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KP3213 CAD/CAM - PowerPoint PPT Presentation

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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Presentation Transcript

• Snapshot Lecture 5

• Objective of Lecture 6

• Understand constraint modeling

• Lecture 6

Constraints and Variational Modelling

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

• Geometry is automatically regenerated based on constraints

D1

D2

D4

D3

Example

• 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 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

User specifies dimension D1, other dimensions calculated sequentially

Solve system of simultaneous equations:

• Ground constraints

• Dimensional constraints

• Geometric constraints

• Horizontal

• Vertical

• Both ends fixed

• Point location

• X of point

• Y of point

• Angle of line

• Horizontal dimension

• Vertical dimension

• Linear dimension

• Angular dimension

• Parallel

• Perpendicular

• Tangent

• Collinear, coincident, coplanar

• Sketch approximate geometry

• Generate solids and features

• Add constraints and dimensions afterwards

• Most CAD systems use “smart” sketching tools

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

• 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

• 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

• 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

• 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

• 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.

• 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.

• 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 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.