Manufacturing of structured surfaces via grinding
Final Report Abstract
In the scope of the research project, the possibilities to manufacture structured surfaces via grinding were investigated. A new and unique approach using a grinding wheel with defined grain pattern was applied and its suitability for the targeted application could be proven. By the use of a grinding wheel with defined grain pattern, the position of each grain on the grinding wheel was defined and hence known. In conjunction with the control of the kinematics, deterministic structures could be manufactured. The control of the kinematics was achieved by analytically describing the motions of the grinding wheel and the workpiece. The derived equations were implemented in a selfdeveloped MATLAB simulation tool. The grinding wheel can be modeled in the simulation; that is the characteristics of the grains (e.g. grain shape, grain dimensions) as well as of the grain pattern (e.g. distance between grain lines, inclination angle of grain lines). Deviations of the set values as well as of the workpiece (e.g. eveness) can also be implemented to closer match real process conditions. Using the simulation tool, resulting properties of the single scratches as well as the structure pattern (distance between the scratches in any direction) can be simulated in dependence of given parameters, or required kinematics to manufacture desired structures can be determined a priori. The manufactured structures are a composition of single grain scratches. The shapes of the single scratches are tied to the shape of the respective material removing grains. The grinding wheel grain pattern, in conjunction with the kinematics, defines the arrangement of the structures. With a specific grinding wheel grain pattern, the possible structures are hence limited. However, the possible variations of the structures are still manifold. The possible structures range from simple straight ones to very complex and dense ones. The manufactured structures show deviations in the uniformity and arrangement compared to the simulated, ideal structures. This can be led back to two main reasons. The first one is the difference in the grain protrusion of the grinding wheel. Although a very smooth touch dressing of the wheel was performed to gain uniform grain heights, a standard deviation of 3 µm of the grain protrusion could not be avoided. The second reason are deviations due to the material removal itself. Elastic and plastic deformations result into deviations in the achieved scratch depths in comparison to the set depth of cut. In addition, the material removal and the corresponding process forces result into deflections of the whole setup. Force measurements during structuring revealed normal forces up to 500 N. Despite all those influences, the manufactured structures were of high uniformity. The largest deviations with respect to ideal consumptions were the resulting lower depths of the scratches compared to the set depth of cut. The ratio of achieved depths to the set cutting depth is an indicator for the efficiency of the material removal process and is typically low in grinding. As a result of the ploughing of the grains, burrs are formed at the sides of the scratches. To further investigate the material removal efficiency and the burr formation, different materials were structured. Those investigations revealed that the softer the material, the higher the efficiency ratio, due to a lower yield point. However, with rising hardness of the material, a lower burr formation resulted. The hardest material investigated did not exhibit any burr formation. This is very favorable for the targeted application of the structures, as sliding components are commonly made of hard or hardened steel. In addition to the proof of the suitability of the researched approach to manufacture structured surfaces, the tribological performance of the manufactured structured was examined using a pin-on-disc tribometer. In doing so, the friction coefficient in dependence of the sliding velocity (the so called Stribeck curve) could be determined. In comparison to a flat, unstructured surface, structured surfaces exhibited better and worse tribological performance. This behavior is in accordance to what can be found in literature, where it is stated that the structuring of workpieces can result into reduced as well as increased friction. To a priori know what structures can achieve less friction and hence better tribological performance, further, e.g. simulation based investigations are necessary.
Publications
- Herstellung von mikrostrukturierten Oberflächen mittels Schleifen. ZWF - Zeitschrift für wirtschaftlichen Fabrikbetrieb 109/4 (2014): S. 242-246
B. Kirsch, J.C. Aurich
(See online at https://doi.org/10.3139/104.111124) - Manufacturing of structured surfaces via grinding. Proceedings from the CIRP Research Affiliate Workshop (2014): S. 3-10
J.C. Aurich, B. Kirsch
