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Experimental evaluation of minimum quantity lubrication in near micro-milling
Journal of Materials Processing Technology 210 (2010) 2163–2170
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Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Experimental evaluation of minimum quantity lubrication in near micro-milling Kuan-Ming Li a,∗ , Shih-Yen Chou b a b
Department of Mechanical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
a r t i c l e
i n f o
Article history: Received 29 March 2010 Received in revised form 23 July 2010 Accepted 30 July 2010
Keywords: Minimum quantity lubrication Near micro-milling Tool flank wear Surface roughness
a b s t r a c t This paper presents the performance of the minimum quantity lubrication (MQL) technique in near micromilling with respect to dry cutting on the basis of tool wear, surface roughness and burr formation. The effects of tool materials, oil flow rate and air flow rate on tool performance in MQL cutting are also studied. It is found that the application of MQL will significantly improve the tool life, surface roughness and burr formation compared to those in dry cutting based on slotting tests with micro-end mills on a meso-scale machine tool. It is also observed that the values of surface roughness are close related to the tool-wear conditions in micro-cutting. Based on the experimental results, it is presumed that the maximum allowable tool flank wear of the 600-m micro-tool is 80 m while the surface finish quickly deteriorates after the tool flank wear reaches 80 m and the tool breaks soon after the tool wear reaches 100 m. The optimal lubrication conditions in this study are oil flow rate of 1.88 ml/h and air flow rate of 40 l/min. It is also found that the air flow rate has a more significant influence on tool life than the oil flow rate under MQL conditions in this study. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The functions of cutting fluids in machining processes are to cool, to lubricate and to make chip-transport easier, and as a result, to improve the tool life, the product surface finish and the dimension accuracy. However, the introduction of cutting fluids often produces airborne mist in the shop floor. The cutting fluid mist brings in the environmental and health concerns. In addition, the costs related to cutting fluids are significant in machining processes (Klocke and Eisenblaetter, 1997). In order to alleviate the environmental and economical impacts, minimum quantity lubrication (MQL) was addressed as an alternative to the conventional flood cooling application a decade ago (Heisel et al., 1994; Klocke and Eisenblaetter, 1997). Minimum quantity lubrication, also known as near dry machining (NDM), refers to the use of cutting fluids in tiny quantities, which is only about ten-thousandth of the amount of cutting fluid used in flood-cooled machining (Machado and Wallbank, 1997; Rahman et al., 2002). Machado and Wallbank (1997) applied 200–300 ml/h of lubricant in a flow of air at a pressure of 2 bar when turning medium carbon steel. The experimental results showed that surface finish, chip thickness and cutting forces variations were all improved compared to those under the conventional flood cooling situation. Varadarajan et al. (2002) conducted experiments on hard turning
∗ Corresponding author. Tel.: +886 2 33664516x4280; fax: +886 2 23631755. E-mail address: [email protected] (K.-M. Li). 0924-0136/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2010.07.031
AISI 4340 with 2 ml/h oil, which was applied in a fast flowing air stream of 20 MPa. Cutting forces, cutting temperatures, surface finish, and tool life were all affected beneficially compared to those obtained in dry or wet cutting. Diniz et al. (2003) applied 10 ml/h oil in an air flow at a pressure of 4.5 bar in turning AISI 52100 steel with CBN tools. Similar values of tool flank wear were observed in dry and minimum quantity lubrication, always smaller than the values for wet cutting. The workpiece surface roughness measured in near dry cutting was close to that obtained from dry cutting and wet cutting. Heinemann et al. (2006) investigated the effect of minimum quantity lubricant on tool life in drilling processes. The cutting fluid flow rate was 18 ml/h. It was found that a continuous supply of minimum quantity lubricant conveyed a longer tool life while a discontinuous supply of lubricant resulted in a reduction of tool life. Chen et al. (2001) investigated the effects of oil–water combined mist on turning stainless steel with the use of 17 ml/h oil and 150 ml/h water mixture. The workpiece surface finish under oil–water combined mist was better than that under dry, oil mist or water soluble oil applications. Lopez de Lacalle et al. (2006) studied the effects of MQL on tool wear in high speed milling aluminum alloys. They also performed computational fluid dynamics (CFD) simulations for estimating the penetration of the cutting fluid to the cutting zone. The results showed that with the help of compressed air, the oil mist with high velocity could be effectively delivered to the inner zones of the tool edges and provide sufficient cooling and lubricating. Jun et al. (2008) studied the effect of cutting fluids on micro-end milling. In addition to the application of MQL in the cutting tests, they also applied large drops of the cutting fluid to
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simulate the wet cutting in micro-end milling. The results showed that lower cutting forces and longer tool life were observed under the minimum quantity lubrication when compared to dry and flood cooling methods. In terms of cutting fluid properties, it was shown that cutting fluids with lower surface tension and higher viscosity result in lower cutting forces. While showing possible benefits for long tool life (Weinert et al., 2004), which is one of the main reasons in achieving high productivity, minimum quantity lubrication has only limited applications so far. This is because of the limited understanding of the optimal near dry lubrication parameters for different machining processes. Moreover, the application effectiveness of cutting fluids in microcutting is not clear (Friedrich, 2000; Jun et al., 2008), not to mention the application of MQL in micro-scale machining operations. This is the first paper to systematically study the effects of MQL in near micro-cutting on a meso-scale machine tool. Micro-machine tools hold several benefits from this miniaturization including reduced thermal distortion, the increase of static stiffness, the improvement of dynamic stability, and the lowering of machine cost (Okazaki et al., 2004; Liang, 2006). However, most of those micro-scale machine tools are developed in the laboratories where the electronic devices are widely adopted for the construction of micro-machine tools such as high-speed spindles, linear stages and high-resolution controllers. In that environment, the traditional flood cooling is not an appropriate method to supply the cutting fluid. Therefore, the minimum quantity lubrication is an suitable alternative to provide both cooling and lubricating effects in micro-cutting with less impact on the electronic devices. The objective of this research is to study the tool wear and surface quality in the near micro-milling process on a meso-scale machine tool. The comparison between minimum quantity lubrication and completely dry cutting process is presented. Experiments on the subject of the lubrication parameters, oil flow rate and air flow rate, are also performed in this study.
