Experimental study of burr formation and tool chipping in the face milling of stainless steel

Journal of Materials Processing Technology 108 (2000) 12±20

Experimental study of burr formation and tool chipping in the face milling of stainless steel Tsann-Rong Lin Department of Mechanical Manufacturing Engineering, National Huwei Institute of Technology, Hu-Wei 632, Taiwan, ROC Received 3 February 1999

Abstract This paper reports the results of experiments to investigate burr formation and tool chipping during the face milling of stainless steel, using a ¯y milling cutter to carry out single-tooth face milling tests. It is found that the burr height is strongly dependent upon the milling process . The results show that ®ve different types of burr can be produced on the exit edge, e.g. knife-type, saw-type, burr breakage curltype and wave-type. A large depth of cut contributes to the formation of the wave-type burr. With high feed rate, the cutting edge of the tool is chipped, with a built-up edge. At high cutting speeds, small masses of material from the rake face of the tool can come off. With large depth of cut, large cavities can be created in the rake face of the tool. # 2000 Published by Elsevier Science B.V. Keywords: Burr; Tool chipping; Face milling; Stainless steel

1. Introduction For machining, tool life is a very important economic factor, particularly for the milling and turning of heatresistant alloys. These hard and dif®cult-to-machine materials generate very high wear rates on both the ¯ank and the face of the tool. Agawal et al. [1] presented the relative performance of coated and uncoated tools in the turning of three cast austenitic stainless steels. Sun et. al., [2] described the interface adhesion behavior between the cutter and workpiece, when austenitic stainless steel is milled by a cemented carbide cutter. It was shown that: (a) at medium cutting speed, an adhesive layer is formed between the rake face and the chip; (b) at low cutting speed, no adhesive layer exists on the rake face and (c) at high speed, a crater is formed on the rake face, and adhesion does not occur. Balazinski and Ennajimi [3] presented the results of experimental studies on the in¯uence of feed variation on tool wear during face milling. Their experimental results from the milling of stainless steel show that it is possible to substantially increase tool life with proper variation of the cutting feed rate throughout the cutting process. There is one thing that almost all machining processes have in common-burrs. A burr is de®ned as plastically deformed material left and attached on the workpiece after machining. Gillespie and Blotter [4] classi®ed machining burrs into four types according to their formation mechanisms, as follows: Poisson burr, roll-over burr, tear burr and 0924-0136/00/$ ± see front matter # 2000 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 5 7 3 - 2

cut-off burr. Nakayama and Arai [5] approached the classi®cation of machining burrs by the combination of cutting edge directly concerned and the burr formation direction. Chern [6] showed that two type of exit burrs (primary and secondary) are formed, depending on the cutting conditions, cutter geometry, part edge geometry and workpiece material properties. Olvera and Barrow [7,8] presented the results of a comprehensive study on burr formation in the square shoulder face milling of medium carbon steel. The study concentrated on the in¯uence of the main cutting parameters, e.g. feed/tooth, cutting velocity, axial depth of cut and exit angle. Narayanaswami and Doinfeld [9] developed an algorithm to minimize burr formation in the face milling of arbitrarily shaped polygons. Simulation results veri®ed that the cutter position, cutter radius and orientation of the part are important parameters which can be varied to minimize the length of edges with a primary burr. Kim and Kang [10] presented milling experiments on aluminum alloy using a diamond endmill, showing that the diamond tool greatly reduces the burr formation over the sintered carbide tool. In all of this previous research [4±9], the burr height was measured under the condition that the object was cut in one pass. The purpose of this paper is to report the results of a study on the burr formation and tool chipping mechanisms over the whole cutting process, when face milling stainless steel material. Five types of burrs are found on the workpieces as a result of machining: knife-type burr, saw-type burr, burr breakage, curl-type burr and wave-type burr.

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2. Experimental details The milling process is one of the most versatile and widely used metalworking processes. During the milling operation, burrs are created where the milling cutter exits the machined part. Face milling experiments were performed on a vertical milling machine having a spindle speed of n ˆ 40ÿ1750 rpm. .The spindle power of the machine used was 5 kW. In order to simplify the tool wear and burr formation analysis, only one insert was used in the milling cutter. Rectangular blocks of 150 mm length, 100 mm width and 26 mm thickness were used for the cutting tests. The cutting tool used was a Mitsubishi carbide UTI20T, this tool being of square shape and of throw-away type. All the tests were repeated at least three and usually four times. After each test, the burr height and tool chipping were measured under a microscope. 3. Results and discussion

Fig. 2. Relationship between burr height and removed volume for different cutting speeds.

