CAD/CAM

LEARNING OBJECTIVES

• Need for CAD standard

• Understand the graphic kernel system and its extensions for developing the graphic software systems

• Requirement of graphic data exchange formats and their details such as IGES, DXF and STEP

• Dimensional measurement interface specification for communication between coordinate measuring machine and the CAD data

STANDARDIZATION IN GRAPHICS-Necessary to standardise certain elements at each stage to minimize company investment on certain software and

hardware without much modification on the newer and different systems.-This means that there should be compatibility between various softwareelements as also between the hardware and software.-Following are some of the interface standards at various levels;• GKS (Graphical Kernel System)• PHIGS (Programmer’s Hierarchical Interface for Graphics)• CORE (ACM-SIGGRAPH)• GKS-3D• IGES (Initial Graphics Exchange Specification)• DXF (Drawing Exchange Format)• STEP (Standard for the Exchange of Product Model Data)• DMIS (Dimensional Measurement Interface Specification)• VDI (Virtual Device Interface)• VDM (Virtual Device Metafile)• GKSM (GKS Metafile)• NAPLPS (North American Presentation Level Protocol• Syntax)

VARIOUS STANDARDS IN GRAPHICS PROGRAMMING

GRAPHICAL KERNEL SYSTEM (GKS)

• It is desirable to have programs interchangeable with a number of systems, and also to make programmers learn the system once and then repeatedly use it on different systems.

Examples are:

Tektronics Plot 10

GINO-F from CAD Centre, Cambridge.

The main objectives that were put forward for GKS are:

1.To provide the complete range of graphical facilities in 2D, including the interactive capabilities,

2.To control all types of graphic devices such as plotters and display devices in a consistent manner,

3. To be small enough for a variety of programs.

LAYER MODEL OF GRAPHICS KERNEL SYSTEM

The coordinate frames available to the user are of three types:

1. World coordinates (WC) which are the user- oriented drawing coordinates.

2. Normalised device coordinates (NDC) which is a uniform system for all work stations.

3. Device coordinates (DC) are the actual coordinate system for the particular work station.

• The GKS is essentially a set of procedures that can be called by user programs for carrying out certain generalized functions (such as arc, circle, ellipse etc.)

• GKS is defined in terms of numbers of levels describing the level of support in term of facilities

GRAPHICS PRIMITIVES IN IBM GKS

OTHER GRAPHIC STANDARDS

• GKS 3D- maintains compatibility with the 2D standard.

• PHIGS (Programmer’s Hierarchical Interface for graphics)

• NAPLPS

NAPLPS NORTH AMERICAN PRESENTATION LEVEL PROTOCOL SYNTEX

• Presentation standard developed by the Canadian government with AT&T and other telcos

• Basis of transferring data from computer to video displays systems such as tele-text and other video presentations systems

• Is a means of encoding graphic data and text in electronically transferable format (ASCII)

• Features:• Very compact (10% more compared with other format)• Resolution will be clearer• Integrated with communication networks• Large range of colors

5.4 EXCHANGE OF MODELLING DATAFIG. 5.4 DATA EXCHANGE BETWEEN VARIOUS SYSTEMS

1. INITIAL GRAPHICS EXCHANGE SPECIFICATION (IGES)

• However, the IGES is the most comprehensive standard and is designed to transmit the entire product definition including that of manufacturing and any other associated information (Fig. 5.6).

• A brief description of the IGES version 3.0 is given below highlighting the philosophy of the conversion methodology.

• Most comprehensive standards and is designed to transmit the entire product definition including that of manufacturing and any other associated information

• IGES file contains 6 sub-sections

• Flag Section – ASCII format. Version 3 onward standardized as follows (ASCII default, binary form and compressed ASCII)

• Start Section – man-readable prologue. Information contain is essentially for the person who would be processing this for other application.

• Global Section – Contains info. about detail of the product, originator, company , date, drafting standards etc.

