Dimensional measurements

X-ray computed tomography

Evaluation of dimensional and material integrity

Driven by the need for quality control of complex three-dimensional engineering components, and enabled by progress in medical imaging and consumer graphics processing, X-ray computed tomography (XCT) has been increasingly used for industrial inspection in the last decade. A rise in the demand to have, more intricate, parts manufactured using additive manufacturing (AM), has limited the use of conventional dimensional measuring systems for verification of these parts. XCT is invaluable in the evaluation, in both dimensional and material integrity, of these complex AM parts.

The most important merit of the technique is that it allows the measurement of the geometry of external (surface) and internal features without the need to disassemble a component. Whilst many XCT instruments are used for non-destructive testing, they are increasingly being used in industry as dimensional metrology tools, akin to the conventional co-ordinate measuring machine.

The XCT system at NPL employs a cone-beam X-ray source with the maximum energy of 225 kV. Details of the system are as follows:

  • Cone-beam X-ray computed tomography
  • Maximum 225 kV
  • Temperature control of ±1 °C
  • 40 cm × 40 cm scintillation detector
  • 200 µm × 200 µm pixel size on detector
  • Magnification up to ~120
  • Spatial frequency: up to 3 µm

Typical penetration depth of materials are given below (an energy of 200 kV is chosen in the estimation):

Material Atomic number Approximate depth 
of penetration (cm)
Carbon 6 14.3
Aluminum 13 9.0
Titanium 22 5.0
Iron 26 2.6
Nickel 28 2.1
Copper 29 2.1

Development of XCT reference samples

NPL has designed a series of reference samples with the aim to:

  • Investigate systematic errors associated with the XCT technology
  • Verify the performance of XCT systems
  • Investigate the factors that influence the quality of AM
  • Evaluate XCT as a method for robust dimensional analysis of AM parts

NPL can provide customers with both qualitative and quantitive analysis of single and multi-material samples. Using our other in-house capabilities, we are in the unique position of being able to correlate XCT images and analysis to other physical dimensions, such as: dimensional through one of our many traceable CMM systems, 3D external surface generation through optical methods (fringe projection or laser line projection), material properties, analysis of inclusions through microwave scanning, acoustical and thermal analysis, is to name only a few from the wide range of available systems.

Examination of the assembled internal structure of manufactured parts, in a range of discrete points in manufacturing. This allows the customer to pinpoint steps in the manufacturing process that may be responsible for particular issues or problems.

NPL has collaborated at both international and national levels with: NMIs, academia and industry on a variety of projects. The list of collaborators includes the following, but not exhaustive: PTB, NIST, NMIJ, BAM, Bath University, Huddersfield University, Padua University, Southampton University, University College London (UCL), Nikon, Simpleware, VolumeGraphics.

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