The in¯uence of feed rate on burr formation of SUS 304 is shown in Fig. 1. It can be clearly seen that the burr height increases with the progress of cutting. This is because as the cutting progresses, the chipping of the tool edge promotes burr formation. At low feed rates (f ˆ 120 and 140 mm/ min), thin chips are produced which, due to the size effect, might give rise to larger burr height. The high feed rates ( f ˆ 180 and 200 mm/min), have the least volume of metal removal of all the feed rate. The medium feed rate (f ˆ 160 mm/min), has the largest volume of metal removal and the smallest burr height. This qualitative analysis of the burr he it is in agreement with the results of Stein and Dornfeld's [11] drilling of stainless steel. The in¯uence of

cuf®ng speed on burr formation is shown in Fig. 2, indicating that the burr height for high cuf®ng speeds is small in compared to the medium or low cuf®ng speed. When the cutting speed is increased, the friction between the chip and tool is reduced. When the tool face friction decreases, there is a corresponding increase in the shear angle and an accompanying decreases in the chip thickness. Thus the plastic strain associated with chip formation is reduced. This reduction affects the size of the burrs produced. The burr height increased due to larger chip load as the depth of cut increased as shown in Fig. 3. Figs. 4±8 show

Fig. 1. Relationship between burr height and removed volume for different feed-rates.

Fig. 3. Relationship between burr height and removed volume for different depths of cut.

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T.-R. Lin / Journal of Materials Processing Technology 108 (2000) 12±20

Fig. 4. Knife-type burr (K): (a) the right side; (b) the reverse side.

the ®ve different types of burrs. The knife-type burr (K) is created by the pushing out of the uncut part when tool chipping has not occurred. The saw-type burr (S) is similar to the knife-type burr, but a small amount of tool chipping has occurred. The burr-breakage (B) is formed when a fracture causing separation of the burr occurs near the middle of the burr. The curl-type burr (C) is due to tool

chipping which is so severe that the chip is pushed and bent over the edge. The wave-type burr (W) is due to stretching that the material undergoes when the burr is formed, which results in the length of the burr at the top being longer than the actual length of the edge machined, and therefore the burr is forced to take a wavy shape to be able to accommodate itself on a shorter edge length. Tables 1±3 show the

Table 1 The type of burrs at different feed-rates Feed-rate (mm/min)

120 140 160 180 200

Removed volume (cm3) 10

20

30

40

50

60

70

80

K K S S S

K‡B K‡B S‡B S‡B S‡B

K‡B K‡B S‡B S‡B‡C S‡B‡C

K‡B K‡B S‡B

K‡B K‡B S‡B‡C

K‡B K‡B S‡B‡C

K‡B K‡B S‡B‡C

K‡B K‡B S‡B‡C

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Table 2 The type of burrs at different cutting speeds Cutting speed (m/min)

60 85 119

Removed volume (cm3) 10

20

30

40

50

60

70

80

S K K

S‡B K S

S‡B K‡B S‡B

S‡B K‡B S‡B

S‡B‡C K‡B S‡B

S‡B‡C K‡B S‡B

S‡B‡C K‡B‡C S‡B‡C

S‡B‡C K‡B‡C

Table 3 The type of burrs at different depths of cut Depth of cut (m/m)

0.6 0.8 1.0

Removed volume (cm3) 10

20

30

40

50

60

70

80

S S W

S‡B W W‡B

S‡B W‡B W‡B

S‡B W‡B‡C W‡B

S‡B‡C W‡B‡C W‡B‡C

S‡B‡C W‡B‡C W‡B‡C

S‡B‡C

S‡B‡C

Fig. 5. Saw-type burr (S): (a) the right side; (b) the reverse side.

W‡B‡C

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T.-R. Lin / Journal of Materials Processing Technology 108 (2000) 12±20

Fig. 6. Burr breakage (B): (a) the right side; (b) the reverse side.

Fig. 7. Curl-type burr (C).

