Unit i computer aided design

ME 2402 COMPUTER INTEGRATED
MANUFACTURING (CIM)
YEAR/SEMESTER : IV/VII
UNIT I – COMPUTER AIDED DESIGN
Prepared By
MANIMARAN.M
ASSISTANT PROFESSOR
K.RAMAKRISHNAN COLLEGE OF ENGINEERING
TRICHY

UNIT I – COMPUTER AIDED DESIGN
Concept of CAD as drafting and designing facility, desirable features
of CAD package, drawing features in CAD – scaling, rotation,
translation, editing, dimensioning, labeling, zoom, pan, redraw and
regenerate, typical CAD command structure, wire frame modeling,
surface modelling and solid modelling (concepts only) in relation to
popular packages.
6/25/2015M.MANIMARAN KRCE TRICHY

What is CAD/CAM?
CAD – design activity that involves the effective use
of the computer to
Create,
Modify and
Document in engineering design.
CAM – an effective use of computers and computers
technology in the
 Manufacturing planning
 Manufacturing control 6/25/2015M.MANIMARAN KRCE TRICHY

The scope of CAD/CAM and CIM

SHIGLEY’S DESIGN PROCESS
PRESENTATION
EVALUATION
ANALYSIS & OPTIMIZATION
SYNTHESIS
PROBLEM DEFINITION
RECOGNITION OF NEED

CAD DESIGN PROCESS
Presentation
Evaluation
Analysis & Optimization
Synthesis
Problem Definition
Recognition of Need
Automated Drafting
Design Review &
Evaluation
Engineering Analysis
Geometric Modelling
CAD

CAD as Drafting and Designing Facility
Draft – the process of preparing drawings with help of
computers.
Advantages of Computer – Aided Drafting
 It is a fast and convenient method
 Drawings can be stored in Database
 Modify the drawings can be done easily and quickly
 Good accuracy

Software Packages for Modelling
There are number of CAD packages available for modelling as followed as
 AutoCAD
 Pro-E
 Unigraphics
 CATIA
 Solid works
 Hyper mesh
 Star ccm +

Characteristics of a CAD packages
According to Newman and Sproull, Any graphic package
should have the below characteristics,
1) Simplicity
2) Consistency
3) Completeness
4) Robustness
5) Performance
6) Economy

DESIRABLE FEATURES OF A CAD PACKAGE
1. Geometric modelling features
2. Editing or Manipulation features
3. Display control features
4. Drafting features
5. Programming facilities
6. Analysis features
7.Connecting features

1. Geometric Modelling Features
A) Features to aid geometric construction methods
 Cartesian and polar coordinates
 Absolute and incremental dimensions
 Units
 Grid and
 Snap
B) 2D analytical features
Points, Lines, Arcs, Circles, and Splines
C) 3D wireframe modelling features
3D lines, 3D faces, Ruled surfaces, Rotational sweep and Tapered
sweep

(contd..)
D) Surface modelling features
should be able to generate various types of sculptured surfaces
such as Bezier, Coons, B – splines
E) Solid modelling features
examples of block, cylinder, sphere, cone, prism etc.,
F) Skinning and profiling features
G) Assembly features
surface filtering and trimming

2. Editing (or) Manipulation features
- Editing or manipulating the existing models at any
stage of the product design
a) Transformation features
Translate, Rotate, Scale, Mirror, copy, et.,
b) To alter the existing drawings
Stretching, Trimming, Undo, Delete, Erase, etc.,
c) Symbols features

3. Display control features
To provide a user friendly interface with effective visuals
characteristics such as
a) Window to identify a set of entities for editing functions
b) Zoom to change the scale of display of the object
c) Pan to move the object on screen without changing the
scale
d) Hidden to remove the hidden lines or surfaces for viewing the
geometry
e) Animation to display the number of images in sequence to imitate
the actual motion
f) Shading to show the 3D view of the image on screen complete
and shade as it appears on the image 6/25/2015M.MANIMARAN KRCE TRICHY

