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Metrology-Grade CT Scanning Comes into Its Own

Bruce Morey
By Bruce Morey Senior Technical Editor, SME Media

Manufacturers are making ever more complicated parts and exploiting new, lighter materials. Internal structures of parts are often intricate, enhancing cooling or reducing weight. This is seen in precision cutting tools and turbine blades boasting internal cooling channels while internal matrix-like structures reduce weight. Mating surfaces between parts need tighter tolerances than ever. As parts—especially in aerospace—become more expensive, destructive testing to “measure inside” these intricate parts is becoming prohibitive.

CT scanning might be especially useful in checking for voids and confirming dimensional analysis in parts made from additive manufacturing.

Fortunately, Industrial Computed Tomography, or CT, scanning is coming into its own, spurred at once by these needs and advances in the technology itself. Proving its worth in casting and injection molding applications, CT scanning may well become even more indispensable in additive manufacturing.

“Today people can make parts but cannot measure them,” said Giles Gaskell, product manager for Wenzel. It is especially an issue in additive manufacturing, he said.

Regulatory pressures like those from the FAA are severe and only going to get more stringent, which is something the AM industry will have to be aware of, Gaskell said. “The advantage of CT scanning is it provides complete coverage compared with other scanning devices, such as lasers or structured light systems.”

Industrial CT works similarly to those used in patient treatment medical applications. A part is placed on a rotational table and flashed with X-rays from one side that are measured on the other. The many images that are taken are then used to reconstruct a volume of image elements, or voxels.

“You can see all the way through the part and the accuracy is usually better than a laser scanner,” he said. “Compared with other scanning systems, you never have to spray the part, there are no limitations with a surface that is shiny or clear. Undercuts and hidden areas are no issue.”

And, of course, you can see interior details as well, such as those pesky internal cooling channels or porosity voids and inclusions in castings. The resolutions vary depending on the size of the measuring volume. A good example of a mainstream system is Wenzel’s exaCT M CT workstation that provides resolutions from 40 to 160 µm.

There are limitations.

Capital cost, part size, and materials that can be measured easily are the top three to which Gaskell pointed. Lighter elements in the periodic table have proven ideal for CT metrology.

“However, as soon as you get to iron in the table, which includes alloys like steel, you get serious limitations on how much thickness you can scan,” he said. “The Holy Grail in measuring aerospace parts are those used at the hot part of the engine. These are usually made of alloys formed from nickel, chrome, cobalt, such as Inconel. Those materials are difficult to penetrate by CT.” The good news is that many of those parts are small, so penetration is sometimes adequate.

Penetration is related to voltage, so the penetrative capability of a CT machine is measured in kilovolts. Systems available from Wenzel top out at 450Kv. The maximum scan volume of the exaCT M CT workstation is 200 mm in diameter by 300 mm in height.

Size and Focus

In this example, a CT scan data displayed using ZEISS software and equipment examines assembly and fit, in this case if a seal is properly positioned on the bottle.

“There are competing techniques, such as ultrasound or eddy-current, for measuring inside parts, but they do not have the accuracies and resolutions that the CT scanner does,” said Herminso Villarraga-Gómez, X-ray CT metrology specialist of Nikon Metrology. Nikon offers a wide range of CT scanners for metrology, from models that are literally room-sized to more conventional versions.

He equates accuracy as dependent on the size of the part and how close the part can be placed next to the source of the X-rays. So, for a part that measures say about 6 × 6 × 6″ (152 × 152 × 152 mm) in volume, the best resolutions one could expect would be about 10–50 µm. He has found that most applications for CT scanning, especially in aerospace and automotive, are in what is termed the micro focus region. These are parts that are about 1–100 mm in size where the voxel resolutions range from 1 to 150 µm.

“Some modern industrial X-ray CT machines can provide measurement uncertainties as small as 4 µm,” Villarraga-Gómez said.

Comparing machines from different manufacturers is not as standardized as it is for the far older CMM technology, he said.

While CMMs can refer to the ISO 10360 for maximum permissible error calculations, there is no international standard for CT scanners. “There is a group of people, the ISO/TC 213/WG 10 working group, working on these standards although they might take some time to be released,” he said.

For now, Villarraga-Gómez said, the only reference document for specification and verification of CT systems for dimensional metrology is the German VDI/VDE 2630-1 published in 2011, which many manufacturers use in reporting CT scanner capabilities.

