According to Carl Zeiss , there are still no generally applicable standards for industrial computed tomography (CT). Manufacturers and users still must agree on de-facto standards for the specification and certification of computed tomography measuring systems.
Carl Zeiss Industrial Metrology (Carl Zeiss IMT) conducts inspections and provides customers with reliable data on the performance of CT measuring processes. If cone-beam computed tomography is to become a qualified measuring procedure, it must be qualified in the same manner as coordinate measuring machines that use contact and optical probing. Test pieces calibrated by certified institutes have been defined for international standards. In turn, these test pieces contain certain geometric elements. CT measurements are performed on the reconstructed object volume either directly or through the intermediate section of the surface extraction. The results of these measurements must be compared with the values of the calibrated elements. The degree of compliance of the results determines the accuracy of the measurement.
For daily use, the main specification of the system is the measuring accuracy or uncertainty. Users must know if the workpieces to be tested meet their tolerance requirements and if the measuring machine is suitable for the corresponding measuring task. The reproducibility of the measured values and the use by any operator are important criteria for the use of a measuring machine. In traditional metrology, standards define the criteria used to assess the accuracy of measuring results and maximum permissible error, as well as how different measuring machines are compared. Linear measuring tolerance and probing toleranceare important specifications. Form deviation can also be specified for scanning measuring machines.
In Germany, the national VDI/VDE 2617 (VDI 86-VDI 01) standards are primarily utilised. The international ISO 10360 standard is being increasingly accepted. For each given measuring task, DIN EN ISO 10360 defines the maximum permissible error (MPE) for each relevant accuracy (DIN 03). MPE values are important criteria when selecting a measuring machine that is used for a variety of measurements on small quantities or single workpieces. The MPE values should be narrower than given tolerances (rule of thumb:1:10). A capability study (similar to a GR&R) for the special measuring task is suitable if a measuring machine is used for the standard measurement of identical or similar workpieces.
However, applying these criteria to computed tomography involves several difficulties. Although VDI/VDE committee 3.33 is currently developing CT-specific standards, conflicting interests are preventing their completion. Carl Zeiss industrial metrology perform comprehensive testing as there are no such standards for CT measuring technology in order to provide metrology customers with reliable data about the performance of the CT measuring procedures. Extensive activities on the metrology project using CT were initiated over the past four years together with GmbH. The following requirements had to be met to implement and establish CT measuring technology at BOSCH.
GmbH have detailed internal directives that define the requirements for measuring machines. These requirements cover all criteria relevant to dimensional metrology and must be met by the supplier of measuring technology. As computed tomography is a new measuring technology, there are no objective standards for measuring uncertainty and machine acceptance testing. GmbH and Carl Zeiss IMT have agreed on five acceptance criteria to evaluate and realise machine acceptance for the METROTOM 1500 measuring system combined with CALYPSO measuring software.
A calibrated reference sphere containing 27 ruby spheres on carbon fiber shafts was used for this inspection. The distance between the sphere centres had to be measured for several pairs of spheres. This enabled the inspection of measuring length in all spatial directions.
It had to be measured 50 times. For all measurements, the deviation over the measured length had to be (5+L/50) µm less than the calibrated value. The standard deviation of the measured length was not permitted to exceed a given maximum value (Cg and Cgk ≥1.33).
Three different operators (two from GmbH and one from Carl Zeiss IMT) had to measure a representative dimension of a volume part in two runs each, in which each run consisted of ten measurements. Each operator had to arrange the workpiece in the measuring range, perform the CT scan and then perform the evaluation using CALYPSO software. As with the previous measuring task, the standard deviation of the single measurements also had to be below a given maximum value, for example (target value: Cg and Cgk ≥1.33 , %GRR ≤10%).
The system measuring uncertainty U was determined with the same volume part based on 20 measurements on selected dimensions. Bosch internal standards provide a detailed definition of the calculation of U. This measuring uncertainty should be less than 0.005mm for the volume part. Calibrated reference sphere consists of 27 ruby spheres to determine the accuracy and repeatability of measurements. The goal of comparative measurement examination was to determine the level of comparability of the results of METROTOM 1500 with the results of the measuring machine used originally. Despite differences caused by the procedure, a high level of comparability had to be achieved with the results. A Zeiss 3D coordinate measuring machine and an optical 2D measuring machine from MYCRONA were used as reference machines. Three representative dimensions were selected for one test workpiece:
- Curve measurement (line form)
- Target value: ΔPRISMO–METROTOM ≤0.01mm
- Inner/outer diameter (Chebyshev minimum circumscribed and maximum inscribed diameter
- Gaussian average diameter)
- Target value: ΔPRISMO–METROTOM ≤0.01mm
Circle position tolerance with X/Y coordinates: comparison of the 2D position data with the results from the smoothed cross-section images of the test workpiece Target value: ΔMYCRONA–METROTOM ≤0.010mm. Comparative measurement of two workpieces with colour-coded display of the results should evaluate the suitability of the measuring machine for re-qualification and for the implementation of a trend check in ongoing production. The CT scans of two different workpieces from different production batches are compared with each other with regard to the complete geometry. This is done by comparing both scans with the CAD data and displaying the respective deviations from the CAD display in a colour-coded illustration.
Computed tomography captures the entire volume of a workpiece in a single scan. This is beneficial for workpieces with complex geometries where traditional measuring machines cannot capture certain dimensions and shadows prevent optical probing of the dimensions. In order to qualify computed tomography for dimensional metrology, it is important to assess the quality of the results. For measuring tasks in an industrial environment, it is important to take the measuring uncertainty into consideration. The operator must know if the dimensions are within the given tolerances and if the measuring machine can provide an answer to the question on whether the tolerances are met. Generally applicable and accepted standards must be defined for this purpose.