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3.1. Orthogonal machining tests
Orthogonal cutting experiments were performed for each insert set. Orthogonal conditions were assured by creating 1.2 mm width disks on a rigid cylindrical workpiece. Experiments were conducted at 70 m/min cutting speed and feed rates of 0.2– 0.275–0.35 mm/rev. In the orthogonal machining tests, it was observed that there was no major difference in the friction angle which was around 198. The results were also showed that there is no significant difference between shear stress values (Table 2).
3.2. Face milling tests
A face mill tool with a diameter of 80 mm which can hold the SPKN type inserts was used. In the experiments a three component dynamometer was used for collecting force data. A single insert for each test was attached to the face mill holder for determining cutting forces acting on the insert. For a cutting depth of 0.5 mm, slot cutting operations were performed for all four sets of plasma boronized inserts and nonboronized inserts.
S. Basturk et al. / CIRP Annals - Manufacturing Technology 59 (2010) 101–104 103
Fig. 3. SEM image of an insert cross-section, (a) SEM Linescan element analysis of insert 4 across the line shown with arrowhead. Illustration of element analysis, (b) results of SEM linescan element analysis for boron particles.
Table 2
Experimental results of orthogonal and oblique machining.
Insert set Orthogonal tests Face milling tests Wear tests
Shear stress (MPa) Average reduction in maximum Test termination VBa (mm)
peaks of resultant force magnitudes (%) time (min)
1 310 – 21 303
2 311 21.3 63 310
3 420 13.6 39 334
4 328 17.5 51 364
5 300 16.2 51 382
a Listed VB values are average values of four repetitions of wear tests at the corresponding wear test termination time.
Experiments were performed at cutting speeds of 50 m/min, 88 m/min, 125 m/min, and 163 m/min. Feed rates were selected as 10–20–30–40 mm/min for all cutting speeds and inserts were compared according to the resultant force data obtained from slot cutting operations. All of the plasma boronized inserts were compared to the reference nonboronized insert set 1. In Fig. 4, a comparison of plasma boronized insert set 4 with nonboronized insert set 1 is given; it is observed that resultant forces are lower for all cutting conditions. The average percentage reductions in resultant forces were calculated by taking the entire data into account and these values were listed in Table 2. According to face milling test results, in comparison to reference nonboronized insert set 1, resultant forces were reduced in the machining of the plasma boronized inserts. Insert set 2 provided the best
Fig. 4. Resultant forces in face milling with nonboronized (insert 1) and plasma boronized (insert 4) tools at various cutting speeds and feed rates.
performance in face milling tests with 21.3% reduction in resultant forces.
3.3. Wear tests
Slot cutting operations were performed for the wear tests for each insert. Cutting speed was set to 70 m/min as in orthogonal experiments, depth of cut is chosen to be 0.2 mm and feed per tooth is 0.2 mm. Wear tests were stopped at certain periods (0.25– 0.5–1–1.5–2–3–4.5–6–9–15–21–27–33–39–51–63 min) and inserts were examined under an optical microscope. Rake and clearance faces were examined and flank wear (VB) was measured. When VB achieved a distinctive value (300 mm) with respect to rest of the group, tests were terminated for a specific insert set. Tests were conducted until 21 min for nonboronized insert set 1, 39 min for inserts of set 3, 51 min for inserts of set 4 and 5. Maximum tool life was observed for inserts of set 2 with 63 min. VB values were listed in Table 2. When the test results were examined inserts of set 4 and set 5 demonstrated almost twice longer tool life than nonboronized inserts of set 1. Inserts of set 2 demonstrated three times longer tool life in comparison to nonboronized insert set 1. In Fig. 6 flank wear evolution of inserts can be seen. Nonboronized insert set 1 in comparison to plasma boronized inserts of all sets worn more rapidly. At 15 min nonboronized inserts of set 1 had a VB value of 252 mm where it was 112 mm for inserts of set 2. As the wear test continued for 21 min VB value for inserts of set 1 reached the value of 303 mm and inserts set 2 had the VB value of 128 mm. After 39 min of machining, inserts of set 2 still had the VB value 202 mm. Tool conditions after the wear test termination for each tool are shown in Fig. 5. In conclusion, plasma boronizing at 600 8C for 6 h increased the tool life almost triple.