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Influence of the number of inserts for tool life evaluation in face milling of steels
International Journal of Machine Tools & Manufacture 44 (2004) 695–700 www.elsevier.com/locate/ijmactool
Influence of the number of inserts for tool life evaluation in face milling of steels ´ .R. Machado a, M.B. Da Silva a, E.O. Ezugwu b,, J. Bonney b A. Richetti a, A a
Machining Research and Education Laboratory, Mechanical Engineering Faculty, Federal University of Uberlaˆndia, Uberlaˆndia, MG, Brazil b Machining Research Centre, Faculty of Engineering, Science and Technology, South Bank University, 103 Borough Road, London SE1 0AA, UK Received 28 January 2003; accepted 5 February 2004
Abstract Tool life tests are often employed to verify the behaviour of one or more inserts in a cutter in order to optimise machining productivity and minimise cost. In milling process, such tests are expensive and require many of tools and a lot of work material to achieve any of the stipulated tool rejection criterion in any of the inserts. In practice, tool life tests are usually carried out using only one or few edges in a face milling cutter in order to minimise cost. The aim of this study is to investigate the effect of the number of tools used in face milling operation and how they relate to the establishment of tool life under specified cutting conditions. Flank wear curves were evaluated for AISI 1045 and 8640 steels using 1, 2, 3 and 6 inserts in a face milling cutter. Test results show that reduction in the number of inserts in the milling cutter led to a reduction in the amount of material removed and also tend to increase tool life when machining at the same feed per tooth. Results obtained using reduced number of inserts in a milling cutter should only be used for comparison between two or more conditions and should not be used to establish tool life. # 2004 Elsevier Ltd. All rights reserved. Keywords: Machinability tests; Face milling; Tool life; Cutting temperature; Volume of material removed
1. Introduction The best machining conditions depend on the cutting tool, workpiece, machine tool, cutting fluids, and cutting parameters, thus machinability trials are therefore essential. This selection process is justified by the problem encountered in choosing from a large number of commercially available tools. Recommendations from manufacturers should only be used as a guide since better conditions may be found for other tools and cutting parameters. Machinability trials on each application are of major importance due to the economic benefits to be gained by manufacturing industries that carry out large amount of machining operations [1]. The use of indexable inserts in face milling operation is now very common. Commercially available milling cutters are variable and comprise 4, 6, 8, 10, 64 or
Corresponding author. Fax: +44-20-7815-7681. E-mail address: [email protected] (E.O. Ezugwu).
0890-6955/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2004.02.007
more inserts per cutter. The cost of machining trials to determine acceptable tool life is significant especially when a large number of inserts per cutter are used. The increased cost is not only due to the number of inserts used but mainly because of the large amount of material required to reach any of the tool rejection criterion for one of the inserts. Cost of the tool life trials becomes more significant when expensive materials are evaluated, e.g. titanium and nickel alloys, stainless steels, composites, etc. [2]. The use of an alternative technique for tool life trials in milling operation can therefore be justified by the high cost of conventional trials. In practice, most of the tool evaluation trials are carried out with reduced number of inserts than the capacity of the milling cutter while maintaining the same feed per tooth for the full capacity cutter. This is done to simulate a real machining condition, where the cutter has all the required number of inserts but, at the same time, reducing the cost to an acceptable level [2,3]. There are,
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A. Richetti et al. / International Journal of Machine Tools & Manufacture 44 (2004) 695–700
and a P30 substrate were used. The final geometry of the inserts during machining are:
Nomenclature AISI HB VB Vf vr ks fz HP Vc ao co
co ¼ 2
American Iron and Steel Institute hardness Brinell flank wear (mm) feed rate (mm/min) entry angle (degree) back rake angle (degree) feed per tooth (mm) horse power cutting speed (m/min) clearance angle (degree) rake angle (degree)
