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Specification and acceptance test

Comparability and traceability

The most important property of a coordinate measuring machine is its contribution to the achievable measurement uncertainty in a measurement process. To select a machine, the user must be able to compare different machines, define purchasing conditions and check the function.

Comparability of the machines

The DIN EN ISO 10360[10] series of standards and the VDI/VDE 2617 guidelines define specifications for this and describe procedures for checking them. In principle, the reverification test of coordinate measuring machines focuses on two characteristics: the probing error and the length measurement error.

Probing error

The probing error test (limit MPE P: Maximum Permissible Probing Error) is used to characterise the behaviour of the sensors used and the reproducibility of a measurement within a small partial measuring range of the coordinate measuring machine. For this purpose, a calibrated sphere is measured with a specified number of measurement points, the span of the individual points around the best-fit element sphere is determined and compared with the limit PF (new notation PForm). The difference between the ball diameter determined from the points and the calibrated ball diameter results in the actual value for PS (new notation PSize). For point sensors with a scanning function, two further characteristic values are defined in a similar way. By scanning several cross sections of a sphere with a predefined path, THP (new notation PForm.Sph.Scan:PP:Tact - PP: predefined path) or THN (new notation PForm.Sph.Scan:NPP:Tact - NPP: not pre-defined path) is measured on the sphere. The evaluation is analogue to the probing error. These characteristics have so far only been defined for tactile sensors (DIN EN ISO 10360 Part 4, will be integrated in Part 5).

Properties of the machines ...

The achievable probing error is determined by the reproducibility of the machine (resolution of the scales, vibration behaviour) and, in the case of different sensors, by various sensor-specific influencing factors. The probing error when using tactile sensors (DIN EN ISO 10360 Part 5) is primarily influenced by the sensor properties of the probe ball shape, shaft deflection as well as non-linearities and backlash of the sensor mechanics.

... and the sensors

The special features of testing optical sensors are described in VDI/VDE 2617 sheets 6.1 and 6.2 and in DIN EN ISO 10360 parts 7 and 8. With optical sensors, the probing error is influenced by the sensor resolution, the optical magnification of the lenses, the depth of field when measuring with the autofocus and, in the case of distance sensors, e.g. also by the reflectance of the material surface. While the spherical standard can be probed bidirectionally from all sides with tactile sensors, only unidirectional probing is possible with some optical sensors. To enable bidirectional probing, a rotary/tilt head can be used. This then also influences the probing error.

The corresponding procedure for checking coordinate measuring machines with X-ray tomography is described in VDI/VDE 2617 Sheet 13 (Fig. 64, see also[8]). Here, the selected magnification, the focal spot size, the set voltage and current of the X-ray tube and other parameters influence the result. It should be emphasised in particular that the material of the sphere used also has a significant influence on the probing error due to the principle-related penetration by the X-rays. The choice of material is therefore limited or determined by the machine manufacturer depending on the machine type.

... and the sensors
<p>Fig. 64: Determination of the probing error (P) by ball measurement: The numerical values given refer to the standard-compliant measurement with 25 points. The graph also shows the result for approx. 20,000 measurement points.</p>

Length measurement error

In order to determine the properties of the coordinate measuring machine as completely as possible, measurements are also required that utilise the machine measuring range as far as possible. In particular, this captures the mechanical guide deviations or the quality of the software geometry correction and the temperature-related length-dependent measurement error.

Temperature and length

The length measurement error (limit MPE E: Maximum Permissible Error of length measurement) is checked by measuring five calibration standards of different lengths (maximum length at least 66 % of the machine measuring range) for seven different spatial positions. The length measurement error values are largely dependent on the measured length. Because the machine geometry is often very well corrected today, the causes lie in particular in the influence of temperature (see p. 119 ff.). It therefore makes sense to show the limits as a linear function of the measured length (L) (Fig. 65). The constant element (K) practically describes the reproducibility.

