Metal monitors: Two CT machines have been installed for Kruth’s research
Engineering researchers are applying a technique more commonly used for medical imaging to establish the geometrical accuracy of the inner features of complex metal parts.
Many components and assemblies, such as hollow hydroformed camshafts, have internal features that are difficult to inspect non-destructively. Now, engineering researchers at Leuven University in Belgium are 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. But the new research, carried out by Professor Jean-Pierre Kruth from Leuven’s mechanical engineering department, is broadening 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.
Kruth said: “Production techniques make it possible to produce complex products, often with internal features or channels. Such products present 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.”
CT presents its own difficulties, however. 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. “There is a lack of understanding in the CT community regarding the accuracy and repeatability problems associated with using the technology for measurement and traceability of the results,” said Kruth.
To enable Kruth and his team to carry out the research, two Nikon Metrology CT machines were recently installed at Leuven. Three groups of components are being targeted – additive manufactured parts, conventionally produced precision components and assemblies, and complex parts also produced by traditional machining. Initial results from using X-ray CT to measure these parts have proved promising, according to Kruth.
Research has indicated that, for some metallic components and depending on the application, measuring uncertainty both internally and for outer dimensions of the part can be less than 10µm using the Nikon Metrology CT system. This means its accuracy lies close to that of a typical coordinate measuring machine (CMM). For example, one of the CMMs in the laboratory has an uncertainty of 5µm plus 5µm per metre of component length.
To help achieve this precision, the Nikon Metrology machines are housed in a temperature-controlled environment, despite each machine having an internal cooling system.
In operation, a source produces X-rays by projecting electrons onto a target. As X-rays penetrate the workpiece, they are attenuated because of absorption and scattering. The amount of attenuation is determined by the distance travelled into the material, its composition and density, and the energy level of the X-rays. After penetrating the workpiece, the attenuated X-rays are typically captured by a flat panel detector, resulting in a 2D greyscale image. 2D images are taken for many rotation steps of the workpiece.
Reconstruction of an industrial component based on the projected image slices leads to a voxel – the 3D analogue of a pixel – model, 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.
One of the CT machines also features a 1D curved linear detector as well as a conventional 2D flat panel detector. Using the linear detector requires the workpiece to be shifted along the rotational axis, to measure successive cross-sections of the object, as with medical CT scanners. Research is under way to determine whether the linear detector, which allows higher power and is less sensitive to X-ray scatter, can be used to inspect large components more accurately than with a flat panel detector.
Other issues that Kruth and his team are investigating include optimising the X-ray illumination parameters and adjusting the grey level thresholding parameters for traceable dimensional measurements, lowering the X-ray spot size for greater accuracy, and increasing the power of the X-ray source for greater penetration into large metallic components.
A deeper European co-operation on the use of CT for metrology has also started, linking Leuven with Nikon Metrology and the National Physical Laboratory in the UK.