Fig. 1. A miniaturized milling machine.
Table 1 Cutting conditions. Work material
SKD 61 steels (hardness: HRC38)
Spindle rotational speed Feed Depth of cut Air supply Lubricant supply
20,000, 30,000 and 40,000 rpm 1.0, 1.5 and 2.0 m/rev 300 m 25 and 40 l/min at 0.5 MPa 1.88, 3.75 and 7.5 ml/h
2. Experimental setup The setup of the meso-scale milling machine in this study is shown in Fig. 1. This desktop milling machine consists of a 3-axis machining table (linear AC motor with 0.6 nm resolution and 25mm travel length in both X and Y directions), a knee-column frame, and a spindle (125 W, 5000–50,000 rpm). 600-m diameter 2-flute flat end mills are used in this study. The micro-tools have two teeth. This represents a machine tool 5000 times smaller than traditional machine tools and 1000 times higher in resolution and accuracy than commercial machine tools. After slotting square grooves on the workpiece with a flat end mill, tool wear and workpiece surface roughness are measured. The oil mist is delivered to the cutting zone by a cutting fluid applicator (Bluebe FK type). In the experiments, the Blube system is used to supply the air–fluid mixture with the oil flow rates of 1.88–7.5 ml/h at a pressure of 0.5 MPa. Bluebe (Accu-lube) lubricant LB-1, a vegetable oil, is chosen as the cutting fluid. The slotting experiments are done on the miniature machine tool at 300 m axial depth of cut while three different spindle speeds and three different feed rates are adopted as shown in Table 1. The flank wear land length is measured for each slot with an optical microscope when slotting SKD61 (with hardness of HRC38) with uncoated carbide end mills on the micro-milling system. The workpiece is 40 mm by 90 mm by 5 mm. Seven slots are cut in the middle of the work piece. Each cutting pass produce a 24 mm long slot. The closed view of the experimental setup is shown in Fig. 2. The cutting fluid is directed to the micro-end mill as shown in the figure. Pretests are performed in order to make sure that the tool will break
in the limited cutting length in dry cutting because the linear stages only provide 25-mm travel length and only 7 slots are cut on one workpiece. After the pretests, the machining tests are performed under different cutting conditions, as shown in Table 1. The same cutting conditions are also applied to dry cutting as comparison. 3. Results and discussions The effects of the MQL on tool flank wear, surface roughness and burr formation are discussed in the following sections. In these sections, the lubricating conditions of MQL are oil flow rate of 7.5 ml/h
Fig. 2. The close view of the experimental setup.
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Fig. 3. Example of tool flank wear measurement (spindle speed = 30,000 rpm, feed = 1 m/rev and cutting length = 96 mm under MQL). (a) New tool and (b) worn tool.
and air flow rate of 40 l/min. As a comparison, dry cutting tests are also carried out. Following these discussions on the capacity of MQL in near micro-milling, the experimental results for different lubricating conditions are presented. By examining the experimental data, the optimal lubricating conditions in near micro-milling for the tool, the work material and the lubricant used in this paper are proposed. 3.1. Tool flank wear Fig. 3 shows the evaluation of the tool flank wear for the micro-tools in this paper. It is usually suggested to take several measurements of the tool wear and use the average value as the flank wear value. Nevertheless, it is seen that the flank wear is almost linearly distributed from the center of the cutter. The value of the flank wear (VB) measured at a distance of one third of the diameter from the center is considered as the representative tool flank wear in the following discussions, as shown in the figure. The value of the tool flank wear is read from the CCD measuring system attached to the microscopic after calibration with a standard scale. The reason for not using the largest value of tool flank wear is to avoid the measurement error due to the loss of the tool cutting edge. Fig. 4 shows the gradual tool flank wears of the micro-tools under different feeds while the spindle speed is 30,000 rpm for both dry and MQL conditions. In the figure, it is shown that the tool life decreases with respect to the decreased feed under both dry and MQL conditions. For dry cutting, the cutting length for the case
Fig. 4. Tool flank wears for different feeds under dry and MQL conditions (spindle rotational speed is 30,000 rpm).
of 2 m/rev feed is more than 168 mm while the cutting lengths before tool breakage for the cases of 1 m/rev and 1.5 m/rev feed are 96 and 120 mm, respectively. This phenomenon is different from the observations in conventional milling processes in which the tool life increases with regard to the decreased feeds. A similar result was also observed by Filiz et al. (2007). It should be also noticed that the results in Fig. 4 are based on the amount of cutting lengths, in which the tools at lower feeds are experienced longer periods of cutting time. The extreme low feed in near micro-milling process is another possible reason to observe these tool wear trends. The nose radius of the cutting tools observed by the microscopic is around 5 m. The feed per revolution and the rounding radius of the cutting edge are comparable. In near microcutting, smaller feed will lead to the significant phenomenon of negative effective rake angle which will impedes the chip flow and thus faster tool wear (Woon et al., 2008). Moreover, the tool flank wears before tool breakages are around 100 m under the feeds of 1 and 1.5 m/rev. It is reasonable to assume that the tool life for the micro-tools used in this study is 100 m. However, the value of 100 m for flank wear compared to 600 m tool diameter is high. It is recommended to set a lower value for the limited flank wear for 600-m diameter micro-tools. For MQL cutting tests, the cutting lengths under all feeds are more than 168 mm. According to the trends in Fig. 4, the cutting lengths under MQL conditions are more than 390 mm before the tool breaks. The progressing tool wears under the feeds of 1.5 and 2 m/rev are comparable throughout the cutting tests. The flank wear under the feed of 1 m/rev is similar to that under the other two feeds before the cutting lengths of 72 mm. After that, the tool wears faster. The maximum difference of tool wears between the feed of 1 m/rev and 1.5 or 2 m/rev is about 10 m after cutting 168 mm long material. It is obvious to see in Fig. 4 that the application of MQL in near micro-milling can significantly extend the tool life under different feeds. For example, after cutting 96 mm long work material, the reductions of tool flank wear lengths in MQL cutting compared to dry cutting are 67.65%, 62.66% and 54.59% under the feeds of 1.0, 1.5 and 2 m/rev, respectively. Although the higher feed leads to a longer tool life in near micro-milling in this study, it is not necessary to select a higher feed for efficient near micro-milling because an excessive feed will cause the tool to break immediately. Tool breakage is one of the main tool failure modes in near micro-cutting with extremely small tools. Fig. 5 shows the progressive tool flank wears of the micro-tools under different spindle speeds while the feed is fixed at 1 m/rev for both dry and MQL conditions. It is observed that the tool flank wear rates for dry cutting are similar under different cutting speeds
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Fig. 5. Tool flank wears for different spindle rotational speeds under dry and MQL conditions (feed is 1 m/rev).
