5th Unit CAD.pdf

Assembly modeling - interferences of positions
and orientation - tolerances analysis - mass
property calculations - mechanism simulation.
Graphics and computing standards– Open GL
Data Exchange standards – IGES, STEP etc –
Communication standards.
UNIT
5

Mate Method
• The most common mating conditions are
coincident, concentric, tangent, parallel faces
and perpendicular faces.

Coincident mating condition
• The coincident condition is satisfied by forcing
n1 and n2 to be opposite each other and the
two faces touch each other such that P1 and
P2 are coincident.

Coincident condition
• Let [T1] and [T2] be the
transformation matrices from
the X1Y1Z1 and X2Y2Z2
coordinate systems.

• The coincident condition
requires four equations that can
be expressed as :
• The coincident condition
requires that the directions of
the two unit normals must be
equal and opposite.
• The two points must lie in the
same plane at which the two
faces mate.
Coincident condition

Concentric mating condition
• The concentric mating condition is achieved
by forcing the shaft and hole axes to be
collinear.

Concentric mating condition
• The equation of the centerline, say,
the hole can be written.
• The constraint equations required
for each concentric condition can be
written as:
• If the shaft axis is collinear with the
hole centerline, points P3 and P4
defining the axis should satisfy Eq.
• Above gives the following two
equations only

Tangent mating condition
• The tangent mating condition is applicable between
two planar/cylindrical or cylindrical/cylindrical faces.
• The tangent mating condition is achieved by forcing a
cylindrical face to be tangent to a planar (flat) face.

Coplanar Mating Condition
• The coplanar mating condition holds between
two planar faces when they lie in the same
plane.

• The constraint equations for
the coplanar condition are
the same as for the
coincident condition except
that the two unit normal's
are in the same direction as
shown in Figure.
• Thus same Eqs. can be used
after replacing the minus sign
in Eqs. with a plus sign.
Coplanar Mating Condition

Parallel Faces Mating Condition
• The parallel faces mating condition is similar to the
coincident mating condition except that the two
mating faces are not in contact with one another.
• This condition is achieved by forcing the normals of
the two faces to be parallel and opposite.

Perpendicular Faces Mating Condition
• The perpendicular faces mating condition is specified
by requiring two faces to be perpendicular to each
other.
• The two shaded faces of Part 1 and Part 2 are
made perpendicular to each other by forcing their
normals n1 and n2 to be perpendicular.

• Tolerance analysis is defined as the process of
checking the tolerances to verify that all the
design constraints are met. Tolerance analysis
is sometimes known as design assurance.

Objective of tolerance analysis
• To determine the variability of any quantity
that is a function of product dimensions and
are called design functions.
• Product dimensions and variables that control
the behavior of a design function are called
design functions variables.
• The variability of design functions is used to
assess the suitability of a particular tolerance
specification.

and orientation - tolerances analysis -
mass property calculations - mechanism
simulation.
Graphics and computing standards– Open GL
Data Exchange standards – IGES, STEP etc –
Communication standards.
UNIT
5

Curve Length
Ken Youssefi
Mechanical Engineering dept. 41
Consider the curve connecting
two points P1 and P2 in space.
The exact length of a curve bounded by
the parametric values u1 and u2, it applies
to open and closed curves.

Cross-Sectional Area
Ken Youssefi
A cross-sectional area is a planar region bounded by a closed boundary.
The boundary is piecewise continuous
The length of the
contour is given by the
sum of the lengths of
C1, C2,…..Cn.
To calculate the area A of the
region R, consider the area of
element dA of sides dxL and
dyL. Integrate over the region.
The centroid of the region is
located by vector rc.

Surface Area
Ken Youssefi
The surface area As of a bounded surface
is formulated the same as the cross-
sectional area. The major difference is that
As is not planar in general as in the case
of B-spline or Bezier surfaces.
For objects with multiple surfaces, the total surface area is equal
to the sum of its individual surfaces.

