ATTACHMENT_PMI_Rec_Pracs_2013_3_6.pdf

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Attached to: The contractor shall provide NIST with the expertise needed to ensure the planning and foundational research for edition 2 of the ISO 10303-242 standard, and to ensure successful deployment of ISO 10303-242.
Solicitation number: SB1341-13-RQ-0357
Issued by: Department of Commerce National Institute of Standards and Technology

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CAx-IF Recommended Practices for the

Representation and Presentation of Product Manufac-turing Information (PMI)

Version 3.6, 02/04/2013

Status: in Work

Contacts:

CAx-IF

Jochen Boy PROSTEP AG / ProSTEP iViP Association Dolivostraße 11 64293 Darmstadt / Germany jochen.boy@prostep.com

Phil Rosché ICF International / PDES, Inc.

5300 International Blvd.

North Charleston, SC 29418 USA rosche@scra.org

Technical

David Briggs Boeing Commercial Airplanes, Information Systems P.O. Box 3707 M/S 2R-97 Seattle, WA 98124-2207 USA david.d.briggs@boeing.com

Ed Paff ITI TranscenData 5303 Dupont Circle Milford, OH 45150 USA ejp@transcendata.com

Bryan Fischer Advanced Dimensional Management LLC 16004 Tualatin-Sherwood Road #163 Sherwood, OR 97140 USA brfischer@advdm.com

Tony Ranger Theorem Solutions Ltd Tame House Fradley Park Staffordshire United Kingdom WS13 8RZ tony@theorem.co.uk

Work in Progress

For Review only!

Representation and Presentation of Product Manufacturing Information (PMI) Version 3.6, 02/04/2013

Document History

Ver-sion

Date Author Description

1.0 May 1, 2006 Dave Briggs, Tom Hendrix

Initial Merge of GD&T and DT Recommended Prac-tices

1.1 May 18, 2006 Steve Yates Completed merging documents

1.2 July 10, 2006 Steve Yates Revised some diagrams and added better specifi-cation for line and rectangular target areas

1.3 Sep. 15, 2006 Steve Yates Revision of Representation diagrams and first Presentation practices inserted

1.4 Dec. 29, 2006 Steve Yates Completed FCF presentation section, published document

2.0 Dec. 23, 2008 Tony Ranger Update shape_aspect to geometry linkage to use geometric_item_specific_usage and shape_aspect to draughting_callout linkage to use draughting_model_item_association

2.1 Jan. 29, 2009 Tony Ranger Complete Recommendations for Presentation Data for Dimensions, Tolerances and Associated Data

2.2 Feb. 19, 2009 Tony Ranger Correct the illustration numbers and references

2.3 Feb. 23, 2009 Jochen Boy Incorporation of “Polyline Presentation “ Recom-mended Practices, Addition of PMI Validation Prop-erties

2.4 Apr. 8, 2009 Jochen Boy Moved document to new template, general clean-up

2.5 Jan. 19, 2010 Jochen Boy Sections 8 to 10: Added missing contents from Pol-yline RecPracs, updated to technical discussions in 2009.

2.6 Apr. 20, 2010 Jochen Boy Sections 9 & 10: Advanced View Implementation, Section Views, and additional ValProps.

3.0 Jan. 20, 2011 Ed Paff Reworked Dimension and Tolerance sections to resolve missing representation information.

3.1 Feb. 3, 2011 Jochen Boy Reworked Polyline and View sections to extend scope and reflect AP242 work.

3.2 Dec. 15, 2011 Ed Paff Updated Dimension and Tolerance sections related to representation information

3.3a Jan. 1, 2012 Ed Paff Updated representation information

3.4 Jan. 18, 2012 Jochen Boy Replaced Sections 7 & 8 for Semantic Presentation and linking Presentation to Representation