Fig. 7. Surface roughness for different spindle rotational speeds under dry and MQL conditions (feed is 1 m/rev).
based on cutting lengths. However, the cutting length for the spindle rotational speed of 40,000 rpm is 72 mm while the cutting length is 96 mm for the spindle speeds of both 20,000 rpm and 30,000 rpm. It is indicated that the higher cutting speed results in a shorter tool life. At the same time, the tool wear rates for MQL conditions are comparable for all cutting speeds in this study. None of the micro-tools breaks after cutting 168 mm long material. The experimental data show clearly that the supply of the oil mist can improve the tool life under all cutting speeds. The reductions of tool flank wear lengths in MQL cutting compared to dry cutting are about 68% under all cutting speeds in this paper after cutting 96 mm long work material. It is practical to presume the maximum tool flank wear of the micro-tools utilized in this study is 100 m according to Figs. 4 and 5.
1.5 and 2 m/rev. The values of surface roughness increase abruptly to 1.0 and 0.8 m for the feed of 1.5 m/rev and for the feed of 2 m/rev before the tool breakage. Comparing Figs. 4 and 6 for dry cutting conditions, it is found that the values of surface roughness gradually increase when the tool flank wears reach in the vicinity of 80 m. The surface roughness is more than 0.8 m and the tools break in the following slotting tests when the flank wears are more than 90 m. Fig. 7 shows the surface roughness of the machined surfaces under different spindle rotational speeds under both dry and MQL conditions. Similar to the results in the previous figure, surface roughness under MQL conditions does not change much under different cutting speeds. The values of surface roughness (Ra ) range below 0.2 m. The values of surface roughness under dry cutting depend on the tool flank wears. The values of surface roughness progressively increase when the tool flank wears achieve 80 m. The surface roughness suddenly deteriorates and the tools break soon after the flank wears attain 90 m. In short, it is conservative to presume the maximum tool flank wear of the micro-tool is 80 m while the surface finish abruptly deteriorates and the tool breaks shortly after the tool flank wear reaches 80 m. The surface profile for the machined surface under MQL condition is shown in Fig. 8. The surface profile is measured by a whitelight-confocal microscope (NanoFocus® surf® ). It is shown
3.2. Surface roughness Fig. 6 shows the effect of the cutting lengths and the feeds on surface roughness of the machined surfaces under both dry and MQL environments. It is observed in the figure that the surface roughness under MQL cutting does not change much for all cutting tests. The values of surface roughness (Ra ) range between 0.1 and 0.2 m. The values of surface roughness do not change with respect to the cutting lengths or the feeds. However, in dry milling, the values of surface roughness increase with regard to the cutting lengths under all feeds. For the case of 1 m/rev feed, the surface roughness of the machined workpiece is below 0.2 m before cutting 48 mm long material. The surface roughness is more than 0.2 m after slotting 72 mm long material and suddenly increases to 1.1 m after cutting 96 mm long material. Similar results are observed for the feeds of
Fig. 6. Surface roughness for different feeds under dry and MQL conditions (spindle rotational speed is 30,000 rpm).
Fig. 8. Surface profile for the machined surface under MQL condition (spindle speed = 30,000 rpm, feed = 2 m/rev and cutting length = 168 mm).
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Table 2 Burr formation for different feeds (spindle rotational speed = 30,000 rpm). Feed (m/rev)
Dry cutting 1 1.5 2 MQL cutting 1 1.5 2
Cutting length (mm) 24
48
72
96
120
144
168
R R R
S W W
S S W
S S S
S S
S
S
R R R
R R R
R R R
R R R
R R R
R R R
R R R
Note: R: no or slight burr formation; W: wavy burr formation observed; S: both wavy and needle-like burr formation observed.