Volume
Ken Youssefi
The volume can be expressed as a triple integral by integrating the
volume element dV
The centroid of the object is
located by the vector rc.
The volume Vm of a multiply connected
object with holes is given by,

Mass & Centroid
Ken Youssefi
The mass of an object can be formulated the same as its volume by
introducing the density.
dm = ρdV
Integrating over the distributed mass of the object,
Assuming the density ρ remains constant through out the object
we have,
∫∫∫ρdV
m =
m
∫∫∫dV
m = ρ = ρV
V
Mass
Centroid
∫∫∫r dm
rc=
m
m
Same formulation as for volume,
replace volume by mass.

First Moment of Inertia
Ken Youssefi
First moment of an area, mass, or volume is a mathematical property that is
useful in various calculations. For a lumped mass, the first moment of the mass
about a given plane is equal to the product of the mass and its perpendicular
distance from the plane. So the first moment of a distributed mass of an object
with respect to the XY, XZ, and YZ planes are given,
Substituting the centroid
equation, we obtain,

Second Moments of Inertia
Ken Youssefi
The physical interpretation of a second mass moment of inertia of an
object about an axis is that it represents the resistance of the object to
any rotation, or angular acceleration, about the axis. The area moment
of inertia represents the ability of the object to resist deformation.
The second moment of inertia about a given axis is the product of the
mass and the square of the perpendicular distance between the mass
and the axis.

Products of Inertia
Ken Youssefi
In some applications of mechanical or structural design it is necessary to know
the orientation of those axis that give the maximum and minimum moments of
inertia for the area. To determine that, we need to find the product of inertia for
the area as well as its moments of inertia about x, y, and z axes.

Mass Properties – CAD/CAM Systems
Ken Youssefi
CAD systems typically calculate the mass properties discussed
so far. Even a 2D package (AutoCAD) calculates some of the
mass properties.
You are responsible for setting up the correct and units for length,
angles and density
Determine the mass properties
SolidWorks

Mass Properties - SolidWorks
Ken Youssefi
Option button allows you
to set the proper units

Mass Properties – Unigraphics NX5
Ken Youssefi
Calculates volume, surface area, circumference,
mass, radius of gyration, weight, moments of area,
principal moment of inertia, product of inertia, and
principal axes.
Calculates and displays geometric properties of
planar figures. This function analyzes figures after
projecting them onto the XC-YC plane (the work
plane). True lengths, areas, etc., are obtained.
2D Analysis

Mass Properties
Unigraphics NX5
Ken Youssefi
Calculates principal moment of inertia,
circumference, are and center of gravity of
Sections. Primarily, used for automotive
body design.

Ken Youssefi
When the software analyzes the selected bodies, the information window displays the
analysis data. The following table provides a brief explanation of the information.
Area/Volume/Mass Obtains the total face area, volume and mass of a 3D object.
Centroid/1st Mom Obtains the center of mass, or Centroid.
Moments of Inertia Obtains the moment of inertia for certain 3D objects of uniform
density about specified axes.
Products of Inertia The Products of Inertia, along with the Moments of Inertia, form
the inertia tensor, and are important in rotational dynamics.
Principal Axes/Moments The Principal Axes/Moments is an orthogonal system of three
axes through the center of mass such that the three products of
inertia relative to the system are all zero.
Radius of Gyration Calculates the radius of gyration.
Information Displays the calculated data for all of the Mass Properties
options previously discussed in the Information window.
Relative Errors Are estimates of the relative tolerances achieved in calculating
the mass properties. Often the relative errors are less than the
specified relative tolerances, indicating that the mass property
values are correct to within tighter tolerances than those
specified. If only a single accuracy value is specified, then +/-
Range Errors are given.

