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Figure 1: Measuring 3D CT METROTOM 1500 from 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 CT measuring systems. With comprehensive inspections, Carl Zeiss Industrial Metrology (Carl Zeiss IMT) provides customers with reliable data on the performance of CT measuring processes.
If cone-beam CT is to become a qualified measuring procedure, it must be qualified in the same manner as coordinate measuring machines (CMMs) that use contact and optical probing. Test piece
s calibrated by certified institutes have been defined for international standards. In turn, these test pieces contain certain geometric elements. Computed tomography measurements are performed on the reconstructed object volume either directly or via 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. After all, the user must know if the workpiece to be test
ed meets tolerance requirements and if the measuring machine is suitable for the corresponding measuring task. Furthermore, 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 compared1. Linear measuring tolerance and probing tolerance are probably the most important specifications. Form deviation can also be specified for scanning measuring machines.
In Germany, the national standards, VDI/VDE 2617 (VDI 86-VDI 01), on accuracy of CMMs—characteristics and their testing, are primarily utilized. The international standard, ISO 10360, on acceptance and reverification tests for CMMs, is being increasingly accepted. For each given measuring task, DIN EN ISO 10360 defines the maximum permissible error for each relevant accuracy (DIN 03). Maximum permissible error values are important criteria when selecting a measuring machine that will be used for a variety of measurements on small quantities or single workpieces. The maximum permissible error values should be clearly narrower than given tolerances (rule of thumb: 1:10). A capability study, similar to gauge repeatability and reproducibility (GR&R), for the special measuring task is a good investment if a measuring machine will be used for the standard measurement of identical or similar workpieces.
However, applying these criteria to CT involves several difficulties. Although the VDI/VDE committee 3.33 “Computed Tomography in Dimensional Metrology” is currently developing CT-specific standards, conflicting interests are preventing their completion.
Carl Zeiss IMT, internationally headquartered in Oberkochen, Germany, will continue performing comprehensive testing as there are no such standards for CT measuring technology for providing metrology customers with reliable data about the performance of the CT measuring procedures. Extensive activities on the metrology project using CT were initiated during the past four years together with Robert Bosch GmbH. The following requirements had to be met to implement and establish CT measuring technology at Bosch.
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About Bosch The Bosch Group is a leading global supplier of technology and services in the areas of automotive and industrial technology, consumer goods, and building technology.
Automotive Technology is the largest corporate division of the Bosch group with approximately 161,100 employees.
Bosch Gasoline Systems is a division of Automotive Technology. Bosch Gasoline
Areas of operation: Engine management Modules and engine components Fuel supply Sensors Ignition Electronic throttle control (ETC) Plug-in connections Transmissions Compressed natural gas (CNG) Flexfuel |
Robert Bosch GmbH, located in Stuttgart-Feuerbach, Germany, has detailed internal directives that define the requirements for a measuring machine. These requirements cover all criteria relevant to dimensional metrology and must be met by the supplier of measuring technology. Because CT is a new measuring technology, there are no objective standards for measuring uncertainty and machine acceptance testing. Bosch and Carl Zeiss IMT have agreed on five acceptance criteria to evaluate and realize 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 centers had to be measured for several pairs of spheres. This enabled the inspection of many measuring lengths in all spatial directions.
Figure 2: Calibrated reference sphere consisting of 27 ruby spheres to determine the accuracy and repeatability of measurements
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. Furthermore, the standard deviation of the measured lengths was not permitted to exceed a given maximum value (Cg and Cgk ≥ 1.33 ).
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Figure 3: Volume part to determine the influence of the operator. |
Three different operators (two from Bosch 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 (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 0.005 mm for the volume part.
The goal of this 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 due to the procedure, a high level of comparability had to be achieved with the results. A Zeiss 3-D CMM and an optical 2-D 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 ≤ 0.01 mm
· Inner/outer diameter (Chebyshev minimum circumscribed and maximum inscribed diameter, Gaussian average diameter); target value: ΔPRISMO–METROTOM ≤ 0.01 mm
· Circle position tolerance with X/Y coordinates: comparison of the 2-D position data with the results from the smoothed cross-section images of the test workpiece; target value: ΔMYCRONA–METROTOM ≤ 0.010 mm.
Figure 4: Test pieces for the curve measurement, determination of diameters and circle position tolerance
This task should evaluate the suitability of the measuring machine for requalification, for the implementation of a trend check in ongoing production. The CT scans of two different workpieces from different production batches will be compared to each other regarding 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 color-coded illustration.
Figure 5: Test workpiece and evaluation for re-qualification
The capability of CT to capture the entire volume of a workpiece in a single scan is unique. This is particularly 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 CT for dimensional metrology, it is important to objectively assess the quality of the results and not be fooled by impressive images. For measuring tasks in an industrial environment, it is extremely important to take the measuring uncertainty into consideration. After all, 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. Without a doubt, generally applicable and accepted standards must be defined for this purpose; the VDI/VDE committee 3.33 is working on this subject. As soon as objective standards are available, CT technology will be more readily accepted by users in industrial metrology.
Source:
1. T. D. Imkamp und J. Wanner: Es geht auch einfach: Genauigkeitsangaben und Leistungstests für Koordinatenmessgeräte, Carl Zeiss Industrielle Messtechnik GmbH, July 2005.
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