Purpose

Scope

Application Subsets

Application Protocols

Implementation Issues

Purpose

Initial Graphics Exchange Specification (IGES) is the standard specified by the American Society of Mechanical Engineers (ASME), standard Y14.26M, for Communication of Product Definition Data.

Scope

Five basic classes of the IGES standard (technical illustrations, engineering drawings, electronic/electronics applications, numerical control manufacturing, and 3D piping) should be considered for this discussion as opposed to the entire IGES standard. IGES is large and complex, with different options that may be used to represent the same Computer Aided Design (CAD) model entity. As a result, CAD software vendors seldom support every IGES entity in the specification, but support a subset of IGES that best matches the features of their CAD system. Invariably, there is a mismatch between the set of entities by one CAD system's pre-processor and another CAD system's post-processor. There is no guarantee that the intersection of the two different CAD systems' supported IGES entities is adequate for the required data transfer.

Application Subsets

The first four classes specify the entities needed for specific application subsets. In this way the recipient of a IGES data file may specify the class of data needed without becoming an expert on the IGES.

The subset concept addresses many of the user's problems, but is not an entire solution. One difficulty is that the subsets address the needs of applications by directly specifying the particular IGES entities to be included in the subsets, but do not include enough information on how to use those entities to transfer all the product data typically needed by that application. Most IGES entities are general purpose in nature. They can be combined to create constructs needed for product data transfer, such as a circuit in an electrical application, but they do not rigorously define how this is done. This can be a problem in transfer, because unless the receiving system knows how the IGES entities were combined to create the construct, and has a rigorous definition of the meaning of the construct, that receiving system will not be able to interpret the construct. The basic data is translated, but not all the information needed to translate product data for the application is transferred.

The four application subsets are described in the following paragraphs:

• Class I: Technical Illustrations Application Subset.

  The Class I application subset is for the exchange of illustrations for technical publications. The emphasis is on the visual appearance of the illustrations, not on the functionality of the entities within the class. Class I is a two dimensional subset with limited non-geometric information (such as subfigures).

• Class II: Engineering Drawings Application Subset.

  The Class II application subset is for the exchange of product data (Technical Data Packages, General Specification for). The emphasis is on completeness, functionality of the drawing model, and visual equivalency for human interpretation. The class contains many geometric entities, annotation entities and attributes such as colour and line fonts, along with organizational information such as levels and subfigures. The geometric entities in this class are three dimensional, though two dimensional data can be transferred by placing all the information on the same plane within the sending CAD system.

• Class III: Electrical/Electronic Applications Subset.

  The Class III application subset is for the exchange of product data for electrical and electronic products. The emphasis is on completeness and functionality of the model for design, manufacturing and testing. Class III supports both the logical product representation and the physical product representation, which can both be in the same file. The logical representation includes netlists and schematics, while the physical representation includes assembly placement and pad layouts. Class III is difficult to use for unambiguous data exchange without further restrictions and interpretations applied to the subset. The IGES/PDES Organization (IPO) Electrical Applications Committee (AEC) is developing a Layered Electrical Products (LEP) AP for the representation of electrical products. The LEP AP is currently planned to be a replacement for MIL-D-28000 Class III.

• Class IV: Geometry for NC Manufacturing Application Subset.

  The Class IV application subset is for the exchange of product data for manufacturing by numerical control. The emphasis is on the completeness and functionality of the part model. Geometry data is either 2-D wireframe, for profiles or sheet metal, or a 3-D wireframe model, for multi-axis machining. Precision and accuracy on the wireframe and surface geometry must be maintained, as well as first order continuity. Geometry and Text form the majority of the data for this class.

Application Protocols

An Application Protocol (AP) is a way to transfer defined product data through IGES. An AP documents the user requirements for an application in a graphical model called an Application Reference Model (ARM). The requirements in the ARM are then represented by specific IGES entities in a given AP (the AIM). APs enable IGES to be used to transfer product data reliably until PDES/STEP is available from the commercial CAD vendors. APs provide a defined and more reliable method for transferring product data through IGES.

An AP is composed of the following elements:

• a scope and requirements section;
• an Application Reference Model (ARM) of the supported information that explains what is covered in the application and how the different elements relate to one another;
• an Application Interpreted Model (AIM) that shows how the information is mapped into IGES entities;
• and Conformance Requirements and Abstract Test Purposes.

APs are very specific in nature. For example the 3D Piping AP (Class V) exclusively supports the exchange of product data for 3D piping system models. It does not support piping engineering drawings. A user wishing to transfer an engineering drawing of a piping system would have to use an Engineering Drawing AP. Also, only CAD/CAM systems supporting piping will be able to support the piping AP. A CAD/CAM system that does not support piping just doesn't have the appropriate constructs within its' database to either output data in the Piping AP, or input the data reliably. APs will provide increased information transfer, but with a much narrowed scope in the information that is transferred.

Implementation Issues

Most IGES entities are general purpose in nature. They can be combined to create constructs needed for product data transfer, such as a circuit in an electrical application, but they do not rigorously define how this is done. This can be a problem in transfer, because unless the receiving system knows how the IGES entities were combined to create the construct, the receiving system may not be able to interpret it. The basic data will be translated, but all the information needed to translate product data for the application will not be available.