MTECH course on CAD /CAM

1. Post Graduate Course on
Computer Aided Design & Computer Graphics
CAD-CAE
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Geometric Computer
Modeling Graphics
Concept Algorithm
CAD
Design & Analysis
Theories/ Tools
…To Automate the Design Process
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2. Geometric Modeling
Geometric modeling plays a crucial role in the overall application of CAD-CAM-CAE
system
Requirements of Geometric Modeling
Information entered through geometric modeling is utilized in number of downstream
applications like:
Design Analysis Evaluation of centroid, area (cross-sectional & surface) & volume
Estimate the mass & inertia properties
Interference checking in assemblies
Analysis of tolerance build-up in assemblies
Kinematic/ Dynamic analysis & simulation
Finite element analysis for stress, vibration, thermal & optimization
Drafting Automatic 2D view generation
Automatic planar cross-sectioning
Automatic dimensioning
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Geometric Modeling
Manufacturing Part classification & Process planning
NC tool path generation & verification
Manufacturing process simulation
Robot program generation
Production &
Preparation of Bill of Material (BOM)
Industrial
Engineering Material requirement planning
Manufacturing resource requirement
Scheduling
Marketing
Computer-Aided Inspection & Quality control etc…..
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4. Geometric Modeling
Hence, It is important that the geometric model generated should be as clear &
comprehensive as possible so that the other modules of CAD-CAM-CAE system are
able to use this information in the most optimal way.
Two Dimensional
(2-D)
Geometric model Wireframe Modeling
Three Dimensional Surface Modeling
(3-D)
Solid Modeling
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Geometric Model
Two Dimensional (2-D)
Utility of 2-D model lies in many of the low end drafting packages which is required
for preparing manufacturing drawings
Their utility is limited because of their inherent difficulty in representing complex
objects.
Three Dimensional (3-D)
The 3-D geometric modeling has the ability to provide all the information required for
CAD-CAM-CAE applications
Basic Requirement
A 3-D geometric model should be an unambiguous representation of an object.
A 3-D model should be complete to all engineering function from documentation
(drafting & shading) to engineering analysis to manufacturing.
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6. Wireframe Modeling
In this method the complete object is represented by number of lines, points, arcs &
curves and their connectivity relationships
Advantages
The construction of a wireframe model is simple
It does not require much computer time & memory.
It can be used for simple NC tool path generation
Disadvantages
It can not be used for calculation of mass, inertia properties
The interpretation of wireframe models having many edges is very difficult
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Surface Modeling
The surface model is constructed essentially from surfaces such a s planes, rotated
curved surfaces & even very complex synthetic surfaces.
Surface creation on existing CAD system usually requires wireframe entities as a
start
Surface & wireframe form the core of all existing CAD system
Advantages
Surface model of an object is a relatively more complete & less ambiguous representation
than its wireframe model
This method is very much useful for specific non-analytical surfaces ( free-form surface/
sculptured surfaces) such as those used for modeling automobile & airplane bodies &
turbine blades etc.
From an application point of view, surface models can be utilized in Finite Element
Modeling, NC tool path generation, sectioning & interference detections.
Disadvantages
The calculation of mass & inertia properties would be difficult
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8. Solid Modeling
Solid model of an object is a more complete representation than surface
model, as all the information required for engineering analysis &
manufacturing can be obtained with this technique.
Advantages
Solid modeling produces accurate design,
provides complete 3D definition
Improves the quality of design
Improves visualization
has potential for functional simulation of the system
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Wireframe Modeling
Point
Lines
Analytic Entities / Arcs
Curves
Circles
Ellipse
Conics Parabolas
Wireframe entities
Hyperbolas
<Geometric Primitives>
Cubic spline
Bezier curves
Synthetic Curves
B-spline
NURBS (Non-Uniform Rational B-spline)
Analytic Curve Synthetic Curve
- are described by analytic equations - are described by a set of data points (i.e.
control points)
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10. Analytic Curve
are defined as those that can be described by analytic equations such as lines, circle,
conics etc.
provide very compact forms to represent shapes & simplify the computation of
related properties such as areas & volume.