3.4 May 15, 2012 Ed Paff Updated figures and text

3.5 Nov. 1 2012 Ed Paff Updated for AP242 DIS

3-6 Feb. 4, 2013 Jochen Boy Extended Graphic Presentation Sections

Version 3.6, 02/04/2013

Contents

Table of Contents

1 Introduction

1.1 Introduction to Previous Issue

1.2 Document Identification

2 Scope

3 Fundamental Concepts

3.1 Dimension and Tolerance

3.2 Dimensions and Dimensional Tolerances

3.3 Geometric Tolerances

3.4 Feature Entities and Attributes

3.5 Identifying Features

3.6 Datum Systems

4 Defining the Dimensioning Standard

4.1 Default Tolerance Decimal Places

5 Implementation Guidelines for Dimensional Tolerances

5.1 Associating Dimensions with Features or Geometry

5.2 Application of Values to Identified Tolerances

5.3 Dimension Modifier

5.4 Applying Number of Decimal Places

6 Implementation Guidelines for GD&T Representation

6.1 Associating Tolerances with Features

6.2 Associating Tolerances with Dimensions

6.3 Associating Tolerances with Part (All-Over)

6.4 Associating Tolerances with Multiple Features

6.5 Implementing DATUMS in a STEP File

6.6 Implementing DATUM TARGETS in a STEP File

6.7 Feature Control Frames

6.8 STEP Supported Tolerance Types

6.9 Implementing Feature Control Frames

7 Presentation of PMI Data

7.1 Basic Principles

7.2 “Minimal” Semantic Presentation

7.3 Linking PMI Representation to Presentation

8 Graphic Presentation

8.1 Definition of the Polylines

Version 3.6, 02/04/2013

8.2 Names for the geometric_curve_set / annotation_fill_areaFehler! Textmarke nicht definiert.

8.3 Styling the Annotation

9 Presentation Organization

9.1 Collecting all Presentation data for a Part ........................ Fehler! Textmarke nicht definiert.

9.2 Annotation Planes and global Draughting Model ............. Fehler! Textmarke nicht definiert.

9.3 Linking Annotations with Geometry

9.4 Saved Views

10 PMI Validation Properties

10.1 Validation Properties on the Part/Product level

10.2 Validation Properties for Saved Views

10.3 Validation Properties for Polyline Annotations

11 Conclusions

Annex A Referenced Documents

Annex B Text Encoding

Annex C Comments against this Document

C.1 Previous Versions

C.2 This Version

Annex D Availability of implementation schemas

D.1 AP242

D.2 AP214 3rd Edition

D.3 AP203 2nd Edition

D.4 AP203 1st Edition with modular extensions

Figures

Figure 1: Choice of Dimensioning Standard (representation)

Figure 2: Choice of Modeling Standard (presentation)

Figure 3: Default Tolerance Decimal Places

Figure 4: Multiple Geometric Items Representing Single Feature

Figure 5: Attaching to Dimensioned Entities

Figure 6: Dimensional Location Examples (with diameter)

Figure 7: Oriented Dimensional Location

Figure 8: Derived Shape Aspect

Figure 9: Dimensional Size

Figure 10: Dimension With Path

Figure 11: Nominal Value

Figure 12: Nominal Value with Qualifier

Figure 13: Nominal Value with Plus/Minus Bounds

Figure 14: Value Range

Version 3.6, 02/04/2013

Figure 15: Tolerance Class

Figure 16: Dimension Modifier

Figure 17: Decimal Places

Figure 18: Defining Toleranced Features using Shape_Aspect

Figure 19: Multiple Feature Construct

Figure 20: STEP entities for defining DATUMs

Figure 21: DATUM referencing mulitple features

Figure 22: DATUM defined by group of features

Figure 23: Datum Target Instantiation

Figure 24: Area Datum Target

Figure 25: Relating Datum Target to Feature

Figure 26: Feature Control Frame

Figure 27: Tolerance without Modification or Datums

Figure 28: Tolerance Zone

Figure 29: Other Tolerance Zones

Figure 30: Intersection Plane Affected Tolerance Zone

Figure 31: Orientation Plane Affected Tolerance Zone

Figure 32: Projected Tolerance Zone

Figure 33: Geometric Tolerance with Modifiers

Figure 34: Geometric Tolerance with Datums

Figure 35: Tolerance with Datum Reference

Figure 36: Tolerance with Datum Reference and Axis System

Figure 37: Common/Multiple Datum

Figure 38: Common/Mulitple Datum Instantiation

Figure 39: Composite Geometric Tolerance

Figure 40: Composite Tolerance Structure

Figure 41: Linking PMI Representation to (Polyline) Presentation

Figure 42: Outline Characters (upper left), Filled Characters (upper right), Stroked Characters and Geometric Elements (below)

Figure 43: Basic Polyline Definition

Figure 44: Filled Polyline Definition

Figure 45: Definition of boundaries using composite_curves

Figure 46: A PMI element defined as a combination of Filled and Basic Polylines

Figure 47: Styling a Polyline Annotation

Figure 48: Definition of the overriding style for a Polyline annotation

Figure 49: The draughting_model as a collector for all presentation dataFehler! Textmarke nicht definiert.

Figure 50: Definition of the view plane

Version 3.6, 02/04/2013

Figure 51: Linking the Annotations together

Figure 52: The use of annotation_planes to group some of the draughting_callouts

Figure 53: Identification of the relevant portion of Geometry

Figure 54: Associating the Annotation with the Geometry

Figure 55: Defining a Saved View with draughting_model and camera_model

Figure 56: Camera definition for a Saved View

Figure 57: Definition of a Section View using a single Plane

Figure 58: Schematic representation of possible clipping cases

Figure 59: Definition of a Saved View using a Combination of Planes

Figure 60: Relating the global and Saved Views

Figure 61: Advanced Saved View Implementation

Figure 62: PMI Validation Property for total number of annotations in the file

Figure 63: PMI Validation Property for number of annotations in a saved view

Figure 64: PMI Validation Property for total number of annotations in the file

Figure 65: Equivalent Unicode String validation property

Tables

Table 1: Dimensional Location Types (no circular cross-section)

Table 2: Dimensional Location Types (circular cross-section)

Table 3: Types of Derived Shape

Table 4: Dimensional Size Types

Table 5: Tolerance Principle

Table 6: Value Qualifiers

Table 7: Dimension Modifiers (type 1)

Table 8: Dimension Modifiers (type 2)

Table 9: Instantiation of DATUM TARGET types

Table 10: Supported Tolerance Types

Table 11: Tolerance Zones

Table 12: Usage of draughting_model_item_association to link Representation with Presentation 46

Table 13: Suggested list of names

Table 14: Global and Saved View draughting_model properties

Table 15: Attribute values for the 'number of view' validation property

Version 3.6, 02/04/2013

1 Introduction

This document is an updated version to that issued in December 2006 entitled “Recommended Practices for Dimensions, Dimensional and Geometric Tolerances”. Most significantly, additional practices are defined to cover presentation definitions for the GD&T data. Some definitions were provided for presentation data in the previous version. Some of these have been modified in the light of various implementations by the CAX-IF and other projects.

This version also incorporates updates to the method by which shape_aspect is linked to the geometric items of the shape definition.

