drawings.

The Y14.41 Standard establishes requirements for preparing, organizing and interpreting 3D digital product images.ETI has developed a Solid Model Tolerancing course intended for designers, engineers, and managers who are considering implementation of a math-based product development process. The course will help you to understand the benefits of a math-based product development. It is not based on any single CAD system or product development process. It covers concepts and standards that can be applied with a number of CAD systems.

To learn more about this class, call 800-886-0909 or email info@etinews.com today.
 

 



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