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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
constraint based 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
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 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
Parametric Definition

User specifies dimension D1, other dimensions calculated sequentially

variational definition
Variational Definition

Solve system of simultaneous equations:

constraint types
Constraint Types
  • Ground constraints
  • Dimensional constraints
  • Geometric constraints
ground constraints
Ground Constraints
  • Horizontal
  • Vertical
  • Both ends fixed
  • Point location
  • X of point
  • Y of point
  • Angle of line
dimensional constraints
Dimensional Constraints
  • Horizontal dimension
  • Vertical dimension
  • Linear dimension
  • Angular dimension
  • Radial dimension
geometric constraints
Geometric Constraints
  • Parallel
  • Perpendicular
  • Tangent
  • Collinear, coincident, coplanar
modelling approach
Modelling Approach
  • Sketch approximate geometry
  • Generate solids and features
  • Add constraints and dimensions afterwards
smart sketching
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 standard1
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
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
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
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
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
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 data1
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 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
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