Prototyping via Fast Machining

 Why make prototype parts out of some proprietary material when you can make it with ABS, nylon, acrylic, plaster, styrene, or some other readily available material? That is a question posed by Pedro Gonzalez, product manager, 3D Milling and Scanning Products, Roland DGA Corp. (www.rolanddga.com), which produces an array of desktop milling machines for what the company calls “Subtractive Rapid Prototyping” (SRP) applications. And, yes, this is equipment that can actually fit on desktop, with, for example, the company’s latest machine, the MDX-40A, having a footprint of 26.3 x 29.9-in. and a height of 21.8 in. In addition to which, the machine requires 110-volt power, so it is simply a matter of plugging and machining. It weighs 143.3 lb., so it needs to be a sturdy desk.

Gonzalez says that an advantage of the use
of milling rather than additive technology for producing prototypes is the fact that while the additive companies seem to offer an either-or (i.e., speed or surface finish), “We can provide both. As we’re using CNC”—computer-numerical control—“technology, the surface finish is going to come out nice. There isn’t the layering or stepping effect of the other processes, but a nice, smooth surface.” He says the accuracy provided is on the order of 0.004 to 0.005 in. Because the machine has a milling spindle that can operate at up to 15,000 rpm, it is capable of producing prototypes at comparatively high speeds.

Amplifying that observation, he says, “Because we are using a solid material rather than something that’s glued or melted together, it is going to be one solid part, more of a true representation of what the final part will be.” What’s more, assuming that the parts are being made with one of the materials that the machine can handle (it is worth noting that the company’s most robust machine, the MDX-540, is capable of machining aluminum, brass and copper), there is, he says, the opportunity to make “truly functional prototypes that can be used for destructive or live testing.”

Of course, he acknowledges that there are some complex shapes that can’t be accommodated by the MDX-40A that comes standard as a three-axis machine, but which is available with an optional fourth-axis (a rotary axis). Still, if it is a shape that can be machined, it can probably be accommodated by the mill.

The machine provides X, Y, Z strokes of, respectively, 12 x 12 x 4.125 in. The feedrates for the X and Y axes are 0.0039 to 1.9 in./second; it is 0.0039 to 1.1 in./second for Z. The table measures 12 x 12 in. and it can accommodate workpieces up to 8.8 lb. As for the optional rotary axis, it can accommodate workpieces that are 4.7 in. in diameter and 10.6-in. long. The machine uses standard tooling that, Gonzalez points out, can be found in any number of tooling catalogs, which means that the costs are low and the availability and selection are high.

Although the MDX-40A is a machine tool—albeit a comparatively small one, but a machine tool nonetheless—Gonzalez notes,
“Our premise is that we are in the prototyping industry.” He explains that “even if someone has no CNC experience, has no idea of what tool to use or how fast to cut, that’s not a problem. All they have to do is import an STL file”—the system supports a variety of design software, such as SolidWorks and ProEngineer, so it is merely a matter of doing the design, then exporting it as an STL file—“answer a couple of questions—how do you want to cut; what kind
of finish do you want—check off the tools they have, and the software will select the kind of tool to use and, based on the material, will determine the feed and speed to use. They can be a complete CNC amateur.” The SRP Player CAM software that comes standard with the machine is easy to use.

Because there is recognition that there are some industrial designers who aren’t CNC amateurs, there is support for G-code programming, which means that there is compatibility with computer-aided manufacturing (CAM) software programs including Mastercam, EdgeCAM, SurfCAM, and GibbsCAM. Gonzalez says, “After a while, some customers say they love our machine but want a more powerful CAM software.”

Although there is a wide array of users of small, fast milling machines—from medical device manufacturers to automotive companies to even NASA—clearly, the additive material technology machines have more visibility. Gonzalez acknowledges that, and admits that there is something to be said for the fact that it is “new technology”—milling machines have been around since at least the early 19th century—that allows you to “load in a material cartridge, press a button, and it builds a part.”

But at the end of the day, the subtractive technology can still more than hold its own.