CT scanning changes the game in metrology

If you’ve been injured in a car accident, developed a blood clot or tumor or felt a weird, unexplainable pain in your abdomen, chances are good that a doctor stuck you inside a big doughnut-shaped machine for a peek at your innards. A short while later, the doctor then shared an amazingly realistic view of your bones and organs and hopefully said your stomachache was caused by nothing more than the tuna fish sandwich you ate for dinner two nights ago, the suspected fractures were only bruises or the tumor was benign.

The first generation of this technology, computerized tomography, was invented in the early 1970s. CT scanning equipment uses an X-ray generator positioned opposite a curved detector, which rotates around a patient to capture hundreds or even thousands of scans per rotation. These 2D images are then fed into a computer and stitched into an accurate 3D representation of the person.

Scanners Are Expensive

But CT scanners have another use, one that might delight anyone who’s had to painstakingly cross-section a part just to measure an internal feature. Jesse Garant, president and owner of Windsor, Ontario-based Jesse Garant Metrology Center, said industrial CT scanning equipment is used for dimensional inspection of everything from aerospace castings to heart stents, offering an unparalleled view of parts without having to destroy them. Scans also allow porosity and wall thickness analysis, failure investigation and reverse engineering.

A quality engineer uses Volume Graphics software to analyze data from a scan of an electric motor. Image courtesy of Jesse Garant Metrology Center

There’s just one problem: CT scanners are expensive. And as parts become larger or denser or require more accurate scans, the expense rises.

“We have eight different machines of all sizes and energies,” Garant said, “ranging from roughly $250,000 for a 100kV scanner up to our custom-built linear accelerator system, which cost over $3 million and generates 3,000kV.”

Having an array of CT scanning equipment allows a metrologist to match a customer’s part to a machine, he said. For example, ¼”-thick steel parts, aluminum parts up to an inch or so thick and plastic parts eight times that thickness are suitable for lower-cost CT scanning machines with 160kV energies. But, Garant noted, the more energy that is available, the more material that can be penetrated. The company’s 3MeV superscanner, for instance, has enough power to scan an aluminum part about 28″ in diameter in 90 minutes. But on the other end of the spectrum, parts thinner than 0.01″ must be scanned with a nanosystem, a machine primarily found at university labs, he said.

“When you enter the nanorealm—say, anything lower than 3µm resolution—you’re typically looking for a lot of precision, lower power and a very small spot size,” Garant said. “For this, a very expensive, specialized CT scanner is needed. But for inspecting larger parts in the 0.25mm to 4mm range, any of the smaller, very high-resolution micro-CT systems found on our floor can be used.”

The Right Stuff

Cincinnati-based Exact Metrology Inc. is another metrology provider with multiple CT scanning systems. President Steve Young described the process as similar to a human CT scan except that the part is spun, unlike with an X-ray source and a detector, and the applied power is enough to kill anything placed in the machine.

CT scanning is suitable for dimensional inspection, reverse engineering and CAD-to-part analysis. Image courtesy of Exact Metrology

“The denser or thicker the workpiece, the more power is needed to scan it,” he said.

As with any manufacturing-related process, however, just owning a machine doesn’t guarantee its successful use. Sources said companies offering CT scanning services should meet the same standards and have the same certifications as any other metrology lab, as well as employ qualified, highly trained personnel, often with more than 10,000 hours of scanning experience.

“CT scanning in particular requires extensive knowledge in many different fields of study,” Garant said. “A skilled operator needs a solid understanding of physics, analytics, metrology and, of course, machine operation. Getting useful results out of one of these machines isn’t just a matter of pushing a button.”

Young agreed that as parts become smaller, accurate scans become increasingly difficult to achieve.

“There are plenty of companies and people advertising dimensional metrology services,” he said. “And CT scanning is becoming one of these offerings, especially when it’s impractical to cut a part open. But when you get into very small parts, it can be quite challenging to take very accurate measurements. You definitely need the right equipment, the right people and the right software.”

Slice and Dice

Despite these challenges, CT scanning offers unprecedented views into what was once uncharted—albeit tiny—territory.

A CT scanning technician analyzes a workpiece. Image courtesy of 3D ProScan