Fig. 9. Surface profile for the machined surface under dry condition (spindle speed = 30,000 rpm, feed = 2 m/rev and cutting length = 168 mm).
in the figure that the machined surface is U-shape after slotting 168 mm long material. This means that the tool wear at the perimeter is more severe when the tool sees larger cutting speeds. However, the tool flank wear is limited in MQL milling so the surface roughness is acceptable along the feed direction. Fig. 9 shows the surface profile under the same cutting conditions as in Fig. 8 but without any lubricant applied. It is found that the center of the slot is damaged by the worn tool and a rough surface is left along the feed direction. Therefore the values of the surface roughness along the feed direction in dry cutting are larger than those observed in MQL machining. The bumpy surface is caused by the tool tip rubbing on the workpiece. Because the edge of the tool wears faster during cutting under higher cutting speeds, the tip of the end mill has a cone shape. However, the tip of the micro-tool does not have much capacity to remove the material. The material removing near the tool tip is a rubbing process rather than a milling process. When the cone shape is considerable compared with that of a new tool, a rough machined surface is observed. 3.3. Burr formation Fig. 10 shows the burr formation in dry cutting in this study. The burr formation is observed in all cutting tests. In the slot milling
tests, it is found that the down-milling burrs (on the right of the slot in the figure) are larger and up-milling burrs are smaller as identified by Lee and Dornfeld (2002) in their work. The first 24mm slots under both dry and MQL conditions only create slight burrs on both down-milling and up-milling sides. Nevertheless, the burr heights in dry cutting increase drastically since slotting 24-mm long material. On the other hand, the burr sizes do not change much with increasing cutting length under MQL conditions. The burr formation under different feeds for dry and MQL machining is shown in Table 2. When wavy burrs are observed on the down-milling side and needle-like burrs are observed on the up-milling side, it is noted as “S” in Table 2. When slight wavy burrs are noticed on the down-milling side and very small burrs are observed on the up-milling side, it is noted as “W” in Table 2. When only very small burrs observed on both side of the slots as shown, it is noted as “R” in Table 2. For dry cutting, it is found that larger burrs are observed for small feed of 1 m/rev, followed by the feed of 1.5 m/rev and 2 m/rev. The same results are also seen in Table 2 in which wavy burrs and needle-like burrs are seen for lower feeds with less material removed. In contrast, the burr formation is not strongly affected by the increasing cutting lengths under MQL slotting. In all cutting tests under MQL conditions, only small burrs are observed. The use of MQL in near micro-milling could reduce the burr formation. It is deduced from the relationships among tool wear, surface roughness and burr formation that the diminished burr formation under MQL conditions is partly attributed to the low tool wear in MQL machining. 3.4. Different tool materials It is known that for tungsten carbide tools, the quantity of cobalt contained in the tool has an effect on its hardness and tool life (Trent and Wright, 2000). Tool hardness decreases with respect to the increased Co content. In this section, the effect of the content of Co in carbide tools on the tool life under both dry and MQL conditions are discussed. The cutting tests are performed under the spindle speed of 30,000 rpm and the feed of 1 m/rev. Two different Co-content tools are listed in Table 3. Tool A and Tool B have the same tool geometry but different Co content, i.e. different tool material. In fact, Tool A and Tool B are under the same specifications by the tool manufacturer. Nevertheless, the tool manufacturer does Table 3 Different tool materials for cutting tests. Tool A
Fig. 10. Burr formation under dry cutting for spindle rotati0nal speed = 30,000 rpm, feed = 2 m/rev, and depth of cut = 300 m.
Tool materials (EDS quantitative results)
Tool B
Elememt
wt%
Element
wt%
C O W Co
9.20 1.39 69.23 20.18
C O W Co
8.80 1.06 76.78 13.35
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Fig. 11. Tool wear progression for speed = 30,000 rpm and feed = 1 m/rev).
different
tool
materials
(spindle
not provide detailed tool geometry and chemical composition. It is assumed that the tool geometry variation is limited and its effect on the tool life is ignored in this study. The element contents of new tools are examined by EDS. It is seen from Table 3 that Tool A has a higher content of Co compared to that of Tool B. The tool wear progression for different lubrication conditions and tools are shown in Fig. 11. It is not surprising to observe that the tool wear rates for Tool A are higher than those of Tool B. The supplies of MQL reduce the tool wear rates for both Tool A and Tool B. Example pictures of worn tools for Tool A and Tool B are shown in Fig. 12. The effect of MQL in near micro-milling for different Co-content carbides is compared by dividing the tool flank wear under MQL by the tool flank wear under dry cutting, as shown in Fig. 13. It is shown in the figure that the tool wear ratio is trending down with a decreasing rate. Moreover, the tool wear ratios are close for Tool A and Tool B. The significant difference of tool wears for cutting length of 24 mm is attributed to the fast initial tool wear in cutting. After that, tool wear behaviors for Tool A and Tool B are similar. It is presumed that the quantity of Co in tungsten carbide tools is not involved in the performance of MQL for tool wear progressions. In the following sections, Tool B is used for discussing the effect of the lubrication conditions of MQL on tool life. 3.5. Different oil flow rates Two parameters of the MQL, oil flow rate and air flow rate, are discussed as following. The effect of the oil flow rates on tool life
Fig. 13. Normalized tool wear progression for different tool materials (spindle speed = 30,000 rpm and feed = 1 m/rev).
Fig. 14. Tool wear progression for different oil flow speed = 30,000 rpm, feed = 1 m/rev and air flow rate = 40 l/min).
rates
(spindle
is presented in this section while the effect of the air flow rates is discussed later. Three different oil flow rates are used in the cutting tests. They are 1.88, 3.75 and 7.5 ml/h. In addition to the dry cutting test, a milling operation with the supply of air without any oil is also performed for comparison. The results are shown
Fig. 12. Pictures of worn tools (spindle speed = 30,000 rpm, feed = 1 m/rev and cutting length = 120 mm under MQL).
K.-M. Li, S.-Y. Chou / Journal of Materials Processing Technology 210 (2010) 2163–2170
Fig. 15. Tool wear progression for different air flow speed = 30,000 rpm, feed = 1 m/rev and oil flow rate = 7.5 ml/h).
rates
(spindle
in Fig. 14. The tool wear progressions are similar when the oil flow rate decreases from 7.5 to 1.88 ml/h. This is attributed to the small tool used in near micro-milling. The small amount oil supplied in cutting, such as 1.88 ml/h, is sufficient for lubrication in this machining operation. However, the use of pure air in near micro-milling does not improve the tool life compared with that of dry cutting. The supply of small amount of oil is necessary to extend the tool life in near micro-cutting. It is inferred that the cooling effect of air does not improve the tool life while the lubricating effect of oil under MQL is important for long tool life of micro-tools.
