In this chapter the change of surface parameters over the grinding experiments is examined and the suitability of the measurement system for the wear evaluation is shown. Thus, the process of flat grinding is used to generate wear at the grinding wheels. The process parameter to be observed is the cutting volume Vw and the related cutting volume V`w.
For the evaluation, the metallic bond is now considered, which is present with diamond grains in different grain sizes (54 µm, 64 µm, 76µm and 91 µm). For the comparability of the surfaces, a characteristic grain arrangement is defined and tracked, which is marked in the illustrations (Figure 5, 6, 7 and 8). This is necessary to ensure an evaluation of the topography compared from one state to the following. This is not necessary for a complete grinding wheel surface evaluation but for surface portions. In this way, areas can be tracked purely optically and local differences can be seen. Moreover, the parameters of the topography are calculated for the complete grinding wheel. A grain rounding can be demonstrated by a decrease in the peak values of the topographies over the course of Vw while the valley structure remains the same (Figure 5). The absolute peak values, which can be viewed using the Sxp value, steadily decrease, which confirms the wear. The change in topography can also be supplemented by an Abbot curve. The slope of the graph at the point of inflection decreases with the main wear of the grain height. The first derivative of the Abbot curve at the inflection point thus forms a decreasing maximum. The decrease in height that occurs here is approximately 50 µm. Other wear mechanisms cannot be clearly identified in this attempts. With this series of measurements, the first wear mechanism can be detected by the developed system.
The wear mechanism of clogging leads to higher peak values in the topography. The valley structure does not remain constant as the material is adhered in valleys of the bond system. The density of valley decreases over the increase of Vw (Figure 6). Here pore spaces are clogged and grains appear taller despite their wear. Moreover, there is a simple method of counting the pores of the topography. If their number decreases some have been clogged. Furthermore, in figure 6 it can be seen that the grain structures remain the same while the valley structures change. With this wear mechanism, the valley structure becomes flatter and the grains appear larger due to the colour resolution. Here, the severity and frequency of the white peaks increases. Despite the lack of distinction between different materials due to the selected measuring method, these mechanisms can be measured with the use of the developed system.
Of course multiple wear mechanism may occur at the same time (Figure 7). Wear is characterized by locally different characteristics. Rather the progress of wear is different due to the different grain structures and the force load of the process. Thus, Figure 7 shows how clogging and grain wear increase at the same time but with different intensity. Grains get smaller locally while the valley structure remains the same. On the other hand, valleys and pores become smaller or completely flat while the grains appear larger (white) due to the colour representation. The grinding wheel is globally worn. Further investigation to increase the wear caused an increase in Vw. This leads to stronger characteristics of the wear phenomena. This ends in overarching clogging, extensive grain breakouts and a complete flattening of grain structures (Figure 8). It can be stated that 3D topographies are basically suitable for the detection of grinding wheel wear and that the measuring system used provides topographies that can be evaluated with repeatable accuracy. The next step is to examine the course of the characteristic values and to prove their correlation with the existing wear.
Thus the different stated surface parameters from the last chapter will be examined. A metal bonding with dg = 76 µm is considered first (Figure 9). The technological consideration represents that the value Spk, which represents the average height of the grains in the overall topography, should decrease with the increasing tool wear and the grain rounding as well as grain breakouts. This time course is confirmed for the examinations with the grinding wheel. The value of Svk represents the pores and chip spaces. Beside a constant change in grain protrusion, fluctuations are to be expected here due to clogging. For this attempts the expected development can be detected (Figure 9, top left). Equivalent attempts were made for all other grinding wheels. For comparison, the metallic bond with a grain size of dg = 64 µm is considered (Figure 10). The predicted decrease in Spk and the increase in Svk can also be detected here. Technologically, this can be derived and is supported by this result (Figure 10, top left). The value Vvv is now considered further. This represents the volume of the pores and chip spaces. If this value rises above average, there is an increase in volume. Technologically, this can be explained by several grain breakouts or the breakout of the bond. These fluctuations can be detected (Figure 9 and 10, top right). However, implementation in an automated evaluation algorithm makes this more difficult. Thus, this value is classified as less suitable. On the other hand, the opposite value Vmp for the volume of the grains shows the expected decrease over the tool wear. The increasing removal and the rounding of the grains ensure a steady decrease in their volumes. The characteristic value evolves in the same way for all measured grinding wheels (Figure 9 and 10, top right). Moreover, in further investigations limit values can be set in order to determine a maximum of grain rounding. The average rounding of the grains is now examined. Thus, the value Spc is detected with increasing tool wear. The peaks of the grain should decrease. Equivalent to other research results there will be less and flatter grains by increasing tool wear. In this case the value will increase until the tool is severely worn. Fluctuations of this value can be explained by grain breakouts, as these can lead to valleys with irregular sharp edges. The steady increase of this value can be seen for the different grinding wheels used (Figure 9 and 10, bottom left). As already noted with the value Spc, the increasing wear of the tools leads to flatter but also less grains. Grain composites are only detected as individual grains and grain break out. This means that the grain density should decrease with wear. This development can be detected for Spd, which represents the density of grains (Figure 9 and 10, bottom left). Fluctuations for this characteristic value occur as well as for Spc and can be explained equally.
The last characteristic values to be examined are represented by Smr1 and Smr2. These represent the material proportion of the peaks and valleys. This is equivalent to the grains on the one hand and to the pores and chip spaces on the other hand. With increasing tool wear, the grains round or break out. This consequently leads to a decrease in material in this area. The material proportion of the valleys is a mathematically inverted value, since this material proportion reflects cavities. These spaces become smaller due to clogging. This leads to a decrease in the proportion of material in the overall grinding wheel topography. From the technological point of view, this means a decrease in both material proportion parameters Smr1 and Smr2. The amount of decrease is a measure of the progressive wear of the tool. This course can be confirmed for the investigations under consideration (Figure 9 and 10, bottom right). The grinding wheels clog and flat areas arise. The development from a grinding process to a friction-like process can be seen. This depends on the increased proportions of friction, which are increasing evenly with the tool wear. This can be further explained by considering the detected process forces (Figure 11).
The increasing tool wear considered above leads to a flat grinding wheel without chip spaces, pores or sharp grains. This development can be seen as a grinding process with increased proportions of friction. This ends with a lower roughness at all but results in increasing grinding forces. Thus, the normal and tangential force have been detected over the hole grinding attempts. The theory states a constant grinding force ratio µ by increasing forces for Ft and Fn. This have been detected for the examined grinding wheels with metal bond (Figure 11, left dg = 76 µm, right dg = 64 µm).
In summary, the developed measuring system is able to measure wear related topography changes with repeatable accuracy within a machine tool. Of the characteristic values considered, all are suitable for evaluating specific tool wear except for Vvv. Overall, the wear characteristics can be sufficiently differentiated. In addition, the changes are large and significant enough for further investigations. This significance of the characteristic values makes it possible to develop and implement algorithms for process control in further steps. A differentiated elaboration using algorithms, divided according to possible process steps like grinding, dressing and sharpening, must be developed.