4. Drafting features
The developed model can be utilized for the purpose
of sharing and communicating the information for
other applications.
It have used different dimension types such as linear,
angular, radius, circular, baseline and limit dimensions
etc.,
In the features, sectioning to get a better visualization
of the model or object. 6/25/2015M.MANIMARAN KRCE TRICHY

5. Programming Features
 Integration with small programs developed in programming
language to avoid repetitive tasks.
 Programming facility allows the user to program specifically for an
application
 Programming facility in a packages,
(i) GRIP in Unigraphics
(ii) vb.net and lisp for Auto CAD
 Programs help the user to input the least amount of information for
any required design

6. Analysis Features
In modelling features, the package should have many analysis features and facilities
such as include,
 Calculation of area
 Calculation of volume
 Calculation of mass
 Calculation of moment of inertia
 Calculation of Centre of gravity
 In FEA, the developed model is converted into finite element mesh and the all the
required data are calculated for analysis.
 Animation facility is also used to see the assembly of the model 6/25/2015M.MANIMARAN KRCE TRICHY

7. Connectivity Features
 The model or object so generated in any of the package
must be capable enough to be used as a input to any
other analysis package with or without converter.
 The data (graphics) losses is minimum while transferring the
data from one package to another.
 It should have very good data interfacing (connectivity)
with other associated modules.

DRAWING FEATURES IN A CAD PACKAGE
Scaling
Rotation
Translation
Editing
Dimensioning
Labeling
Zoom
Pan
Redraw and
Regenerate 6/25/2015M.MANIMARAN KRCE TRICHY

Drawing features in CAD
Scaling transformation used to change, increase or decrease the
size of an object without alter the shape.
Rotation the movement of an object in such a way that the distance
between a certain fixed point and any given point of that
body.
Translation moving an object across the screen from one position to
another position.
Editing changes the properties of a specific object by the editing
commands of package.

(contd..)
Dimensioning provides the dimension of the selected entity.
1. Linear dimension
2. Aligned dimension
3. Radius dimension
4. Angular dimension
Labelling it should be labelled for better documentation and
reference.
Pan moves the drawing display along the direction of the
mouse swipe.
Redraw refreshes the display in the current viewport.
Regenerate regenerates the entire drawing from the current view port.

(Contd..)
Zoom Magnifying a portion of current drawing on the screen.
It helps to improve the accuracy of Drawing by enlarging the
details
1. All zooms
2. Centre zooms
3. Dynamic zooms
4. Extends zooms
5. Previous zooms
6. Scale zooms
7.Window zooms
8. Object zooms
9. Real time zooms

Typical CAD Command Structure
Drawing commands
Editing commands
Text commands
Display commands
Dimensioning commands
Setting commands
Hatching commands
3D Surface commands
3D Solid commands

GEOMETRIC MODELLING
 Geometry modelling is the use of a CAD system to develop a
mathematical description of the geometry of an object.
 The geometric modelling can be classified into
1. Wire frame modelling
2. Surface modelling
3. Solid modelling

WIRE FRAME MODELLING
Wire frame models can be consider as networks of
interconnected lines to represent the edges of the physical
objects

Merits and Demerits of wire frame modelling
 It is simple and easy to create
 This is basis for surface model
 Accurate information about the location of surface
 Automatic generations of cutter paths to drive NC machine tool
 CPU time required to edit or update a wire frame model is small
compare with surface and solid modelling.
 Calculation of section properties and mass properties are
impossible
 It has limited use in manufacturing and analysis
 Presentation of circular holes and curved surfaces are poor
 Interference checking will be difficult

SURFACE MODELLING
A surface model can be built by defining the surface on the
wire frame

Surface Entities
1.Plane surface
This is a simplest surface.
A planer surface is generated
through number of coplanar points.
2. Rules surface
This is a linear surface
A surface is interpolated b/w two
guiding curves “rails”

(contd..)
3. Surface of revolution
It is generated Axi-symmetric surfaces
It is generated by rotating a plane curve
in a space about the for certain angle of
rotation
4. Translated surface
Generated by extruding a 2D entity along the specified direction.