Ken Youssefi
Measure Bodies
Output

Importance of Data Exchange
 Computer data base are now replacing paper blue prints
in defining geometry and non-geometry for all phases of
product design and manufacturing.
 It is important to find effective procedures for exchanging
these databases.
 Transferring data between dissimilar CAD/CAM system
must embrace the complete product description stored in the
data base.
 The geometric data is used in all downstream
applications of CAD
 Finite element modeling and analysis, Process
planning, Estimation
 CNC programming, Robot Programming, CMM
59

TYPES OF MODELING DATA
• Shape data:
a) Geometrical
e.g. Font, color, layer & annotation
b) Topological
e.g. Hole, flange, web etc.
• Non-shape data:
Includes graphics data
e.g. shaded images, resolutions of storing the database
numerical values.
60

• Design data:
It deals with information that designers generate from
geometric models for analysis.
e.g. mass property data & finite element mesh data.
• Manufacturing data:
Consists of information as tooling, NC tool paths,
tolerance, process planning & tool design and bill of
materials.
Data formats that are designed to communicate data
among CAD/CAM systems must exchange these four types of
data.
61

NEED FOR DATA EXCHANGE
• To integrate and automate the design and manufacturing
process to obtain maximum benefits from CAD / CAM
systems.
• Data transfer between compatible CAD/CAM system can be
done directly.
• When dissimilar CAD/CAM systems are used, data
communication becomes difficult as each system stores
drawings and modelling representations in its own way.
62

Development of Graphic Standards
• A Graphic Standards Planning Committee (GSPC) was
formed in 1974 by ACM-SIGGRAPH (Association of
computing Machinery’s Special Interest Group on Graphics
and Interactive Techniques).
• A committee for the development of computer graphics
standard was formed by DIN in 1975.
• A significant development in CAD standards is the
publication of Graphical Kernel System (GKS) in 1982.
63

Direct and Indirect Data Exchange
 Translation of modelling data
stored in a product database
directly from one CAD/CAM
system format to another in one
step.
 Small number of systems involved.
 Runs quickly, smaller data files.
 Two translators are needed to
transfer data between system 1 and
2.
64

 Creates a neutral file
which is independent of
any existing or future
CAD/CAM system and
hence acts as an
intermediary and a focal
point of communication
among dissimilar
CAD/CAM systems.
 Runs slowly, larger data
files.
 Large number of systems
involved.
Indirect Data Exchange
65

Characteristics of a data exchange format
• Data exchange format will support common entities of
each of the 4 types of data.
• Compact form to store and retrieve data.
• Future versions of the data exchange format (standard)
must remain.
• Compatible with old & existing version.
66

CAD Standards
• 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) 67

Standards in Graphics Programming
68

Graphical Kernel System (GKS)
• GKS is an ANSI (American National Standards Institute)
and ISO (International Organization for Standardization)
standard.
• It is a subroutine package which provides functions for
controlling graphical output and input.
• It is used by application programs as a standard
interface to graphics devices.
69

Principle concepts of GKS
Coordinate systems and transformations
The pictures are drawn by the application program by sending
coordinate information expressed in the world coordinate system
to GKS and by issuing instructions for mapping world co-
ordinates to a GKS standard co-ordinate system called the
normalized device co-ordinate system (NDC).
Output Primitives
The graphic program generate pictures by calling GKS
functions which display lines, characters, filled areas etc.
Segments:
GKS saves the lists of output primitives and their attributes in
records called segments.
It helps to manipulate entire pictures or parts of pictures.
70

Input:
On receiving the appropriate commands from the GKS
the input devices attached to the workstation provide
information or input data such as
Location of a point on the screen, selection of a menu
item.
State:
A lot of data or state is maintained by GKS during its
operation. (e.g. line width)
Errors:
GKS checks for errors and alerts the application
program on occurrence of error.
71

Output primitives of GKS
 The basic function of the GKS is to display primitive
geometric objects (lines, filled regions and text characters)
 The characteristics of the primitive are specified by the
parameters in each function call.
 The various properties of graphical primitives are collectively
known as attributes. (colour of line, dashed or solid line,
thickness)
Polyline:
Which draws a sequence of connected straight line segments.
Function: CALL gpl (n, px, py)
Arguments: n – number of points, px and py – two arrays that
specify the coordinates of the end points. 72

