High-spend machining of aluminium using diamond endmills

Int. J. Mnch.ToolsManufact.Vol.37, No. 8, pp. 1155-1165,1997 © 1997Publishedby ElsevierScienceLtd All rightsreserved.Printedin GreatBritain 0890--6955/97517.00+ .00

~ Pergamon

PII: S0890-6955(96)00011-9

HIGH-SPEED MACHINING OF ALUMINIUM USING DIAMOND ENDMILLS JEONG-DU KIMt and YOUN-HEE KANGt Received 20 April 1995; in final form 20 December 1995 ) Abstract--Milling experiments on aluminum alloy were carded out using a laboratory-designed diamond endmill. The machining of aluminium alloy using a conventional tool at a conventional cutting speed generally results in short tool-life, poor surface quality and poor edge finishes because of the formation of built-up edges and burrs. The machining technology of aluminum alloy surfaces with a good surface finish and edge finishing is very important and required in many industries. The experimental results reveal the possibility of machining a mirror-like aluminum alloy without built-up edges and burrs using the designed diamond endmill at high speed. © 1997 Published by Elsevier Science Ltd. All fights reserved

1. INTRODUCTION

High-speed machining is recognized as one of the key manufacturing technologies for higher productivity and better surface roughness [1]. Also, diamond tools are widely used in machining a mirror-like surface of nonferrous metals such as copper and aluminium [2, 3]. Aluminium used in the field of aviation and die/mold production is required to have a very precise machined surface. However, the machining of aluminium using conventional tools all conventional cutting speeds shortens the tool life and has an adverse effect on the surface quality and edge quality, because of the formation of built-up edges and burrs [4]. Therefore aluminium parts require deburring and finishing processes such as polishing after cutting; vast amounts of time and money are spent on such processes. Burrs are damaging even during machining because they hit the cutting tool edge and cause groove wear. This groove wear, in turn, accelerates burr growth [5]. This aspect is currently an obstacle to improving productivity and automation of machine parts production in many manufacturing fields. Therefore machining technology which does not form built-up edges and hence produces very good surface finishes is important and very much in demand. To solve this problem the milling of aluminum alloy using a diamond endmill and a high-speed spind]Le was experimented upon and some characteristics of the machining process were analyzed. It was possible to obtain mirror-like surfaces of aluminum alloy which did not require polishing after milling in the experiments. 2. DESIGN OF THE DIAMOND ENDMILL

The diamond endmill has a sharper edge, a lower friction coefficient when in contact with metals, and a higher thermal conductivity than conventional endmills. Thus the machining of a ductile material such as aluminium occurs at a low cutting temperature at high speed, which can improve surface accuracy and surface roughness by restraining the formarion of built-up edges [6]. The designed diamond endmill was manufactured for the machining of aluminium alloy both in the bottom and lateral surfaces of the workpiece.

tDepartment of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Taejon, Korea 1155

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Jeong-Du Kim and Youn-Hee Kang

The configuration of the diamond endmill is shown in Fig. 1, and its specification in Table 1. The material of the tool holder is sintered carbide, which has a high stiffness factor to prevent deflection because of the small diameter of the tool. The tip has a cutting edge of polycrystal diamond. The diamond endmill was designed as a solid type, where the tip is brazed to the tool holder. To maintain the strength of the cutting edge and to prevent chipping, the axial and radial rake angle was set to 0 °. The total length, flute length and face relief angle have been set to 50 mm, 4 mm and 0.5 °, respectively. 3. EXPERIMENTAL EQUIPMENT AND METHOD

The high-speed milling system consisted of a high-speed spindle attaching to a conventional milling machine (an economical arrangement). The high-speed spindle was magnetically actuated. The rotational speed of the spindle ranged from 2500 to 20 000 rpm. The workpiece is made of an aluminium alloy, A12024, and the shape is a rectangular block (45 x 28 x 28 ram3). Kerosene was used as a cutting fluid--it is one of the most popular coolants for mirror-like aluminium machining. The surface finish of the machined surface was measured using Mitsutoyo Surftest-402. The stylus was traced over the machined surface in the direction of feed motion. The arithmetic mean deviation Ra (/xm) with sampling length lc = 0.8 mm was applied as the criteria for evaluating the surface finish. The specifications of the experimental equipment are shown in Table 2. The details of the

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diamond Fig. 1. The configuration of the designed tool.

