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Industrial computed tomography: increased accuracy

Industrial computed tomography facilities are increasingly applied for non-destructive mea-surements of the geometry of components. Within an industrial project PTB reduced the occurring measurement deviations.

Contour of a water jacket of a cylinder head. The colour-coded surface represents the deviations from the nominal contour in mm.
(Source: Fa. Rautenbach, Wernigerode)

As a method computed tomography (CT) with x-rays was originally developed for medical applications. However, it has also been used for non-destructive error detection for a long time. Nowadays, industrial CT-facilities are also applied for dimensional measurements with high point densities, e.g. to determine wall thicknesses or for comparisons with CAD data.

In particular, CT can determine inner geome-tries, which were accessible classically only after object segmentation. However, complex influence factors cause measurement errors which were hard to quantify so far.

The dimensional measurement properties of CT have been assessed in cooperation with a medium-sized foundry company.

Test bodies made of aluminium, ceramics, granite and aluminium cast parts (e.g.: cylinder heads of max. 200 mm penetrated thickness) were calibrated tactilely with coordinate measurement machines and subsequently investigated on the 450 kV CT-facility of the industrial partner. Systematic measurement deviations are corrected now with the procedure resulting from the experimental tests. For single measurement points the mea-surement deviations already could be reduced to 0.1 mm. Thus, for specific applications CT is now capable to substitute measurements on coordinate measurement machines. Here enhanced applications of CT are foreseeable.

In a new project by the German Federal Ministry for Commerce and Labour (BMWA) PTB is aiming at increasing the accuracy of CT-facilities in cooperation with ten German industrial firms and the German Federal Institute for Materials Research and Testing (BAM). On one hand CT-facilities with standard x-ray tubes for measuring large components are under investigation. On the other hand CT facilities are examined which use microfocus-x-ray tubes of up to 225 kV and smallest volume-elements (voxels) of currently (3 µm)3 to measure small components with aspired uncertainties in the µm range.

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