University of Engineering and Technology Lahore, Pakistan Department of Mechanical Engineering C A D 31/03/2012 by Dr. Muhammad Mahmood Aslam Bhutta University of Engineering and Technology Lahore, Pakistan Lahore Pakistan

All the mathematical sciences are founded on relations between physical laws and laws of numbers, so that the aim of exact science is to reduce the problems of nature to the determination of quantities by operations with numbers. James Clerk Maxwell, 1856

In the late 70 this approach (the use of supercomputers to solve aerodynamic problems began to pay off. One early success was the experimental NASA aircraft called HIMAT (Highly Maneuverable Aircraft Technology), designed to test concepts of high maneuverability for the next generation of fighter planes. Wind tunnel tests of a preliminary design for HiMAT showed that it would have unacceptable drag at speeds near the speed of sound; if built that way the plane would be unable to provide any useful data. The cost of redesigning it in further wind tunnel tests would have been around $150,000 and would have unacceptably delayed the project. Instead, the wing was redesigned by a computer at a cost of $6,000. Paul E. Ceruzzi, Curator, National Air and Space Museum, in Beyond the Limits, The MIT Press, 1989

Geometry Kernel A geometry kernel offers basic geometric objects, such as points, lines, segments, rays, planes, circles, ..., and geometric primitives operating on these objects, e.g., the computation of the area of the triangle defined by three points and the computation of the intersection of two lines.

ACIS As a geometric kernel, ACIS is second generation system, coming after the first generation Romulus ACIS stands for Alan, Charles, Ian's System (Alan Grayer, Charles Lang and Ian Braid (Three-Space Ltd.) Alan, Charles, Ian and Spatial The system was later on sold to Spatial Technology, (now Spatial Corp). The first version of ACIS was released in 1989 and was quickly licensed by HP for integration into its ME CAD software.

STL An STL file is a triangular representation of a 3D surface geometry. The surface is logically converted into a set of oriented triangles (facets). Each facet is described by the unit outward normal and three points listed in counterclockwise order representing the vertices of the triangle. The STL standard includes two data formats - ASCII and binary. While the ASCII form is more descriptive, the binary form is far more common due to the very large resulting size of the CAD data when saved in the ASCII format.

IGES Initial Graphics Exchange Specification (IGES) (pronounced eye-jess) It defines a neutral data format that allows the digital exchange of information among Computer-aided design (CAD) systems. The IGES project was started in 1979 by a group of CAD users and vendors, including Boeing, General Electric, Xerox, Computer vision and Applicon, with the support of the National Bureau of Standards (now known as NIST) and the U.S. Department of Defense (DoD).

GEOMETRIC REPRESENTATION SCHEMES IN CAD SYSTEMS

1. Wire frame representation 2. Surface modeling schemes 3. Solid modeling schemes, including: a. Constructive solid geometry (CSG) modeling b. Boundary representation solid modeling c. Sweep representations

Geometric modeling schemes Wire frame model Surface model (c) Constructive solid geometry (CSG) model, (d) Boundary representation (B-rep) model (e) Sweep representations

Wire frame representations contain only vertices (points) and edges (lines), which are the intersections of the surfaces of the object. No other information about the surfaces of the object is carried Wire frame models lack the surface definition for many of the analyses that might be required on the defined objects.

Visualization of complex wire frame models can be confusing, as automatic hidden line removal and surface renderingare not possible. .

A major disadvantage is that a unique object representation is not achieved, and wire frame models can often be interpreted in a number of different ways . The identification of manufacturing-related features is usually not possible and interpretation of the model into its manufacturing requirements is generally impossible to do automatically