In product development, computer-aided design (CAD) automates the design process while computer-aided manufacturing (CAM) automates the manufacturing of CAD-generated product designs. Both systems are now widely used and have transformed much of our country's manufacturing processes. Before their advent, parts were described with technical part prints and then manufactured using traditional machine tools and machining processes.

Then machine tools became computerized and numerically controlled, or as we now call them, CNC machine tools. Shortly after that, CAD began to take the place of drafting as a way to produce technical part prints. The use of CAD then led to the use of CAM, which utilizes data from CAD-generated prints to manufacture a part.

CAD and CAM are fundamentally different. While CAD defines design features, CAM identifies manufacturing features. In other words, with CAD systems, you are creating something digital—the product's computer image—and with CAM, you are creating something physical—the actual product.

Because of these basic differences, these systems have divergent focuses. For example, workflow and time are not important factors in CAD. Meanwhile, CAM is concerned with workflow because the sequence in which various processes are carried out can be crucial. Additionally, the concept of time is significant in CAM because a part undergoes changes as it moves through different machining processes over time.

Focus on CAD

Initially, CAD systems were simply electronic drawing boards and were more commonly called computer-aided drafting systems because they only automated the drafting stage of a product's design. Nowadays, CAD is capable of automating most of, if not the entire, design process. And this can truncate design times, which will translate to lower costs, better quality and bolstered product performance.

Despite the great potential of today's CAD systems, many users are still employing their systems as electronic drawing boards. One of two reasons may be behind this—first, manufacturers may still be operating the old systems, and second, they may not be using all the capabilities of their current systems and are instead continuing old habits.

There are two main categories of CAD—2D CAD and 3D CAD systems. Two-dimensional CAD systems are usually little more than electronic drawing boards, providing different views of the product in various 2D planes. They fail to portray product properties, such as volume, surface area and center of gravity, and can be prone to interpretation errors downstream. But using 2D CAD is generally faster than manual drawing, and it's easier to use than 3D CAD. It's suitable for products with very simple geometries.

In comparison, 3D CAD depicts product designs in three dimensions, providing a virtual prototype. 3D CAD software uses one of three modeling methods—wireframe modeling, surface modeling and solid modeling. The simplest of the three, wireframe modeling shows the part shape using interconnected line elements that convey information about edges, corners and surface discontinuities. Surface modeling, meanwhile, depicts the outside part geometry and in so doing, clears up some of the ambiguities of wireframe models. And finally, solid modeling, which is the most sophisticated form of 3D modeling, defines both the exterior surface and the interior volume and mass of an object.

Focus on CAM

A CAM system uses CAD-generated data to create the code needed to operate a CNC machine. CAM software facilitates the programming of machine tools. It lets users define part geometry and set machining strategies, create and confirm toolpaths, and share programming data with other shop-floor machines.

The two basic types of CAM systems are process-oriented and geometry-oriented. Process-oriented CAM systems are geared toward the process part of manufacturing engineering, which includes the effective use of tooling and machining operations, advantageous tool changes and the management of complex processes. In contrast, geometry-oriented CAM systems lean toward the geometric aspects of manufacturing engineering, including complex part geometries, sizeable CAD models, and running process details through geometric conditions.

These two basic types of CAM each have their strengths and weaknesses. Process-oriented CAM systems can deal with exceedingly complex processes, but they have a relatively simple concept of geometry. In comparison, geometry-oriented CAM systems can take on very complex geometries, but they generally have rather simple process abilities.

Process-oriented CAM systems are superb for production machining work, where the geometry is not as big of an issue as the overall process complexity and the need to maximize the process to compress overall machining time. For example, a plate with a lot of 2.5D pockets and holes may seem to have a complicated geometry, but the part's overall process complexity is actually more significant, including pocketing and hole-making operations as well as related tooling and tool changes. Thus, this particular workpiece will be best handled by a process-oriented CAM system.

Geometry-oriented CAM systems excel in mold, tool and die work, where the more difficult part of the job is the part geometry, not process optimization, since typically only a few parts are being machined. Prime candidates for this type of CAM system would include a complex mold cavity for an injection molded part and the stamping die for a sheet metal automotive panel. In such cases, the machining process is not as challenging as the geometric aspects of the part.

CAD/CAM

Many CAD vendors market fully integrated CAM systems, aptly called CAD/CAM systems. These CAD/CAM packages deliver many advantages. For starters, they feature a common user interface that allows CAD operators to quickly learn the software. Moreover, users can easily transfer CAD data to the CAM system without worrying about translation errors or other difficulties. And finally, some integrated systems provide full associativity, which means that any modification to the CAD model will prompt the associated toolpath to be automatically updated.

However, there are some disadvantages to using integrated CAD/CAM systems. For one thing, such systems are usually pricier than standalone or add-on packages. And second, these systems may not be as flexible or feature-laden as standalone packages because oftentimes, the manufacturers of CAD/CAM packages place more emphasis on the CAD part of their product.

For standalone packages, CAD and CAM interoperability is still problematic, despite all the progress that has been made in this area. Indeed, as design data travels from CAD to CAM, a lot of design subtleties are still unable to survive the journey. CAD and CAM data remain incompatible, and fixing this comes at a huge cost—tens of billions of dollars a year.

Ultimately, experts expect CAD and CAM to become one. This may not happen in the foreseeable future, however. For now, these two powerful tools are still separate and fundamentally different. While CAD systems enable designers to generate geometric models of parts they want to make, CAM systems interpret these digital descriptions and produce the instructions for machine tools. Together, they have revolutionized our manufacturing processes.

Editor: Katrina C. Arabe

Sources: Process Versus Geometry
John Callen
Modern Machine Shop, April 15, 2003
http://www.mmsonline.com/articles/040302.html

Introduction to CAD/CAM for SMEs
QMI Solutions Ltd.
http://www.qmi.asn.au/services/techassist/cad_brief.html

Advanced Design Insights
Jeffrey Rowe
Advanced Manufacturing, Jan. 2003
http://www.advancedmanufacturing.com/

Exploring Advanced Manufacturing Technology: Digital Manufacturing
Steve Krar and Arthur Gill
Advanced Manufacturing, March/April 2003
http://www.advancedmanufacturing.com/

Primer Links

Organizations

The IEEE (Institute of Electrical and Electronic Engineers) Computer Society http://www.computer.org

The National Science Foundation http://www.nsf.gov/

Publications

CAD User http://www.caduser.com/

Cadalyst http://www.cadonline.com/index.cfm

CADinfo.NET http://www.cadinfo.net/