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KP3213 CAD/CAM. Snapshot Lecture 5 Objective of Lecture 6 Understand constraint modeling Learn CAD Standard Lecture 6. Constraints and Variational Modelling. Constraint-based Modelling. User constrains geometry based on Design Intent

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  • Snapshot Lecture 5

  • Objective of Lecture 6

    • Understand constraint modeling

    • Learn CAD Standard

  • Lecture 6

Constraints and Variational Modelling

Constraint-based Modelling

  • User constrains geometry based on Design Intent

  • Design variations can be generated by changing a few key dimensions

  • Geometry is automatically regenerated based on constraints







Design Intent

  • The part should be twice as long as it is wide

  • The hole should be centred in both directions

  • The hole diameter should be 50mm

Parametric and Variational Modelling

  • Parametric modelling

    • constraints defined sequentially

    • each constraint calculated based on previously defined constraints

    • order of constraint specification is important

  • Variational modelling

    • constraints solved simultaneously

    • order of constraint specification doesn’t matter

Parametric Definition

User specifies dimension D1, other dimensions calculated sequentially

Variational Definition

Solve system of simultaneous equations:

Constraint Types

  • Ground constraints

  • Dimensional constraints

  • Geometric constraints

Ground Constraints

  • Horizontal

  • Vertical

  • Both ends fixed

  • Point location

  • X of point

  • Y of point

  • Angle of line

Dimensional Constraints

  • Horizontal dimension

  • Vertical dimension

  • Linear dimension

  • Angular dimension

  • Radial dimension

Geometric Constraints

  • Parallel

  • Perpendicular

  • Tangent

  • Collinear, coincident, coplanar

Modelling Approach

  • Sketch approximate geometry

  • Generate solids and features

  • Add constraints and dimensions afterwards

Smart Sketching

  • Most CAD systems use “smart” sketching tools

  • Design intent is inferred, and constraints added automatically

  • For example, two lines that are nearly perpendicular “snap” perpendicular, with a constraint

CAD Standard

CAD Standard

  • Too many ways, need for standardization

  • Understand the graphic kernel system and its extension for developing graphic software system

  • Understand IGES, DXF and STEP

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


  • GKS Graphical Kernel System

  • PHIGS Programmer’s Hierarchical Interface for Graphics

  • 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 Syntex

GKS Graphical Kernel System

  • It is very difficult for a program developed for a particular system to run on different system

  • However, substantial portions of it is similar

  • Same codes are rewritten many times

  • Program interchangeable is created

  • ISO has standardized GKS for 2D in 1982 with objectives:

    • Complete range of 2D facilities including interactive

    • Control graphics devices such as plotter and display devices

    • Small enough for variety of program

  • GKS 3D standards developed to cover which were not covered by GKS

  • GKS has been enhanced to provide separate stand for 3D

PHIGS Programmer’s Hierarchical Interface for Graphics

  • Accepted by CAD vendors as system capable of 3D

  • Features in PHIGS not covered by GKS is as follows:

    • Very high interactivity

    • Hierarchical structuring of data

    • Real time modification pf graphics data

    • Support of geometry animation

    • Adaptability to distributed user environment

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

IGES Initial Graphics Exchange Specification

  • 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.

STEP Standard for the Exchange of Product Model Data

  • ISO standard 10303 (Product Data Representation and Exchange)

  • Able to share data across application, vendor platform between contractor, suppliers and customers

  • Scope:

    • Standard method representing info. Necessary for completely defining a product throughout the entire life cycle

    • Standard method for exchanging data electronically between two different systems

  • Overview – general introduction and overview part one of standard 10303

  • Description methods - the application protocols planned in STEP are far-reaching compared to existing standards

  • Implementation methods – Specifications how STEP information physically represented for exchange environment. Refer to actual implementation levels.

STEP Standard for the Exchange of Product Model Data

  • Conformance and tools – Provide specifications and conformance testing of the processors used for STEP information.

  • Integrated generic resources – Contains the specifications of the information models that support various application areas. Includes geometric and topological representation, product structure organization, material, visual representation, etc.

  • Application information models – These specify the information models to be used for specific application

  • Application protocols – Defines the context for the use of product data for a specific industrial needs.

DXF Drawing Exchange Format

  • 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.

Book Review for Week 3

  • CAD/CAM Principles and Application 2nd edition by PN Rao Published by Mc Graw Hill

    • Chapter 8 Finite Element Analysis. Find other books that is relevant to FEM because we will do some detailed calculation

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