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Fig. 2 Influence of process parameters on surface roughness
In order to describe the effects of surface roughness regarding tribological effects, the material ratio curve (Abott curve) is used. The roughness values from the Abott curves (Rk, Rpk and Rvk) are mainly used by bearing manufactures to describe the tribological effects within the bearing. Conventional parameters as Rz or Ra are not suitable for that. Micro contacts between roughness peaks, described by Rpk, or micro dimples for lubrication support (Rvk) can be illustrated by the Abott curve. The Abott curves corresponding to the results above, are depicted in Fig. 3. A very smooth surface is represented by a very flat Abott curve with small inclinations. Typical turned surfaces have got large inclination within the curve. Honed surfaces are very flat with a large depth of profile at the beginning (0–20 %). . The curves in Fig. 3 are based on 3D optical measurements. From the Abott curves the surface roughness values, core roughness Rk, peak height Rpk and depth of valley Rvk, can be identified. As seen from Fig. 2 the most important process parameters are feed and cutting edge radius. Feed also increases the core roughness Rk comparable to Rz. This leads to a higher inclination angle of the Abott curve. More important than feed for the functional roughness parameters is the cutting edge radius. Fig. 3 depicts the Abott curves for five different cutting edge radii and constant machining parameter vc = 200m/min and f =0.07mm. An increase radius from rβ=40 to 70μm results in a flatter curve. The core roughness decreases, as well as the peak height. If rβincreases more, the curves get shifted to higher depth of profiles with slight higher inclination angles. To machine smooth surfaces, the minimum uncut chip thickness should be chosen close to 0μm, according to the theoretical roughness model by Brammertz [14]. This can be realized by sharp cutting edges. Due to the fact that large cutting edge radii also increase the tool life [16], feed and cutting edge radius have to be adjusted to each other. For the presented machining operation a cutting edge radius of rβ=70μm and a feed of f = 0.07mm represent optimal settings.
Fig. 3 Influence of process parameters on the Abott curves
The second surface integrity parameter determining the endurance of roller bearings is residual stress. Experiments demonstrate that large cutting edge radii are useful to create large compressive residual stresses within the near surface area. The cutting edge radius is the most significant among the analyzed parameters. The residual stress depth profiles are shown in Fig. 4. The maximum compressive stresses for peripheral and axial direction are comparable as common in turning processes. At the surface the experiments show slight tensile stresses in peripheral direction. With an increasing cutting edge radius the maximum compressive stresses raise, as well as the affected surface area. The compressive stresses increase for Smax,peri= 570MPa (rβ=40μm) to Smax,peri= 1050MPa (rβ=105μm). These correlate very well with the resulting cutting forces. Whereas, the most significant force component is the passive force, which is affected mainly by the cutting edge radius. Therefore, hard turning with a large cutting edge radius causes high passive forces, which lead to high mechanical loads on the subsurface area. These loads lead to high compressive stresses. To increase the endurance of roller bearings, a compromise between surface roughness and compressive stresses has to be found.
Fig. 4 Effect of cutting edge radius on residual stress profiles
In hard turning operations often so called white layers occurs. In general these layers are below 2μm and occur due to large thermal and mechanical loads. White layers are hard areas, followed by a softer part of the material. The experimental results show an increase of the surface hardness from 720HV0.025 ((rβ=50μm) to almost 900HV0.025 for a large cutting edge radius. Again the radius is the most significant parameter affecting the surface hardness compared to cutting speed and feed. Figure 5 illustrates the surface hardness measured in radial direction as well as the depth profile determined in cross sections. The hardness profiles illustrate, how the hardness value at the surface increases with a larger cutting edge radius. At the same time the area beneath the surface also gets softer due to the effect of tempering. Also the affected area increases with higher cutting edge radii.