The document has been amended where appropriate to include the additional content. For com-pleteness of the definition, the introduction to the previous issue is included as section 1.1. Where entity diagrams have been modified within the text, the originals are provided in Appendix 1.

It should be noted that this document does not adhere, in itself, to any specific standard for the definition or presentation of GD&T data. The document provides recommendations as to how specific data constructions may be defined using STEP entities within a STEP part 21 file. If the original data adheres to a standard in the way it is defined, then the STEP definition should also adhere to that standard.

1.1 Introduction to Previous Issue

This document describes the recommended practices for implementing dimensions, dimensional and geometric tolerances. This document incorporates the content of “Recommended Practices for Dimensions and Dimensional Tolerances” written by Markus Hauser, Mike Strub and Tom Hendrix, dated April 18, 2000 and “Recommended Practices Guide for Geometric Tolerances” writ-ten by David Briggs and Tom Hendrix dated March 14, 2003. These two documents were com-bined to ensure a consistent approach to dimensioning and tolerancing. This document conforms to the agreements reached by the Tolerance Harmonization team representing AP203, AP214, AP224, AP238 and AP240.

Tolerances treat the uncertainty with which the realized shape or measurements of a real manu-factured object compare to their design ideals. If all parts could be manufactured perfectly as de-signed, there would be no need for tolerancing practices. However, it is certain that this cannot be done for finite cost in any but the most trivial cases. In the drawing world, tolerances are noted on the drawing per standard notations such as ANSI Y14.5 or ISO 1101.

This document is not intended as a primer on geometric tolerancing. The explanations included are only provided to relate common tolerancing techniques to the STEP entity structures. This is not a comprehensive coverage of any existing draughting standard but does provide a capability to exchange a variety of typical models. Future versions of this document will address additional ca-pabilities, in particular the STEP Structures required to convey the Presentation of Geometric and Dimensional Tolerances, as well as the application of Tolerances at the Assembly level.

1.2 Document Identification

For validation purposes, STEP processors shall state which Recommended Practice document and version thereof have been used in the creation of the STEP file. This will not only indicate what information a consumer can expect to find in the file, but even more important where to find it in the file.

This shall be done by adding a pre-defined ID string to the description attribute of the file_description entity in the STEP file header, which is a list of strings. The ID string con-sists of four values delimitated by a triple dash (‘---‘). The values are:

Version 3.6, 02/04/2013

Document Type---Document Name---Document Version---Publication Date

The string corresponding to this version of this document is:

CAx-IF Rec.Pracs.---Representation and Presentation of Product Manufac-turing Information (PMI)---3.6---2013-02-01

It will appear in a STEP file as follows:

FILE_DESCRIPTION(('...','CAx-IF Rec.Pracs.---Representation and Presentation of

Product Manufacturing Information (PMI)---3.6---2013-02-01',),'2;1');

2 Scope

There are two main methods of tolerancing: dimensional tolerancing and geometric tolerancing.

Dimensional tolerancing is the less complex of the two methods of applying tolerances. It is also called “direct tolerancing of dimensions” because a tolerance can be specified only where a di-mension is defined. Direct dimensioning and tolerancing address the acceptable range of values of an individual dimension of a manufactured object. Direct tolerancing amounts to generalizing the single value of a dimension to be a range.

Geometric tolerances are the more complex of these two types. A geometric tolerance specifies a geometric region, such as an area or a volume, in which the realized feature must lie in order to meet the design criteria. Geometric tolerancing separates the specification of tolerance from the dimensioning, thus allowing more flexibility and allowing more precise controls that relate more directly to the form, fit and function of the part. This document covers the recommended usage and implementation of geometric tolerances defined in Application Integrated Construct (AIC) 519.

This document covers the application of tolerances to boundary representation solid models. The application of tolerances to wireframe or other geometric models is not covered here.

3 Fundamental Concepts

A tolerance describes a constraint on the acceptable deviation of a manufactured object from the ideal design. Tolerances are applied to the geometric aspects or features of a part, such as edges, faces and holes.

The fundamental principles of geometric tolerances can be found in national and international standards such as ANSI Y14.5M-1994 or ISO 5459-1981.

There are several subtypes of the geometric_tolerance entity, which are not mutually exclu-sive. For example, tolerances that reference datums are of type geometric_tolerance_-with_datum_reference. Tolerances that include a modifier such as maximum material condi-tion are of type geometric_tolerance_with_modifiers. Many typical engineering toleranc-es combine these. In these cases, complex entities instances will occur in the Part 21 file.

This document defines two aspects for the definition of such data within a STEP Part 21 file. The two aspects are representation and presentation. “Representation” provides the semantic aspects of the data. Presentation describes how the data can be displayed in a viewing application.

Two forms of presentation data have been defined for GD&T data within STEP. These are pol-yline presentation and semantic presentation. For Polyline presentation the display characteristics of the data are defined with simple geometric entities – polylines and arcs. Recommended prac-tices for this are defined in “Recommended Practices for Geometric Dimensions and Tolerances (GD&T) – Polyline Presentation” June 2008, Jochen Boy, ProSTEP.

This document describes how semantic presentation data can be defined. Semantic data provides the information in an intelligent form – geometric entities used such as lines are defined as leader

Version 3.6, 02/04/2013

3.1 Dimension and Tolerance

Dimension is a term for a specification of the value of a parameter of some aspect of the shape of a mechanical part or assembly. A dimension can be implied by the geometric model, or it can be explicitly modeled, which is what this guide covers. The term dimension can also refer to the nu-merical value itself; however in this document the term value is used.

