3D Printer Shoot Out

If you need fast, inexpensive prototype parts, 3D printing may fit the bill. But what kind of accuracy can you expect from these desktop modeling systems? A new benchmark study provides some detailed answers.

Conducted by Todd Grimm, an engineer whose consulting firm specializes in rapid pro-totyping, the study examined the dimensional accuracy of Z Corporation's ZPrinter 310, 3D Systems' InVisionSR, and Stratasys' Dimension SST. All three machines build physical prototypes by depositing layers of a build material, with each layer corresponding to a slice of the part's CAD model. The machines differ, however, in the specific type of build material they use and in how they deposit it. They also differ in their overall resolution-a combination of their layer thickness, the machine's motion-control capabilities, and printhead design.

To see which machine performed the best, Grimm asked users of these 3D printers to build a pair of two-piece assemblies: One consisted of the top and bottom halves of a 2x2.5x1.0-inch battery box with hinge and snap-fit features. The other consisted of a round base and cap for a 2.0-inch tall, 1.5-inch-diameter light fixture. Grimm then had a quality control specialist measure the parts using laser-scanning and computer-aided-inspection techniques.

What he found after comparing these measurements to the CAD dimensions fed into the machines surprised him. The machine with the best build resolution, the InVisionSR, actually came in at the bottom of the pack in terms of absolute accuracy and the breadth of its tolerance deviation. Grimm found that most of the measurements for this system fell within -0.010 to +0.005 inch with an overall tolerance deviation between plus or minus 0.020 inch.

The ZPrinter occupied the next rung up on the accuracy ladder with the results showing that the majority of measurements fell within plus or minus 0.005 inch with a tolerance deviation of plus or minus 0.015 inch.

And the Dimension machine took the top spot with results that suggest it can usually hold plus or minus 0.003 inch and a narrow tolerance band between plus or minus 0.010 inch. "Its results rival those of rapid prototyping systems that cost much more," he says.

While Grimm's study can certainly help engineers know what to expect from prototypes from these three systems, the study also provides a couple of broader lessons. One key finding is that the resolution doesn't directly equate to dimensional accuracy, contrary to conventional wisdom. "Good resolution and layer thickness don't necessarily guarantee good accuracy," he says, noting that the build materials and binders used in 3D printing can play a significant role in accuracy. "Materials shrink in ways that are hard to predict."

Another lesson has to do with measurement error. Coordinate measuring machines (CMM) may have a limited value as a way to assess the accuracy of rapid prototyping systems. Grimm argues that CMM picks up too few points to capture the variation inherent in rapid prototyped parts. "With CMM you can move your inspection point by just a tiny amount and find that the dimensions of a seemingly uniform feature have changed by several thousandths," he explains. Here the use of laser scanning and Polyworks computer-aided-inspection software offer a much more detailed picture of accuracy. "This analysis used more than 200,000 data points per part," he says.

The final, perhaps most important lesson, has to do with putting accuracy in its proper place. While it is helpful to know whether a particular desktop modeler can faithfully produce precision features, all of these machines have a limited ability to make functional prototypes. So they often don't need to be as accurate as high-end rapid proto-type machines-much less precision machine tools. "All the machines are accurate enough for their intended use," Grimm says. "And even the least accurate of the three makes very nice parts."