Analytic curves are usually not sufficient to meet today’s geometric design
requirements of complex mechanical parts like automobile bodies, aeroplane wings,
propeller blades, bottles etc.
That require synthetic curves & surfaces (free-form surfaces)
Synthetic Curve
are defined as those that can be described by a set of data points (i.e. control points)
such as Splines, Bezier curve etc.
Synthetic curves provide designers with greater flexibility & control of a curve shape
by changing the positions of the one or more data points or control points.
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Synthetic Curve
The need for synthetic curves in design arises on two occasions:
i) when a curve is represented by a collection of measured data points (in case of
Reverse Engineering) [graphical visualization of experimental data]
ii) when an existing curve must change to meet new design requirements.
Synthetic Curve Construction Techniques :
Interpolation technique Curve passes through the data points
Approximation technique Curve do not passes through the data points
Mathematically, synthetic curves represent a Curve-fitting problem to construct a smooth curve
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12. Surface Modeling
Plane surface
Analytic Surface Ruled surface/ Lofted surface
Entities Surface of revolution
Tabulated cylindrical surface
Surface entities
Bi-cubic Hermite spline surface
Synthetic Surface B-Spline surface
Entities
Bezier surface or patch
Coons patch
Curve segment : is the fundamental building block for curve entities
Surface patch : is the fundamental building block for surfaces
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Plane Surface
is the simplest surface which requires three non-coincident
points to define a plane.
The plane surface can be used to generate cross-sectional
view by intersecting a surface model with it.
Ruled Surface
is a surface constructed by transitioning between two or more curves by using
linear blending between each section of the surface
It interpolates linearly between two boundary curves that define the surface.
Lofted Surface
is a surface constructed by transitioning between two or more
curves by a smooth i.e. higher order blending between each
section of the surface.
Used for modeling engine manifolds, turbine blades etc. 14
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14. Surface of Revolution
is an axi-symmetric surface that can model axi-
symmetric objects.
is generated by a rotating a planar wireframe entity
in space about the axis of symmetry a certain
angle.
Tabulated Cylindrical Surface
is a surface generated by translating a planar curve
a certain distance along a specified direction (axis of
cylinder).
Plane of the curve is perpendicular to the axis of the
cylinder.
is used to generate surfaces that have identical
curved cross-sections.
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Solid Modeling
Solid model are known to be informationally more complete, valid &
unambiguous representation of objects than its wireframe or
surface model.
The completeness & unambiguity of a solid model are attributed to
the fact that CAD database stores both its geometry & topology.
Solid models are complete & unambiguous but they are not unique
because same object may be constructed in various ways.
Solid modeling has been acknowledged as the technological
solution to automating & integrating design & manufacturing
functions like…
Design / Engineering Analysis Drafting CAPP, CNC tool path generation MRP
Computer-Aided Inspection & Quality control
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16. A solid model of an object consists of both the geometrical & topological
data of the object.
Geometry
- Geometry of an object defines the actual dimensions of its entities.
Topology
- Topology of an object defines the connectivity & associativity of the entities.
- it determines the relational information between entities.
From user point of view,
Geometry is visible.
Topology is considered to be non-graphical relational
information that is stored in solid model databases &
are not visible to users.
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Geometry The geometry that defines the object is the
- the length of lines L1, L2, L3
- the angle between the lines
- radius ‘R’ & the centre ‘P’ of semi-circle
Topology The topology of the object can be stated as
- L1 shares a vertex (point) with L2 & C1
- L2 shares a vertex with L1 & L3
- L3 shares a vertex with L2 & C1
L1 - L1 & L3 donot overlap
- ‘P’ lies outside the object L1
R
R
L2 C1
P L2 P
C1
L3
L1 L3
L1
R R
L2 L2 P
P
C1 C1
L3 L3
Same Geometry but Different Topology Same Topology but Different Geometry18
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18. Solid Modeling
Modern CAD systems offer two approaches to creating solid models :
- allows designers to use pre-defined shapes (primitives) as
building blocks to model solid object.
