QMT Features: April 2010
X-ray CT-based CMMs?
High-resolution CT-based defect analysis and dimensional measurement systems now allow extremely precise and complete 3D representation of objects opening up their field of application. By Dr. Oliver Brunke

With industrial X-ray computed tomography (CT), even low-contrast defects in cast parts, such as cracks, pores and blowholes, can be localized and measured in three dimensions. Analysis of the defects can be performed using either multi-positional 2D cross-section planes or the 3D volume view.

Complete 3D mapping means that CT can also be used for the non-destructive 3D measurement of cast parts that cannot be inspected using conventional coordinate measuring machines on account of their complex internal geometry. Therefore, CT has numerous practical uses in addition to non-destructive quality control; for instance, it can be used for optimizing and reducing the time required for development and initial sampling processes, comparing components with the target CAD model or reverse engineering in which 3D component data is used to construct a three-dimensional CAD model (fig 1).

Fully automated scanning and analysis processes mean that the creation of first article inspection reports, even for complex components, is possible in less than an hour.

Non-destructive 3D analysis
Over the past few years, industrial computed tomography has made great advances in increasingly high resolutions and ever greater reconstruction speeds for 3D volume data. Thanks to GPU-based image reconstruction, CT results are now available within minutes.

Full three-dimensional scanning of samples and the possibility of creating cross-sections from any angle opens up new analysis-related and time-saving potential for foundry-based quality controls. With automatic porosity analysis (fig. 2), for example, the size of the inclusions can be shown on a table or marked in different colours on the component itself, thus giving an indication as to the quality of the cast process, for instance, or component stability. It can also be used to verify correct assembly or to determine the position of cast components following an inconclusive 2D X-ray inspection.

High-resolution CT-based dimensional measurement
On account of the large number of internal contours in cast parts made from plastic or light alloys, the use of conventional measurement methods to inspect such parts is often not possible in a non-destructive way and takes a considerable amount of time. The fact that CT offers extremely precise and complete 3D representation of objects opens up its field of application, meaning that it is also suitable for coordinate measuring.

This is because, in contrast to conventional tactile or optical coordinate measuring systems, CT measuring systems are also able to completely capture the hidden contours of specimens, such as cavities and undercuts. In addition, a CT scan of a specimen results in a very high number of measurement points, typically in the order of magnitude of 105 to 106, which can then be used (through the application of statistical methods) to achieve a measurement resolution that is typically significantly below 1/10 of the voxel size (voxel = volumetric pixel); which, depending on the object size, is within the micrometer range.

Fig. 3 shows, by way of an example, a target/actual comparison of the variances between a specimen and the CAD model. The swiftly generated results from these measurements can be used as a basis for correcting the manufacturing process in a timely manner and optimizing series production.

The precision of the measured raw data (CT projection data) determines the accuracy of all subsequent evaluations. In addition to a stable system structure that is optimized for the specific application at hand, data processing is the key to successful CT-based measurement. For optimal measurement results, therefore, the reconstruction algorithm used to calculate the volume data has to take into account and correct the unavoidable physical effects of CT scanning, such as 'beam hardening'. The phoenix|x-ray CT product line from GE Sensing & Inspection Technologies features a variety of software modules for optimized volume data reconstruction.

It also offers a comprehensive range of software packages for surface data analysis, including element modelling, DIN/ISO-compliant geometric dimensioning and tolerancing calculation (GD&T), and even the fully automated generation of initial sample inspection reports. The measurement data generated using dedicated, optimized surface extraction is traced back to normal standards using DKD-certified (German metrology accreditation body) specimens.

