QMT Features: March 2014
X-rays for internal metrology
The Katholieke Universiteit in Leuven is using X-ray CT to look inside industrial components

Many components and assemblies have internal features that are difficult to inspect non-destructively, as conventional metrology requires them to be sectioned. Now, the PMA division of Leuven’s university is using X-ray Computed Tomography (CT) machines to research measuring the interiors of such components in 3D.

CT has been widely used for many years in medicine for imaging and diagnosis, and to inspect materials to identify the presence of internal features, such as unwanted inclusions in a casting. Now, research is being carried out by Prof Jean-Pierre Kruth and his team at the Katholieke Universiteit in Leuven (KU Leuven), the oldest and largest university in Belgium, to broaden the application of CT into the field of dimensional metrology. With CT, components can be inspected externally, as traditionally done with a touch probe or laser scanner, but internal geometry can also be measured non-destructively in the same set-up.

KU Leuven, is close to the European headquarters of Nikon Metrology and the two organisations are collaborating closely in the development of CT as a tool for geometrical measuring and quality control. Two Nikon Metrology CT machines were recently installed at KU Leuven, enabling Prof Kruth’s production engineering, machine design and automation (PMA) division of the mechanical engineering department to carry out in-depth research. CT machines are also used in the metallurgy department at the Leuven university, mainly for materials examination. This department has recently upgraded one its CT machines with a new Nikon Metrology 180 keV X-ray source and control software.

One of the PMA division’s X-ray machines, a 225 keV model XT H 225, includes a microfocus X-ray source, linear scales, better cooling and other enhancements that provide increased accuracy, making it suitable for CT metrology. The second machine is a large cabinet microfocus XT H 450, the highest power CT machine currently installed in Belgium and the Netherlands, providing sufficient X-ray penetration for thicker metal parts to be inspected. As a guide, 450 keV microfocus source can penetrate 35 mm of steel or 110 mm of aluminium.

Complex problems
Prof Kruth said:”Today, production techniques including five-axis milling, additive manufacturing and hydroforming make it possible to produce complex products, often with internal features or channels.
“Such complex products presented us with a challenge, as it is impossible to non-destructively inspect the internal features without X-raying the parts.

“Often, one-off prototypes or small batches of components are produced. Sectioning even one component to inspect it conventionally would result in an unacceptable scrap level in percentage terms.
“CT presents its own difficulties, however, as metal in particular is dense and the X-rays tend to scatter and be absorbed unless the power is high. Moreover, the standard machine platforms are not developed with sufficient rigidity and accuracy for precision measuring, as they are traditionally used for material inspection.
“In fact there is a general lack of understanding within the CT community regarding the accuracy and repeatability problems associated with using the technology for measurement and traceability of the results.”

The researchers targeted three groups of components – additive manufactured parts, conventionally produced precision components and assemblies, and highly complex parts also produced by traditional machining.
One example, a servo valve used in the F16 fighter, has hundreds of intersecting channels whose dimensions need to be measured. It also needs to be checked for internal burrs where holes intersect.
100% inspection, which is demanded for many such safety-critical parts, is impossible without some form of non-destructive testing.

Initial results from using X-ray CT to measure these parts have proved very promising. For some metallic components, depending on the application, measuring uncertainty both internally and for the outer dimensions of the part can be below 10 microns using the Nikon Metrology CT system. This is close to a typical coordinate measuring machine. For example, one of the CMMs in the laboratory has an uncertainty of 5 microns plus 5 microns per metre of component length.

How it works
In operation, a source produces X-rays by projecting electrons onto a target. As X-rays penetrate the workpiece, they are attenuated due to absorption and scattering. The amount of attenuation is determined by the distance travelled into the material and by its composition and density, as well as by the energy level of the X-rays. After penetrating the workpiece, the attenuated X-rays are typically captured by means of a flat panel detector, resulting in a 2D grayscale image.

The component is rotated to give multiple 2D slices and these are reconstructed to create a 3D voxel model (a voxel is a 3D pixel), where the grey value of the voxels is a measure of the linear attenuation coefficient of the material. The big advantage of CT is that it eliminates the superimposition of structural images outside the area of interest. The voxel data is post-processed using algorithms to detect the edges and features of the workpiece, allowing dimensional measurement and quality control.

The XT H 450 installed in PMA’s laboratory also has a 1D curved linear detector. Using this requires the workpiece to be shifted along the rotational axis in order to measure successive cross-sections of the object in a similar way as medical CT scanners. Research is currently being carried out to determine whether the linear detector, which allows higher power (higher voltage, current or exposure time and hence larger material penetration) and is less sensitive to X-ray scatter, can be used to inspect large components more accurately than with a flat panel detector.
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