Dressing with Small Depths of Dressing Cut — Touch Dressing
Grinding wheel topography influences the quality of the ground workpiece surface to a major extent. For example, individual grits projecting from the envelope of the grinding wheel bond lead to high workpiece roughness in the ground component. Such conditions can be observed especially in the case of single-layered, galvanically bonded cBN grinding wheels. The reason for this is to be found in a distribution of the selected grain size for the grinding wheel layer that is accept
able in certain areas. For example, according to the standard of the European Association of Abrasive Manufacturers (the FEPA standard), about ten percent cBN crystals with an average grain diameter of up to 227 pm may be contained in the standard granulation B 151.
Such a grain size distribution leads to protrusion of individual cBN grains from the grain bond in the manufacture of single-layered, galvanically bonded cBN grinding wheels. These cBN grains form kinematic cutting edges during the grinding process and impress themselves correspondingly on the workpiece surface, causing a large amount of workpiece roughness which often does not correspond to the desired surface roughness.
Dressing with very small depths of dressing cut, so-called touch dressing, makes it possible to level in particular grits lying exposed on the grinding wheel layer periphery. In this way we can obtain a workpiece roughness which is reduced according to the amount of dressing [HEUE92, STUC88, TREF94]. In order to exert the strongest possible normal force on the grain, so that it splinters, one often works with velocity relationships of qd ~ 1. Here, the individual depth of dressing cut is customarily in the range of 1 to 3 pm.
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projection of all static cutting edges of a galvanic bonded grinding wheel to a steel plate
Fig. 6-13 illustrates the principle of dressing with small depths of dressing cut with the help of a representation of the grinding wheel topography in a small steel plate. This method presumes an exact knowledge of the position of the dressing
tool with respect to the grinding wheel surface. For this purpose, so-called first contact control systems have been tried and tested, which can detect with high precision the first contact between a dressing tool and grinding wheel with the help of Acoustic Emission.
After dressing the exposed grits of galvanically bonded grinding wheels, we obtain workpiece roughness values that are essentially stable, even with increasing specific material removal. The reason for this is in the high grain holding forces of the galvanic bond, by means of which only a minimal amount of grain breakaway from the grinding wheel layer takes place. With optimally designed grinding processes, single-layered grinding wheels wear out primarily from the formation of wear surfaces. As long as the grain protrusion is sufficient and the frictional heat produced does not lead to impermissible influence of the workpiece, the grinding wheels can be used; the surface quality improves and asymptotically approaches a boundary value at a constant specific material removal rate Q’w.
For multi-layered superabrasive grinding wheels, a combination of normal dressing in the common sense with a collectively higher dressing amount and dressing with small cutting depths can be sensible (Fig. 6-14). Profiling with larger depths of dressing cut can become necessary, for example in the initial profiling of a grinding wheel, or if, within a machining cycle, a larger grinding wheel profile loss is allowed while high demands are placed on workpiece roughness. This kind of dressing can cause chip space reduction. In this way, an additional sharpening process becomes necessary.
The second strategy of dressing with small cutting depths has the advantage that the chip space of superabrasive grinding wheels are not appreciably altered, so that, after the dressing process, additional sharpening of the grinding wheel layer is superfluous. Dressing with small depths leads to an improvement in workpiece roughness, which can be held within tolerant levels under frequent use. This is represented in Fig. 6-14, bottom. The duration between two successive dressing cycles depends on the tolerance width of the required workpiece roughness as well as the wear behaviour of the grinding wheel. In general, each dressing cycle take place in quicker succession in comparison with dressing with larger dressing amounts.