Primitive Approach
- Boolean operation are used to combine the primitives to
create complex objects.
- is limited by the restricted shapes of primitives
- Features are more flexible as they allow the construction
Feature Approach
of more complex solid than what the primitive offer.
Feature is defined as a geometric shape (i.e. feature profile or 2-D sketch) and feature
operation to build parts.
Feature Feature Profile (Sketch) + Feature Operation
The entities required for Feature profile (Sketch) are wireframe & surface entities
Feature Operation is an activity that converts the Sketch (2-D) into 3-D shape 19
Constraints
Geometric Constraints Dimension Constraints
Fix
Coincidence
Concentricity
Tangency
Parallelism
Perpendicular
Horizontal
Vertical
Mid point
Equidistant point
Geometric constraints is defined as a geometric condition that relates two or
more sketch entities.
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20. Constraint-based Feature Modeling
Select a Sketch Plane
Sketching the feature profile (2D profile)
Apply Geometric & Dimensional Constraints
Apply feature operations
Create Base Feature
Combine feature to build part
- is considered the best tool to create solids because of its ability to edit & change the
shape of the solid in the future by using its relations & constraints. It has flexibility to
create complex shape.
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Common Feature Operations & Features
Extrude Protruded Feature
Revolve
Sweep
Loft
Feature Rib
Operations
Pocket
Hole
Slot
Shell
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22. Common Feature Operations & Features
Fillet
Chamfer
Rectangular Pattern
Pattern
Feature Circular Pattern
Operations
Translation
Rotation
Transformation
Scaling
Mirror
Thread
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Common Features
Extruded or Protruded feature
Extrude command is used to create a solid object by
extruding a given closed profile.
It requires a closed profile (sketch of cross-section of
solid object) & an extrusion vector (length & direction).
The extrusion direction is always perpendicular to the
sketch plane of the profile.
Revolved feature
Revolve command is used to create an axi-symmetric
objects by revolving the selected geometry about an axis.
It requires a profile (sketch of cross-section of solid
object) & a revolution vector (axis & angle of revolution).
The axis of revolution is always in the sketch plane of the
profile.
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24. Common Features
Sweep feature
is a generalization of extrusion.
Creates a feature by moving a sketch (closed or open)
along a linear or non-linear path, which is not
necessarily perpendicular to the sketch plane.
Loft feature
Use to blend multiple cross-section (two or more) along
the linear or non-linear guide curve to create a solid.
Hole feature
is equivalent to subtracting a cylinder from a solid.
Simple and Taper hole
Counter-bored hole
Type of Hole feature
Counter-sunk hole
Threaded hole
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Common Features
Slot feature
Removes material from solid.
Rectangular slot
Type of Slot feature T- slot
Dovetail slot
Shell feature
Shell operation is used to create hollow or thin-walled
solids by removing material out.
The input to a shell operation is the faces to be removed
& a wall thickness.
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26. Common Features
Rib feature
is a special type of extruded feature.
Creates an extension of geometric element by creating a
feature connecting the two faces of the part.
The input to create a rib is a contour & a thickness
Pattern feature
Rectangular Pattern Used to create multiple copy of given
feature in rectangular (linear) array.
Circular Pattern Used to create multiple copy of given
feature in circular array.
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Common Features
Fillet
are used to smoothen (round) the sharp edges of solid.
input to create a constant radius fillet feature is to select
the edges to be filleted & fillet radius.
Chamfer
use to remove sharp edges (or corners) from parts
input to create a chamfer is angle-distance, distance-
distance.
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28. Boundary representation (B-rep)
Constructive Solid Geometry (CSG)
Sweeping
Half spaces
Analytic Solid Modeling (ASM)
Solid Representation Schemes
Cell decomposition
Spatial enumeration
Octree encoding
Primitive Instancing
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Boundary representation (B-rep)
B-rep is based on the topological notion that an object is bounded
by a set of faces.
A face is a closed, orientable & bounded (by edges) surface. Each face is
bounded by edges & each edge (bounded curve) is bounded by vertices.