The CT-based 3D measurement process chain
Data acquisition. The actual physical measurement is taken by scanning a series of 2D X-ray projection images. To do this, the specimen is positioned on a granite-based precision manipulation system and, during the measurement, is completely rotated through 360° on a precision turntable. A 2D projection image is typically taken every <0.5°. The quality of the raw data, and naturally the accuracy of all subsequent evaluations, is significantly influenced by the sharpness of the X-ray images, which is heavily dependent on the quality of the x-ray source and detector in addition to the precision and stability of the manipulation device. It is true to say, therefore, that the more effectively the CT measurement system performs this first step, the more precisely the measurement task can be performed.

Volume reconstruction. The volumetric data set for the specimen is generated from the raw data using a numeric reconstruction method filtered back-projection . For optimum measurement results, the reconstruction algorithm should take into account and correct the physical effects, such as beam hardening or thermal expansion, during the raw data capture in step 1.

Surface data generation. For the subsequent processing of the measurement results, the surface is extracted from the volume data as a generic ASCII point cloud or STL surface for import into 3D inspection software, such as the Polyworks Inspector™.
Evaluation and analysis (virtual coordinate measuring machine). After importing the surface data from the specimen into the 3D analysis software, the additional measuring steps can be performed (fig. 4). These steps include a target/actual comparison between the surface data and the CAD model with a variance analysis or measurements using ruled geometries.

Substantiation of CT measurement accuracy
To substantiate the measurement accuracy of the phoenix|x-ray computed tomographs of GE Sensing & Inspection Technologies, and therefore their suitability for use as 3D coordinate measuring machines, a valve block made from aircraft aluminum with an edge length of 130 mm was subjected to a CT scan at Continental AG in Frankfurt followed by a reference measurement using high-precision tactile 3D coordinate measurement technology. The phoenix v|tome|x L computed tomograph of GE Sensing & Inspection Technologies was used for the CT scan and a Hexagon/Leitz 3D PMM 8.6.6 coordinate measuring machine for the reference measurement.
Table 1 contains extracts from the specimen inspection report and demonstrates the excellent concordance between both the KMM and CT measurement methods: The proven diameter and length variance is <6 µm.

Table 1: Excellent correspondence between the measurement data from the
3D coordinate measuring machine and high-resolution computed tomography
 Ø Z 1 [mm]
 Distance A 1 [mm]
 Target CAD

  28.000  70.500
 0.050 0.100
  Actual CT
  Actual CMM

  28.034 70.447
How advances in CT systems are  benefiting foundries
A new tomograph from the phoenix|x-ray product line of GE Sensing & Inspection Technologies, which has recently been placed on the market and which features a revolutionary unipolar 300 kV/500 W microfocus x-ray tube, is enabling CT analyses of difficult-to-penetrate components at particularly high levels of magnification (fig. 5). It is the first time that a resolution of up to 1 µm has been achieved with a 300 kV X-ray tube. The CT system also uses a new temperature-stabilized digital detector with even higher contrast resolution. As a result of recent hardware and software developments, the v|tome|x L 300 offers effective CT results in a very short time.
The user-friendly and highly efficient phoenix|x-ray CT software datos|x includes a wide variety of modules for optimizing CT results for superior precision and quality.

The new bhc|module, for instance, corrects beam hardening automatically. This compensation for undesirable artifacts significantly increases the precision of pore analyses or surface extraction for subsequent dimensional measurements. Hardware innovations are also ensuring more precise measurement results. For instance, a multiline configuration for flat panel detectors effectively reduces scatter interference, permitting the performance of high-resolution scans on highly dispersive aluminum components with high X-ray energy without the need for installation of an additional line detector.

The phoenix v|tome|x L 300 can be used for full, high-resolution 3D scanning of samples weighing up to 50kg and measuring up to 500 mm in diameter, making it ideal for quality control in foundries (fig. 6). The system also has a special metrology package that contains everything needed for precise and intuitive dimensional measuring, from calibration instruments to surface extraction modules. l

Dr. Oliver Brunke, product manager, GE Sensing & Inspection Technologies GmbH, phoenix|x-ray, Wunstorf,
email: Oliver.Brunke@ge.com

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