CAD/CAM overview
By Roger Matar
Umass Lowell, Robotics I 91-548
Computer Aided Design (CAD) and Computer Aided Manufacturing (CAM) are two classes of application programs that help the user to design and build simple or complex products, assemblies, and plants. These programs have been on the market for 20 years. At first they were very expensive and hard to learn. For instance a CAD program such as McDonald-Douglas from Boeing used for designing aircrafts was priced at half million dollar and required a workstation computer and lots of hours to learn. Nowadays, with the advent of fast personal computers, user friendly GUI interfaces, and much more efficient calculation algorithms, CAD/CAM has become a household name in the engineering and manufacturing field. In fact, because of these tools, an engineer has become a designer, eliminating the need for a full time drafter.
CAD software is based on representing geometric forms in vectors as opposed to a bitmap image database. Vector representation has two major advantages: first, it reduces the size of the database to an absolute minimum. For example, a line is represented by the Cartesian Coordinate values of the two end points. In contrast, a bitmap line representation can be very large, a large set of several bitmap pixels. Second, a vector representation allows mathematical matrix calculations, mostly multiplications, which are at the heart of CAD engines. Below is a picture showing a line drawn in 2D CAD.
CAD has evolved from being a simple 2D drafting tool such as the line drawn above, to a full blown 3D solid modeler tool. In 3D modeling, a object is maintained by saving its 3D Cartesian Coordinate information at all times. There is however a difference between a Constructive Geometric System (CGS) and a Parametric System. In CGS, an object is constructed by adding and subtracting from a solid. This is accomplished by using Boolean operations as if a solid mass of material is machined in several steps and the only way to move a hole for instance is to fill the old hole and drill a new one. When a file is saved, it will have only the end geometry. The Parametric system on the other hand, is more versatile because it keeps track of the full history of how a part was built. Therefore a hole is moved by simply changing its coordinates. This is similar to a spreadsheet because the dimensions affect the geometry and vise versa. For instance, if a wheel is design to be 12 inches in diameter and is set to be tangent to the floor, and if the diameter is later changed to 16 inch diameter, the center of the wheel will automatically move up to maintain tangency. The diameter and the tangency are 2 constraints on the wheel. An object can be fully constrained or under constrained depending on the design intent. If the design intent if to allow a part to move in the X direction, the object will have to be constrained in the Y and Z in translation, as well as in the X, Y, and Z in rotation.
The image below show a 3D solid model designed using a parametric CAD program.
A solid model maintains volumetric information as well. The user supplies the density of the material and the program calculates the mass of the object as well as other mechanical characteristics. Below is another solid modeling sample of a multi-part assembly.
With a simulation program, a part is animated to show the material being removed from a solid workpiece as shown below.
The image below shows the end of the machining simulation.
Other machining processes will have different parameters. For example, a turning machine will require the tool tip profile and the part rotational speed, and wire electrical discharge machine will require the wire diameter and both the wire feed rate.
Another major innovation on the market today is Rapid Prototyping in which a CAD designed part is physically modeled using stereo lithography machining. Basically a CAD model is first graphically sliced into multi-layers, usually in the Z (vertical) axis. The machine then builds the model layer by layer using a polymer based fast curing compound or other material. The process usually takes less than an hour. The software will figure out a way to introduce small ribs into the areas that would be suspended in air during the building process. Rapid Prototyping has sometimes taken the name of 3D plotting. It is a very inexpensive way to produce a prototype for demonstration and fit and function tests.
General references:
1 – www.thomasregister.com/newsletter.html (free registration needed)
2 – The association for Manufacturing Technology http://www.mfgtech.org/
3 – IEEE Robotics & Automation Society http://www.ncsu.edu/IEEE-RAS/
4 – Managing Automation http://www.managingautomation.com/
5 – The Robotics Institute http://www/ri.cmu.edu
6 – Robotics Industries Association http://www.robotics.org/