How to Make Prototypes At Half the Cost of SLA.

* Once Anderson Design decided to make its own prototype models directly from Pro-Engineer CAD data, the question was how to do it affordably. Anderson, a general-purpose industrial design firm in Plainville, Conn., formerly made urethane foam models by hand. It wanted to upgrade to computer-driven model making but also wanted to do it in-house rather than going to an outside service. The firm looked into the currently popular rapid-prototyping systems, such as a stereolithography apparatus (SLA), and also at traditional CNC machining systems. According to Glen Nielsen, director of 3-D design, Anderson found its solution in a type of CNC gantry router that interfaced with CAD systems. It had a large cutting area and was half the price of the alternatives.

Lower cost and simplicity were key reasons for this choice. Says Nielsen, "It is a quick and inexpensive way to get the feel and look of the final product." From Anderson's point of view, drawbacks of SLA included the inability to produce aesthetically critical parts with complex surfaces, since the technology makes tiny steps or facets in a curved surface. Also, the least costly SLA system ran about $100,000 and has only a 12 x 12 x 10 in. working area. SLA models cost nearly twice as much to produce as models from high-density foam (7 to 15 lb/cu ft). Nielsen concedes that its CNC router produces only solid foam models that can replicate the outer surface of a product. For complex shapes with internal hollows, Anderson does contract out for SLA models.

Anderson also decided against traditional CNC machines. Not only do they start at around $50,000 without the necessary programming software, but a unit with the desired working area would have cost at least twice as much.

Router makes prototypes

Anderson Design opted for a Techno Series III CNC router from Techno-Isel of New Hyde Park, N.Y. It works basically like a CNC machine tool, but it has a smaller, faster spindle motor suited for cutting plastic and composite models. Also, the cutting head is mounted on a bridge-type moving gantry which moves the router over the workpiece. In contrast, a CNC metal cutting machine has a large cast-iron frame that holds the head stationary while the table moves the workpiece. The three-axis Techno CNC router costs under $19,000 and is upgradable to four axes for around $5000. Its working area of 24 x 36 in. and z-axis height of 6 in. is large enough for most of Anderson's foam models. The latter are cut from 4-in.-thick sheets measuring 4 x 8 ft. Including clamps, hand tools, lights, vacuum systems, cutting tools, and CNC programming software CMastercam from CNC Software of Tolland, Conn.), Anderson Design spent a total of about $40,000 to bring automated model making in-house.

Within three days after installation, Anderson Design was making models. A big factor was the easy-to-learn Mastercam software. According to Nielsen, it is well suited for industrial design models because it can generate the most complex contours with little programming effort. It includes IGES, DXF, and CADL converters, so that geometry can be uploaded from many GAD systems.

The machine offers 0.0001-in. resolution and repeatability and 0.0004-in, absolute accuracy. According to Nielsen, this degree of resolution has allowedAnderson to use the system in unanticipated ways. Many of the company's projects involve "reverse engineering" or replicating surfaces and dimensions from products already on the market. Nielsen says Anderson found most 3D digitizing techniques to be impractical. Laser scanning, for instance, generates thousands of data points-too many for the designer to use.

By modifying the Techno router to function as a coordinate-measuring machine, Anderson came up with a better way to get surface data into the CAD system. After securing an object to the machine's table, the operator manually moves the machine's crosshead until a flexible touch probe mounted in the tool holder contacts the object. The machine's display shows the probe's position coordinates. The values are recorded manually and later entered into the CAD system.

"We have found that between 70 and 80 planned points give a better indication of the surface than the thousands of points that are captured with a laser scanner," says Nielsen.