Tolerance is a general term for permitted variations in the shape of manufactured parts. Tolerance treats the how the realized form or measurements of a real manufactured object compare to their design ideals.

3.2 Dimensions and Dimensional Tolerances

The dimensions and dimensional tolerances addressed in this document are:

• directional dimensions

• location dimensions such as angular, curved, or linear distances

• size dimensions such as angular, thickness, or other

• the association of dimensions with geometry

• the representation of dimensional tolerances including:

• plus-or-minus deviations

• maxima, minima, and nominal dimensions

• limits and fits

• significant digits

• the association of dimensional tolerances with dimensions

3.3 Geometric Tolerances

The geometric tolerances addressed in this document are:

• Angularity

• Circular runout

• Circularity/Roundness

• Coaxiality/Concentricity

• Cylindricity

• Flatness

• Parallelism

• Perpendicularity

• Position

• Profile of a line

• Profile of a surface

• Straightness

• Symmetry

Version 3.6, 02/04/2013

• Total runout

Tolerance modifiers (Maximum and minimum material condition, regardless of feature size and projected tolerance zone) are also addressed.

3.4 Feature Entities and Attributes

Dimensions and dimensional tolerances are applied to aspects of the product shape. The product and product shape are modeled as in other geometry and PDM applications.

GDT Features in STEP are modeled as shape_aspect entities. The term “feature” in some di-mensioning standards is reserved for definitional elements that lie in the surface of the part. In such cases, a term such as “derived element”, is used for derived geometry. In computing sys-tems and in some dimensioning standards, either is called a feature, and these are distinguished as “integral” feature and “derived” feature. In this recommendation, all are modeled as shape as-pect. For integral features, shape_aspects.product_definitional=’.TRUE.’. For derived elements it is shape_aspects.product_definitional=’.FALSE.’.

3.5 Identifying Features

The surface of a part can be partitioned into features, to which dimensions are applied. Normally a feature boundary corresponds to a locus of discontinuities of surface curvature, as when a straight side encounters a corner fillet. For the purposes of GDT, every point on the surface is either in the interior of one feature or on the boundary of two or more features. When a finished part is meas-ured, each point of the surface belongs to exactly one feature.

It is sometimes convenient to treat as a single feature a union of these natural geometric features.

Unions can be disjoint, for example:

• a pattern of holes

• a surface that is “interrupted” by a slot.

• the two sides of a slot

Unions can be made of contiguous features for example:

• the all-around shape of an irregular hole.

Similarly it may be necessary to identify a restricted region of a feature, for example:

• Where a tighter tolerance is required

• to indicate how a finished piece is mounted on a inspection workbench

3.5.1 Data Elements of the Representation of GD&T Features and Derived El-ements

It is recommended that when available the advanced_face be used for representing a feature, since its topology is well-defined.

Derived center elements such as points, curves, and surfaces may be unbounded and can be rep-resented by geometry primitives. In GDT the derived elements are considered to be implicitly bounded where they intersect another feature of a part. Any geometric_representation_-item or topological_representation_item could potentially be incorporated into a feature or derived element’s representation.

Version 3.6, 02/04/2013

3.6 Datum Systems

Some types of tolerances refer to one or more datums in order to represent the requirements on the shape. Datum systems are related datums that provide a reference system for describing re-quirements on the product shape.

3.6.1 Datums

A datum is a theoretically exact geometric reference, i.e. an exact point, line or plane, to which toleranced features are related. A datum is the origin from which the location or geometric charac-teristics of features of a part are established. A datum may be based on one or more datum fea-tures of a part.

3.6.2 Datum Features

Datum Features are tangible features of a part, for example a face that provides a reference sys-tem for measurements of the actual part. Datum Features must lie on the physical boundary of the shape. Consequentially, Datum Feature entities are related to topological entities that represent those boundaries in the solid model such as an advanced_face.

3.6.3 Datum Targets

A datum Target designates a specific point, line or area of contact on a part that is used in estab-lishing a data reference frame (definition from ANSI Y14.5). It differs from a datum_feature in that it identifies a restricted region of a feature, i.e. a point, line or area of a surface rather than a topological feature. Typically, two or more datum target elements are used to define a datum

4 Defining the Dimensioning Standard

Different dimensioning standards define the visual representation and interpretation of the symbol-ogy of the tolerances. In order for the receiving system to create the correct visual representation of the tolerance, the dimensioning standard must be known. This information is captured as an applied_document_reference which applies the referenced document, i.e., the dimensioning standard, to the product_definition of the part. In addition an application_context is required when there is geometric dimension and tolerance information present in the physical file.

These concepts are shown in Figure 1 and Figure 2.

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Figure 1: Choice of Dimensioning Standard (representation) of_product product product_ definition_ formation product_ definition

#103 object_role role_ association document applied_ document_ reference formation items S[1:?] assigned_ document role item_with_role

‘ASME Y14.41’

‘ISO 16792’

document_type kind product_data_type

‘configuration controlled document version’

‘configuration controlled document’ product_ definition_ context

#104 frame_of_reference name

‘mandatory’ product_ definition_ context_ association

#105 frame_of_reference product_ definition_ context_ role

#106 role definition name

‘additional context’ product_ definition_ context

#102 application_ context

#101 frame_of_reference frame_of_reference

(initial context in the ARM) application document_ product_ equivalence relating_document of_product product product_ definition_ formation related_product id

‘ASME Y14.41-2003’

