BMFR 4113 – CAD/CAM

LECTURE 4

CAD Standards

Text: PN Rao

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 10GINO-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

GGDPPurpose: Output routine. Outputs one of the four generaliseddrawing primitives.

Format: CALL GGDP (n, px, py, primid, ldr, datrec)n Integer*2 Number of pointspx(n) Real. List of X coordinates (WC)py(n) Real. List of Y coordinates (WC)primid Integer*2. GDP identifier (Fig. 5.3)

1 = bar2 = arc3 = pie slice4 = circle

ldr Integer*2. Length of data recorddatrec Character*80 Data record

GRAPHICS PRIMITIVES IN IBM GKS

Other graphic standards

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

• PHIGS (Programmer’s Hierarchical Interface for graphics)

• NAPLPS

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.

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

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 APPLICATION PROTOCOL AP 203 EXPLICIT DRAUGHTING

STEP APPLICATION PROTOCOL AP207 CONFIGURATION CONTROLLED

DESIGN

EXAMPLE FOR STEP FILE GENERATION SHEET METAL DIE PLANNING AND DESIGN

3.DRAWING EXCHANGE FORMAT (DXF)

The DXF format has been developed and supported by Autodesk for use with the AutoCAD drawing files.

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 CYCLE

Figure Shows: a circularity tolerance specifies a tolerance zone bounded by two concentric

circles within which the feature must lie.

TOL/CIRLTYFunction: Specifies a circularity tolerance for a

feature, and assigns to it a label.Default: NoneFormat: T(label) = TOL/CIRLTY, tolzon,

var_1, F(label)Where: var_1 can be: MMC or: LMC

or: RFS label is an alphanumeric label

Assigned to the tolerance, and is up to 10 characters in length.

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