Table 1. The specifications of the diamond endmill Tool material

Sintered carbide

Tip material Diameter Shank diameter Total length Flute length Number of flutes Helix angle Axial rake angle Radial rake angle Face relief angle

Polycrystal diamond 6 mm 6 mm 50 mm 4 mm 2 0° 0° 0° 0.5 °

Table 2. The specifications of the experimental equipment Milling machine High-speed spindle Surface roughness tester Workpicce Cutting fluid

Whachon Nihon Seimitsu, 2500-2000 rpm, magnetic spindle Mitsutoyo Surftest-402 A12024 I¢~rosene

High-speed machining of aluminium

1157

workpiece are slhown in Table 3. A carbide tool was used as a comparison with the diamond tool in all experiments. The specifications of the carbide tool are shown in Table 4. In order to observe the effect of cutting conditions (cutting speed, axial depth of cut, radial depth of cut and feed per tooth) on surface roughness, the milling experiments were carded out using the designed diamond endmill and the high-speed spindle under various cutting conditions. A microscope was used for the observation of built-up edges at the cutting edge and burrs at the edge of the machined surface. To observe the micro-machined surface and micro-burr, SEM photography was also employed. The experimental conditions are shown in Table 5. A minuscule level of feed per tooth and axial depth of cut was used for micro-machining the aluminium alloy. Face milling as shown in Fig. 2(a) for variable cutting speeds, axial depths of cut and feed per tooth was carded out and sidedown milling for variable radial depths of cut was carried out according to the designed experimental conditions [see Fig. 2(b)]. Other conditions were set to the same degree in each experiment. Table 3. The details of workpiece A12024 Composition

Cu, 3.8-4.9%; Si, < 0.5%; Fe, < 0.5%; Mn, 0.3-0.9%; Mg, 1.21.8%; Zn, < 0.25%; Cr, < 0.1%; A1, the remainder Brinell hardness HB = 47 45 x 28 x 28 mm 3

Hardness Size

Table 4. The specifications of the carbide endmill Tool material

Sintered carbide

Shank diameter Diameter Total length Flute length Number of flutes Helix angle Axial rake angle Radial rake angle Face relief angle

6 mm 6 mm 50 mm 13 mm 2 15° 0° 0° 0.5 °

Table 5. Cutting conditions for the experiment Cutting speed (m/min)

Feed per tooth (/xm/tooth) Radial depth of cut (mm) Axial depth of cut (mm)

47-377

0.38-4.35

1-6

0.04-1

/ (a) Face milling

(b) Side milling

Fig. 2. The experimental process for the milling method.

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Jeong-Du Kim and Youn-Hee Kang

Slnlered Carbide EndmlU Low Cutting Speed {47m/rain] Conventional milling |

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edge

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Diamond Endmlll 1 High Cutting Speed(377rrVmln) J High speed diamond milling

Ra 60rim

axial depth of cut : 0.2mm radial depth of cut : 6 m m feed per tooth : 2.181am/tooth Fig. 3. The advantages of using high-speed and diamond endmills.

High-speed machining of aluminium

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4. EXPERIMENTAL RESULTS AND DISCUSSION

4.1. The advantages of using a high-speed spindle and diamond endmill Machined surfaces and cutting edges after milling magnified 50 times through a microscope are shown in Fig. 3. In the case of using a sintered carbide endmill at a low cutting speed (47 m/min) (conventional milling), it is known that the surface roughness value was Ra 1608 nm and a built-up edge formed at cutting edge. It is considered that irregular shapes in machined surface were formed by the over-cutting of variable built-up edges. When using the sintered carbide endmill at a high cutting speed (377 m/min) (high-speed milling), the surface roughness was Ra 130 nm. As can be seen from the figure the builtup edge did not form, but chipping occurred at the cutting edge after milling. This is considered to be a consequence of increasing the cutting speed. When using the diamond endmill at a high cutting speed (high-speed diamond milling), surface roughness value was Ra 60 nm and the built-up edge did not form, but chipping occurred at the cutting edge, too. This machined surface roughness is better than that when using the sintered carbide endmill at a high cutting speed, which is considered to be due to the sharpness of the diamond tool edge and the low friction coefficient of diamond with metals. 4.2. The effects of cutting tool and cutting speed on burr formation Figure 4 shows', at × 50 magnification the burrs which were formed at the edge of the machined surface. In the case of using the sintered carbide tool at a low cutting speed, the thickness and height of the burrs are about 0.3 and 0.5 mm, respectively. In the case of high-speed cutting using the same cutting tool, these values are about 0.17 and 0.2 mm, respectively; they have been significantly reduced, but not completely eliminated. In contrast, when using the diamond endmill, burrs were not observed with the naked eye both at low and in high cutting speeds. Therefore it can be said that the size of the burr is Diamond