however, some restrictions related to the acceptance of these test results since the dynamics of the milling process would completely change with variation in the number of inserts in the cutter. The wear mechanisms, impact stress, mechanical fatigue and thermal crack formation are not expected to exhibit similar behaviour with a real milling operation [4,5]. This paper investigates the influence of the number of inserts for tool life evaluation in face milling of steels with uncoated and coated tools. 2. Experimental procedure AISI 1045 steel with average hardness of 229 HB (square section bar of 76:2 76:2 500 mm) and AISI 8640 steel with average hardness of 299 HB (square section bar of 110 110 490 mm) were machined on a CNC milling machine tool with 22 HP. The chemical compositions of AISI 1045 and AISI 8640 steels are given in Tables 1 and 2, respectively. A constant feed per tooth (fz) was used for all the milling trials irrespective of the number of inserts in the cutter. This is ensured by adjusting the feed velocity relative to the number of tools, i.e., for six insert cutter, the feed velocity was twice more than for three insert cutter, three times greater than for two insert cutter and six times greater than for one insert cutter. A constant depth of cut of 1.0 mm was used in all the machining trials. The AISI 1045 steel was machined with a 100 mm diameter cutter with capacity for six inserts. A schematic illustration of the cutter is shown in Fig. 1, with v an insert exiting angle of approximately 40 . Cemented carbide inserts with ISO designation SPUN 12 03 08
v
ao ¼ 9
v
ks ¼ 7
v
vr ¼ 75
v
These tests were carried out with 1, 2 (equally spaced v v at 180 ), 3 (equally spaced at 120 ) and six inserts in the cutter at cutting speeds of 300, 325 and 350 m/min and feed per tooth of 0.075, 0.100 and 0.125 mm. Combination of these parameters gave 36 tests in total. The AISI 8640 steel was machined with an 80 mm diameter cutter also with a capacity for six inserts. A schematic illustration of the cutter position during v machining is given in Fig. 2. A 90 exiting angle was v obtained, out of the critical range (45+20 ) where the foot forming phenomenon cause tool damage [1]. Coated carbide inserts of ISO designation SEMN 12 04 AZ class with P45 and M35 substrates were used. The final geometry of the inserts during machining are: v
v
v
v
co ¼ 9 ao ¼ 20 ks ¼ 17 vr ¼ 45 For AISI 8640 steel, tests were carried out with 1, 2 v v (equally spaced at 180 ), 3 (equally spaced at 120 ) and six inserts in the cutter at a cutting speed of 200 m/min and a feed per tooth of 0.150 mm. This combination involved only four tests which were used to verify results obtained when machining AISI 1045 steel. The position of the inserts was verified using a dial indicator. The acceptable radial deviation was 0.01 mm. Flank wear (VB) was recorded at various intervals during machining until the 0.7 mm tool rejection criterion was reached in any of the inserts tested. Flank wear was recorded at 40 times magnification with a microscope that allows the measurement without removing the inserts from the milling cutter. This microscope consist of a moving support to the cutter and assembled on an XY coordinates table controlled by a dial indicator. 3. Results and discussions Flank wear curves were evaluated for AISI 1045 steel, in a first stage by varying the number of inserts in the cutter, cutting speed and feed per tooth. Figs. 3–8 show the maximum flank wear curves at various cutting conditions. The different flank wear rate/progression for 1, 2, 3 and 6 inserts in the cutter is clearly demonstrated in Figs. 3–8. This behaviour is probably a result of a combination of two thermal effects, assuming that cutting temperature increases with the number of inserts in the milling cutter. The first is the
Table 1 Chemical composition of AISI 1045 steel (wt%) C
Si
Mn
P
S
Cr
Ni
Mo
Al
Cu
Fe
0.48
0.25
0.67
0.019
0.038
0.12
0.09
0.03
0.032
0.11
Bal.
A. Richetti et al. / International Journal of Machine Tools & Manufacture 44 (2004) 695–700
697
Table 2 Chemical composition of AISI 8640 (wt%) C
Si
Mn
P
S
Cr
Ni
Mo
Al
Cu
Fe
0.43
0.26
0.81
0.014
0.014
0.47
0.47
0.19
0.019
0.09
Bal.
Fig. 1. Schematic illustration of cutter position when machining AISI 1045 steel. Fig. 4. Flank wear curves for AISI 1045 steel when machining at a speed of 300 m/min and a feed per tooth of 0.125 mm.
Fig. 2. Schematic illustration of the cutter position when machining AISI 8640 steel.
reduction in the strength of the workpiece associated with temperature, which facilitate the machining operation and the second is the thermally related wear mechanism(s) that tend to lower tool life. In both cases, the shortest tool life was always obtained with six inserts in the cutter. This situation enable machining of the material with higher frequency and the generation of higher temperatures in the workpiece due to the heat generated by each tool to facilitate the cutting
Fig. 3. Flank wear curves for AISI 1045 steel when machining at a speed of 300 m/min and a feed per tooth of 0.100 mm.
Fig. 5. Flank wear curves for AISI 1045 steel when machining at a speed of 325 m/min and a feed per tooth of 0.075 mm.
process. The generation of higher tool temperatures significantly reduces tool life as a result of thermally activated wear mechanisms. Analysis of the curves in
Fig. 6. Flank wear curves for AISI 1045 steel when machining at a speed of 325 m/min and a feed per tooth of 0.100 mm.
