CT scanning is especially useful in checking for voids and confirming dimensional analysis in parts made that can not be measured. Especially in the case where additive manufacturing is utilized
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
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.
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. 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.
You can also 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.
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. 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.
Size and Focus
In this example, a CT scan data displayed using 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. Accuracy is typically 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. 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
Comparing machines from different manufacturers is not as standardized as it is for the far older CMM technology. 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. For now, 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. The terms accuracy and uncertainty need to be better understood” in the context of CT scanning.
The CT Scanner technology is 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
Tools and parts are getting increasingly complex, with internal channels that must meet precise tolerance requirements, such as this end mill.
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.
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. The growth in additive manufacturing parts is further driving the benefits of CT scanning. As 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.
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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