B-rep model of an object consists of faces, edges, vertices.
Vertices (v)
- The database of a boundary
model contains both its
Edge (E)
topology & its geometry.
Topology & Geometry are
interrelated & cannot be
Face (F)
separated entirely from each
other
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30. B - rep
Advantages
B-rep is very appropriate to construct solid models of unusual or
complex shapes ( like turbine blades, auto body etc.) that are
difficult to build using primitives.
It is relatively simple to convert a B-rep model into a wireframe
model because the model’s boundary definition is similar to the
wireframe definition.
Disadvantages
It requires large amounts of storage because it stores the explicit
definition of model boundaries i.e. faces, edges & vertices which
tend to grow fairly fast for complex model.
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Constructive Solid Geometry (CSG)
CSG model is based on the topological notion that a physical object
can be divided into set of primitives (basic elements & shapes) that
can be combined in a certain order following a set of rules (Boolean
operation) to form the object.
Primitives are considered as building block
Primitives are simple, basic shapes which can be combined by a mathematical
set of Boolean operations to create the complex solid object.
Primitives
Block Cylinder Cone Sphere Wedge
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32. Boolean Operations
Union Intersection Difference
(U) (∩) (-)
Used to combine or Intersecting two Used to subtract one
add two primitives/ primitives give a primitive from other &
objects shape equal to their results in shape equal
common value to the difference in
their volume
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CSG
Advantages
It is easy to construct out of primitives & Boolean operation.
It is concise & requires minimum storage.
Disadvantages
Major disadvantage of CSG is in its inability to represent sculptured
surfaces i.e. non-analytical (synthetic surface) cannot be modeled
using CSG scheme.
- Modern CAD systems provide both approaches to increase their modeling domain
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34. Coordinate Systems
World Coordinate System Working Coordinate System Screen Coordinate System
Device coordinate system
Model coordinate system
Master coordinate system
Database coordinate system
World Coordinate System
is defined as the reference space of the model with respect to which all the geometrical data is
stored in CAD database.
Default coordinate system used by a particular software.
can be displayed on the computer screen.
Input information is transformed to coordinates relative to the world coordinate system before
being stored in the database.
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Working Coordinate System
It is convenient in the development of geometric models & the input of geometric data to refer to
an auxiliary coordinate system instead of world coordinate system. The user can define a
Cartesian coordinate system whose XY plane is coincident with the desired plane of
construction (sketch plane).
While the user can input data in reference to the working coordinate system, the CAD software
performs the necessary transformations to the world coordinate systems before storing the data.
Screen Coordinate System
is defined as a 2D device –dependent coordinate system whose origin is usually located at the
lower left corner of the graphics display.
The physical dimensions of a device screen & type of device determine the range of the SCS.
A 1024×1024 display has an SCS with a range of (0,0) to (1024, 1024).
This SCS is used by the CAD/CAM software to display relevant graphics by converting directly
from world coordinates to SCS coordinates. A transformation operation from world coordinate
systems coordinates to SCS coordinates is performed by the software displaying the model views
and graphics.
For a geometric model, there is a data structure to store its geometric data (relative to world
coordinate system) & a display file to store its display data (relative to SCS)
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36. Solid Model-based Down-stream Application
Solid Model-based Assembly Design
Solid Model-based Drafting
Solid Model-based Mechanism Design
Finite Element Modeling from Solid Models
CNC machining based on Solid Modeling …
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37. Solid Model-based Down-stream Application
Solid Model-based Assembly Design
Solid Model-based Drafting
Solid Model-based Mechanism Design
Finite Element Modeling from Solid Models
CNC machining based on Solid Modeling …
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38. Solid Model-based Down-stream Application
Solid Model-based Assembly Design
Solid Model-based Drafting
Solid Model-based Mechanism Design
Finite Element Modeling from Solid Models
CNC machining based on Solid Modeling …
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39. Solid Model-based Down-stream Application
Solid Model-based Assembly Design
Solid Model-based Drafting
Solid Model-based Mechanism Design
Finite Element Modeling from Solid Models
CNC machining based on Solid Modeling …
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