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KP3213 CAD/CAM

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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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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
• Design variations can be generated by changing a few key dimensions
• Geometry is automatically regenerated based on constraints
D5

D1

D2

D4

D3

Example
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
Standardization
• 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