The three main types of surface topography are generated by using conventional machining processes, which are classified according to how they are caused.
These can be classified into three groups:
- The waviness which results from factors such as deflections (machine or work), unbalanced grinding wheel, irregularities in tool feed, vibrations, chatter or extraneous influences.
- “The roughness and irregularities” which are left by machining (e.g. spark, cutting tool) as a part of the production process. This is as a result of the built of edge formation, tool tip irregularities are described with it.
- The form is the third component of the surface, which is left after elimination of waviness and roughness. An overview of those surface components with respect to vertical and lateral dimensions is shown in Figure 1. Roughness measurement can be seen in the bottom left part of this graph, where detailed information in vertical and lateral dimensions is required. This is a popular application in surface metrology.
Today, the most frequently employed technique to calculate surface roughness is still the application of stylus contact-based surface measurement instruments. Though stylus-type instruments are well-accepted throughout industry and suitable for various applications, problems can occur because of the mechanical contact with the sample, as instrument or surface damage may happen.
So, for applications where tactile methods can have challenges, e.g. risk of contamination, complex structures or recessed surfaces, non-contact optical instruments continue to evolve to better meet these ever-growing measurement needs.
Presently, it is common to measure as many parameters as possible with a single measurement system due to the advancements of optical surface metrology tools. One frequently used method to measure flatness together with roughness is the use of several objectives together with an xy stage (for stitching).
This method can be time-consuming and limited to the properties of chosen objectives (limited field of view, working distance, crash with a sample). So, most of the technical drawings need a profile-based 2D parameter set, for which areal information from a high magnified objective is not needed.
Another characteristic is the specified profile length within a technical drawing – in some instances when employing an optical instrument, the field of view of an objective is smaller than the profile length necessary to conform to international standards.
Zig-zag profiles may be drawn across the surface but due to either the nature of the roughness or the directionality of the surface, this is not always permitted by international standards because of higher levels of surface roughness or directional surface structures.
To overcome these constraints and supply further advantages, the Polytec TopMap family has been extended by a multi-sensor system which can be utilized easily, to measure form deviation plus roughness parameters all in a single measurement instrument setup.
Figure 1. Classification of surface components with respect to vertical and lateral dimensions.
Figure 2. Principle of scanning white-light interferometry (Michelson setup).
Multi-Sensor Concept of Polytec
The TopMap white light interferometers from Polytec are chiefly designed for bigger area measurements. For example, a single measurement volume (without stitching) of 30 mm x 40 mm x 70 mm (X x Y x Z) can be attained with nanometer vertical resolution.
An advantage of white light interferometry is the extremely high vertical resolution of the measurement system, which does not rely on the magnification of the objective. This makes it viable to resolve even large surfaces with extremely high resolutions in a very short space of time, whereas some methods (such as confocal or focus variation) must apply objectives with high magnification and combine many regions to cover the surface.
It is also worth noting that white light interferometers are excellent for measuring highly polished, smooth, and lapped surfaces, unlike the focus variation method which needs a surface to contain higher levels of image contrast. So, for applications where high lateral resolution is needed, the multi-sensor concept integrated with chromatic confocal probing was created.
Figure 3. Combining the advantages of both worlds, large areal measurements and roughness measurements.
As an optical technique, in terms of spot size and resolutions, chromatic confocal probing is an optical point-based sensor most like the stylus profilers. By scanning the point sensor with help of a laterally translated stage, surface data is gathered, just like the tactile techniques.
This configuration makes tracing complex shapes viable. Even when employed in conjunction with time-sensitive areal measurements, no vertical scanning unit is needed, which makes chromatic confocal technology static and with no moving parts in the optical head.
The form deviation on the whole surface can be characterized with the white light interferometer, then additional measurements with the chromatic confocal technology can be carried out to assess roughness for applications where numerous measurement parameters are required. The position, length and the shape of the profile can be easily chosen by the operator – similar to stylus measurements.
Measurements with TopMap Pro.Surf+
Even new optical techniques produce new opportunities and there is still a discussion on the comparison of results gathered by stylus versus optical instruments. Each data acquisition technique has its benefits and drawbacks based on the mechanical, optical, or electromagnetic characteristics of the workpiece, the end surface performance and functionality required, resulting surfaces are designed to be different from one another.
As the optical characteristics of the surfaces are not always identical to that of the mechanical characteristics, the comparison of different non-contact and contact measurements can clarify how these differences can impact the end numerical results.
Figure 4. Overview of the roughness standard Right above: Acquired 3D surface data (one single measurement) and 5 profiles on it.
To compare the results of Pro.Surf+ with the results gathered by tactile techniques, a roughness standard (Halle Standard, KNT 4058/01 class A) was measured for comparison purposes. The roughness standard made of hardened stainless steel of dimensions 40 mm x 20 mm x 11.3 mm was measured by Pro.Surf+, as shown in Figure 4. Five individual profiles are determined as measured by a contact stylus instrument.
The comparison of the results can be seen in Figure 5: The measurement results on the calibration protocol are Ra 0.197 µm Rz: 1.46 µm. Measurement results by Pro.Surf+: Ra 0.197 µm Rz: 1.43 µm. The measurement data of Pro.Surf+ is in good agreement with the reference profile of the stylus instrument. There is a slight difference between Rz values, although Ra values are identical.
Figure 5. Measured profile with Pro.Surf+ versus reference profile (tactile calibration measurement).
The definition of parameters can explain this: Ra relies on the average properties of surfaces, but Rz is calculated with minimum and maximum characteristics. The measured heights of tactile and optical instruments may be different, which can lead to slightly different measured height values in the peaks and valleys of the structured surface, as previously discussed.
Exploring New Possibilities
It is possible to upgrade the standard Pro.Surf to a Pro.Surf+ due to the modular concept of the TopMap family, and new capabilities with the roughness module of Pro.Surf+ can be observed in Figure 6.
Figure 6. Modular concept of TopMap Pro.Surf+
There is a massive demand to understand surface characteristics of products within industry quickly but with high levels of accuracy and a high degree of surface information because of the latest developments in manufacturing technologies. Also, it is not easy to identify a measurement method which fulfils all the requirements in a single measurement system.
Depending on the application, every technique has its advantages and disadvantages regardless of optical or tactile method. Because of new concepts such as “multi-sensor” methods, tools in optical surface metrology are enhanced to solve most of the needs simultaneously.
This is now viable using this novel combination, i.e. of large-area white light interferometry and combination chromatic confocal measurement technologies, particularly for applications where roughness and form must be measured within a single measurement cycle.
This information has been sourced, reviewed and adapted from materials provided by Polytec.
For more information on this source, please visit Polytec.