‘’ id product_ related_ product_ category products S[1:?] name

‘document’ name ‘part definition’ product_ definition_ shape name

‘nominal model’

‘model based product geometry’ life_ cycle_ stage ‘design’

NOTE:

#104, #105 and #106 come from gdt_representation_view_context in the ARM application_ context

#107 application ‘model based 3d annotation presentation’ life_cycle_stage

‘design’ name

NOTE:

There can be other

‘additional contexts’ and type information present in the file name ‘equivalence’ related_product

Figure 2: Choice of Modeling Standard (presentation)

Note the green and blue strings in both figures represent the acceptable values for ASME and ISO respectively. Also, the product_definition_shape.name attribute should contain the string ‘nominal model’ only if it represents a design intended to be manufactured. If this is not the case (i.e. the model is simplified, represents a draft, is maximized or minimized in some aspect beyond the allowed tolerances, etc.) this string should be ‘’ (empty).

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4.1 Default Tolerance Decimal Places

The construct to convey the default tolerance decimal places to be used for the product is illustrat-ed below in Figure 3.

Figure 3: Default Tolerance Decimal Places

The number of decimal places specified above (i.e. 3) will be applied to all dimension values un-less otherwise specified (see section 5.4).

5 Implementation Guidelines for Dimensional Tolerances

5.1 Associating Dimensions with Features or Geometry

Dimensional tolerances are attached to the items of geometry or topology that the dimension ap-plies to. This is done by means of a subtype of the shape_aspect_relationship Entity, i.e.

dimensional_location, or by means of a dimensional_size entity. The identification of the type of tolerance is done via a specified string in the “name” attribute of the Dimensional Location or Dimensional Size entities. Note that Angular Dimensions are an exception to this, being defined by the Angular Location and Angular Size entities. The mappings for the Dimensional Tolerance types to the required string are shown in Table 1 and Table 3.

Note the combination of the shape_aspect entity, geometric_item_specific_usage entity and the geometric_representation_item, i.e. advanced_face, are unique. That is to say if another geometric dimension or tolerance references the same advanced_face entity, the same shape_aspect entity and geometric_item_specific_usage should be used as well.

This rule should be followed for all such relationships throughout this document.

Note if a single feature is actually made up of multiple geometric_representation_item enti-ties, the structure illustrated in Figure 4 should be used. An example use of this concept is a cylin-drical hole represented by 2 faces, where each face represents half of the cylindrical hole.

Version 3.6, 02/04/2013

© CAx Implementor Forum http://www.cax-if.de/ 8 http://www.cax-if.org/ shape_ aspect identified_item geometric_ representation_ item e.g. advanced_face item_ identified_ representation_ usage identified_item geometric_ representation_ item e.g. advanced_face item_ identified_ representation_ usage definition

Figure 4: Multiple Geometric Items Representing Single Feature

The attribute shape_aspect.description shall contain the string ‘unified geometric item’ to indicate the separate geometric_representation_item entities are unified to define the fea-ture. This rule should be followed for all such relationships throughout this document. A preferred alternate approach for this relationship, favored by some postprocessors, would be to combine the multiple advanced_face entities representing the single geometric feature into a single advanced_face. This operation would be performed by the preprocessor before exporting to

STEP. Otherwise postprocessors might combine neighboring advanced_face entities of the same underlying surface into one.

Note the id attribute for shape_aspect, dimensional_location, dimensional_size and geometric_tolerance shall be unique in the context of a product_definition_shape.

5.1.1 Dimensional Location

For the dimensional_location entity, the items between which the dimension applies are de-fined by shape_aspects. This is shown in Figure 5.

Figure 5: Attaching to Dimensioned Entities

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NOTE: this is only applicable if the shape aspects are unambiguous, otherwise refer to section 5.1.4.

NOTE: that this illustration is a change from that of the original document which can be found in Appendix 1. It shows the use of the new geometric_item_specific_usage entity to link the shape_aspect to the geometric_representation_item.

Note also that the same structure is applied for the specialized subtypes of Dimensional Location, namely “Dimensional Location with Path” and “Directed Dimensional Location”. The former of the-se provides a “path” for the measurement to follow by means of a shape_aspect (see Figure 10), the latter provides additional semantics to the “Related” and “Relating” attributes of the dimensional_location, i.e. The measurement is to occur from the “relating” shape aspect to the “related” shape aspect.

Dimensions that map to a Dimensional Location are:

Dimensional Location dimensional_location.name

Curved Distance 'curved distance'

Linear Distance 'linear distance'

Table 1: Dimensional Location Types (no circular cross-section)

The values listed in Table 1 are to be used when the objects being dimensioned do not have a circular cross-section. For a dimension between objects with a circular cross-section the values in Table 2 should be used to differentiate between the desired locations. An example of this type of dimension is illustrated in Figure 6.

Figure 6: Dimensional Location Examples (with diameter)

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Dimensional Location dimensional_location.name

From center to outer ‘linear distance centre outer’

From center to inner ‘linear distance centre inner’

From outer to center ‘linear distance outer centre’

From outer to outer ‘linear distance outer outer’

From outer to inner ‘linear distance outer inner’

From inner to center ‘linear distance inner centre’

From inner to outer ‘linear distance inner outer’

From inner to inner 'linear distance inner inner'

Table 2: Dimensional Location Types (circular cross-section)

NOTE: By default a dimensional location between objects with a diameter is defined from each center point so the value ‘linear distance’ is used.

5.1.2 Angular Location

To define an angular dimension between 2 features, the subtype angular_location of dimensional_location should be used. There is an additional attribute named angle_selection that defines the measured angle as an enumeration with valid values of .EQUAL., .LARGE. and .SMALL..