Temperature and length
<p>Fig. 65: Illustration of the results of the acceptance test of the length measurement error - example MPE E: (0.25 + L/600) µm; L in mm</p>

Observe operating parameters

Depending on the sensors used, different types of length standards can be used. The measurement results may depend heavily on the parameters set for the machine (e.g. probing speed, filter) and the sensors (e.g. X-ray parameters, light settings for image processing). It is therefore important to ensure that the parameters specified in the data sheet are set for all reverification tests. In the event of deviations, additional contributions to the probing or length measurement error or even more favourable values can be expected.

Stylus: Step gages

For tactile sensors, the length measurement error is tested by measuring lengths at parallel or step gages (DIN EN ISO 10360 Part 2, VDI/VDE 2617 Sheet 2.1). The new version of DIN EN ISO 10360 published in 2009 also defines two new characteristics. To assess the rotation around the vertical axis, the length measurement error is also determined using a stylus with a lateral distance of 150 mm between the centre of the stylus ball and the ram. In addition, an additional characteristic for reproducibility is defined with the repeatability margin of the length measurement error for every three measurements.

Image processing: Glass scales

For the reverification test of the length measurement error with image processing sensors, the gage blocks are replaced by glass scales with lines of vapour-deposited chrome (DIN EN ISO 10360 Part 7, VDI/VDE 2617 Sheet 6.1). The measurement is preferably carried out bidirectionally, analogue to the measurement of the step gage, in order to capture the influence of backlash and edge detection methods. Spatial measurements of the length measurement error are strongly recommended if three-dimensional measurements are to be made in practical use of the measuring machines.

Distance sensors and CT: multi-sphere standards

With distance sensors (point, line and area sensors), bidirectional probing, which is actually useful, is not possible without the use of rotary/tilt heads. In this case, ball plates/ sphere plates or linear sphere plates can be used according to DIN EN ISO 10360 Part 8 and VDI/VDE 2617 Sheet 6.2. However, in order to ensure comparability with tactile measurements on gage blocks, a mathematical correction must be made for this measurement method. This ensures that systematic measurement errors of the surface points are captured during the inspection in the same way as in the bidirectional measurement of gage blocks. Such deviations arise, for example, during the prior qualification of the sensor (scanning sphere diameter) or due to properties of the sensors (edge location definition for image processing and tomography scans, penetration depth of the laser for distance sensors). With sphere measurement, these influencing factors are partially cancelled out by averaging. The mathematical correction is made by adding the probing error or the deviation of additional bidirectionally measured short lengths in a suitable manner (analogous to the procedure for coordinate measuring machines with X-ray tomography, Fig. 66). The averaging effect achieved by probing the spheres with possibly many measurement points is thus corrected in the result.

Distance sensors and CT: multi-sphere standards
<p>Fig. 66: Determination of the length measurement error E with sphere standard: (a) multi-sphere distance standard for tomography machines, (b) measurement of a two-point diameter D<sub>a1</sub> on a calibrated sphere (D<sub>r</sub>) to realise bidirectionality, (c) measurement of a sphere centre distance L<sub>a1</sub> on a calibrated multi-sphere standard (L<sub>ri</sub>) for lengthd) addition of the diameter deviation to the deviation of the distance between the sphere centres in all standard orientations and lengths</p>

"Accuracy" cannot be specified

Similar procedures are used for the reverification test of machines with X-ray tomography[8] (VDI/VDE 2617 Sheet 13). Because volumes can be completely captured with X-ray tomography, it is advisable to use three-dimensional calibration standards (Fig. 66). Only one measurement is then necessary to determine the lengths in all orientations. The choice of material and design of the calibration standards is of special feature due to the radiographic properties. Although the standards and guidelines for the specification of coordinate measuring machines have been in use for over 20 years, machines with incorrect specification information can still be found. For example, twice the standard deviation is given as the "accuracy". Such specifications are usually more favourable than the parameters for the probe and length measurement error determined in accordance with the standards and say nothing about the systematic measurement errors.