Seeing through metal: CT scanning for metrology

Cincinnati-based Exact Metrology invests in X-ray computed tomography equipment to collect dimensional measurements of complex 3D workpieces.

October 14, 2016

Metrology fixtures are rarely simple. Measuring critical dimensions on workpieces typically involves holding the part at an angle to give touch probes or scanning lasers access to the needed location.

But on this particular Tuesday, Steve Young is using a milk crate to hold his workpiece while getting 1µm level resolutions from his metrology equipment. The 10" diameter metal piece sits haphazardly in the blue plastic crate while the scan head measures features. The president of Exact Metrology in Cincinnati, Ohio, says simple fixturing is only one of the benefits of using X-ray computed tomography (CT) scanning in industrial measurement.

“We’ve known of industrial CT scanning for awhile. People kept coming to us and asking, ‘Can you see this feature? Can you see that feature? Can you scan this?’ And we couldn’t,” Young says. “As we got more and more requests, we had to ask ourselves why we were sending this work away?”

Developed for the medical world, CT scanning constructs a 3D image by stacking hundreds of thin 2D images on top of each other. Young says engineers have theorized that such system could be useful in manufacturing, but until recently, the resolution wasn’t high enough. Medical CT scanners tend to be accurate within about 1mm – several orders of magnitude less than what companies need in the industrial world.

“The GE metrology addition that we have was the first one for the U.S,” Young says. “When we went around looking for one to buy, we needed a metrology level system, and most of what we saw wasn’t that precise.”

Seeing inside
A CT scan of a casting generates a 3D image showing porosity in the metal, lumping various porous regions in the part by color.

With manufacturers reducing metal use for cost and weight reasons, many new designs are defined by their cavities, not their external dimensions. Hollow castings, injection-molded parts, metal-injection-molded parts, sintered components, and 3D-printed items all tend to have thin-walled features that must be accurate to within microns. Testing those parts for dimensional accuracy can be challenging as touch probes and scanners can’t always be angled into workpieces in a way that catches those readings.

“There was a company spending seven days to inspect some small, high-precision plastic parts. There were so many nooks and crannies and cavities to it that they had to use some CMMs to do most of the outside. Then they had to cut the part to do other tests. Then they would use calipers to do some other measurements,” Young says. “I could put that in the CT scanner – I would have to write the scan program, but I could do that ahead of time. The scan would take 30 minutes, then we’d need another five for the report to come out. I’ve got 35 minutes out of it. Yeah, I had to write the scan program, but they had to program their CMMs, so that’s a wash.

“I can give you every radius, every feature you want measured in those 35 minutes,” Young adds. “And depending on the size of the part or the resolution you need, I could put four of those parts in the scanner at the same time, so I’d measure four parts in 35 minutes instead of spending seven days on one.”

The day that Young was using a milk crate to hold the metal casting being measured – as long as the piece doesn’t move as the scan head rotates around it, the X-ray will see through the crate’s plastic. A series of plastic parts was in the hopper for scanning later in the day. Four identical plastic parts, each about the size of a 12oz beer bottle, were bundled together in pink plastic foam. That foam package was wrapped with masking tape, creating a small bundle for the scanner. The tape and foam would disappear when seen by the X-ray, leaving only the plastic workpieces to be measured.

He adds that the bulk of the work that Exact has received since it purchased its scanner has been to measure aluminum and iron castings, but engineers have also scanned multiple plastic components.

Material challenges
CT X-ray scanners build 3D models by collecting data from hundreds of 2D images, each effectively a slice of the workpiece being measured. Porosity in an aluminum casting, near a bolt hole is shown in this 2D slice image.

CT metrology is best suited to monolithic castings because the technology works best when the X-ray power level is constant. A cast-iron engine block, for example, needs a high-power beam to penetrate the metal to generate images. Plastic or rubber parts attached to the block would disappear, much in the same way the milk crate fixture disappears in the image.

Conversely, with plastic parts that have metal inserts, the low power levels needed to see complex features in the plastic means not getting high-resolution images of the metal parts.

“If you’re scanning something that has three or four different types of plastics in it, it’s doable, but once you start mixing soft and hard materials, you get holes in the data,” Young explains. He recently scanned a pump housing for a small watercraft that was primarily plastic but had brass inserts. “The amount of energy that I needed to get through the brass, there’s so much power that it just blows away the image on the plastic. Brass and plastic are so vastly different on the density scale, I couldn’t see the plastic.”

When doctors and researchers developed the technology, Young says beam density wasn’t a major concern. Skin, bone, muscle tissue, and other materials within the body have similar densities, so one scan can typically capture everything that doctors needed to see. Tool steel and light plastics, on the other hand, are radically different.

One solution is to run multiple scans to collect data for different materials then put them together in the software, but more scans adds time and cost.

“It’s not perfect for everyone. You put brass inserts in there, and I can’t give you the resolution. But if it’s all plastic, I’m golden,” Young says. He adds that as CT scanner manufacturers continue to develop the technology, he expects material density to become less of a problem.

Leaking engine
Quality managers can use 2D data showing porosity in a casting to measure the distance between defects, such as the dark blobs show, and intentional features, such as the bolt hole on the upper left side of the component.

As is typical in manufacturing, Young says companies stick to existing systems and technologies until a specific need arises – such as when a part fails and the causes of that flaw are not obvious with traditional metrology systems.

“Very few people come to us saying they have a good part and they just want to confirm it,” Young says. “Most of the time, they have something that once they tear it apart, they can’t tell where the problem was.”

One example was a lawnmower engine leaking fuel. Engineers tested the threads on the fuel connections, the seatings of valves, and every location where fuel moved from one system to the next, and the seals all looked good. In the CT scanner, however, the culprit was clear.

“You could see where the bolt hole was on the flange, and there was massive porosity around there,” Young says. “The problem wasn’t the fittings, it was the metal from the casting. The fuel would seep through the casting around there because it was so thin. The air bubbles in the metal, and how close they were to the surface, were causing a big issue.”

He adds that the manufacturer probably would have discovered the porosity issue with the casting eventually by process of elimination and destructive testing. But the scanner identified the problem much more quickly and gave the company actionable data on exactly where the problem was – giving engineers a target to fix on the aluminum casting.

Young says that as manufacturers become aware of CT scanning metrology solutions, popularity will rise. While the equipment is expensive, users can get a quick return on investment by reducing the amount of time spent on scans and increasing accuracy.

“We jumped into it because we felt that CT was the next wave of the technology for scanning,” Young concludes. “It was a gut feeling that customers were going to want more detail in those complex internal features.”

Exact Metrology Inc.

About the author: Robert Schoenberger is the editor of TMV and can be reached at 216.393.0271 or