NOTE: If the specified angle is equal to or less than 180 degrees, the interpretation is direct. If the specified angle is larger than 180 degrees, the interpretation is the reflex angle.

5.1.3 Oriented Dimensional Location

To define a dimensional_location in a certain orientation the structure illustrated in Figure 7 should be used. The direction defined by the x axis of the axis2_placement_3d entity is the orientation of the dimensional_location entity. This concept is useful to distinguish between the two dimensions illustrated below:

The dimension in cyan is the default orientation between the outer sides of the two cylindrical fea-tures. However, the dimension in green has a user defined orientation.

Version 3.6, 02/04/2013

Figure 7: Oriented Dimensional Location

5.1.4 Derived Shapes

The preceding figure (Figure 5) shows the attachment of the Representation Items that form either end of the dimension_location to the Geometric or Topological entities at those points. In some cases, a Representation Item may not exist for that location in the model, e.g. the centre of a hole, so in this case a derived_shape_aspect will need to be created to provide an anchor point. In these cases the additional entities required are shown in Figure 8.

Figure 8: Derived Shape Aspect

Note that this illustration is a change from that of the original document which can be found in Ap-pendix 1.

Under certain conditions, a specialized subtype of the derived_shape_aspect should be used.

These conditions are listed in Table 3. Under any other conditions, the plain derived_shape_-aspect entity should be used. It is recommended, but not obligatory, that a meaningful string for the type of derivation be entered in the “name” attribute of this entity.

Condition STEP Entity Used

Apex of a Cone apex

Centre of a Symmetrical Feature centre_of_symmetry

Geometric Alignment of two Features geometric_alignment

Perpendicular to Feature perpendicular_to

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Condition STEP Entity Used

Spatial Extension to Feature extension

Tangential to Feature tangent

Parallel Offset from Feature parallel_offset

Table 3: Types of Derived Shape

Note the derived geometric_representation_item entities should be placed into a constructive_geometry_representation entity. This concept is covered in the Recom-mended Practices for Supplemental Geometry.

IMPORTANT: One area where this concept will be used frequently is dimensioning between holes or shafts and identifying locations on the surfaces to dimension between. In this case the simplest method to use would be to define two cartesian_point entities as derived_shape_aspect entities related to the advanced_face entities referencing the cylindrical_surface entities.

Referencing supplemental geometry in dimensions is an important concept and will be critical to ensure association of all geometric dimensions and tolerances to their respective features.

5.1.5 Dimensional Size

The Dimensional Size Entity is used where the measurement only applies to one object, rather than being a measurement between two distinct geometric or topological features. Note that this “one object” can, under certain circumstances, be a composite of several shape_aspect entities.

This will be illustrated in a later section. Figure 9 shows how a Dimensional Size Entity is attached to the representation object of which it is a dimension.

Figure 9: Dimensional Size

Note that this illustration is a change from that of the original document which can be found in Ap-pendix 1.

Note also that the same structure is applied for the specialized subtype of Dimensional Size, namely “Dimensional Size with Path”. This provides a “path” for the measurement to follow by means of a shape_aspect (see Figure 10).

Dimensions that map to Dimensional Size are:

Dimensional Size dimensional_size.name

Curve Length 'curve length'

Diameter 'diameter'

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Dimensional Size dimensional_size.name

Spherical Diameter ‘spherical diameter’

Radius 'radius'

Spherical Radius ‘spherical radius’

Toroidal Minor Diameter ‘toroidal minor diameter’

Toroidal Major Diameter ‘toroidal major diameter’

Toroidal Minor Radius ‘toroidal minor radius’

Toroidal Major Radius ‘toroidal major radius’

Toroidal High Major Diameter ‘toroidal high major diameter’

Toroidal Low Major Diameter ‘toroidal low major diameter’

Toroidal High Major Radius ‘toroidal high major radius’

Toroidal Low Major Radius ‘toroidal low major radius’

Thickness 'thickness'

Table 4: Dimensional Size Types

5.1.6 Angular Size

To define an angular dimension on a single feature, the subtype angular_size of dimensional_size should be used. There is an additional attribute named angle_selection that defines the measured angle as an enumeration with valid values of .EQUAL., .LARGE. and .SMALL..

NOTE: If the specified angle is equal to or less than 180 degrees, the interpretation is direct. If the specified angle is larger than 180 degrees, the interpretation is the reflex angle.

5.1.7 Dimensional Location/Size with Path

For some measurements, a path for the measurement needs to be defined, for example, when measuring the linear distance between two points on a curved surface. In this case, the meas-urement would need to follow the curve of the surface and not be the shortest straight line distance between the two points. In order to convey this information, this specialized subtype has an addi-tional attribute, “path”, which points to a shape_aspect defining the path the measurement is to take. The instantiation of this is shown in Figure 10.

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Figure 10: Dimension With Path

5.2 Application of Values to Identified Tolerances

Once you have identified the tolerance and attached it to the relevant pieces of geometry or topol-ogy as shown in the previous section, you can assign the tolerance value. The types of values and modifiers/qualifiers that can be applied are:

• nominal value

• nominal value with qualifier

• nominal value with plus/minus bounds

• value range

• tolerance class

5.2.1 Nominal Value

The structure for a dimension with a nominal value is illustrated in Figure 11.

Note that the dimensional_location referred to in the diagram could also be a dimension-al_size entity.

Note the length_measure_with_unit could also be a plane_angle_measure_with_unit entity.

