LECTURE NOTES DEPARTMENT OF MECHANICAL ENGINEERING ME 6501-COMPUTER AIDED DESIGN UNIT-I (COMPUTER AIDED DESIGN) 1.1 Introduction In general, a Computer Aided Design (CAD) package has three components: a) Design, b) Analysis, and c) Visualization, as shown in the sketch. A brief description of these components follows. a) Design: Design refers to geometric modeling, i.e., 2-D and 3-D modeling, including, drafting, part creation, creation of drawings with various views of the part, assemblies of the parts, etc. b) Analysis: Analysis refers to finite element analysis, optimization, and other number crunching engineering analyses. In general, a geometric model is first created and then the model is analyzed for loads, stresses, moment of inertia, and volume, etc. c) Visualization: Visualization refers to computer graphics, which includes: rendering a model, creation of pie charts, contour plots, shading a model, sizing, animation, etc.
Each of these three areas has been extensively developed in the last 30 years. Several books are written on each of these subjects and courses are available through the academic institutions and the industry. Most commercial CAD packages (software) consist of only a single component: design or analysis or visualization. However, a few of the vendors have developed an integrated package that includes not only these three areas, but also includes the manufacturing software (CAM). Due to the large storage requirement, integrated packages use either an UNIX workstation or a mainframe platform, and not the popular PC platform. With the improvement in PC computing speed, it’s only a matter of time before we see an integrated package run on a PC. CAD has revolutionized the modern engineering practice; small and large companies use it alike, spending several billion dollars for the initial purchase or lease alone. CAD related jobs are high in demand and the new graduates have advantage over their senior colleagues, as they are more up to date and more productive. In this course, we will limit our coverage to the design only. Those of you interested in analysis area, look into the course ME 160 – Introduction to Finite Element Analysis. Concurrent Engineering Concurrent Engineering is another powerful CAD concept that has evolved in the 90’s. According to this concept, there is an instantaneous communication between the designer, analyst, and manufacturing. Changes made at any of these work centers are immediately passed on to the others and the product is modified without delay. Often, the customer, management, and the marketing people join in and become part of the process. Concurrent engineering saves the valuable time and helps get the product out in the market quicker. Products that use to take years from the date of its concept to the actual production now take only a few weeks, and the final product is better and cost-effective.
Some large organizations have invested in Rapid Prototyping process. In this process, the part is created by a CAD package and downloaded into the rapid prototyping machine; the machine immediately manufactures the part, using a plastic material. This is a good example of concurrent engineering, sometimes referred as Art to Part concept. CAD Hardware There are basically two types of devices that constitute CAD hardware: a) Input devices, and b) Output devices. A brief description follows. Input Devices These are the devices that we use for communicating with computer, and providing our input in the form of text and graphics. The text input is mainly provided through keyboard. For graphic input, there are several devices available and used according to the work environment. A brief description of these devices is given here. Mouse: This is a potentiometric device, which contains several variable resistors that send signals to the computer. The functions of a mouse include locating a point on the screen, sketching, dragging an object, entering values, accepting a software command, etc. Joystick and trackballs are analogous to a mouse device, and operate on the same principle. Digitizers: Digitizers are used to trace a sketch or other 2-D entities by moving a cursor over a flat surface (which contains the sketch). The position of the cursor provides a feedback to the computer connected with the device. There are electrical wires embedded in orthogonal directions that receive and pass signals between the device and the computer. The device is basically a free moving puck or pen shaped stylus, connected to a tablet. Light Pens: Lockheed’s CADAM software utilized this device to carry out the graphic input. A light pen looks like a pen and contains a photocell, which emits an electronic signal. When the pen is pointed at the monitor screen, it senses light, which is
converted to a signal. The signal is sent to the computer, for determination of the exact location of the pen on the monitor screen. Touch Sensitive Screens: This device is embedded in the monitor screens, usually, in the form of an overlay. The screen senses the physical contact of the user. The new generation of the Laptop computers is a good example of this device. Other Graphic Input Devices: In addition to the devices described above, some CAD software will accept input via Image Scanners, which can copy a drawing or schematic with a camera and light beam assembly and convert it into a pictorial database. The devices just described are, in general, independent of the CAD package being used. All commercial CAD software packages contain the device drivers for the most commonly used input devices. The device drivers facilitate a smooth interaction between our input, the software, and the computer. An input device is evaluated on the basis of the following factors: • Resolution • Accuracy • Repeatability • Linearity Output Devices After creating a CAD model, we often need a hard copy, using an output device. Plotters and printers are used for this purpose. A plotter is often used to produce large size drawings and assemblies, where as, a laser jet printer is adequate to provide a 3-D view of a model. Most CAD software require a plotter for producing a shaded or a rendered view. CAD Software CAD software is written in FORTRAN and C languages. FORTRAN provides the number crunching, where as, C language provides the visual images. Early CAD packages were turnkey systems, i.e., the CAD packages were sold as an integrated