CAD 1-7 CAD CAM Data Exchange PDF

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1-7 CAD CAM Data Exchange
Dr. Chin Chee Wen...

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EME 3066 CAD CAM 1.7 CAD/CAM Data Exchange 1.7.1 Introduction The transfer of modelling data between dissimilar CAD/CAM systems must embrace the complete description of a product stored in its database. Four types of modelling data make up this description. These are shape, non-shape, design and manufacturing data. Shape data consists of both geometrical and topological information. Non-shape data includes shaded images and measuring units of the database. Design data includes FEM/FEA and mass properties. Manufacturing data includes tolerancing, NC tool paths, process planning and bill of materials.

Realizing the importance of product data exchange among different CAD/CAM systems, all CAD/CAM vendors have been collaborating to set, exchange standards to make it manageable for users to communicate their product information effectively. Older standards such as IGES focus on geometric data, while newer standards such as STEP embrace the four types of product data just described. Other de facto standards such as ACIS and DXF have been popular due to their common use in industry.

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1.7.2 Types of Translators The need to exchange modelling data arises when there are several CAD/CAM systems on the market and each CAD/CAM system stores the model data in a unique manner. For example, certain parts of a particular assembly may be subcontracted and the subcontractor may have a different CAD/CAM system and it is necessary to bring the all of all components in the assembly together.

To exchange data, there are two approaches: direct and indirect. In the first approach, the data stored by one CAD/CAM system is directly translated to another system in one step. In the indirect method, the data is exchanged in two steps. Data from one system is translated into a neutral database structure which is independent of any CAD/CAM system. Figure 1 shows how both systems work. Direct translators are typically written by computer specialists; they are dedicated translator programs. Two translator programs are required to link two CAD/CAM systems: one for transferring data from system 1 to 2 and another from system 2 to 1. Indirect translators utilize two types of translators (known as pre-processor and post-processor). Pre-processor translates data from the system to the neutral format while the post-processor translates the neutral data into the CAD/CAM system format. Each type of translator has its advantages and disadvantages. Direct translators provide a satisfactory solution when only a small number of systems are involved, but as the number increases, the number of translator programs that need to be written becomes prohibitive. If there are n CAD/CAM systems, then the total number of translators, N that must be written is given by:

Adding one system to the existing n systems would require writing 2n additional translators. In Figure 1(a), eight more translators are required if system 5 is added. On the plus side, direct translators run more quickly than the indirect ones, and the data files they produce are smaller than the neutral files created by indirect translators. As for indirect translators, if there are n CAD/CAM systems, then the total number of indirect translators that must be written is given by:

N

2n

Adding system 5 (see Figure 1(b)) to n existing systems would only require writing two 1.7 CAD CAM Data Exchange

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EME 3066 CAD CAM additional translators regardless of the value of n. Moreover, indirect translator philosophy provides stable communication between CAD/CAM systems, protects against system obsolescence, and eliminates dependence on a single system supplier. Data stored in neutral files can potentially be archived. Some aerospace industry need to keep databases for over 20 years.

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1.7.3 IGES IGES (Initial Graphics Exchange Specification) is the first standard exchange format developed for modelling data exchange initiative. IGES is the ANSI Standard Y14.26M. It supports solid modelling including both B-rep and CSG schemes.

IGES has three data types: geometric, annotation, and structure. Table 1, 2 and 3 shows IGES geometric, annotation and structure entities respectively. Geometric entities define the product shape and include curves, surfaces and solids. Nongeometric entities provide views and drawings of the model to enrich its representation and include annotation and structure entities. Annotation entities include various type of dimensions (linear, angular, ordinate, etc), centerlines, notes, general labels, symbols, and cross hatching. Structure entities include views, drawings, attributes (such as line and text fonts, colors and layers), properties (e.g. mass properties), subfigures and external cross reference entities (for surfaces and assemblies), symbols (e.g. mechanical and electrical symbols) and macros (to define parametric parts). 1.7.4 STEP STEP (Standard for Exchange of Product data) is an exchange for product data in support of industrial automation. STEP is an ISO standard. It has absorbed PDES (Product Data Exchange Standard) which was an ANSI standard. In order to support industrial automation, STEP files are fully interpretable by computer. For example, tolerance information can be directly interpretable by a computer rather than a computerized text form which requires human intervention to interpret. In addition, this information is associated with those entities in the model affected by the tolerance. The main difference between STEP and IGES is that the central unit of data exchange in IGES model is the entity, while the central unit of data exchange in STEP model is the application, which contains various types of entities. Therefore, when data is exchanged between systems, it is done in terms of “application” units. This approach maintains all the 1.7 CAD CAM Data Exchange

