Microscopic processes can, however, also be used to make quantitative statements on the state of the grinding wheel effective area. For this purpose, grains or different wear characteristics were statically counted [Buettner 1968, Bohlheim 1995]. The measurements are either carried out directly on the grinding wheel surface or indirectly on a print of the surface.
3.3.6 Piezo and Thermoelectric Measurements
Piezoelectric and thermoelectric processes for the determination of the active cutting edge number are based on the measurement of force signals or temperature peaks of single active cutting edges [Daude 1966, Kaiser 1977, Shaw and Komanduri 1977, Verkerk 1977, Damlos 1985]. The piezoelectric process is only applicable for small contact surfaces, since it has to be ensured that no more than one cutting edge is engaged in the contact zone at any time. Small samples of a width of circa 0.3 mm [Kaiser 1977] are ground with relatively small feed. In the thermoelectric process, a thermocouple wire of a small diameter is divided by a thin insulating layer from the surrounding material within the workpiece. Every active cutting edge or bond ridge destroys the insulating layer and creates a thermocouple junction through plastic deformation. This leads to the emission of a measurable thermoelectric voltage signal from which a corresponding temperature can be established. Therefore, the material has to be electrically conductive and sufficiently ductile. Hence, this process cannot be applied to ceramics [Shaw and Komanduri 1977]. A contact resistance measurement can be applied for diamond and cubic boron nitride (CBN) grinding wheels with an electrically conducting bond for the purpose of distinguishing between active cutting edges and active bond ridges [Kaiser 1977].
3.3.7 Photoelectric Method
Photoelectric methods according to the scattered light principle are based on light reflected by the cutting area and collected using a radiation detector. In this method, the scattered light distribution and the duration and number of light impulses in the direction of the regular direct reflection are evaluated [Werner 1994].