Sintered carbide

Low

cu~ing speed (47m/min)

bF0.3mm

b~0.5mm

bt=-0.17ram

bh=0.2mm

High cutting speed (377m/rain)

axial depth of cut : 0.2mm radial depth of cut : 6mm feed per tooth : 2.18~m/tooth bh= height Of burr

Fig. 4. Burr formations according to cutting speed and tool.

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Jeong-Du Kim and Youn-Hee Kang

greatly influenced by the physical properties of the cutting tool, i.e. edge sharpness, friction coefficient, etc., rather than the cutting conditions such as cutting speed. Though the burr formation was not observed with the naked eye, the possibility of micro-burr formation was expected. Figure 5 shows the SEM photograph of burrs when using the diamond endmill. Figure 6 shows the thickness of burrs which was measured in Fig. 5 according to the cutting speed. As can be seen from the figure, the thickness of burrs decreases as the cutting speed increases, and, moreover, up-milling is more desirable than down-milling. Figure 7 shows the height of the burrs in Fig. 5. Figure 8 shows the height of burrs according to the cutting speed. The height of burrs decreases as cutting speed increases, and up-milling is clearly superior to down-milling. This fact indicates that, at the beginning of cutting, the oxide and/or adsorbed gas on the tool face prevents the adhesion of chips to the tool, reduces the friction, lowers the cutting force and shear strain and, as the result, reduces the burr size. However, as cutting progresses, this particular lubrication is diminished [5]. On this point, up-milling is superior to down-milling. 204 m/min

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axial d~th of cut : 0.2ram radial depth of cut : 6ram fi~edper tooth : 2.18 ¢tnv'tooth tool : diamond, workpiece:A12024

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Fig. 5. SEM photograph of burr by diamond milling. 0.1

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t78 m/min

95 m/min

377 m/min

axialdepthofctrt : 0.2ram radialdepthof cut : 6ram feedper tooth : 2.18 ~tm/tooth tool : diamond,workpiece:A12024 Fig. 7. SEM photograph of burr height by diamond milling. O.l

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Cutting speed (m/rain) cuttingconditkm: see Fig.5 Fig. 8. Relationship between height of burr and cutting speed. 4.3. The effects of cutting condition on surface roughness Figure 9 shows the bottom surface roughness in the feed direction according to cutting speed for the conventional sintered carbide endmill and the designed diamond endmill. From the figure it can be seen that surface roughness decreases as cutting speed increases. When using the diamond endmill, the overall magnitude of the surface roughness value is about one-third of that when using the sintered carbide endmill. Also, for both cases surface roughness does not improve or only improves very slowly at speeds above 170 m/min. Thus for economical machining of the aluminum alloy, it can be recommended to use a cutting speed of about 170 m/min. Figure 10 shows the surface roughness in the feed direction according to the axial depth of cut. Overall, file surface roughness increases as the axial depth of cut increases. A constant cutting speed of 188.5 m/min was used considering the experimental result shown in Fig. 9. In the case of using the diamond endmill, it is apparent that three characteristic regions exist for the value of axial depth of cut, i.e. region a, b and c. With a depth of cut between 0.08 and 0.5 mm, it is shown that surface roughness is almost constant (region

1162

Jeong-Du Kim and Youn-Hee Kang

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AxialdqXhofcut (ram) a,b,c region:variation of surface roeshness a~ordin8 to axial depth o f cut cuttin8 speed = 188,Sin/rain feed per tooth = 4.35~z/tcoth ratlial depth of cut = 6ram Fig. 10. Relationship between surface roughness and axial depth of cut.