Figure 11: Nominal Value

The name attribute in shape_dimension_representation carries significance in its value as defined below in Table 5 for the tolerance principle used.

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Tolerance Principle shape_dimension_representation.name

Independency (ASME) [ ] ‘independency’

Envelope Requirement (ISO) [ ] ‘envelope requirement’

Use default as specified in section 4 ‘’ (empty string)

Table 5: Tolerance Principle

5.2.2 Nominal Value with Qualifier

In this case, a nominal value is applied to the dimension, i.e. A set value for the measurement, without any tolerance bounds. However, the value can be limited as to whether it is a Maximum value or Minimum Value. There may be other limits allowed, and as the limit maps to a String in the Part21 Instantiation, there is no restriction on what value can be entered. An example of this type of dimension is illustrated below:

The entity used to apply a qualifier to a dimension value is type_qualifier and it has a single attribute of type string as illustrated in Figure 12. A qualified_representation_item is re-quired to reference this new entity.

Note the dimensional_location referred to in the diagram could also be a dimen-sional_size entity.

Note the length_measure_with_unit referred to in the diagram could also be a plane_an-gle_measure_with_unit entity.

Figure 12: Nominal Value with Qualifier

The types of qualifiers and the valid values for the name attribute are illustrated in Table 6.

Value Qualifier type_qualifier.name

Maximum Value ‘maximum’

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Value Qualifier type_qualifier.name

Minimum Value ‘minimum’

Average Value ‘average’

Table 6: Value Qualifiers

5.2.3 Nominal Value with Plus/Minus Bounds

If a tolerance is represented as a Nominal value with a set of plus and minus deviations or bounds to that tolerance, then the entities shown in Figure 13 are used to instantiate the STEP file. An example of this type of dimension is illustrated below:

Figure 13: Nominal Value with Plus/Minus Bounds

The values of the plus/minus tolerance are found in the measure_with_unit entities referred to by the “upper” and “lower” attributes of the tolerance_value entity.

Note that for clarity, even though it is given that the tolerance_value is an upper or lower limit, the value should still be given as an offset from the nominal. E.g. the lower limit should usually be a negative number and the upper limit a positive number.

Note that it is possible to have two positive numbers or two negative numbers (see part 47 section 6.5.3).

5.2.4 Value Range

An alternative way of presenting, rather than representing the dimension, is as a range of values that is the extents of the tolerance applied. As originally mapped in the Standard, this would give

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© CAx Implementor Forum http://www.cax-if.de/ 17 http://www.cax-if.org/ two values, one for the upper limit and one for the lower limit. Upon implementation, however, it was found that the majority of CAD systems hold the tolerance value as a nominal, with a plus/minus deviation, and handle the presentation of this data separately, thus the lack of a nomi-nal value through this method caused problems for re-importing the data, as this value was lost. In order to alleviate this, a third value, representing the nominal value was added to the mapping. An example of this type of dimension is illustrated below:

The STEP instantiation of this is shown in Figure 14.

Figure 14: Value Range

To determine which of the measure_representation_items denotes which value, they shall be identified by one of the following strings used for the representation_item.name attribute:

“nominal value”, “upper limit”, and “lower limit”.

5.2.5 Tolerance Class

An example of a tolerance class dimension is illustrated below:

The implementation of this type of dimension uses the limits_and_fits entity as illustrated in

Figure 15. The form_variance attribute represents the fundamental deviation or "position letter" of the ISO 286 limits-and-fits tolerance classification. Only symbols defined in ISO 286 shall be provided. The symbols “A” to “ZC” for holes or “a” to “zc” for shafts may be used to classify devia-tion. The zone_variance attribute represents the kind of fitting to which the tolerance applies.

The value of this attribute need not be specified. Normally the tolerance applies to an external or internal feature. Where applicable the following values shall be used:

"hole": the tolerance applies to an internal feature

"shaft": the tolerance applies to an external feature

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Whether the tolerance applies to an interior or exterior feature can be determined from the position letter of the fundamental deviation, therefore zone_variance need not be specified. The grade attribute represents the quality or the accuracy grade of a tolerance. Only symbols defined in ISO 286 shall be provided. The grade is one of the 18 international standard tolerance grades defined in ISO 286. The source attribute is not populated at this time.

dimension measure_ representation_ item shape_ dimension_ representation dimensional_ size items S[1:?] representation dimensional_ characteristics_ representation plus_minus_ tolerance limits_and_ fits toleranced_dimension range

Figure 15: Tolerance Class

5.3 Dimension Modifier

Examples of dimensions with modifiers are illustrated below:

To describe these dimension modifiers, a descriptive_representation_item entity can be used as illustrated in Figure 16.

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Figure 16: Dimension Modifier

Note the dimensional_location referred to in the diagram could also be a dimen-sional_size entity.

Note the length_measure_with_unit referred to in the diagram could also be a plane_an-gle_measure_with_unit entity.

Note representation_item.name should be set to ‘dimensional note’

As you may notice, there are two types of modifiers used for dimensions, the first type is limited to the kind of dimension it is. There can be only one descriptive_representation_item (type

1) entity associated with a dimension. The types of modifiers and valid values for the type 1 de-scription attribute are shown in Table 7.

Dimension Modifier (type 1) descriptive_representation_ item.description

Basic (ASME) / Theoretical (ISO) ‘theoretical’

Reference (ASME) / Auxiliary (ISO) ‘auxiliary’

Table 7: Dimension Modifiers (type 1)

There can be multiple type 2 modifiers associated with a single dimension so the compound_representation_item entity is used to group them. The types of modifiers and valid values for the type 2 description attribute are shown in Table 8.

