Meeting tight part tolerances with any machine tool often requires some form of in-process control, but that’s especially true in a hybrid additive/subtractive machine. And, as Mitsui Seiki USA CEO Robb Hudson said, “Additive is not a perfectly predictable process, particularly in hybrid machines that use blown powder as opposed to a powder bed. Atmospheric conditions affect your build rate and a lot of other variables come into play. So for any precision component, you always build more than you need and then machine to the proper dimensions.”
Hudson pointed out that if you’re concerned with more than adjusting a length or a diameter, as you would be with any prismatic part, the measurements and adjustments quickly become complex. Luckily, he said, “laser scanning and probing capabilities have come a long way in just the last three to four years, and the software driving that hardware gets better every year, yielding much more sophisticated and complete solutions, allowing us to do this in-process monitoring more readily and without sacrificing cycle time. And by doing it in the machine, you reduce handling to get to a better part quicker.
“In some cases, we scan the entire part in the fixture, using either a touch probe or a laser probe, capturing literally tens of thousands of data points,” Hudson continued. “The machine then compares that point cloud to the original CAD model and makes fully adaptive process changes on the fly.” He pointed out that this is more complex than some of the other adaptive technologies that have been demonstrated over the years because more data is being collected and more accurately. “We have a lot more computational horsepower now, so we can handle many more data points, which yield a more accurate point cloud. And when we’re talking about Mitsui Seiki type accuracies, we can machine down to four- to five-micron accuracy with some of these scanning systems,” said Hudson.
Meanwhile, there are efforts to make the additive process itself more accurate. Nitin Chaphalkar, product manager for DMG Mori USA (Hoffman Estates, IL), said it’s important to maintain constant temperature of the melt pool: “If the melt pool temperature varies as you build the part, different areas will solidify at different rates, which will result in residual stresses and distortions of the part. Varying melt pool temperatures will also cause non-uniform material properties.” DMG Mori has a closed loop system that tracks the melt pool temperature and adjusts the laser power, to keep the melt pool temperature constant. This yields uniform material properties all along the part.
Hudson agreed, adding that “melt pool temperature may need to change if you are going from a thick wall feature to a thin wall feature in a relatively small work area.”
He described a “symbiosis” in which machine tool builders work closely with the makers of controls, lasers, nozzles, and powder delivery systems to deliver systems with ever better accuracy. “Mitsui Seiki’s Vertex machine achieves sub-15 micron volumetric accuracy without the addition of 3D scanning technology,” he says. “So as we see powder delivery and nozzle technology improve, you’ll be able to take advantage of the accuracy of the machine motion on the additive side. So we’ll be able to put material exactly where we want it, targeting within a few microns where we have that nozzle in space.”
Still, the ultimate in part accuracy will likely always involve at least some subtractive machining and some form of in-process control, and Hudson said that’s OK for high-value parts: “People often make the argument that they don’t want their million-dollar machine turning into a piece of metrology hardware. But there are plenty of applications, particularly in very high-value parts like in aerospace, where adding 10 or 15 minutes of cycle time to a machining process that takes 25 to 30 hours is inconsequential. You’re working on a part worth tens of thousands of dollars and the last operation can ruin it.”