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sible tool flank wear of 100 m in length before tool breakage is observed in dry cutting. Surface roughness measurements when milling SKD 61 steels show that minimum quantity lubrication is better than that of dry cutting in terms of surface roughness. The values of surface roughness (Ra ) under MQL are smaller than 0.2 m and do not change much with respect to the cutting speeds or the feeds. In dry cutting, the values of surface roughness increase abruptly when the tool flank wears reach 80 m. It is conservative to presume that the maximum tool flank wear of the micro-tool is 80 m while the surface finish suddenly gets worse after the tool flank wear reaches 80 m. Burr formation can be alleviated by the use of minimum quantity lubrication. It is found that from the relationship between tool wear and burr formation, the diminished burr formation under MQL conditions is attributed to the low tool wear in near micro-cutting. The effect of MQL in near micro-milling on different Co-content tungsten carbides is negligible in this study. It is observed that the quantity of Co in tungsten carbide tools is not involved in the performance of MQL for tool wear progressions. The small amount oil of 1.88 ml/h supplied in cutting tests is sufficient for lubrication in near micro-cutting while the use of pure air in near micro-milling does not improve the tool life. It is inferred that the lubricating effect of oil under MQL is important for long tool life of micro-tools. It is observed that use of 40 l/min air flow in MQL is sufficient in near micro-slotting tests. The effect of air flow rate in near micromilling under MQL is to increase the oil mist velocity and to reduce the oil drop size. Acknowledgements
3.6. Different air flow rates The effect of air flow rate on the tool life in MQL cutting is shown in Fig. 15. It is shown that the tool wear rate for dry cutting is maximum followed by the use of MQL with 25 l/min air flow and subsequently 40 l/min air flow. The tool wear for the use of 40 l/min air flow is not significant after slotting 168 mm material. It is clear that use of 40 l/min air flow in MQL is sufficient in this study. The outcome of air flow rate to the effect of MQL in near micro-milling is to increase the oil mist velocity and to reduce the oil drop size. Both could provide better penetration of the oil mist to the cutting zone (Lopez de Lacalle et al., 2006). Smaller oil drops also bring more heat away the cutting zone by evaporation. It is deserved to be mentioned that the cutting heat in near micro-milling is only a small amount. The better penetration of small oil drops providing better lubrication is assumed to be the major function of higher air flow rate for reducing tool wear. Comparing the oil flow rates and air flow rates used in previous and this section, it is found that the air flow rate has a more considerable effect on tool life than the oil flow rate in this study. 4. Conclusions This paper compares the mechanical performance of MQL to completely dry condition for the near micro-milling of SKD 61 steels based on experimental investigations in terms of tool wear, surface finish and burr formation. Micro-tools with 600 m diameter are used to slotting the hardened steels. The following conclusions can be drawn from the findings of this study. Minimum quantity lubrication in near micro-cutting helps to reduce flank wear for all cutting conditions chosen in this study and hence is expected to improve tool life. The reductions of tool flank wear lengths in MQL cutting compared to dry cutting are about 60% under all cutting conditions in this paper. The maximum permis-
The authors would like to express their appreciation to National Science Council in Taiwan for their financial support of this research. References Chen, D.C., Suzuki, Y., Sakai, K., 2001. A study of turning operation by oil–water combined mist lubrication machining method. Key Engineering Materials 202–203, 47–52. Diniz, A.E., Ferreira, J.R., Filho, F.T., 2003. Influence of refrigeration/lubrication condition on SAE 52100 hardened steel turning at several cutting speeds. International Journal of Machine Tools and Manufacture 43 (3), 317–326. Filiz, S., Conley, C.M., Wasserman, M.B., Ozdoganlar, O.B., 2007. An experimental investigation of micro-machinability of copper 101 using tungsten carbide micro-endmills. International Journal of Machine Tools and Manufacture 47 (7–8), 1088–1100. Friedrich, C.R., 2000. Near-cryogenic machining of polymethyl methacrylate for micromilling tool development. Materials and Manufacturing Processes 15 (5), 667–678. Heinemann, R., Hinduja, S., Barrow, G., Petuelli, G., 2006. Effect of MQL on the tool life of small twist drills in deep-hole drilling. International Journal of Machine Tools and Manufacture 46 (1), 1–6. Heisel, U., Lutz, M., Spath, D., Wassmer, R., 1994. Application of minimum quantity cooling lubrication technology in cutting process. Production Engineering II (1), 49–54. Jun, M.B.G., Joshi, S.S., DeVor, R.E., Kapoor, S.G., 2008. An experimental evaluation of an atomization-based cutting fluid application system for micromachining. Journal of Manufacturing Science and Engineering, Transactions of the ASME 130 (3), 0311181–0311188. Klocke, F., Eisenblaetter, G., 1997. Dry cutting. CIRP Annals: Manufacturing Technology 46 (2), 519–526. Lee, K., Dornfeld, D.A., 2002. An experimental study on burr formation in micro milling aluminum and copper. United states, Society of Manufacturing Engineers, West Lafayette, ID, 1–8. Liang, S.Y., 2006. Mechanical machining and metrology at micro/nano scale, Xinjiang, China. International Society for Optical Engineering, Bellingham, WA, United States, 628002. Lopez de Lacalle, L.N., Angulo, C., Lamikiz, A., Sanchez, J.A., 2006. Experimental and numerical investigation of the effect of spray cutting fluids in high speed milling. Journal of Materials Processing Technology 172 (1), 11–15. Machado, A.R., Wallbank, J., 1997. Effect of extremely low lubricant volumes in machining. Wear 210 (1–2), 76–82.
