1. Modelling Techniques in CAD are:
2D Drafting and
3D modelling
• 3D is the most advanced method of geometric modeling in three
dimensions. Solid modeling is the representation of the solid parts of the
object on your computer.
• 3D modelling is applying a consistent set of principles for mathematical
and computer modeling of three-dimensional solids.
• Normally, in 3D modelling, the geometries are created using 3 approaches:
1.Wireframe Modeling
2.Surface Modeling
3.Solid Modeling
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3. Wireframe modelling
In wireframe modelling, the
model is created by making use of
set of lines or wires and hence the
name wireframe modelling. In this,
the skeleton of the geometry is
displayed and the whole geometry
is displayed with a single colour.
The wireframe models are
ambiguous models since there is
no hidden line removal.
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5. Surface modelling
In Surface modelling, the
drawbacks of wireframe model are
overcome by creating the lateral
surfaces of the geometry. This
method can be utilized to construct
the geometry corresponding to the
body (outer shell/cover) of
automobile bodies, ship hulls, aircraft
bodies etc. The disadvantage is that
SOLID MODELLING
9. SOLID MODELLING
Solid Modelling
• A solid model of an object is a more complete representation than its
surface (wireframe) model.
• In solid modelling, the actual 3D geometry is constructed by assigning
material properties to it.
• The solid models thus constructed will have mass properties, volumetric
properties and physical properties according to the material applied.
• They also display photorealistic appearance on the screen.
• Wireframes and surface representations contain only geometrical data,
but, the solid model uses topological information in addition to the
geometrical information to represent the object unambiguously and
completely.
10. Solid Model
• A solid model of an object is a more complete representation than its surface (wireframe)
model. It provides more topological information in addition to the geometrical information
which helps to represent the solid unambiguously.
• In solid modelling, the actual 3D geometry is constructed by assigning material properties .
• The solid models thus constructed will have mass properties, volumetric properties and
physical properties according to the material applied. They also display photorealistic
appearance on the screen.
SOLID MODELLING
13. SOLID MODELLING
Closure under rigid motion and regularised booleon operations:
• Movement and Booleon operations should produce other valid solids
Boundary determinism:
• The boundary must contain the solid and hence should determine the solid
distinctly
Conciseness, ease of creation and efficiency in application:
• Conciseness refers to compact database to represent the object
• Ease of creation implies user friendly software to create the solid model
• The solid model must be usable for manufacturing / analysis related operations
14. Solid modelling approaches
1. Constructive Solid Geometry or CSG or C-Rep
2. Boundary Representations or (B-Rep) and
3. Hybrid(C-rep & B-rep combined)
1. Constructive Solid Geometry (CSG) or (C-Rep)
• The CSG approach allows the user to build the
model out of standard geometric primitives such as
rectangular blocks or prisms, pyramids, spheres,
cones, cylinder, etc.
• This "building-block geometry" approach begins
with a set of primitive solid objects such as the
block, sphere, cylinder, cone, torus, and wedge.
Each of these primitives is easily described by a
small set of user-defined parameters specifying the
primitive's geometry, location, and orientation.
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15. • A desired solid can be obtained by
combining two or more solids. After creating
the required geometry, the geometries are
transformed to the required locations
followed by the Boolean operations such as
union, subtraction, intersection, etc to get
the end product.
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16. Booleon Operations:
New solid models can be created from
the primitives or other solid models by
the following operations:
• Union : Two solids are joined and the
common volume of one of the primitives
is neglected in the resulting solid.
• Subtraction or Difference: One solid is
subtracted from the other and the
resultant solid retains only the uncut
portion of the solid.
• Intersection : When two solids are
combined, the resultant solid represents
the common volume of the two solids.
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23. Pure (Parameterized) Primitive Instancing
(PPI)
• Involves describing the model by expressing its
dimensions in terms of a single parameter.
• When it is required to get a similar geometry on the
screen, the value of the single parameter is entered
to get the new model on the screen which is
geometrically and topologically similar to the one
which is stored in the database. Eg: When it is
required to create different sizes of bolts & nuts, all
the dimensions of the bolt and nut are expressed in
terms of a single parameter (d) and are saved in
the database. When it is needed to create bolts &
nuts of different sizes, the file is recalled and the
value of the single parameter ‘d’, is entered to get
bolt and nut of desired size easily
SOLID MODELLING
