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Table 1 Material properties AISI 52100
Multiple Ti (C, N) and Al2O3 coated DNMA150616 cemented carbide cutting inserts are used. For the coating a CVD process is applied. The tool holder DDJNL2020 creates a nominal rake angle of γ=-6°, a clearance angle of α=6°and a tool cutting edge angle of χ=93°. Due to the combination of a nose radius of r ε= 1.6mm and a depth of cut of ap=0.1mm very low surface roughness values are achieved. Within the experiments the cutting speed vc, feed f and cutting edge radius rβare varied (Table 2). The cutting speed and feed are chosen from conventional cutting parameters for hard turning mentioned within the literature. The literature also mentions large cutting edge radii to be positive to induce high compressive stresses, therefore cutting edge radii of 40μm or greater are chosen for the experiments. The latter is limited to symmetric cutting edges K =1. The cutting edge radius was produced by a brushing operation. Each cutting edge rounding was measured after the coating process using a GFM MicroCAD. Due to the fluctuation of the brushing process, cutting edge radii with a difference of ±5% are used for the experiments. To minimize the influence of tool wear on the experimental results a new cutting edge is used for each parameter combination. During the experiment the cutting forces are measured with a three-components dynamometer Kistler type 9121. Dry cutting is applied.
The analysis of the surface integrity is done on several measurement devices. To detect the effect on surface roughness, the inner rings are analyzed tactile at five different positions, using a Mahr profilometer. Additional optical measurements are carried out using a confocal Nanofocus Microscope μSurf once per workpiece. Following DIN EN ISO 4287, in both cases a measurement length of 5.6 mm, a Gaussian filter and a cut-off λ=0.8mm is used. The measurements are applied in feed direction. The tactile measurement is used to create standardized values according to DIN EN ISO 4287. To analyze the material ratio curve more information about a surface area can be measured by optical surface measurements. Residual stress measurements are carried out using a X-ray diffractometer GE XRD 3000 P with Cu-Kα-radiation (35 kV and 30 mA). After the analysis of the residual stress the microstructure of the near surface area is characterized and micro hardness measurements are conducted. The microstructure is analyzed within the specimen of the bearing. The cross-section area is etched and analyzed with SEM. Micro hardness measurements are also applied in the cross-section with Vickers hardness method. All experiments are repeated one time.
3 Results and discussion
Roller bearings are highly loaded parts, which require a very low surface roughness. Tactile surface roughness measurements show that feed and cutting edge radius are the two main factors for surface quality. As known from literature, higher feed values increase the surface roughness due to larger feed marks. With a decreasing feed, the influence of cutting edge radius increases. First the ratio between cutting edge radius and the uncut chip thickness gets improved to create better surface roughness values. At a minimum point the geometric influence of feed and tool nose radius gets dominant and surface roughness increases. For cutting edge radius rβ=70μm within the conducted experiments this effect can be shown, by the surface roughness Rz decreases for feeds f ＜0.07mm and increases for higher feed values (Fig. 2). Following [14], this can be explained by an uncut chip thickness below the minimum uncut chip thickness. For large cutting edge radii the same effect occurs, whereas the minimum uncut chip thickness and therefore the critical feed increases as well. From the experiments it is not possible to identify the uncut chip thickness. However, the literature describes an increasing hmin with larger cutting edge radii. In Fig. 2 this effect is plotted by the grey lines. It can be seen, that the theoretical roughness, calculated by Brammertz [14], changes because of the increasing minimum uncut chip thickness. A further increase of the feed leads to an increase of the surface roughness. The results demonstrate a strong interaction between feed and cutting edge radius. On the other hand, cutting speed does not affect the surface roughness significantly. For a cutting speed of vc=100m/min the surface roughness changes from Rz =1.32 to Rz =1.74μm for vc= 300m/min (f = 0.07mm;rβ =70μm).