COMPUTER-AIDED DESIGN
AND MANUFACTURING



Computer-aided design (CAD), also known as computer-aided design and drafting (CADD), involves the entire spectrum of drawing with the aid of a computer—from straight lines to custom animation. In practice, CAD refers to software for the design of engineering and architectural solutions, complete with two- and three-dimensional modeling capabilities.

Computer-aided manufacturing (CAM) involves the use of computers to aid in any manufacturing process, including flexible manufacturing and robotics. Often outputs from CAD systems serve as inputs to CAM systems. When these two systems work in conjunction, the result is called CADCAM, and becomes part of a firm's computer-integrated manufacturing (CIM) process.

CADCAM systems are intended to assist in many, if not all, of the steps of a typical product life cycle. The product life cycle involves a design phase and an implementation phase. The design phase includes identifying the design needs and specifications; performing a feasibility study, design documentation, evaluation, analysis, and optimization; and completing the design itself. The implementation phase includes process planning, production planning, quality control, packaging, marketing, and shipping.

CAD systems can help with most of the design phase processes, while CAM systems can help with most of the implementation processes. The contributions of CAD and CAM systems are described below.

CAD SYSTEMS

CAD systems are a specialized form of graphics software, and thus must adhere to basic principles of graphics programming. All graphics programs work in the context of a graphics device (e.g., a window on a monitor, a printer, or a plotter). Graphics images are drawn in relation to a 2-D or 3-D coordinate system, of which there are several types.

A device coordinate system is 2-D and maps images directly to the points (pixels) of the hardware device. In order to facilitate device-independent graphics, a virtual device coordinate system abstracts the 2-D points into a logical framework.

Of course, the devices being designed are generally 3-D objects, which also require a world coordinate system for representing the space in which the objects reside, and a model coordinate system for representing each of the objects in that space. CAD software includes algorithms for projecting the 3-D models onto the 2-D device coordinate systems and vice versa.

CAD systems include several primitive drawing functions, including lines, polygons, circles and arcs, rectangles, and other simple shapes. From these primitives, 3-D composites can be constructed, and include cubes, pyramids, cones, wedges, cylinders, and spheres. These shapes can be drawn in any color, and filled with solid colors or other patterns (called hatching). In addition, basic shapes can be altered by filleting (rounding) or chamfering (line segmentation).

Based on the manipulation of basic shapes, designers construct models of objects. A skeletal wire form model is a 3-D representation that shows all edges and features as lines. A more realistic-looking model is called a solid model, which is a 3-D model of the object being designed as a unitary whole showing no hidden features. The solid model represents a closed volume. It includes surface information and data determining if the closed volume contains other objects or features.

Solid modeling involves functions for creating 3-D shapes, combining shapes (via union, intersection, and difference operations), sweeping (translational and rotational) for converting simple shapes into more complex ones, skinning (for creation of surface textures), and various boundary creation functions. Solid modeling also includes parameterization, in which the CAD system maintains a set of relationships between the components of an object so that changes can be propagated to following constructions.

Common shapes are constructed into features (e.g., slots, holes, pockets), which can then be included in a solid model of an object. Feature representation helps the user define parts. It also simplifies CAD software design because features are easier to parameterize than explicit interactions. Objects built from features are called parts. Since a product being designed is composed of several parts, many CAD systems include a useful assembly model, in which the parts are referenced and their geometric and functional relationships are stored.

CAD models can be manipulated and viewed in a wide variety of contexts. They can be viewed from any angle and perspective desired, broken apart or sliced, and even put through simulation tests to analyze for strengths and defects of design. Parts can be moved within their coordinate systems via rotation operations, which provide different perspectives of a part, and translation, which allows the part to move to different locations in the view space. In addition, CAD systems provide valuable dimensioning functionality, which assigns size values based on the designer's drawing.