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Okazaki, Y., Mishima, N., Ashida, K., 2004. Microfactory—concept, history, and developments. Journal of Manufacturing Science and Engineering, Transactions of the ASME 126 (4), 837–844. Rahman, M, Senthil Kumar, A., Salam, M.U., 2002. Experimental evaluation on the effect of minimal quantities of lubricant in milling. International Journal of Machine Tools and Manufacture 42 (5), 539–547. Trent, E.M., Wright, P.K., 2000. Metal Cutting. Butterworth-Heinemann, Boston. Varadarajan, A.S., Philip, P.K., Ramamoorthy, B., 2002. Investigations on hard turning with minimal cutting fluid application (HTMF) and its comparison with dry and
wet turning. International Journal of Machine Tools and Manufacture 42 (2), 193–200. Weinert, K., Inasaki, I., Sutherland, J.W., Wakabayashi, T., 2004. Dry machining and minimum quantity lubrication. CIRP Annals: Manufacturing Technology 53 (2), 511–537. Woon, K.S., Rahman, M., Fang, F.Z., Neo, K.S., Liu, K., 2008. Investigations of tool edge radius effect in micromachining: a FEM simulation approach. Journal of Materials Processing Technology 195 (1–3), 204–211.
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Experimental evaluation of minimum quantity lubrication in near micro-milling Kuan-Ming Li a,∗ , Shih-Yen Chou b a b
Department of Mechanical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
a r t i c l e
i n f o
Article history: Received 29 March 2010 Received in revised form 23 July 2010 Accepted 30 July 2010
Keywords: Minimum quantity lubrication Near micro-milling Tool flank wear Surface roughness
a b s t r a c t This paper presents the performance of the minimum quantity lubrication (MQL) technique in near micromilling with respect to dry cutting on the basis of tool wear, surface roughness and burr formation. The effects of tool materials, oil flow rate and air flow rate on tool performance in MQL cutting are also studied. It is found that the application of MQL will significantly improve the tool life, surface roughness and burr formation compared to those in dry cutting based on slotting tests with micro-end mills on a meso-scale machine tool. It is also observed that the values of surface roughness are close related to the tool-wear conditions in micro-cutting. Based on the experimental results, it is presumed that the maximum allowable tool flank wear of the 600-m micro-tool is 80 m while the surface finish quickly deteriorates after the tool flank wear reaches 80 m and the tool breaks soon after the tool wear reaches 100 m. The optimal lubrication conditions in this study are oil flow rate of 1.88 ml/h and air flow rate of 40 l/min. It is also found that the air flow rate has a more significant influence on tool life than the oil flow rate under MQL conditions in this study. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The functions of cutting fluids in machining processes are to cool, to lubricate and to make chip-transport easier, and as a result, to improve the tool life, the product surface finish and the dimension accuracy. However, the introduction of cutting fluids often produces airborne mist in the shop floor. The cutting fluid mist brings in the environmental and health concerns. In addition, the costs related to cutting fluids are significant in machining processes (Klocke and Eisenblaetter, 1997). In order to alleviate the environmental and economical impacts, minimum quantity lubrication (MQL) was addressed as an alternative to the conventional flood cooling application a decade ago (Heisel et al., 1994; Klocke and Eisenblaetter, 1997). Minimum quantity lubrication, also known as near dry machining (NDM), refers to the use of cutting fluids in tiny quantities, which is only about ten-thousandth of the amount of cutting fluid used in flood-cooled machining (Machado and Wallbank, 1997; Rahman et al., 2002). Machado and Wallbank (1997) applied 200–300 ml/h of lubricant in a flow of air at a pressure of 2 bar when turning medium carbon steel. The experimental results showed that surface finish, chip thickness and cutting forces variations were all improved compared to those under the conventional flood cooling situation. Varadarajan et al. (2002) conducted experiments on hard turning
∗ Corresponding author. Tel.: +886 2 33664516x4280; fax: +886 2 23631755. E-mail address: [email protected] (K.-M. Li). 0924-0136/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2010.07.031
AISI 4340 with 2 ml/h oil, which was applied in a fast flowing air stream of 20 MPa. Cutting forces, cutting temperatures, surface finish, and tool life were all affected beneficially compared to those obtained in dry or wet cutting. Diniz et al. (2003) applied 10 ml/h oil in an air flow at a pressure of 4.5 bar in turning AISI 52100 steel with CBN tools. Similar values of tool flank wear were observed in dry and minimum quantity lubrication, always smaller than the values for wet cutting. The workpiece surface roughness measured in near dry cutting was close to that obtained from dry cutting and wet cutting. Heinemann et al. (2006) investigated the effect of minimum quantity lubricant on tool life in drilling processes. The cutting fluid flow rate was 18 ml/h. It was found that a continuous supply of minimum quantity lubricant conveyed a longer tool life while a discontinuous supply of lubricant resulted in a reduction of tool life. Chen et al. (2001) investigated the effects of oil–water combined mist on turning stainless steel with the use of 17 ml/h oil and 150 ml/h water mixture. The workpiece surface finish under oil–water combined mist was better than that under dry, oil mist or water soluble oil applications. Lopez de Lacalle et al. (2006) studied the effects of MQL on tool wear in high speed milling aluminum alloys. They also performed computational fluid dynamics (CFD) simulations for estimating the penetration of the cutting fluid to the cutting zone. The results showed that with the help of compressed air, the oil mist with high velocity could be effectively delivered to the inner zones of the tool edges and provide sufficient cooling and lubricating. Jun et al. (2008) studied the effect of cutting fluids on micro-end milling. In addition to the application of MQL in the cutting tests, they also applied large drops of the cutting fluid to
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simulate the wet cutting in micro-end milling. The results showed that lower cutting forces and longer tool life were observed under the minimum quantity lubrication when compared to dry and flood cooling methods. In terms of cutting fluid properties, it was shown that cutting fluids with lower surface tension and higher viscosity result in lower cutting forces. While showing possible benefits for long tool life (Weinert et al., 2004), which is one of the main reasons in achieving high productivity, minimum quantity lubrication has only limited applications so far. This is because of the limited understanding of the optimal near dry lubrication parameters for different machining processes. Moreover, the application effectiveness of cutting fluids in microcutting is not clear (Friedrich, 2000; Jun et al., 2008), not to mention the application of MQL in micro-scale machining operations. This is the first paper to systematically study the effects of MQL in near micro-cutting on a meso-scale machine tool. Micro-machine tools hold several benefits from this miniaturization including reduced thermal distortion, the increase of static stiffness, the improvement of dynamic stability, and the lowering of machine cost (Okazaki et al., 2004; Liang, 2006). However, most of those micro-scale machine tools are developed in the laboratories where the electronic devices are widely adopted for the construction of micro-machine tools such as high-speed spindles, linear stages and high-resolution controllers. In that environment, the traditional flood cooling is not an appropriate method to supply the cutting fluid. Therefore, the minimum quantity lubrication is an suitable alternative to provide both cooling and lubricating effects in micro-cutting with less impact on the electronic devices. The objective of this research is to study the tool wear and surface quality in the near micro-milling process on a meso-scale machine tool. The comparison between minimum quantity lubrication and completely dry cutting process is presented. Experiments on the subject of the lubrication parameters, oil flow rate and air flow rate, are also performed in this study.
