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3. WLI non-contact three-dimensional roughness measurement
（1）Surface texture measuring method
As mentioned above, classification of measuring methods of surface texture is defined by ISO and JIS. (ISO 25178-6:2010, JIS B 0681-6:2014) In these, methods including conventional stylus scanning are defined as methods to measure two-dimensional contour curves (Line Profiling Method) and methods shown in Fig. 5 are defined as methods to measure three-dimensional roughness (form). Among these methods, the measuring method employed for QV WLI corresponds to the Coherence Scanning Interferometry.
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Fig.5
Surface texture measuring method classification JIS B0681-6 (2014) |
Fig.6
Formtracer Extreme
SV-C4500CNC HYBRID TYPE1
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Besides the stylus scanning method, the form tracer extreme SVC4500CNCHYBRID TYPE1 equipped with a chromatic point sensor (CPS) using axis color aberration of white light source for the displacement amount, which corresponds to the color aberration confocal microscopy method, is also available from Mitutoyo.
（2）Type of three-dimensional surface texture parameter
To the three-dimensional surface texture parameter, parameters including the height parameter which expands the two-dimensional contour method to three-dimensional, the autocorrelation length Sal called space parameter used to evaluate the periodicity of a plane region and the texture aspect ratio are added.
Parameters relating to function include a parameter called the volume parameter to evaluate the volume of convex and concave parts of surface roughness and many parameters required for tribology analysis are defined such as a parameter that evaluates the lubricant oil holding property.
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Table 2
List of areal surface texture parameters JIS B0681-2:2018 |
For an example of a space parameter, the following description shown in Fig. 7 explains the texture aspect ratio Str analyzed using our 3D surface property analyzing software MCubeMap. When observing the texture aspect ratio, the Str value goes closer to 0 when a form has a periodicity in a specific direction such as the form shown on the left. The Str value will go closer to 1 when a form is equal to the average in the area as shown on the right.
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Fig.7
Texture aspect ratio Str |
4. Comparison between stylus surface roughness measuring instruments and QV WLI
Based on the already explained measuring methods above, data correlation will be your concern in order to replace surface texture evaluation of non-contact measuring method at the actual quality management sites.
Since measuring methods are defined by ISO and JIS, there may be no meaning in discussing data correlation. However, we compared and verified using each measuring method as a vendor who has both stylus surface roughness measuring instruments and WLI roughness measuring instruments in our product line-up. Please refer to them as references.
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Table 3
Specifications of BoaTex-WLI |
As you can see, the profile and the measurement value are almost the same on the standard sample having the equivalent roughness (Ra3.0 μm) of a calibrated stylus surface roughness measuring instrument. Yet, on the standard sample having random shape, since a WLI type can measure a fine groove in which the tip of the stylus cannot be inserted, measurement values using WLI tend to be more precise. Please note this point when comparing between stylus surface roughness measuring instruments and WLI.
5. WLI application measuring instrument: Engine bore inner wall property measuring system BoaTex-WLI
We have advantages such as having our own basic WLI technology such as the interference optical system lens. Lastly, as an application example of surface texture evaluation technology, we introduce BoaTex-WLI developed as a surface texture measuring system of the inner wall of an engine bore.
（1）BoaTex-WLI outline
BoaTex-WLI is a system that can directly measure the surface texture of the bore inner wall in nano order. It is possible to evaluate the three-dimensional surface texture of the bore’s circumferential direction and axial direction by rotating the newly developed WLI optical head. In addition, since a high accuracy XY stage is provided, it is possible to automatically analyze multiple bores of inline-four or V-type engines from cross hatch to pit.
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Fig.8
Outline of BoaTex-WLI |
（2）BoaTex-WLI specifications
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Table 4
Specifications of BoaTex-WLI |
（3）BoaTex-WLI analysis example
It is possible to perform pit analysis (pit area, pit volume), cross hatch analysis and roughness analysis using the 3D surface texture analyzing software MCubeMAP supporting ISO 25178.
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Fig.9
Analysis example of BoaTex-WLI |
3. Conclusion
Mitutoyo is providing many precision measuring instruments from small tools such as micrometers and calipers to machines such as CMM with which high accuracy measurement of coordinate positions and element positions is available as well as high-accuracy form measuring machines offering high accuracy measurement of machining shape. In recent years, demand is rapidly increasing for acquisition of highly efficient point cloud data by non-contact measuring methods such as laser displacement meters and color aberration confocal displacement meters in addition to white-light interferometers. Mitutoyo will continue to provide the leading edge of measurement technology.
Cited from "Tool Engineer 2018 September" published by TAIGA Publishing Co.,Ltd.
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