b). A depth of cut less than 0.08 mm causes the surface roughness to decrease rapidly as the depth of cut decreases (region a). Finally, for depths of cut above 0.5 mm, surface roughness increases rapidly (region c). From the figure it is possible to approximate the surface roughness curve as a exponential for region a, a constant value for region b and a polynomial for region c. The trend shown in region c is inferred as the result of tool deflection. As shown later, in this region the formation of built-up edges is more likely than in other regions (regions a and b). Figure 11 shows the surface roughness according to the radial depth of cut. Surface roughness is almost constant for both cases and the magnitude is far lower in the case of the diamond endmill than the conventional endmill. Thus the radial depth of cut does not have an effect on the surface roughness. Figure 12 shows the surface roughness according to feed per tooth. Surface roughness

High-speed machining of aluminium

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Fig. 11. Relationship between surface roughness and radial depth of cut. 180

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a ~ l d q ~ of cut = O.2mm Fig. 12. Relationship between surface roughness and feed per tooth.

slowly increases as feed per tooth increases, but the rate of increase is smaller than that in the case of the axial depth of cut. In the case of using the diamond endmill, the surface roughness varies little as feed per tooth increases. The slope of the sintered carbide endmill is steeper than that of the diamond endmill. 4.4. Mirror-like milling o f aluminium

From the experiments, the best conditions for high surface quality were selected as follows: a cutting speed of 377 m/rain, a feed per tooth rate of 0.38/~m/tooth; a radial depth of cut of 6 mm; and 0.04 mm for the axial depth of cut. These are shown in Table 6. Table 6. Cutting conditions for the mirror-like surface Cutting speed (m/mJLn)

Feed per tooth (/.~m/tooth) Radial depth of cut (ram) Axial depth of cut (mm)

377

0.38

I ~ 3?db[

6

0.04

1164

Jeong-Du Kim and Youn-Hee Kang

Figure 13(a) shows the machined surfaces using diamond and sintered carbide endmills according to the cutting conditions listed in Table 6. A mirror-like surface was obtained using the diamond tool, where the surface roughness of Ra 30 nm and a burr-free surface was possible. In contrast, when using the sintered carbide endmill, surface roughness is very poor and many burrs are formed. Figure 13(b) shows the SEM photographs of the each regions designated in Fig. 13(a). A and B are the machined surfaces for the sintered

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(b) SEM photograph Fig. 13. Machined surface by diamond endmill and by sintered carbide endmill and its SEM photograph.

High-speed machining of aluminium

1165

carbide and diamond endmills, respectively, in which the machining scratch or groove for both cases is compared. C and D are the SEM photographs of burrs formed during machining. It can be ,;een that the diamond endmill has greatly reduced the burr formation as predicted. This is considered mainly due to the differences in material properties between the two endmills, i.e. cutting edge sharpness, friction coefficient, thermal conductivity, etc. These properties generally cause the built-up edge which results in poor surface quality. 5. CONCLUSIONS The finishing milling experiments of an aluminum alloy using a specially designed diamond endmidl and high-speed spindle were carded out, and the results obtained are as follows: (1) A mirror-like aluminium surface without burrs can be obtained by high-speed diamond milling, which does not require additional finishing processes such as polishing, lapping, etc. (2) For the finishing milling of aluminum alloy, the most dominant machining condition affecting the surface quality is axial depth of cut. (3) For a high quality of machined aluminum surface, it is recommended that the machining condition should be a high cutting speed, a low level of axial depth of cut and a low feed speed per tooth. Radial depth of cut has little effect on the surface quality. (4) The most dominant factors of deburring are the sharpness of the tool edge and the friction coefficient with the workpiece. (5) The size of burrs in up-milling is smaller compared to down-milling. REFERENCES [1] H. Schulz and T. Moriwaki, High-speed machining, CIRP Ann. 41(2), 637-643 (1992). [2] T. Nishiguchi, Y. Maeda, M. Masuda and M. Sawa, Mechanism of micro chip formation in diamond turning of A1-Mg alloy, CIRP Ann. 37, 117-120 (1988). [3] Y. Takeuchi, K. Kato and S. Kawakita, Generation of sculptured surfaces by means of an ultraprecision milling machine, C1RP Ann. 42(1), 611-614 (1993). [4] Nam-Sub Seo, Metal Cutting Theory. Dong myoung sa, Korea (1985) [5] K. Nakayama and M. Arai, Burr formation in metal cutting, CIRP Ann. 36(1), 33-36 (1987). [6] Young-Ha Yum, The Cutting Theory of Machine Tool. Dong myoung sa, Korea (1992)