Dimension Modifier (type 2) descriptive_representation_ item.description

Controlled Radius [CR] ‘controlled radius’

Square [ ] ‘square’

Statistical Tolerance [ ] ‘statistical tolerance’

Continuous Feature [ ] ‘continuous feature’

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Table 8: Dimension Modifiers (type 2)

5.4 Applying Number of Decimal Places

The displayed value of the Dimensional Tolerance can be truncated to a number of decimal plac-es, by specifying a Value Format Type Qualifier to the Representation Item which holds the toler-ance value. This is done using a complex entity for the measure_representation_item, con-taining the qualified_representation_item entity, which in turn, points to a value_format_type_qualifier, which specifies the number of places to the right and left of the decimal point by its format_type attribute.

dimension measure_ representation_item && qualified_ representation_item && measure_with_unit && plane_angle_measure_with_unit && representation_item shape_ dimension_ representation dimensional_ location items S[1:?] representation dimensional_ characteristics_ representation value_format_type_qualifier qualifiers

S[1:?] format_type

‘NR2 x.y’

‘NR2S x.y’

Figure 17: Decimal Places

The values for x and y in Figure 17 specify the number of places to the left and right of the decimal point respectively. A typical value would be ‘NR2 2.2’.

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6 Implementation Guidelines for GD&T Representation

6.1 Associating Tolerances with Features

In STEP, the tolerance entities are associated with a shape_aspect that identifies the toleranced feature. The feature is identified by a shape_aspect which has a representation. In the case of a solid boundary representation model, the feature of the part is represented by one or more topological_representation_items such as advanced_face entities. For example, a through hole in a solid model might be represented by two semi-circular surfaces, each an advanced_face entity. These topological_representation_items are collected together by a shape_representation which is representation of the shape_aspect for the feature.

This shape_representation shall share the same geometric_representation_context as the solid. See Figure 18 for an example of how the tolerance entities are related to the shape elements of the toleranced feature.

Figure 18: Defining Toleranced Features using Shape_Aspect

Part 21 Example

#187=ADVANCED_FACE('feature face',(#181),#186,.T.);

#252=PRODUCT_DEFINITION_SHAPE('',$,#17);

#315=SHAPE_ASPECT('',$,#252,.T.);

#316=GEOMETRIC_ITEM_SPECIFIC_USAGE(‘’,’flatness tolerance face’, #315, #199, #187);

#320=FLATNESS_TOLERANCE('#320',$,#319,#315);

6.2 Associating Tolerances with Dimensions

Instead of referencing a shape_aspect entity, a tolerance may reference a dimensional_location entity or a dimensional_size entity to represent the concept illus-trated below:

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6.3 Associating Tolerances with Part (All-Over)

To support the concept of all-over as illustrated below, a product_definition_shape entity may be referenced from a geometric_tolerance entity instead of the shape_aspect entity.

6.4 Associating Tolerances with Multiple Features

There are many cases where a Tolerance or Datum needs to be associated with more than one feature, for example, when a face based on the datum geometry has been split by a slot or other feature. In these cases, the associated features are combined under a composite_group_shape_aspect. Figure 19 shows the STEP Entities required to implement this functionality. In order to distinguish this case from the “Pattern of Features” case (see below), the name attribute of the composite_group_shape_aspect shall be set to the string “multiple elements”.

Figure 19: Multiple Feature Construct

Part 21 Example

#10341=PRODUCT_DEFINITION_SHAPE('',$,#17);

#10396=SHAPE_ASPECT('',$,#10341,.T.);

#10403=GEOMETRIC_ITEM_SPECIFIC_USAGE(‘’, ‘single feature’, #10396, #5500, #5423);

#10400=SHAPE_ASPECT('',$,#10341,.T.);

#10403=GEOMETRIC_ITEM_SPECIFIC_USAGE(‘’, ‘single feature’, #10400, Version 3.6, 02/04/2013

#5500, #5411);

#10404=COMPOSITE_GROUP_SHAPE_ASPECT(‘’,'multiple elements', #10341,.T.);

#10405=SHAPE_ASPECT_RELATIONSHIP('',$,#10404,#10396);

#10406=SHAPE_ASPECT_RELATIONSHIP('',$,#10404,#10400);

6.4.1 Associating tolerances with a Pattern of Features

One of the uses of the Multiple Feature construct is allowing the exchange of Tolerances applied to a pattern of features. In order to differentiate this from the normal case, the string “pattern of features” is used to populate the composite_group_shape_aspect.description attribute.

e.g. #299=COMPOSITE_GROUP_SHAPE_ASPECT(‘’,'pattern of features', #246,.T.);

6.4.2 Applying All Around Modifier

There is a subtype of the composite_shape_aspect entity called all_around_shape_aspect which can be used to represent the concept illustrated below:

6.4.3 Applying Between Modifier

There is another subtype of the composite_shape_aspect entity called between_shape_aspect which is used to convey the concept illustrated below:

6.5 Implementing DATUMS in a STEP File

This section of the document deals with the implementation of Datums for geometric tolerancing in the STEP file. For a description of the meaning of Datums in the context of tolerancing see section 3.6.

In STEP, a datum_feature is a specialization of the shape_aspect entity. It needs to attach to the feature of the model which represents the datum. This attachment is shown in Figure 20. Note that the Datum Identifier is stored as an alphanumeric string in the datum.identification at-tribute.

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