Fill Area:
GKS provides several functions that control how a fill area is
filled.
Filling Methods:
Solid (set fill area colour), Pattern (Set fill style), Hatch
and Hollow
Generalised Drawing Primitives:
Arc, circle, ellipse and spline.
Text:
Font type, colour, height of the text box, spacing.
73

Input methods in GKS environment
Locator: A means of entering the location in world coordinates.
Valuator: Real values in terms of distances.
Choice: Integer options such as 0, 1, 2, 3.
Pick: To select an object or segment in a drawing already created.
String: Character values.
Stroke: To provide continuously the location values in coordinates.
74

PHIGS
 Programmer’s Hierarchical Interactive Graphics Standard
(PHIGS) is an extension of GKS.
 Increased capabilities for object modeling, colour
specifications, surface rendering and picture manipulations
are provided.
 It provides very highly interactive graphics environment.
 It allows building, manipulating, modifying and storing 3D
geometric models.
 PHIGS+ provide 3D surface shading.
 It does not specify methods for storing and transmitting pictures.
75

PHIGS
Structure Networks
• Structure networks are formed through a relation among
various structures in the centralized structure store (CSS).
• They are hierarchical and acyclic.
Logical Input Devices
• PHIGS allows graphics input for application programs via six
classes of input devices. They are
Locater, stroke, valuator, choice, string, hierarchical pick.
Structure Manipulation
• A structure can be deleted by a function called DELETE
STRUCTURE (entire structure is wiped out from the CSS).
• Another function is DELETE STRUCTURE NETWORK
76

PHIGS
Search and Enquiry
 It is used for determining the element contents and its
characteristics.
 The complete details of a specified structure element can be
determined by calling functions like INQUIRE ELEMENT
TYPE AND SIZE and INQUIRE ELEMENT CONTENT.
 Functions such as INQUIRE PATHS TO ANCESTORS and
INQUIRE PATHS TO DECENDANTS can determine the
relationship of a structure to other structure in the CSS.
 The ELEMENT SEARCH function allows searching within a
structure for a particular element type or one of a set of element
types.
77

PHIGS
Structure archival and retrieval
 An archive file is a medium for storing structure definitions.
 Functions like ARCHIVE STRUCTURES, RETRIEVE
STRUCTURES, DELETE STRUCTURES are available in
PHIGS for archiving structures into an ‘archive file’ from the
CSS, or retrieving it from the file itself, respectively.
Structure traversal and display
 Traversal is an operation used to describe a method of
structure element processing.
 The traversal of a network leads to the display of graphical
output from the structure network.
78

and orientation - tolerances analysis -
mass property calculations - mechanism
simulation.
Graphics and computing standards –
Open GL Data Exchange standards – IGES, STEP
etc –Communication standards.
UNIT
5

IGES GENERAL DESCRIPTION
• IGES is a document describing what should go into a data file.
• IGES defines a neutral file format which describes an ‘IGES model’
of modelling data of a given product.
• IGES file format can be interpreted by dissimilar CAD/CAM systems
and so product data can be exchanged between them.
• Translators in the softwares that convert one format to another.
Pre-processors:
• Software that translates the file format of a given CAD/CAM
system to the IGES format.
Postprocessors:
• Software that translates the IGES format to a given
CAD/CAM system format.
89

ENTITY
• It is a fundamental unit of information in the IGES file.
• All product data are expressed as a list of predefined entities.
• Each entity is represented in a standard format.
• Each entity defined by IGES is assigned a specific entity type number
to refer to it in the IGES file.
• Each entity has two types of data -directory data & parameter data.
• Directory data gives the entity type number.
• Parameter data gives the parameters required to uniquely and
completely define the entity.
90

TYPES OF ENTITIES
There are two types of entities.
• Geometric entities:
Definition of the product shape including curves & surfaces.
• Non geometric entities:
Defines views and drawings of the model to enrich its
representation.
Annotation: represents drafting data constructed by
using other entities. (centre line, section, arrow)
Structure: associativity, drawing view, external
reference properties, subfigure.
91