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A. Richetti et al. / International Journal of Machine Tools & Manufacture 44 (2004) 695–700
Fig. 7. Flank wear curves for AISI 1045 steel when machining at a speed of 350 m/min and a feed per tooth of 0.1 mm.
Fig. 9. Length of cut for 1 (a) and 6 (b) inserts in the cutter in one rotation.
Fig. 8. Flank wear curves for AISI 1045 steel when machining at a speed of 350 m/min and a feed per tooth of 0.125 mm.
Figs. 3–8 indicates that machining with six inserts in the cutter gave higher tool wear that can be attributed to higher cutting temperatures generated during machining relative to machining with cutters with fewer inserts. The longest tool life obtained when machining with the one insert cutter may be associated with the shortest active cycle of the cutter relative to the idle cycle (smaller cutting frequency), resulting in a lower cutting temperature in comparison with other cutters tested.
This will also lead to very minimum (if any) reduction in the strength of the work material. Furthermore, the adjusted feed velocity is lower, contributing to maintaining the workpiece temperature at lower levels. The lower cutting temperature will minimise thermally activated wear mechanisms during machining. In this case, the wear reduction effect associated with reduction in cutting temperature was greater than retention of the strength of the work material, thus promoting longer tool life. A comparison of the results using the same feed per tooth and different cutting speeds highlight the effect of cutting temperature (e.g. Figs. 3 and 6 as well as Figs. 4 and 8). Tool life decreased considerably at higher cutting speed when machining with a cutter containing only one insert. This indicates that the thermally activated wear mechanisms commence after exceeding a critical cutting speed. For milling cutters containing 2, 3 and 6 inserts, reduction of tool life at higher cutting speeds is more gradual, suggesting that the thermally activated wear mechanisms were always present
Table 3 Tool life and volume of material removed when face milling AISI 1045 steel Machining conditions Vc (m/min) fz (mm) 1 2 3 4 5 6 7 8 9
300 (0.075) 300 (0.100) 300 (0.125) 325 (0.075) 325 (0.100) 325 (0.125) 350 (0.075) 350 (0.100) 350 (0.125)
Tool life (min) for VB ¼ 0:7 mm (volume of removed material (cm3) for VB ¼ 0:7 mm) Cutter with 1 insert
Cutter with 2 inserts
Cutter with 3 inserts
Cutter with 6 inserts
12.7 (69.3) 12.7 (92.4) 8.7 (79.2) 8 (47.3) 6.5 (51.3) 5.8 (57.2) 5 (31.9) 3.5 (29.7) 4 (42.5)
9 (98.2) 6.3 (91.7) 5.6 (102) 5 (59.2) 4 (63.2) 3.5 (69) 4 (51) 3 (51) 1.8 (38.2)
9.3 (155.2) 4.5 (98.1) 4.3 (117.3) 5.8 (102.9) 5 (118.2) 4 (118.2) 4.3 (82.2) 2.8 (71.4) 2 (63.6)
4.3 (141) 2.2 (96) 1.8 (98.4) 3.2 (113.4) 3 (142.2) 2.5 (147.6) 2.8 (107.4) 2 (102) 1.5 (95.4)
A. Richetti et al. / International Journal of Machine Tools & Manufacture 44 (2004) 695–700
because of the greater cutting frequency, independent of cutting speed. Milling operations, however, present more complexity than the analysis of tool wear and heating of the workpiece during machining. This is evident from the fracture observed in many of the worn tool edges, which adversely affect tool performance. Thermal cracks were also observed on the worn tools due perhaps to thermal fatigue mechanism. This will consequently lead to tool embrittlement, which promote premature fracture of the insert edges during machining. The use of higher feed velocities when machining with more inserts in the cutter to maintain the same feed per tooth suggests a proportional increase in the length of cut as shown schematically in Fig. 9. This increase in the length of cut could contribute to lowering tool life. Table 3 shows results of tool life and volume of material removed when face milling AISI 1045 steel under various machining conditions. Tool life tends to decrease with increasing cutting speed and feed per tooth. It has been reported that increase in cutting speed and feed per tooth accelerate thermally activated wear mechanisms in addition to generating more intense mechanical impact [6]. These promote an increase in the thermal gradient which tend to lower tool life as thermal cracks generation rate increases [7]. The volume of removed material during machining is also affected by the number of inserts in the milling cutter. Table 3 also shows that in spite of the lower tool life obtained when machining with more inserts in the cutter, the volume of material removed increased with more inserts in the cutter. This suggests that the effect of increasing the feed velocity, to maintain the same feed per tooth, was greater than its effect in lowering tool life. In fact, more materials were removed at the same cutting time when machining at higher speeds despite the effect of the more adverse thermally activated wear mechanisms. Furthermore, prolonged machining with more inserts in the cutter tend to ensure that the work material remains hot, thereby decreasing its shear strength and consequently facilitating the cutting process. It should, however, be noted that some tests did not present coherent results due probably to the random fracturing of the cutting edge. Further analysis of the material removed per tooth shows that the amount of material removed per tool decreases with increasing number of inserts in the cutter, although the total volume of material removed tend to increase. In order to validate these results, tests were carried out with AISI 8640 steel using 1, 2, 3 and 6 inserts in an 80 mm diameter cutter. In these tests, the wear progression for each insert was evaluated and the volume of material removed until tool rejection for one of the tools was determined. The flank wear curves when machining AISI 8640 steel (Fig. 10a–d) presented a
699
Fig. 10. Flank wear recorded when machining AISI 8640 steel with (a) 1 insert (Vc ¼ 200 m=min, fz ¼ 0:150 mm and Vf ¼ 119 mm=min); (b) 2 inserts (Vc ¼ 200 m=min, fz ¼ 0:150 mm and Vf ¼ 239 mm=min); (c) 3 inserts (Vc ¼ 200 m=min, fz ¼ and (d) 6 inserts 0:150 mm and Vf ¼ 358 mm=min) (Vc ¼ 200 m=min, fz ¼ 0:150 mm and Vf ¼ 716 mm=min).