Fig. 1. A miniaturized milling machine.
Table 1 Cutting conditions. Work material
SKD 61 steels (hardness: HRC38)
Spindle rotational speed Feed Depth of cut Air supply Lubricant supply
20,000, 30,000 and 40,000 rpm 1.0, 1.5 and 2.0 m/rev 300 m 25 and 40 l/min at 0.5 MPa 1.88, 3.75 and 7.5 ml/h
2. Experimental setup The setup of the meso-scale milling machine in this study is shown in Fig. 1. This desktop milling machine consists of a 3-axis machining table (linear AC motor with 0.6 nm resolution and 25mm travel length in both X and Y directions), a knee-column frame, and a spindle (125 W, 5000–50,000 rpm). 600-m diameter 2-flute flat end mills are used in this study. The micro-tools have two teeth. This represents a machine tool 5000 times smaller than traditional machine tools and 1000 times higher in resolution and accuracy than commercial machine tools. After slotting square grooves on the workpiece with a flat end mill, tool wear and workpiece surface roughness are measured. The oil mist is delivered to the cutting zone by a cutting fluid applicator (Bluebe FK type). In the experiments, the Blube system is used to supply the air–fluid mixture with the oil flow rates of 1.88–7.5 ml/h at a pressure of 0.5 MPa. Bluebe (Accu-lube) lubricant LB-1, a vegetable oil, is chosen as the cutting fluid. The slotting experiments are done on the miniature machine tool at 300 m axial depth of cut while three different spindle speeds and three different feed rates are adopted as shown in Table 1. The flank wear land length is measured for each slot with an optical microscope when slotting SKD61 (with hardness of HRC38) with uncoated carbide end mills on the micro-milling system. The workpiece is 40 mm by 90 mm by 5 mm. Seven slots are cut in the middle of the work piece. Each cutting pass produce a 24 mm long slot. The closed view of the experimental setup is shown in Fig. 2. The cutting fluid is directed to the micro-end mill as shown in the figure. Pretests are performed in order to make sure that the tool will break
in the limited cutting length in dry cutting because the linear stages only provide 25-mm travel length and only 7 slots are cut on one workpiece. After the pretests, the machining tests are performed under different cutting conditions, as shown in Table 1. The same cutting conditions are also applied to dry cutting as comparison. 3. Results and discussions The effects of the MQL on tool flank wear, surface roughness and burr formation are discussed in the following sections. In these sections, the lubricating conditions of MQL are oil flow rate of 7.5 ml/h
Fig. 2. The close view of the experimental setup.
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Fig. 3. Example of tool flank wear measurement (spindle speed = 30,000 rpm, feed = 1 m/rev and cutting length = 96 mm under MQL). (a) New tool and (b) worn tool.
and air flow rate of 40 l/min. As a comparison, dry cutting tests are also carried out. Following these discussions on the capacity of MQL in near micro-milling, the experimental results for different lubricating conditions are presented. By examining the experimental data, the optimal lubricating conditions in near micro-milling for the tool, the work material and the lubricant used in this paper are proposed. 3.1. Tool flank wear Fig. 3 shows the evaluation of the tool flank wear for the micro-tools in this paper. It is usually suggested to take several measurements of the tool wear and use the average value as the flank wear value. Nevertheless, it is seen that the flank wear is almost linearly distributed from the center of the cutter. The value of the flank wear (VB) measured at a distance of one third of the diameter from the center is considered as the representative tool flank wear in the following discussions, as shown in the figure. The value of the tool flank wear is read from the CCD measuring system attached to the microscopic after calibration with a standard scale. The reason for not using the largest value of tool flank wear is to avoid the measurement error due to the loss of the tool cutting edge. Fig. 4 shows the gradual tool flank wears of the micro-tools under different feeds while the spindle speed is 30,000 rpm for both dry and MQL conditions. In the figure, it is shown that the tool life decreases with respect to the decreased feed under both dry and MQL conditions. For dry cutting, the cutting length for the case
Fig. 4. Tool flank wears for different feeds under dry and MQL conditions (spindle rotational speed is 30,000 rpm).