FILE STRUCTURE AND FORMAT
• An IGES file is a sequential file consisting of a sequence of record
sections.
• The file format treat the product definition to be exchanges as a file
of entities each having a standard format.
Format:
ASCII - a) Fixed 80 characters
b) compressed format
Binary
• Depending on the format the record length may be fixed or
variable.
92

Standard for the Exchange of Product Data (STEP)
 The IGES files and DXF files were developed to exchange
product definition data instead of product data.
 By product data we mean the data relevant to the entire life
cycle of a product (e.g., design, manufacturing, quality
assurance, testing, and support).
 Even though the specification of the IGES or DXF file has been
broadened to some of these product data, the data carried by
those files are inherently insufficient to be the product data
supporting the entire life cycle.
94

 As a result, a new effort called product data exchange
specification (PDES) was initiated in the United States in
1983.
 The ISO's Technical Committee were formed in July 1984 to
establish a single worldwide standard for the exchange of
product model data, STEP (Standard for the Exchange of
Product model data).
STEP
95

STEP Architecture
STEP architecture has the following main components:
 Using EXPRESS language
 Data schemes including attributed such as geometry, topology,
features and tolerance.
 Application interface called standard data access interface
(SDAI), which is a standard interface to enable
applications to access and manipulate STEP Data.
STEP database has the following forms:
 ASCII format file data exchange
 Working from file, usually in binary format, that can be
shared by multiple systems.
 Shared database, involving object oriented database
management system or relational database system.
 Knowledge base, with a database management system as a base
coupled to an expert shell.
96

STEP Architecture
Application layer: Description and information of various
application areas.
Logical Layer: Provide a computer independent description of the
data constructs.
Physical layer: Data structure and data format and maintain the file
size and processing time. 97

STEP data export in a CAD modeling package has the following options:
(i) Wire frame edges
(ii) Surfaces
(iii) Solids
(iv) Shells
(v) Datum curves and points
98

Data Exchange Format (DXF)
 DXF also called as drawing interchange format files,
developed by Autodesk and introduced in 1982 (AUTOCAD).
 This format has been the very first of the data transfer
formats used in CAD.
 It gives flexibility in managing data and translating
AutoCAD drawings into file formats that could be read and
used by other CAD/CAM/CAE systems.
 As AutoCAD® becomes more powerful and supports more
complex object types, DXF has become less useful.
99

 A DXF file is an ASCII text and consists of five sections:
Header, table, block, entity and terminate.
 Header section: Describes the AutoCAD drawing environment that
existed when the DXF file was created.
 Table section: contains information about line types, layers, text
styles, and views that may have been defined in the drawing.
 Block section: contains a list of graphic entities that are defined as a
group.
 Entity section: The specific data of each entity of a block are stored in
the corresponding Entity section .
 Terminate section: Indicates the end of the file.
DXF- File Structure
101

Dimensional Measurement Interface Specification (DMIS)
 DMIS is established by CAM-I for manufacturing.
 The database in the form of geometric instructions and
manufacturing information, which is being used by some of the
part programming for automatically converting into CNC part
programs.
 From the same database, inspections programs can also be
generated for the CMM.
 The type of instructions needed for CMM are
Inspection probe selection, speed for positioning the probe, the
path to be followed by the probe, speed and angle.
 DMIS provides a complete vocabulary for passing inspection
program to the dimensional measuring equipment and to pass results
back to the computer. 102

Measurement in product development cycle (DMIS)
103

Summary
 Standardisation of graphic systems can be at the graphic databases,
graphic data handling systems.
 GKS is used to standardise the graphics system calling procedures at the
lowest level so that programmers and programs can be easily migrated
between different systems.
 The neutral CAD database is an important requirement to help with the
transfer of information between various CAD/CAM systems.
 IGES is used for transferring information between various CAD
systems for modeling as well as drafting data.
 STEP is being used extensively in view of its varied and better facilities
for exchanging product model data.
 DXF developed by Autodesk is used for lower end drafting and model
information exchange.
104

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