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uniform and steady growth region from zero up to 0.2 mm in most of the tests. In this flank wear region, cutting forces and temperatures caused by the friction between the clearance face and the workpiece surface did not present significant influence on the wear process, hence the reduced wear at the initial stage of milling. This behaviour was not observed when machining AISI 1045 steel. This could be due to the use of coated cemented carbide which gave improved wear resistance. Wear values in excess of 0.2 mm increased the cutting forces and temperatures as well as the wear rates. Fracture of the cutting edge accelerated the wear rate in some of the tests. This failure mode is associated with thermal cracks (fatigue cracks), promoted by the cyclic variation of the tool temperature during machining. These cracks embrittle the cutting tool, resulting in fracture under thermal and mechanical impacts encountered in an intermittent machining operation. Results obtained when machining AISI 8640 steel showed similar trend to those obtained when machining AISI 1045 steel, i.e., tool life decreased and the volume of removed material increased when machining with more inserts in the cutter (Table 4). Increasing the feed rate to maintain the same feed per tooth increases the cutting temperatures, thus accelerating wear. Discrepancies observed in the volume of removed material in Table 4 were probably caused by the fracture of the cutting edge. All the machining data obtained indicate that the use of fewer inserts in a cutter for machinability trials should be used only for comparison of two or more machining conditions. The economic viability of these alternative tests must be analysed prior to commencement. Determination of tool life can only be effectively carried out under the same real conditions because the change in the number of inserts in the cutter can com-
Table 4 Volume of removed material and tool lives recorded when machining AISI 8640 steel with various number of inserts in the cutter at Vc ¼ 200 m=min and fz ¼ 0:15 mm Cutter with Cutter with Cutter with Cutter with 1 insert 2 inserts 3 inserts 6 inserts Tool life for 9.42 VB ¼ 0:7 mm 60.074 Volume of removed material (cm3) Vf (mm/min) 119
4.18 48.4
239
4.72
1.87
89.174
72.124
358
716
pletely alter the wear conditions and the anticipated results from such exercise.
4. Conclusions 1. Results from the milling test using lesser number of inserts than the cutter capacity should be used as comparison index of the machinability between two or more machining conditions. 2. Machinability experiments with inserts less than the full capacity must not be used to determine tool life as they do not reproduce the real machining conditions. 3. The total volume of material removed tends to increase with the number of inserts in the cutter for the same machining time. 4. Flank wear rate tends to increase when milling with more inserts in the cutter due to higher cutting temperatures generated.
Acknowledgements The authors are grateful for the technical support from Mr. Reginaldo Ferreira de Souza and to CAPES, CNPq and FAPEMIG for financial support.