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EME 3066 CAD CAM meaningful associativities and relationships between the application entities which make industrial automation possible. The goal of STEP is to represent all product information in a common data format, throughout a product’s entire life cycle. The data being transferred is geometry (e.g. curves, surfaces, solids), but also analysis, manufacturing, implementation and testing procedures. Essentially, STEP is a common structure, operating as a template, for sharing data among multiple users, across all functional areas. STEP uses three-layer architecture, as shown in Figure 2 that forms the core of STEP structure. The three layers are the application layer, the logical layer and the physical layer. The application layer is the interface between the user and STEP. It contains all the descriptions and information of various application areas. The logical layer provides consistent, computer-independent description of the data constructs that contain the information to be exchanged. The physical layer deals with data data format for the structures and exchange file itself. The main goal here is to establish and maintain efficiency in the file size and processing time.

Figure 2 STEP three-layer architecture 1.7.5 ACIS and DXF ACIS is used as a kernel in a number of commercial CAD/CAM systems. The maker of ACIS, Spatial provides a translator for these system used. Spatial’s translator allows the exchange of solid, surface and wireframe data via a variety of neutral and native formats, including IGES, STEP, Pro/E, SolidWorks, CATIA, Parasolid (PS), Unigraphics (UG), and Inventor. These major systems offer these formats in their translation menus. DXF (Data eXchange Format) is a de facto standard due to its popularity. DXF is an AutoCAD format, which is supported and maintained by Autodesk Inc. DXF 3D translates CAD models while DXF/DWG translate drawing files. 1.7 CAD CAM Data Exchange

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EME 3066 CAD CAM DXF file comes in two formats: ASCII and binary. The ASCII version is the most widely used in industry. A DXF file consists of four sections: Header, Tables, Blocks, and Entities. Header: AutoCAD system settings such as dimension style and layers. Tables: line styles and user-defined coordinate systems Blocks: drawing blocks (instances) Entities: entity definition and data 1.7.6 Processors IGES or STEP is just a format specification to exchange product data among dissimilar CAD/CAM systems. This format must be interpreted, understood and implemented by CAD/CAM vendors into program called processors or translators. The processors translate from their systems to the standard format and vice versa. The software that translates from the native file format to a standard format is called a preprocessor. The software that translates in the opposite way is called a post-processor. Figure 3 shows file exchange using a translator. The source system is the originating or sending CAD/CAM system, and the target system is the receiving one. The archival database allows databases to be kept as long as needed.

Figure 3 Pre- and Post-Processors of a translator A newly developed processor must be carefully tested before it is used in a production environment. The common methods of testing are (see Fgure 4): (i) Reflection test. In this test, a neutral file created by the system’s pre-processor is read by its own post-processor to create a native file of the translated model. This test is used to establish that a system’s processors can read and write common entities, making them symmetric. (ii) Transmission test. A neutral file of a model created by the system’s pre-processor is used to recreate the model on the target system. This test essentially determines the capabilities of the pre- and post-processor of the source and target system respectively. (iii) Loopback test. A neutral file created by the source system is read by the target system which in turn creates another neutral file and then transfers this file back to the source system to read it. This test checks the pre- and post-processors of both the source and the target systems. 1.7 CAD CAM Data Exchange

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Figure 4 Methods of verification of processors

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