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DRON-3M diffractometer. Analysis of residual stresses was done on the same diffractometer, but Co K source with diffraction angle of 111º in plane {311} was used instead. sin2 method with = –30, –20, –10, 0, 10, 20,
30º was used for stress estimation. Material removal by electropolishing with step of 20 μm was applied in order to measure profiles of residual stresses. Scanning electron microscope HRSEM FEI Nova NanoLab 600 was used for inspection of wear morphology and focused ion beam milling of worn-out tools. Energy dispersive X-Ray analysis was performed with ISIS 300 Microanalysis System at 15 kV. Atomic force microscope AFM Dimension 3100 in tapping mode was applied for the study of topography of worn tools. MikroCAD 3-D system was used for evaluation of edge radius for new and worn tools. SEM imaging has revealed that PCBN tools have limited-to-moderate adhesion of workpiece material and thus the tools were etched prior 3-D optical microscopy and AFM, which is the common practice.
3. Results and discussion
3.1. Performance of uncoated and coated PCBN tools
Figure 2.a shows cutting forces for different tools in their unworn state. General tendency is that coated and uncoated PCBN tools give close to similar forces, yet coated tools have force level about 10 % higher. For the case of coated tools lower force level due to lower friction coefficient and higher cutting temperatures is expected. But in this study the observed opposing behavior can mostly be attributed to variations in tool microgeometry. 3-D optical measurements have revealed that uncoated tools have edge radius r =15-18 m, while for coated PCBN tools r =20-22 m.
(a) SM of microstructure and (b) XRD phase analysis of PCBN tool material
RNGN120300E25 inserts with honed edge radius were used throughout the tests, which along with corresponding toolholder provided 6 inclination and –6 rake angles. Cutting conditions were selected to cover finishing operations. Three selected speeds are vc=250, 300 and 350 m/min and three feed rates are f=0.1, 0.15 and 0.2 mm/rev. Depth of cut was fixed for all tests, equaling ap=0.3 mm. All tests were performed with use of Sitala D 201-03 (Shell) 8% semi-synthetic coolant supplied at 5 bar and 40 l/min.
Surface roughness, cutting forces, tool wear and wear morphology were analyzed for all tests. 9121 type Kistler dynamometer was used for recording forces. XRD analysis of tool material was done with Cu K source on
Comparison of cutting force components (vc=350 m/min, f=0.15 mm/rev); (b) surface roughness under the variation of feed
Similar effect of tool microgeometry was observed when studying the quality of machined surface. Surface
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roughness (see Figure 2.b) has higher values for coated tools as a result of increased edge radius. This effect is normally associated with minimum chip thickness h1min, when the tool ceases to remove material below a certain value of h1 leading to the plastic deformation of the being removed material. This effect becomes especially strong for round tools where h1min region falls on surface-generating part of the edge. For the selected tools feed has a strong effect on h1min by thinning the chip cross-section. Side-flow was observed on the machined surface at conditions of small feed which was a result of plastic deformation and flow of material located below h1min value. The most frequent occurrence of side flow was recorded for coated PCBN tools, leading to increase in roughness (Figure 2.b). This particular behavior can be attributed to workpiece material softening due to low thermal conductivity of TiN coating (28 W/m K) [9] and consuquent flow towards the minor cutting edge. For larger feeds roughness increases as expected from the viewpoint of process kinematics.