References [1] A.J. Pekelharing, Annals of CIRP 33 (1) (1984) 47–50. [2] L. Bohes, R. Borges, E.D.S. Neto, Fresamento de Superligas Constituı´das de Alta Percentagem de Nı´quel e Outros Elementos, VI Congresso Nacional de Ingenieria Mecanica, Chile, 1994, pp. 365–370, (in Portuguese). [3] J. Panato, M.B. Da Silva, A.R. Machado, Efeito do Nu´mero de Ferramentas Utilizado nos Testes de Usinabilidade no Processo de Fresamento, XIII COBEM—Congresso Brasileiro de Engenharia Mecaˆnica, Belo Horizonte, MG, Brazil, 12–15 December, 1995, (CD-ROM). [4] H. Chandrasekaran, H. Thoors, Tribology in interrupted machining: role of interruption cycle and work material, Wear 179 (1994) 83–88. [5] J. Tlusty, Dynamics of high speed milling, The Winter Annual Meeting of ASME, New Orleans, LA, USA, 9–14 December, 1984, pp. 101–126. [6] S. Eldem, G. Barrow, Tool life in interrupted turning operations, Israel Journal of Technology 14 (1976) 172–178. [7] S.M. Bhatia, P.C. Pandey, H.S. Shan, Failure of Cemented Carbide Tools in Intermittent Cutting, Precision Engineering, , IPC Business Press, 1979, pp. 148–152.
Influence of the number of inserts for tool life evaluation in face milling of steels ´ .R. Machado a, M.B. Da Silva a, E.O. Ezugwu b,, J. Bonney b A. Richetti a, A a
Machining Research and Education Laboratory, Mechanical Engineering Faculty, Federal University of Uberlaˆndia, Uberlaˆndia, MG, Brazil b Machining Research Centre, Faculty of Engineering, Science and Technology, South Bank University, 103 Borough Road, London SE1 0AA, UK Received 28 January 2003; accepted 5 February 2004
Abstract Tool life tests are often employed to verify the behaviour of one or more inserts in a cutter in order to optimise machining productivity and minimise cost. In milling process, such tests are expensive and require many of tools and a lot of work material to achieve any of the stipulated tool rejection criterion in any of the inserts. In practice, tool life tests are usually carried out using only one or few edges in a face milling cutter in order to minimise cost. The aim of this study is to investigate the effect of the number of tools used in face milling operation and how they relate to the establishment of tool life under specified cutting conditions. Flank wear curves were evaluated for AISI 1045 and 8640 steels using 1, 2, 3 and 6 inserts in a face milling cutter. Test results show that reduction in the number of inserts in the milling cutter led to a reduction in the amount of material removed and also tend to increase tool life when machining at the same feed per tooth. Results obtained using reduced number of inserts in a milling cutter should only be used for comparison between two or more conditions and should not be used to establish tool life. # 2004 Elsevier Ltd. All rights reserved. Keywords: Machinability tests; Face milling; Tool life; Cutting temperature; Volume of material removed
1. Introduction The best machining conditions depend on the cutting tool, workpiece, machine tool, cutting fluids, and cutting parameters, thus machinability trials are therefore essential. This selection process is justified by the problem encountered in choosing from a large number of commercially available tools. Recommendations from manufacturers should only be used as a guide since better conditions may be found for other tools and cutting parameters. Machinability trials on each application are of major importance due to the economic benefits to be gained by manufacturing industries that carry out large amount of machining operations [1]. The use of indexable inserts in face milling operation is now very common. Commercially available milling cutters are variable and comprise 4, 6, 8, 10, 64 or
Corresponding author. Fax: +44-20-7815-7681. E-mail address: [email protected] (E.O. Ezugwu).
0890-6955/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2004.02.007
more inserts per cutter. The cost of machining trials to determine acceptable tool life is significant especially when a large number of inserts per cutter are used. The increased cost is not only due to the number of inserts used but mainly because of the large amount of material required to reach any of the tool rejection criterion for one of the inserts. Cost of the tool life trials becomes more significant when expensive materials are evaluated, e.g. titanium and nickel alloys, stainless steels, composites, etc. [2]. The use of an alternative technique for tool life trials in milling operation can therefore be justified by the high cost of conventional trials. In practice, most of the tool evaluation trials are carried out with reduced number of inserts than the capacity of the milling cutter while maintaining the same feed per tooth for the full capacity cutter. This is done to simulate a real machining condition, where the cutter has all the required number of inserts but, at the same time, reducing the cost to an acceptable level [2,3]. There are,
696
A. Richetti et al. / International Journal of Machine Tools & Manufacture 44 (2004) 695–700
and a P30 substrate were used. The final geometry of the inserts during machining are:
Nomenclature AISI HB VB Vf vr ks fz HP Vc ao co
co ¼ 2