Using this guideline, Nikon publishes a maximum permissible error, or MPE of (9 + L/50) microns for its workhorse MCT225 cabinet CT scanner, a 225 kV system with a temperature controlled enclosure. “In-house calibration to provide MPE budgets might work well but international standardization is still needed,” he said. “The terms accuracy and uncertainty need to be better understood” in the context of CT scanning.

Nikon also offers large-envelope systems that are configurable to accept larger and more complex parts, such as the M2 high-precision X-ray/ CT inspection system. The granite base provides the foundation for precise and stable eight-axis manipulators that can support dual sources 225 kV, 320 kV, 450 kV power ranges, according to Nikon. Villarraga-Gómez reports a 750kv system still in development. He noted that word of the system “has attracted a lot of customer interest”, showing the growing demand for measuring larger parts or made of denser materials.

Workflow chart for the full CT process that includes evaluation of dimensional measurements.

Small Parts, High Volume

“This market has changed leaps and bounds, not just in terms of the product per se, but how it has been perceived and how the industry is more comfortable embracing CT metrology technology,” said Raghuram Bhogaraju, CT applications specialist, Carl Zeiss Industrial Metrology.

While Zeiss also offers a cabinet-based CT scanning machine that resembles a CMM in usage, “a CMM with an X-ray” as Bhogaraju described it, Zeiss has also addressed the in-line, high-volume production market with its VoluMax line. “Metrotom is a high-accuracy, high-resolution system, that can be automated if need be,” he said. It boasts resolution ranges of 3.5–6 µm with an MPE specification down to (2.9 + L/100) microns as measured with the VDI/VDE 2630-1 standard.

“Volumax, on the other hand, is a project centric system,” Bhogaraju said. “So, say, I need to measure cylinder heads for porosity inspection, we will fine-tune the machine and write custom software catered toward that particular project. We have a good mix of X-ray sources that can get resolutions down to 5 µm to large systems that go to 400 µm. We can do a cylinder block in 60 seconds with Volumax,” he said.

However, Volumax does not have an uncertainty specification. Instead, it refers to a known gage, perhaps one characterized on a Metrotom. The inspection is then optimized for cycle time with Volumax acting like an inspection gage.

A lot of the casting and injection molding industries currently using 2D X-ray systems are now moving towards 3D and no longer looking just for defects—they want to do some real measurements,” he said. “They can replace some of their lower-end borescopes and optical and vision systems that they would normally use in an in-line procedure with CT scanning using Volumax.”

Services and Power

Tools and parts are getting increasingly complex, with internal channels that must meet precise tolerance requirements, such as this end mill.

Many providers of CT machines interviewed also offer service bureaus that are busy with customer orders. Given the relative newness of the technology and its expense makes sense for companies new to the technology or with applications that require measuring just a few parts to use a bureau.

One company, Jesse Garant Metrology Center, has made CT metrology as a service its primary business model. The firm grew from its founding in 2009 as a manufacturing consulting firm into an industrial CT service company into today’s full-service metrology bureau, CEO Jesse Garant said.

“We added a structured light division and laser CMM, primarily to support our CT inspection requirements for secondary validation,” Garant said, noting that his company purpose-builds some  CT metrology scanners.

Garant, too, sees the CT scanning market maturing.

“A lot of our customers over the years have used CT more as a reactive tool for failure investigation. Now, they are using it more as a proactive inspection tool, either in new product development, in production environments, or in fulfilling First Article Inspection AS9102 Form 3 requirements,” he said.

He also sees the growth in additive manufacturing parts—General Electric’s innovative LEAP engine development program is an example—somewhat uniquely requiring CT scanning. AM parts are getting bigger as new techniques are developed.

CT scanners are especially useful in precision, high-value parts with internal details often found in aerospace applications.

Higher penetration in CT scanning comes through higher voltages, and Jesse Garant boasts an astounding 3 MV system that generates internal data in 3D for parts up to 48″ (1219 mm) in diameter by 66″ (1676 mm) in height.

“Higher powers also allow you to go up in the periodic table. Today, our newer, higher energy systems scan 5–10 times faster with resolutions that are double to triple what they were just a decade ago,” Garant said. “That means our cost per inspection is drastically reduced which moves CT from a failure investigation tool to a quality assurance tool.”

With CT scanning, there is no one size that fits all, but the options continue to grow. The future of true see-through metrology is looking bright. Some adopters will learn to harness and use higher powers for faster scan times. Others will expand CT equipment with existing power limits into new uses and applications as users find more value in measuring inside their critical components.

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