Correlation of 2D and 3D surface roughness measurements - Part 3 - Other surface dimensions

Advantages of other surface measurements

Based on the previous investigations, it could now be concluded that the area measurements do not offer any particular advantages, as their detection is somewhat more complicated and the devices are more expensive or the existing stylus devices cannot detect them and would have to be replaced. On the one hand, however, this can be countered by the fact that the measurement of areas offers much greater reliability than simply measuring a randomly selected line across an area. On the other hand, the metrological detection of the selected surface segment allows further evaluations that are not possible with a stylus section and can be very important for the characteristics and behavior of a surface in use.

A few of these additional possibilities will now be presented. First of all, there is the precise description of the directional characteristics via the autocorrelation length_Sal to the texture aspect ratio or the isotropy coefficient_Str and the texture direction_Std. With these measures, the direction of the grooves of a surface can be determined and evaluated numerically in terms of their shape and angle. A precise description of these dimensions is given in DIN EN ISO 25178, Part 2, whereby the draft from 2020 [1] offers better image quality than the predecessor document from 2012.

Fig. 36: Surface of a vibratory ground workpiece; image with Keyence VR-5000 profilometer with lines of a 2D multiple measurement (explanation in the text) For example, the surface in Figures 20 and 22 has an isotropy coefficient of_Str = 0.134. This coefficient can be between 0 and 1, where 0 means that the surface has completely uniform grooves (seen in the longitudinal direction) in only one direction and 1 describes a surface that is absolutely identical in all directions, e.g. as produced by spark erosion. This means that the aforementioned surface with a value significantly below 0.5 can be clearly described by one direction, whereby in this case the angle to the x-axis is_Std = 91.7°. In contrast, a surface of the same material produced by mass finishing with vitrified bonded abrasives of medium abrasive effect (Fig. 36) has a value of_Str > 0.3, i.e. still an orientation in a specific direction, which can be explained by the manufacturing process but could not have been determined by purely visual assessment of the surface. The direction indicated here is close to 0° or 180°. The 2D multiple line measurement shown in Figure 36 is another option for the optical detection of surfaces, which, incidentally, is significantly faster with a profilometer than with a white light interferometer, but is limited in its area of application (e.g. no very smooth surfaces). Once the surface has been recorded and called up in the evaluation software, the number and distance of adjacent measurement lines can be specified with a mouse click and the result along the selected center line (red) can be output in a table of measured values including scatter and standard deviation.

The significance of the various volumes is similar to that of a two-dimensional load-bearing component and, since they affect the entire surface under consideration, they also have a significance for the function of the surface that goes far beyond the statements of 2D measurements. For example, _Vmp (Fig. 37) denotes the volume of the uppermost peaks, which goes beyond the level of the section line _c1 and which should have no significance for the function of a surface, e.g. as a storage area. Similarly, _Vvv below the intersection line _c2 denotes the lowest depths of the troughs, which should have little significance as a lubricant depot. The important volumes of the load-bearing mounds without peaks and the troughs between the two cutting lines that can be used as lubricant depots are labeled _Vmc and _Vvc.

Fig. 37: Labeling of the volumes, based on cutting lines c1 and c2, similar to the definition of the dimensions Rk etc. according to DIN EN ISO 13565; according to [1] A comparison of two surfaces produced by belt grinding and vibratory grinding (Fig. 20 and 36), it can be seen that the volume of the peaks with a contact ratio of just under 2.6 % (analogous to the p =_Smr1 = 10 % in Fig. 37) for the mass finishing part and just under 10 % for the belt grinding part is close together in both cases at 0.06 and 0.05 ^ml/m2 respectively (Table 3). This means that the peaks play a lesser role in the mass finishing part than in the belt grinding part. The volume of the deepest troughs _Vvv is also similar for both surfaces with comparable contact ratios_Smr2 (analogous to q =_Smr2 = 80 % in Fig. 37). This shows that the volume of the load-bearing mounds _Vmc as well as the volume as a lubricant depot _Vvc is greater with vibratory finishing than with the belt-ground surface, despite the lower roughness _Rz = 2.9 to 5.8 µm (belt grinding). This is due to the many small grinding marks of the abrasive grains, which act in different directions, in contrast to the abrasive grains of the abrasive belt, which leave larger surface areas between the individual grooves running in a specific direction (see e.g. Fig. 28). This comparison would probably not have been possible when looking at the profile contact area curves, as the curve of the vibratory grinding part is flatter than that of the belt grinding part. However, at a cutting depth of c = 5 µm, the profile contact ratio of the mass finishing part is almost twice as high as that of the belt grinding part at _Rmr = 27 %.

Finally, the developed transition surface ratio _Sdr should be explained. If an isolated roughness hill is regarded as a spherical section, this is the ratio of the lateral surface of this spherical section to its base surface (Fig. 38). As can easily be seen from the sketch, the surface area of the upper part of the sphere is larger than the base area at a depth h calculated from the uppermost point. The difference between these values can be expressed as a percentage and the value is equal to zero if the lateral surface were the same size as the base surface, i.e. if there were no section of the sphere. Any lateral surface that is larger than the base surface has a value greater than zero, all the more so as the height of the hill increases and the extent of the hill decreases.Applied to a real rough surface, this means that it is regarded as a collection of any number of small hills and this measure can be used to differentiate between different surfaces. For example, the value for the vibratory ground surface is _Sdr = 0.012 % and that of the belt-ground surface is _Sdr = 0.034 %, i.e. almost three times as much. This is of course in line with the greater roughness Rz (see Table 3), but is apparently in contrast to the volume values. From these considerations it can be seen that, depending on the subsequent use of the manufactured surfaces, different roughness parameters must be used and evaluated in order to "tailor" a surface to these requirements.

Conclusion:

In addition to the established and widely used roughness measurements from scanning with tactile measuring devices, there have been sufficient principles and standards for a few years now to be able to reliably measure, evaluate and interpret surface roughness parameters. It may be that simple values from 2D scanning are sufficient for checking ongoing large-scale production if the surface has already been adapted to the production process and subsequent use and optimized with additional parameters as described above. However, it is of the utmost importance that the possibilities of modern measuring methods are used more and more for the development of durable and reliably functioning components in order to avoid faulty developments and product failures. In individual cases, this is already being practiced successfully, especially in research facilities, but a more widespread use of such measurements and their application would offer greater security for today's production or manufacturing facilities of companies and also improve communication in design, in communication with suppliers or in the case of expert opinions.

Literature

[1] E DIN EN ISO 25178-2: Geometric product specification (GPS) - Surface finish: Planar - Part 2: Terms, definitions and surface characteristics, Beuth-Verlag, Berlin, 2020
[2] Wiehr, C.: Anwenderunterstützung bei der Nutzung und Überprüfung von optischen 3D-Oberflächenmessgeräten. Dissertation, Technical University of Kaiserslautern, 2019
[3] Ströer, F.; Seewig, J.; Depiereux, F.: Roughness measurement tactile or optical? Comparable results. QZ 59(2014)5, 70-72
[4] Plein, C.: Comparative surface topography. Student research project. Duale Hochschule, Lörrach, 2018
[5] Schorr, D.: Measurement of the Steinbeis Transfer Center Tribology in Application and Practice, Karlsruhe, 2018
[6] https://commons.wikimedia.org/w/index.php?curid=44998701