American Iron and Steel Institute hardness Brinell flank wear (mm) feed rate (mm/min) entry angle (degree) back rake angle (degree) feed per tooth (mm) horse power cutting speed (m/min) clearance angle (degree) rake angle (degree)
however, some restrictions related to the acceptance of these test results since the dynamics of the milling process would completely change with variation in the number of inserts in the cutter. The wear mechanisms, impact stress, mechanical fatigue and thermal crack formation are not expected to exhibit similar behaviour with a real milling operation [4,5]. This paper investigates the influence of the number of inserts for tool life evaluation in face milling of steels with uncoated and coated tools. 2. Experimental procedure AISI 1045 steel with average hardness of 229 HB (square section bar of 76:2 76:2 500 mm) and AISI 8640 steel with average hardness of 299 HB (square section bar of 110 110 490 mm) were machined on a CNC milling machine tool with 22 HP. The chemical compositions of AISI 1045 and AISI 8640 steels are given in Tables 1 and 2, respectively. A constant feed per tooth (fz) was used for all the milling trials irrespective of the number of inserts in the cutter. This is ensured by adjusting the feed velocity relative to the number of tools, i.e., for six insert cutter, the feed velocity was twice more than for three insert cutter, three times greater than for two insert cutter and six times greater than for one insert cutter. A constant depth of cut of 1.0 mm was used in all the machining trials. The AISI 1045 steel was machined with a 100 mm diameter cutter with capacity for six inserts. A schematic illustration of the cutter is shown in Fig. 1, with v an insert exiting angle of approximately 40 . Cemented carbide inserts with ISO designation SPUN 12 03 08
v
ao ¼ 9
v
ks ¼ 7
v
vr ¼ 75
v
These tests were carried out with 1, 2 (equally spaced v v at 180 ), 3 (equally spaced at 120 ) and six inserts in the cutter at cutting speeds of 300, 325 and 350 m/min and feed per tooth of 0.075, 0.100 and 0.125 mm. Combination of these parameters gave 36 tests in total. The AISI 8640 steel was machined with an 80 mm diameter cutter also with a capacity for six inserts. A schematic illustration of the cutter position during v machining is given in Fig. 2. A 90 exiting angle was v obtained, out of the critical range (45+20 ) where the foot forming phenomenon cause tool damage [1]. Coated carbide inserts of ISO designation SEMN 12 04 AZ class with P45 and M35 substrates were used. The final geometry of the inserts during machining are: v
v
v
v
co ¼ 9 ao ¼ 20 ks ¼ 17 vr ¼ 45 For AISI 8640 steel, tests were carried out with 1, 2 v v (equally spaced at 180 ), 3 (equally spaced at 120 ) and six inserts in the cutter at a cutting speed of 200 m/min and a feed per tooth of 0.150 mm. This combination involved only four tests which were used to verify results obtained when machining AISI 1045 steel. The position of the inserts was verified using a dial indicator. The acceptable radial deviation was 0.01 mm. Flank wear (VB) was recorded at various intervals during machining until the 0.7 mm tool rejection criterion was reached in any of the inserts tested. Flank wear was recorded at 40 times magnification with a microscope that allows the measurement without removing the inserts from the milling cutter. This microscope consist of a moving support to the cutter and assembled on an XY coordinates table controlled by a dial indicator. 3. Results and discussions Flank wear curves were evaluated for AISI 1045 steel, in a first stage by varying the number of inserts in the cutter, cutting speed and feed per tooth. Figs. 3–8 show the maximum flank wear curves at various cutting conditions. The different flank wear rate/progression for 1, 2, 3 and 6 inserts in the cutter is clearly demonstrated in Figs. 3–8. This behaviour is probably a result of a combination of two thermal effects, assuming that cutting temperature increases with the number of inserts in the milling cutter. The first is the
Table 1 Chemical composition of AISI 1045 steel (wt%) C
Si
Mn
P
S
Cr
Ni
Mo
Al
Cu
Fe
0.48
0.25
0.67
0.019
0.038
0.12
0.09
0.03
0.032
0.11
Bal.
A. Richetti et al. / International Journal of Machine Tools & Manufacture 44 (2004) 695–700
697
Table 2 Chemical composition of AISI 8640 (wt%) C
Si
Mn
P
S
Cr
Ni
Mo
Al
Cu
Fe
0.43
0.26
0.81
0.014
0.014
0.47
0.47
0.19
0.019
0.09
Bal.
Fig. 1. Schematic illustration of cutter position when machining AISI 1045 steel. Fig. 4. Flank wear curves for AISI 1045 steel when machining at a speed of 300 m/min and a feed per tooth of 0.125 mm.
Fig. 2. Schematic illustration of the cutter position when machining AISI 8640 steel.
reduction in the strength of the workpiece associated with temperature, which facilitate the machining operation and the second is the thermally related wear mechanism(s) that tend to lower tool life. In both cases, the shortest tool life was always obtained with six inserts in the cutter. This situation enable machining of the material with higher frequency and the generation of higher temperatures in the workpiece due to the heat generated by each tool to facilitate the cutting
Fig. 3. Flank wear curves for AISI 1045 steel when machining at a speed of 300 m/min and a feed per tooth of 0.100 mm.
