This is the accuracy standard for a CMM if equipped with a scanning probe. The test procedure under this standard is to perform a scanning measurement in 4 planes on the standard sphere and then, for the least squares sphere center calculated using all the measurement points, calculate the radial range (dimension ‘A’ in Figure 5) within which all measurement points exist. Based on the least squares sphere center calculated above, calculate the radial distance between the calibrated standard sphere radius and the maximum measurement point and the minimum measurement point, and take the larger distance (dimension ’B’ in Figure 5). Add an extended uncertainty that combines the uncertainty of the stylus tip shape and the uncertainty of the standard test sphere shape to each A and B dimension. If both calculated values are less than the specified values, this scanning probe test is passed. Maximum Permissible Scanning Probing Error MPE THP [ISO 10360-4: 2000]
Stylus
45°
Calibrated value of standard sphere radius
Least square sphere
Scan plane 2
Scan plane 1
Scan plane 3
Least square sphere center
Scan plane 4
A
Measurement point locus
B
Figure 5 Target measurement planes for the maximum permissible scanning probing error and its evaluation concept
Maximum Permissible Single Stylus Form Error P FTU, MPE [ISO 10360-5: 2010]
This measurement was included in the dimensional measurement in ISO 10360-2: 2001. However, it is specified as "CMMs using single and multiple stylus contacting probing systems" in ISO 10360-5: 2010. The measurement procedure has not been changed, and the following procedure should be performed. Measure the defined target points on a standard sphere (25 points, as in Figure 6) and use all the results to calculate the center position of the sphere by the least squares method. Then, calculate the radial distance from the center position of the sphere by the least squares method for each of the 25 measurement points and obtain the radial difference Rmax - Rmin. If this difference, to which a compound uncertainty of forms of the stylus tip and the standard test sphere are added, is equal to or less than the specified value, it can be judged that the probe has passed the test.
22.5 ゜ a
22.5 ゜
22.5 ゜
22.5 ゜
22.5 ゜
Figure 6 Target points of measurement for Single Stylus Form Error
Measurement Uncertainty of the CMM
Measurement uncertainty is an indication used for evaluating reliability of measurement results. In ISO 14253-1: 1998, it is proposed to consider the uncertainty when evaluating the measurement result in reference to the specification. However, it is not easy to estimate the uncertainty of the measurement performed by a CMM. To estimate the uncertainty of the measurement, it is necessary to quantify each source of uncertainty and determine how it propagates to the measurement result. The CMM is subject to all types of settings that determine how the measurement should be performed, such as measurement point distribution, or datum definition, according to the drawing instruction or operator's intention. This fact makes it harder to detect the sources of uncertainty influencing the result. Taking circle measurement as an example, just a difference of one measurement point and its distribution causes the necessity of recalculation of the uncertainty.
Also, there are many sources of uncertainty to be considered with the CMM and their interactions are complex. Because of the above, it is almost impossible to generalize on how to estimate measurement uncertainty of the CMM.
Measurement task
Data processing
Positioning of measuring points
CMM
Environment
Uncertainty of center position
L
Uncertainty of circle profile
Probe
Workpiece
Uncertainty of measurement
Example of circle measurement by CMM
Major contributions that cause uncertainty in CMM measurement results
L-24
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