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Fig. 8. Wave-type burr (W).

types of the burrs at different feed rates, cuf®ng speeds and depths of cut over the whole cutting process. In the low feed rates, the knife-type burr is prevalent; whereas at the medium and high feed rate, the saw-type burr is prevalent, as shown in Table 1. Regardless of the feed rate, burr breakage occurs after the second pass cutting process. Table 2 shows the main types of burr, knife-type and saw-type burr, at different cuf®ng speeds. Table 3 shows that wave-type burr occurs at medium and large depths of cut. Under all the cutting conditions, the curl-type burr occurs at the end of the tool life. Figs. 9±11 show the relationships between tool edge chipping depth and removed volume for different feed rates, cutting speeds and depths of cut. At medium feed

rates, high cutting speeds and small depths of cut, the tool edge chipping depths are the smallest of all. The tendency shown in Figs. 9±11 is the same as that in Figs. 1±3. Fig. 12 shows the chipping shape of tool face at the different feed rates. At the low feed rate, chipping rarely occurs. At the medium feed rate, the cutting edge of the tool is chipped with no built-up edge. At the high feed rates, the cutting edge of the tool is chipped with a built-up edge. At the medium and high cutting speeds, small pieces of material in the rake face of the tools can come off, as shown in Fig. 13. For the medium and large depths of cut, large cavities can be found in the rake face of the tools, as shown in Fig. 14. This is due to chips from the workpiece striking the rake face of the tool.

Fig. 9. Relationship between edge chipping depth and removed volume for different feed-rates.

Fig. 10. Relationship between edge chipping depth and removed volume for different cutting speeds.

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The cavity size at the large depth of cut is greater than that for the medium depth of cut. Fig. 15 shows the overall process of tool chipping at high cutting speed. It can be clearly seen that the rake face cavities occur in the early stages of milling, and the cavity becomes larger as the milling process produces chipping in the cutting edge of tool. 4. Conclusions From the above-mentioned experimental results, the following conclusions are obtained.

Fig. 11. Relationship between edge chipping depth and removed volume for different depth of cut.

1. Five different types of burrs are produced on the exit edge in the cutting direction in a face milling operation, e.g. knife-type, saw-type, burr breakage, curl-type and wave-type. 2. The tendency of the burr formation is closed to the tool edge chipping depth.

Fig. 12. Tool failure for different feed-rates: (a) 120; (b) 160 and (c) 200 mm/min.

Fig. 13. Tool failure for different cutting speeds: (a) 60; (b) 85 and (c) 119 m/min.

Fig. 14. Tool failure for different depths of cut: (a) 0.6; (b) 0.8 and (c) 1.0 mm.

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Fig. 15. The process of tool failure.

3. At high feed rates, the cutting edge of the tool is chipped with a built-up edge. 4. At high cutting speeds, small pieces of material in the rake face of the tool can come off. 5. The cavity in the rake face increases as the depth of cut increases. Acknowledgements This research was supported by the National Science Council of the Republic of China under Grant no. NSC 88-2212-E-150-004. References [1] S. Agawal, A.K. Chakrabarti, A.B. Chattopadhyay, A study of the machining of cast austenitic stainless steels with carbide tools, J. Mater. Proc. Technol. 52 (1995) 610±620. [2] F. Sun, Z. Li, D. Jiang, B. Chen, Adhering wear mechanism of cemented carbide cutter in the intervallic cutting of stainless steel, Wear 214 (1998) 79±82.

[3] M. Balazinski, E. Ennajimi, In¯uence of feed variation on tool wear when milling stainless steel 17-4PH, ASME J. Eng. Ind. 116 (1994) 516±520. [4] L.K. Gillespie, P.T. Blotter, The formation and properties of machining burrs, ASME J. Eng. Ind. 98 (1) (1976) 66±74. [5] K. Nakayama, M. Arai, Burr formation in metal cutting, Ann. CIRP 36 (1) (1987) 33±36. [6] G.L. Chern, Analysis of burr formation and breakout in metal cutting, Ph.D. Dissertation, Department of Mechanical Engineering, University of California, Berkeley, 1993. [7] O. Olvera, G. Barrow, An experimental study of burr formation in square shoulder face milling, Int. J. Mach. Tools Manufact. 36 (9) (1996) 1005±1020. [8] O. Olvera, G. Barrow, In¯uence of exit angle and tool nose geometry on burr formation in face milling operations, Proc. Inst. Mech. Eng. B 212 (1998) 59±72. [9] R. Narayanaswami, D. Dornfeld, Burr minimization in face milling: a geometric approach, ASME J. Manufact. Sci. Eng. 119 (1997) 170±177. [10] J.D. Kim, Y.H. Kang, High-speed machining of aluminium using diamond endmills, Int. J. Mach. Tools Manufact. 37 (8) (1997) 1155± 1165. [11] J.M. Stein, D.A. Dornfeld, Burr formation in drilling miniature holes, Ann. CIRP 46 (1) (1997) 63±66.