Fig. 5. Flank wear curves for AISI 1045 steel when machining at a speed of 325 m/min and a feed per tooth of 0.075 mm.
process. The generation of higher tool temperatures significantly reduces tool life as a result of thermally activated wear mechanisms. Analysis of the curves in
Fig. 6. Flank wear curves for AISI 1045 steel when machining at a speed of 325 m/min and a feed per tooth of 0.100 mm.
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A. Richetti et al. / International Journal of Machine Tools & Manufacture 44 (2004) 695–700
Fig. 7. Flank wear curves for AISI 1045 steel when machining at a speed of 350 m/min and a feed per tooth of 0.1 mm.
Fig. 9. Length of cut for 1 (a) and 6 (b) inserts in the cutter in one rotation.
Fig. 8. Flank wear curves for AISI 1045 steel when machining at a speed of 350 m/min and a feed per tooth of 0.125 mm.
Figs. 3–8 indicates that machining with six inserts in the cutter gave higher tool wear that can be attributed to higher cutting temperatures generated during machining relative to machining with cutters with fewer inserts. The longest tool life obtained when machining with the one insert cutter may be associated with the shortest active cycle of the cutter relative to the idle cycle (smaller cutting frequency), resulting in a lower cutting temperature in comparison with other cutters tested.
This will also lead to very minimum (if any) reduction in the strength of the work material. Furthermore, the adjusted feed velocity is lower, contributing to maintaining the workpiece temperature at lower levels. The lower cutting temperature will minimise thermally activated wear mechanisms during machining. In this case, the wear reduction effect associated with reduction in cutting temperature was greater than retention of the strength of the work material, thus promoting longer tool life. A comparison of the results using the same feed per tooth and different cutting speeds highlight the effect of cutting temperature (e.g. Figs. 3 and 6 as well as Figs. 4 and 8). Tool life decreased considerably at higher cutting speed when machining with a cutter containing only one insert. This indicates that the thermally activated wear mechanisms commence after exceeding a critical cutting speed. For milling cutters containing 2, 3 and 6 inserts, reduction of tool life at higher cutting speeds is more gradual, suggesting that the thermally activated wear mechanisms were always present
Table 3 Tool life and volume of material removed when face milling AISI 1045 steel Machining conditions Vc (m/min) fz (mm) 1 2 3 4 5 6 7 8 9
300 (0.075) 300 (0.100) 300 (0.125) 325 (0.075) 325 (0.100) 325 (0.125) 350 (0.075) 350 (0.100) 350 (0.125)
Tool life (min) for VB ¼ 0:7 mm (volume of removed material (cm3) for VB ¼ 0:7 mm) Cutter with 1 insert
Cutter with 2 inserts
Cutter with 3 inserts
Cutter with 6 inserts
12.7 (69.3) 12.7 (92.4) 8.7 (79.2) 8 (47.3) 6.5 (51.3) 5.8 (57.2) 5 (31.9) 3.5 (29.7) 4 (42.5)
9 (98.2) 6.3 (91.7) 5.6 (102) 5 (59.2) 4 (63.2) 3.5 (69) 4 (51) 3 (51) 1.8 (38.2)
9.3 (155.2) 4.5 (98.1) 4.3 (117.3) 5.8 (102.9) 5 (118.2) 4 (118.2) 4.3 (82.2) 2.8 (71.4) 2 (63.6)
4.3 (141) 2.2 (96) 1.8 (98.4) 3.2 (113.4) 3 (142.2) 2.5 (147.6) 2.8 (107.4) 2 (102) 1.5 (95.4)
A. Richetti et al. / International Journal of Machine Tools & Manufacture 44 (2004) 695–700
because of the greater cutting frequency, independent of cutting speed. Milling operations, however, present more complexity than the analysis of tool wear and heating of the workpiece during machining. This is evident from the fracture observed in many of the worn tool edges, which adversely affect tool performance. Thermal cracks were also observed on the worn tools due perhaps to thermal fatigue mechanism. This will consequently lead to tool embrittlement, which promote premature fracture of the insert edges during machining. The use of higher feed velocities when machining with more inserts in the cutter to maintain the same feed per tooth suggests a proportional increase in the length of cut as shown schematically in Fig. 9. This increase in the length of cut could contribute to lowering tool life. Table 3 shows results of tool life and volume of material removed when face milling AISI 1045 steel under various machining conditions. Tool life tends to decrease with increasing cutting speed and feed per tooth. It has been reported that increase in cutting speed and feed per tooth accelerate thermally activated wear mechanisms in addition to generating more intense mechanical impact [6]. These promote an increase in the thermal gradient which tend to lower tool life as thermal cracks generation rate increases [7]. The volume of removed material during machining is also affected by the number of inserts in the milling cutter. Table 3 also shows that in spite of the lower tool life obtained when machining with more inserts in the cutter, the volume of material removed