The movement of these images is a form of animation. Often, CAD systems include virtual reality technology, which produces animated images that simulate a real-world interaction with the object being designed. For example, if the object is a building, the virtual reality system may allow you to visualize the scene as if you were walking around the inside and the outside of the building, enabling you to dynamically view the building from a multitude of perspectives. In order to produce realistic effects, the system must depict the expected effects of light reflecting on the surface as it moves through the user's view space. This process is called rendering.

Rendering technology includes facilities for shading, reflection, and ray tracing. This technique, which is also used in sophisticated video games, provides a realistic image of the object and often helps users make decisions prior to investing money in building construction. Some virtual reality interfaces involve more than just visual stimuli. In fact, they allow the designer to be completely immersed in the virtual environment, experiencing kinesthetic interaction with the designed device.

Some CAD systems go beyond assisting in parts design and actually include functionality for testing a product against stresses in the environment. Using a technique called finite element method (FEM), these systems determine stress, deformation, heat transfer, magnetic field distribution, fluid flow, and other continuous field problems.

Finite element analysis is not concerned with all design details, so instead of the complete solid model a mesh is used. Mesh generation involves computing a set of simple elements giving a good approximation of the designed part. A good meshing must result in an analytical model of sufficient precision for the FEM computation, but with a minimum number of elements in order to avoid unnecessary complexity.

In addition to FEM, some CAD systems provide a variety of optimization techniques, including simulated annealing and genetic algorithms (borrowed from the field of artificial intelligence). These methods help to improve the shape, thickness, and other parameters of a designed object while satisfying user-defined constraints (e.g., allowable stress levels or cost limitations).

When a designer uses CAD to develop a product design, this data is stored into a CAD database. CAD systems allow for a design process in which objects are composed of sub-objects, which are composed of smaller components, and so on. Thus CAD databases tend to be object-oriented. Since CAD designs may need to be used in CAM systems, or shared with other CAD designers using a variety of software packages, most CAD packages ensure that their databases conform to one of the standard CAD data formats. One such standard, developed by the American National Standards Institute (ANSI), is called Initial Graphics Exchange Specification (IGES).

Another data format is DXF, which is used by the popular AutoCAD software and is becoming a de facto industry standard. The capability to convert from one file format to another is called data exchange, and is a common feature of many CAD software packages.

Modern CAD systems offer a number of advantages to designers and companies. For example, they enable users to save time, money, and other resources by automatically generating standard components of a design, allowing the reuse of previously designed components, and facilitating design modification. Such systems also provide for the verification of designs against specifications, the simulation and testing of designs, and the output of designs and engineering documentation directly to manufacturing facilities. While some designers complain that the limitations of CAD systems sometimes serve to curb their creativity, there is no doubt that they have become an indispensable tool in electrical, mechanical, and architectural design.

CAM SYSTEMS

The manufacturing process includes process planning, production planning (involving tool procurement, materials ordering, and numerical control programming), production, quality control, packaging, marketing, and shipping. CAM systems assist in all but the last two steps of this process. In CAM systems, the computer interfaces directly or indirectly with the plant's production resources.

Process planning is a manufacturing function that establishes which processes and parameters are to be used, as well as the machines performing these processes. This often involves preparing detailed work instructions to machines for assembling or manufacturing parts. Computer-aided process planning (CAPP) systems help to automate the planning process by developing, based on the family classification of the part being produced, a sequence of operations required for producing this part (sometimes called a routing), together with text descriptions of the work to be done at each step in the sequence. Sometimes these process plans are constructed based on data from the CAD databases.

Process planning is a difficult scheduling problem. For a complex manufacturing procedure, there could be a huge number of possible permutations of tasks in a process requiring the use of sophisticated optimization methods to obtain the best process plan. Techniques such as genetic algorithms and heuristic search (based on artificial intelligence) are often employed to solve this problem.

The most common CAM application is numerical control (NC), in which programmed instructions control machine tools that grind, cut, mill, punch, or bend raw stock into finished products. Often the NC inputs specifications from a CAD database, together with additional information from the machine tool operator. A typical NC machine tool includes a machine control unit (MCU) and the machine tool itself. The MCU includes a data processing unit (DPU), which reads and decodes instructions from a part program, and a control loop unit (CLU), which converts the instructions into control signals and operates the drive mechanisms of the machine tool.