• Directory Entry Section – Each entiry present is fixed in size contains 20 fields and 8 character each. To provide an index for the file and to contain attribute information such as color, line type transformation matrix etc.

• Parameter Data Section – contains data associated with entities. A free format is allowed for maximum convenience. Also contains pointers.

• Terminate Section – Contains the sub-totals of the records present. This would always contains a single records. It is possible that some design technique is lost.

DEVELOPMENTS IN THE DRAWING DATA EXCHANGE FORMATS

FIG 5.6 DATA INTERCHANGE METHOD BETWEEN TWO DIFFERENT CAD SYSTEMS USING NEUTRAL DATA FORMAT SUCH AS IGES OR STEP

FIG. 5.7 COMPONENT DRAWING FOR IGES FILE GENERATION

PARTIAL LISTING OF THE IGES FILE FOR DRAWING SHOWN IN FIG. 5.7

PROBLEMS WITH IGES

• Many incompatible “flavours”

• Unreliable translation, particularly for complex geometry

• No formal information modelling basis

• Insufficient support for conformance testing

2.STANDARD FOR THE EXCHANGE OF PRODUCT MODEL DATA (STEP)

The broad scope of STEP is as follows:1. The standard method of representing the information necessary to completely define a product throughout its entire life, i.e., from the product conception to the end of useful life.

2. Standard methods for exchanging the data electronically between two different systems.

STEP ARCHITECTURE

Layer 1: Implementation methods(EXPRESS)

Physical files

Layer 2: Resource information models

Layer 3: Application protocols

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STEP APPLICATION PROTOCOL AP 203 EXPLICIT DRAUGHTING

STEP APPLICATION PROTOCOL AP207 CONFIGURATION CONTROLLEDDESIGN

EXAMPLE FOR STEP FILE GENERATION SHEET METAL DIE PLANNING AND DESIGN

• DXF has been developed and and supported by AutoDesk for use with the AutoCAD files

• The drawing interchange file is ASCII

• Overall organization:

• HEADER section – contains general information about the drawing similar to the global section of IGES. Consists of AutoCAD database version number of system variables.

• CLASSES section – Holds information for the application-defines classes.

• TABLES section – contains definition of symbol tables such as line-tables, layer table text style table etc.

• BLOCK section – contains symbols

• ENTITIES section – contains graphical objects

• OBJECTS section – Contains non-graphical. Examples are dictionaries that contain mline (multiple line styles and groups.

DXF DRAWING EXCHANGE FORMAT

DMIS

Dimensional Measurement Interface Specification (DMIS) is a new standard in communication being established by CAM-I for manufacturing.

This standard tries to establish a means of knowing what has been made by the CAM process.

The objectives – is to provide a bi-directional communication of inspection data between computer systems and inspection equipment.

INTRODUCTION OF MEASUREMENT IN PRODUCT DEVELOPMENT CYCLE

INTRODUCTION OF MEASUREMENT IN PRODUCT DEVELOPMENT CYCLEFIGURE SHOWS: A CIRCULARITY TOLERANCE SPECIFIES A TOLERANCE ZONE BOUNDED BY

TWO CONCENTRIC CIRCLES WITHIN WHICH THE FEATURE MUST LIE.

SUMMARY• GKS is used to standardise the graphic system calling procedures at the lowest

level so that programmers and program can be easily migrated between different systems

• The neutral CAD database is an important requirement to help with the transfer of information between various CAD/CAM systems

• IGES is used for transferring information between various CAD systems for modelling as well as drafting data

• STEP is being used extensively in view of its varied and better facilities for exchanging product model data

• DXF - ?

• DMIS - ?

REVIEW QUESTIONS

• What is the importance of standards in CADCAM?

• Write briefly on any one of the known graphic standards

• What is meant by DMIS? Explain its importance in the manufacturing of mass consumption items

• How is IGES different from GKS?

• Write a short note about DXF standard

• Explain the relevance of IGES in the Malaysian manufacturing scene.

THANK YOU