increased with more inserts in the cutter. This suggests that the effect of increasing the feed velocity, to maintain the same feed per tooth, was greater than its effect in lowering tool life. In fact, more materials were removed at the same cutting time when machining at higher speeds despite the effect of the more adverse thermally activated wear mechanisms. Furthermore, prolonged machining with more inserts in the cutter tend to ensure that the work material remains hot, thereby decreasing its shear strength and consequently facilitating the cutting process. It should, however, be noted that some tests did not present coherent results due probably to the random fracturing of the cutting edge. Further analysis of the material removed per tooth shows that the amount of material removed per tool decreases with increasing number of inserts in the cutter, although the total volume of material removed tend to increase. In order to validate these results, tests were carried out with AISI 8640 steel using 1, 2, 3 and 6 inserts in an 80 mm diameter cutter. In these tests, the wear progression for each insert was evaluated and the volume of material removed until tool rejection for one of the tools was determined. The flank wear curves when machining AISI 8640 steel (Fig. 10a–d) presented a
699
Fig. 10. Flank wear recorded when machining AISI 8640 steel with (a) 1 insert (Vc ¼ 200 m=min, fz ¼ 0:150 mm and Vf ¼ 119 mm=min); (b) 2 inserts (Vc ¼ 200 m=min, fz ¼ 0:150 mm and Vf ¼ 239 mm=min); (c) 3 inserts (Vc ¼ 200 m=min, fz ¼ and (d) 6 inserts 0:150 mm and Vf ¼ 358 mm=min) (Vc ¼ 200 m=min, fz ¼ 0:150 mm and Vf ¼ 716 mm=min).
700
A. Richetti et al. / International Journal of Machine Tools & Manufacture 44 (2004) 695–700
uniform and steady growth region from zero up to 0.2 mm in most of the tests. In this flank wear region, cutting forces and temperatures caused by the friction between the clearance face and the workpiece surface did not present significant influence on the wear process, hence the reduced wear at the initial stage of milling. This behaviour was not observed when machining AISI 1045 steel. This could be due to the use of coated cemented carbide which gave improved wear resistance. Wear values in excess of 0.2 mm increased the cutting forces and temperatures as well as the wear rates. Fracture of the cutting edge accelerated the wear rate in some of the tests. This failure mode is associated with thermal cracks (fatigue cracks), promoted by the cyclic variation of the tool temperature during machining. These cracks embrittle the cutting tool, resulting in fracture under thermal and mechanical impacts encountered in an intermittent machining operation. Results obtained when machining AISI 8640 steel showed similar trend to those obtained when machining AISI 1045 steel, i.e., tool life decreased and the volume of removed material increased when machining with more inserts in the cutter (Table 4). Increasing the feed rate to maintain the same feed per tooth increases the cutting temperatures, thus accelerating wear. Discrepancies observed in the volume of removed material in Table 4 were probably caused by the fracture of the cutting edge. All the machining data obtained indicate that the use of fewer inserts in a cutter for machinability trials should be used only for comparison of two or more machining conditions. The economic viability of these alternative tests must be analysed prior to commencement. Determination of tool life can only be effectively carried out under the same real conditions because the change in the number of inserts in the cutter can com-
Table 4 Volume of removed material and tool lives recorded when machining AISI 8640 steel with various number of inserts in the cutter at Vc ¼ 200 m=min and fz ¼ 0:15 mm Cutter with Cutter with Cutter with Cutter with 1 insert 2 inserts 3 inserts 6 inserts Tool life for 9.42 VB ¼ 0:7 mm 60.074 Volume of removed material (cm3) Vf (mm/min) 119
4.18 48.4
239
4.72
1.87
89.174
72.124
358
716
pletely alter the wear conditions and the anticipated results from such exercise.
4. Conclusions 1. Results from the milling test using lesser number of inserts than the cutter capacity should be used as comparison index of the machinability between two or more machining conditions. 2. Machinability experiments with inserts less than the full capacity must not be used to determine tool life as they do not reproduce the real machining conditions. 3. The total volume of material removed tends to increase with the number of inserts in the cutter for the same machining time. 4. Flank wear rate tends to increase when milling with more inserts in the cutter due to higher cutting temperatures generated.
Acknowledgements The authors are grateful for the technical support from Mr. Reginaldo Ferreira de Souza and to CAPES, CNPq and FAPEMIG for financial support.
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