(contd..)
5. Bezier surface
This is a synthetic surface that
approximate data points.
The Bezier surface is generated from
the basis of Bezier curve.
This type of surfaces can permit twist in
the surface
6. B-Spline surface
Generated from the basis B-spline curve
Another synthetic surface and local
control surface 6/25/2015M.MANIMARAN KRCE TRICHY

(contd..)
7. Coons patch surface
It is used to create a surface using
curves that form closed boundaries.
The patches may be triangular or
rectangular type
8. Fillet surface
It is obtained by blending two
surfaces with a fillet radius

Application of surface modelling
 It can used sheet metal works and thin moulded plastic parts
 Body panels of passenger cars
 Structural components of aircraft and marine structure
 Plastic container, telephones, impellers of pumps and turbine,
glass marking

Merits and Demerits of surface modelling
Merits
 To check the aesthetic look of the product
 Complex surface can be created very easily (shoes, car panels, doors )
 It can be used to perform Interference checking
Demerits
 It require more computational time compared with wire frame
 Mass properties (weight, volume and Moment Inertia) cannot be
derived from surface models
 More skill is required for surface modelling

SOLID MODELLING
 Solid modeling is based on complete, valid and unambiguous
geometric representation of physical object.
 Complete  points in space can be classified.(inside/ outside)
 Valid vertices, edges, faces are connected properly.
 Unambiguous  there can only be one interpretation of object

Constructive solid geometry (CSG)
 Objects are represented as a combination of simpler solid objects
(primitives) such as cube, cylinder, cone, torus, sphere etc.
 The physical operations are modelled by combining these primitives
by a set of Boolean operations.

Tree structure of model
Binary tree scheme is used to store
the model in the data structure
Any node may have one parent
and two child node.
The root node (R) has no parent
and leaf node (L) has no children.
Object = (A∩B)-(CU(DUE)

Advantages of CSG
 CSG is powerful with high level command.
 Easy to construct a solid model – minimum step.
 CSG modeling techniques lead to a concise database
less storage.
 Can be converted to the corresponding boundary
representation.
 Less skill is enough
Disadvantages of CSG
 More computational effort and time are required whenever
the model is to be displayed on the screen
 Getting fillet chamfer and taperness in the model is very
difficult 6/25/2015M.MANIMARAN KRCE TRICHY

Boundary Representation (B-rep)
 The solid model created by using B-rep technique may be stored in the
graph based on data structure system
 Vertices are connected to form edges and these edges are connected
together to form the face which makes the complete solid
B-rep Vs CSG
 In the CSG the number of basic primitives available are limited
 The performance of B-rep scheme is very much superior to that of CSG for
complex engineering models.
 Conversion of CSG to B-rep is possible, but B-rep to CSG is not possible

Euler’s Equation
For bodies without holes should satisfy EULER’S RULE
V – E + F = 2
V- No. of Vertices
E – No.of Edges
F – No. of Faces

(contd..)
For bodies with holes and re-entrant face, should satisfy Euler
Poincare formula
V – E + F – H + 2P = 2B
H – No. of holes
P – No. of passages
B – No. of separate bodies

Advantages of B-rep
 Computational effort and time required to display the
models are less
 Combining wire frame and surface models are possible
 Complex engineering objects can be modelled very easily
Disadvantages of B-rep
 The data to be stored is more and it requires more memory
 complex curve and surfaces only approximated

Advantages of solid modelling
 Mass properties of physical model can be calculated
quickly
 Cross section views of model can be obtained easily
 It can be used for interference/clearance checking of
moving parts
 Aesthetic look of finished object can be visualized in the
computer
 Solid model is very much useful for finite element analysis
 They can help to produce NC machining instructions
automatically.

THANK YOU…
6/25/2015
M.MANIMARAN KRCE TRICHY

Unit i computer aided design

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