The part program is a set of statements that contain geometric information about the part and motion information about how the cutting tool should move with respect to the workpiece. Cutting speed, feed rate, and other information are also specified to meet the required part tolerances. Part programming is an entire technical discipline in itself, requiring a sophisticated programming language and coordinate system reference points. Sometimes parts programs can be generated automatically from CAD databases, where the geometric and functional specifications of the CAD design automatically translate into the parts program instructions.

Numerical control systems are evolving into a more sophisticated technology called rapid prototyping and manufacturing (RP&M). This technology involves three steps: forming cross sections of the objects to be manufactured, laying cross sections layer by layer, and combining the layers. This is a tool-less approach to manufacturing made possible by the availability of solid modeling CAD systems. RP&M is often used for evaluating designs, verifying functional specifications, and reverse engineering.

Of course, machine control systems are often used in conjunction with robotics technology, making use of artificial intelligence and computer controlled humanoid physical capabilities (e.g., dexterity, movement, and vision). These "steel-collar workers" increase productivity and reduce costs by replacing human workers in repetitive, mundane, and hazardous environments.

CAM systems often include components for automating the quality control function. This involves evaluating product and process specifications, testing incoming materials and outgoing products, and testing the production process in progress. Quality control systems often measure the products that are coming off the assembly line to ensure that they are meeting the tolerance specifications established in the CAD databases. They produce exception reports for the assembly line managers when products are not meeting specifications.

In summary, CAM systems increase manufacturing efficiency by simplifying and automating production processes, improve the utilization of production facilities, reduce investment in production inventories, and ultimately improve customer service by drastically reducing out-of-stock situations.

PUTTING IT ALL TOGETHER: COMPUTER
INTEGRATED MANUFACTURING

In a CADCAM system, a part is designed on the computer (via CAD) then transmitted directly to the computer-driven machine tools that manufacture the part via CAM. Within this process, there will be many other computerized steps along the way. The entire realm of design, material handling, manufacturing, and packaging is often referred to as computer-integrated manufacturing (CIM).

CIM includes all aspects of CAD and CAM, as well as inventory management. To keep costs down, companies have a strong motivation to minimize stock volumes in their warehouses. Just-in-time (JIT) inventory policies are becoming the norm. To facilitate this, CIM includes material requirements planning (MRP) as part of its overall configuration. MRP systems help to plan the types and quantities of materials that will be needed for the manufacturing process. The merger of MRP with CAM's production scheduling and shop floor control is called manufacturing resource planning (MRPII). Thus, the merger of MRP with CADCAM systems integrates the production and the inventory control functions of an organization.

Today's industries cannot survive unless they can introduce new products with high quality, low cost, and short lead time. CADCAM systems apply computing technology to make these requirements a reality, and promise to exert a major influence on design, engineering, and manufacturing processes for the foreseeable future.

SEE ALSO: Computer-Integrated Manufacturing ; Manufacturing Resources Planning ; Robotics

Michel Mitri

Revised by Rhoda L. Wilburn

FURTHER READING:

Bean, Robert. "CAD Should Enable Design Creativity: Engineers Need CAD Tools as Easy as the 'Paper Napkin.'" Design News, 10 January 2005.

Grabowski, Ralph, and R. Huber. The Successful CAD Manager's Handbook. Albany, NY: Delmar Publishers, 1994.

Lee, Kunwoo. Principles of CAD/CAM/CAE Systems. Reading, MA: Addison Wesley, 1999.

McMahon, Chris, and Jimmie Browne. CAD/CAM: Principles, Practice, and Manufacturing Management. 2d ed. Upper Saddle River, NJ: Prentice-Hall, 1999.

Port, Otis. "Design Tools Move into the Fast Lane." Business Week, 2 June 2003.

Sheh, Mike. "A Quantum Leap in Engineering Design." Business Week, 2 June 2003.



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