X-Ray Computed Tomography (CT) has been emerging as a valuable technology for use in the quality assurance of manufactured parts. Most manufacturing and quality engineers have familiarity with X-Ray processes for crack detection and porosity checks in castings but Computed Tomography for dimensional metrology is relatively new and emerging as an alternative to the traditional tactile Coordinate Measuring Machine (CMM).
CT is an exciting new technology when used for the dimensional inspection of parts since it provides the ability to visualize and measure both external and internal features and in many instances can measure features where tactile or optical sensors have significant limitations.
The emergence and use of Additive Manufacturing (AM) increases the complexity of internal part features as complex part designs previously unable to be manufactured are now producible. These next generation parts needs to be inspected and CT represents the most logical non-destructive solution.
A typical CT machine comprises of an X-Ray generator that sends out either a fan or cone shaped X-Ray beam. The part is positioned on a rotary table between the X-Ray beam generator and a flat panel detector. A series of individual 2D images (tomograms) are produced by rotating the part and are subsequently processed and reconstructed to create a comprehensive 3D data set comprising of voxels (3D pixels).
The X-Ray image can be imported into metrology software and rendered into a 3D part model which can be compared directly with the 3D CAD model and the inspected part geometry extracted and compared to the design intent data and G D & T applied just as with a CMM. In fact manufacturers who supply both CT and CMMs utilize the same core metrology software for both measurement technologies.
CT generated data sets are clean and require no data merging or smoothing providing ‘watertight’ STL data sets unlike laser and optical scanning of external part surfaces now also common place in managing manufacturing quality.
Dimensional measurement using CT is complex with the accuracy of results dependant on a number of influencing factors. The German PTB have issued guidelines which describes typical influences on the measured results when performing dimensional measurements. The guideline provides recommendations regarding the quality assurance of dimensional measurements with CT.
CT Advantages Over CMM
Industrial CT is generally faster than tactile CMM since a single scan can yield all required measurements. Complex GD&T call-outs such as concentricity, coaxiality and cylindricity take less time to measure than CMM. CT also has high data density providing detailed volumetric models which provide permanent visual records. CT data can be easily manipulated in a 3D work-space allowing detailed visualization and analysis of non-conformance.
As a CMM generally uses tactile sensing they can only measure external surfaces. For internal measurements, prior disassembly or destruction of the part is necessary, which is sometimes not possible or practical. CMM measurement is also typically not a viable metrology option for soft or flexible parts. Highly complex part geometries CMM measurements can take longer than CT and may also require expensive part fixturing.
CMM Advantages Over CT
Coordinate measurement machines are highly precise, accurate, and repeatable and easier and more straightforward to operate and less expensive to maintain. With supplied CAD data available, actual to nominal comparisons between predetermined points are quick and readily identify out of tolerance dimensions.
A CMM is also more suitable for metrology of parts made from high density materials, as long as the features to be measured are external and accessible. For onsite measurement, portable tactile CMMs can be easily transported to remote or difficult locations.
Portability is an issue with industrial CT since the machines tend to be large. Parts must be able to fit inside machines, so there are size restrictions with what parts can be measured. Parts made from high density metals may also not be suitable for CT since x-rays must be able to penetrate the material. Developments with high energy CT can overcome this limitation by allowing for larger part sizes and denser materials. CT machines are also expensive to purchase, operate, and maintain, requiring specialized staff and require investment in computing power for rendering detailed 3D models from massive data sets.
To ensure that CT measurements are traceable back to measuring standards a statement of measurement uncertainty must be given together with the actual measurement result. A generally accepted method for uncertainty evaluation is the use of calibrated work-pieces however, the influencing factors throughout the CT measurement procedure which contribute to the uncertainty are not quantified individually and remain unknown. The quality and reliability of the measurement, expressed in measurement uncertainty, depends on both hardware and software as well as the user-set scan parameters. Scan parameters, such as current, tube voltage or exposure time, all can influence the measurement results as well as surface determination and geometrical evaluation of the measured features which add to measurement uncertainty.
CT is an exciting technology when used for the dimensional inspection of components since it provides the ability to visualize and measure both external and internal features and in many instances can measure features that tactile or optical sensors have limitations.
Three methods for assessing the task specific uncertainty for CT measurements are currently under discussion: Assessment by model equations analytically calculated according to the Guide to the Expression of Uncertainty in Measurement (GUM), Monte Carlo simulations or empirical methods, namely the uncertainty evaluation by use of calibrated work-pieces according to ISO 15530-3. The third approach is generally accepted and already established for CT measurements; in this method, calibrated work-pieces are repeatedly measured under the same conditions. To ensure traceability, a high accuracy tactile CMM is commonly used as the reference method for the calibration artifact.
The CMM has remained unchallenged as the ‘go-to’ dimensional measurement device for over 30 years. The CT challenger is beginning to find its market position and its presence can only grow in coming years particularly as Additive Manufacturing increases its role in the manufacture of high value complex part geometries.
Vaunell Itaas of Jesse Grant Metrology Center has contributed to this article