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Chapter: Mechanical : Computer Aided Design : Geometric Modeling

Important Questions and Answers: Geometric Modeling

Mechanical - Computer Aided Design - Geometric Modeling




1.     What are the two types of equations for curve representation?



(1) Parametric equation x, y, z coordinates are related by a parametric variable (u or θ)


(2) Nonparametric equation x, y, z coordinates are related by a function


Example: Circle (2-D)


Name some types of curves used in geometric modelling.


            Hermite curves


            Bezeir curves


            B-spline curv es


            NURBS curv es


3   What are the desirable properties of Bezier Curve?

  4 Write any Two Drawbacks of Bezier Curves.

5.  List the advantages of B -spline curves.

6.  What are the functions of Geometric Modelling in design analysis?


Evaluation of are a, volume, mass and inertia properties Interference chec king in assemblies


Analysis of tolerance build-up in assemblies Kinematic analys is of mechanisms and robots Automatic mesh generation for finite element analysis


7.  What are the functions of Geometric Modelling in Manufacturing??


Parts classification Process planning


NC data generatio n and verification Robot program ge neration Scheduling


8.     List the Properties of a Geometric Modeling System.



The geometric model must stay invariant with regard to its location and o rientation The solid must have an in terior and must not have isolated parts


The solid must be finite a nd occupy only a finite shape


The application of a trans formation or Boolean operation must produce another solid The solid must have a finite number of surfaces which can be described


The boundary of the solid must not be ambiguous


9.  What are called 2 ½ - D Wire frame models?



Two classes of shape for which a simple wire-frame representation is often adequate are those shapes defined by projecting a plane profile along its normal or by rotating a planar profile about an axis. Such shapes are not two-dimensional, but neither do they require sophisticated three-dimensional schemes for their representation. Such representation is called 2 ½ - D.


10. Draw an example for 2 ½ - D Wire frame model.



11.                        Catalog Techniques In Surface Modelling.


i.           Surface Patch


ii.           Coons Patch


iii.           Bicubic Patch


iv.          Be’zier Surface


v.           B-Spline Surfaces


12.                        What are the Solid Modeling Techniques?


The various methods for representing the solids are:


1.     Half-space method


2.     Boundary representation method (B-rep)


3.     Constructive solid geometry (CSG and C-rep)


4.     Sweep representation


5.     Analytical solid modeling (ASM)


6.     Primitive instancing


7.     Spatial partitioning representation


a.     Cell decomposition


b.     Spatial occupancy enumeration


c.      Octree encoding

13.                        Write short note on Be’zier Surface.



The Be’zier s urface formulation use a characteristic polygon

Points the Be zier surface are given by



14.                        Write any topological te rms used Boundary representation method (B-rep).



Vertex (V) : It is a unique point (an ordered triplet) in space


Edge (E): It is finite, non-self intersecting, directed space c urve bounded by t wo vertices that are not necessarily distinct


Face (F) : It is defined as a finite connected, non-self-intersecting, region of a closed oriented surface bounded by one or mor e loops


Loop (L) : It is an ordered alternating sequence of vertices and edges


Genus (G) : It is the topological name for the number of handles or through holes in an object


Body/Shell(B) : It is a set of faces that bound a single connected c losed volume. A m inimum body is a point




15. What is called singular body in b-rep?



A minimum body is a point; topologically this body has one face, one vertex, and no edges. It is called a seminal or singular body


16.                        Sketch some open polyhedral objects used in B-rep.


17. Write Euler’s formula for open and closed objects used in b-rep.


Euler – Poincare Law for closed objects  :


F – E + V – L = 2 (B – G)


Euler – Poincare Law for open objects   :


F – E + V – L =  B – G


18.            What are Advantages and Disadvantages of b-rep?




Appropriate to construct solid models of unusual shapes


Relatively simple to convert a b-rep model to wireframe model


Disadvantages of b-rep


Requires more storage


Not suitable for applications like tool path generation


O   Slow manipulation


19.            How solids are created using CSG?


20. Give an example for CS G Tree.


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Mechanical : Computer Aided Design : Geometric Modeling : Important Questions and Answers: Geometric Modeling |

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