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Extensive research on the effect of hard turning process parameters on the surface integrity has been carried out in the past. However, it mainly focused on single aspects of surface integrity, such as residual stress, white layer or surface roughness. Following, the common opinion on surface integrity created by hard turning processes is discussed.
The combination of feed f and cutting edge radius rb affects the resulting surface quality significantly [11]. The main effect on surface roughness has feed [12]. With feed values of less than f＜0.1mm roughness values of Rz＜1μm can be achieved on high precision lathes [13]. The theoretical surface roughness can be described for conventional turning processes depending on feed and corner radius rε. The difference between the real surface roughness and the theoretical roughness increases for small feed values [14]. In this case, the effect of the minimum uncut chip thickness hminbecomes more important, so that surface roughness increases [2]. Due to the ploughing effect described by Albrecht [15, 16], material is pressed underneath the cutting edge instead of being cutted out of the surface as a chip. If the uncut chip thickness is equal to the separation point height in the ploughing effect, the minimum uncut chip thickness is reached. An increasing cutting edge radius leads to a changing of the separation point and so the minimum uncut chip thickness increases [16]. From that aspect, the influence of the cutting edge radius
Fig. 1 Surface integrity affecting the roller bearings endurance
becomes significant for the resulting surface quality. For hard turning processes the minimum uncut chip thickness is very close to the occurring chip thicknesses. Therefore, the cutting edge radius has to be considered more closely in machining of hardened parts [17]. Large cutting edge radii (rβ＞ 50μm) smoothen the surface and produce better surface qualities for high feed values of f ＞0.1mm . In case of small feeds, the effects reverses and large cutting edge radii lead to a decreasing surface quality [18]. Cutting speed does not affect the residual stress within a common process window [19, 20]. An increasing feed leads to higher maximum compressive stresses, however, in axial direction the surface residual stresses shifts towards tensile stress [20, 21]. Similar to the feed, an increasing cutting edge radius also leads to a higher maximum compressive stress in axial and circumferential direction [18, 20, 21]. The microstructure can also be affected by the cutting edge radius. An increasing cutting edge radius leads to an increasing white layer thickness [20, 21] because of the increasing temperature within the contact area [22]. Due to the different effects of machining parameters on perse surface integrity aspects, it is almost impossible to choose the optimal process parameters to increase the endurance of roller bearings. A smooth surface roughness, high maximum compressive stresses and the absence of white layers are important values to increase the performance of roller bearings (Fig. 1). These three key properties are significantly affected by the cutting edge radius, however, in a conflictive direction. Previous research did not consider how the cutting edge radius affects the surface integrity and how it interacts with cutting speed and feed with respect to increase roller bearings endurance. Presented results often focus on single characteristic variables of surface integrity. Often the researchers do not use comparable cutting conditions, as cutting material, coating, heat treatment of the bearings or cutting edge preparation, which makes it impossible to predict the effects of machining parameters on the surface integrity. Therefore, this research focuses on the interaction of process parameters and surface integrity to increase the endurance of roller bearings by hard turning.
2 Experimental setup
The effects of process parameters and cutting edge geometry on the surface integrity of hard turned roller bearings are analyzed within this paper. Inner rings of roller bearings type 206NU are finished on a high precision lathe Hembrug Slantbed Microturn 100. The material of the bearings is AISI 52100 with a hardness of 62 HRC. The material properties are given in Table 1. To machine the rings with a very high accuracy, a clamping with a hydraulic expansion mandrel is applied. This clamping strategy provides an equal deformation around the whole part.