of 2 m/rev feed is more than 168 mm while the cutting lengths before tool breakage for the cases of 1 m/rev and 1.5 m/rev feed are 96 and 120 mm, respectively. This phenomenon is different from the observations in conventional milling processes in which the tool life increases with regard to the decreased feeds. A similar result was also observed by Filiz et al. (2007). It should be also noticed that the results in Fig. 4 are based on the amount of cutting lengths, in which the tools at lower feeds are experienced longer periods of cutting time. The extreme low feed in near micro-milling process is another possible reason to observe these tool wear trends. The nose radius of the cutting tools observed by the microscopic is around 5 m. The feed per revolution and the rounding radius of the cutting edge are comparable. In near microcutting, smaller feed will lead to the significant phenomenon of negative effective rake angle which will impedes the chip flow and thus faster tool wear (Woon et al., 2008). Moreover, the tool flank wears before tool breakages are around 100 m under the feeds of 1 and 1.5 m/rev. It is reasonable to assume that the tool life for the micro-tools used in this study is 100 m. However, the value of 100 m for flank wear compared to 600 m tool diameter is high. It is recommended to set a lower value for the limited flank wear for 600-m diameter micro-tools. For MQL cutting tests, the cutting lengths under all feeds are more than 168 mm. According to the trends in Fig. 4, the cutting lengths under MQL conditions are more than 390 mm before the tool breaks. The progressing tool wears under the feeds of 1.5 and 2 m/rev are comparable throughout the cutting tests. The flank wear under the feed of 1 m/rev is similar to that under the other two feeds before the cutting lengths of 72 mm. After that, the tool wears faster. The maximum difference of tool wears between the feed of 1 m/rev and 1.5 or 2 m/rev is about 10 m after cutting 168 mm long material. It is obvious to see in Fig. 4 that the application of MQL in near micro-milling can significantly extend the tool life under different feeds. For example, after cutting 96 mm long work material, the reductions of tool flank wear lengths in MQL cutting compared to dry cutting are 67.65%, 62.66% and 54.59% under the feeds of 1.0, 1.5 and 2 m/rev, respectively. Although the higher feed leads to a longer tool life in near micro-milling in this study, it is not necessary to select a higher feed for efficient near micro-milling because an excessive feed will cause the tool to break immediately. Tool breakage is one of the main tool failure modes in near micro-cutting with extremely small tools. Fig. 5 shows the progressive tool flank wears of the micro-tools under different spindle speeds while the feed is fixed at 1 m/rev for both dry and MQL conditions. It is observed that the tool flank wear rates for dry cutting are similar under different cutting speeds
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Fig. 5. Tool flank wears for different spindle rotational speeds under dry and MQL conditions (feed is 1 m/rev).
Fig. 7. Surface roughness for different spindle rotational speeds under dry and MQL conditions (feed is 1 m/rev).
based on cutting lengths. However, the cutting length for the spindle rotational speed of 40,000 rpm is 72 mm while the cutting length is 96 mm for the spindle speeds of both 20,000 rpm and 30,000 rpm. It is indicated that the higher cutting speed results in a shorter tool life. At the same time, the tool wear rates for MQL conditions are comparable for all cutting speeds in this study. None of the micro-tools breaks after cutting 168 mm long material. The experimental data show clearly that the supply of the oil mist can improve the tool life under all cutting speeds. The reductions of tool flank wear lengths in MQL cutting compared to dry cutting are about 68% under all cutting speeds in this paper after cutting 96 mm long work material. It is practical to presume the maximum tool flank wear of the micro-tools utilized in this study is 100 m according to Figs. 4 and 5.
1.5 and 2 m/rev. The values of surface roughness increase abruptly to 1.0 and 0.8 m for the feed of 1.5 m/rev and for the feed of 2 m/rev before the tool breakage. Comparing Figs. 4 and 6 for dry cutting conditions, it is found that the values of surface roughness gradually increase when the tool flank wears reach in the vicinity of 80 m. The surface roughness is more than 0.8 m and the tools break in the following slotting tests when the flank wears are more than 90 m. Fig. 7 shows the surface roughness of the machined surfaces under different spindle rotational speeds under both dry and MQL conditions. Similar to the results in the previous figure, surface roughness under MQL conditions does not change much under different cutting speeds. The values of surface roughness (Ra ) range below 0.2 m. The values of surface roughness under dry cutting depend on the tool flank wears. The values of surface roughness progressively increase when the tool flank wears achieve 80 m. The surface roughness suddenly deteriorates and the tools break soon after the flank wears attain 90 m. In short, it is conservative to presume the maximum tool flank wear of the micro-tool is 80 m while the surface finish abruptly deteriorates and the tool breaks shortly after the tool flank wear reaches 80 m. The surface profile for the machined surface under MQL condition is shown in Fig. 8. The surface profile is measured by a whitelight-confocal microscope (NanoFocus® surf® ). It is shown
3.2. Surface roughness Fig. 6 shows the effect of the cutting lengths and the feeds on surface roughness of the machined surfaces under both dry and MQL environments. It is observed in the figure that the surface roughness under MQL cutting does not change much for all cutting tests. The values of surface roughness (Ra ) range between 0.1 and 0.2 m. The values of surface roughness do not change with respect to the cutting lengths or the feeds. However, in dry milling, the values of surface roughness increase with regard to the cutting lengths under all feeds. For the case of 1 m/rev feed, the surface roughness of the machined workpiece is below 0.2 m before cutting 48 mm long material. The surface roughness is more than 0.2 m after slotting 72 mm long material and suddenly increases to 1.1 m after cutting 96 mm long material. Similar results are observed for the feeds of
Fig. 6. Surface roughness for different feeds under dry and MQL conditions (spindle rotational speed is 30,000 rpm).
Fig. 8. Surface profile for the machined surface under MQL condition (spindle speed = 30,000 rpm, feed = 2 m/rev and cutting length = 168 mm).
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Table 2 Burr formation for different feeds (spindle rotational speed = 30,000 rpm). Feed (m/rev)
Dry cutting 1 1.5 2 MQL cutting 1 1.5 2
Cutting length (mm) 24
48
72
96
120
144
168
R R R
S W W
S S W
S S S
S S
S
S
R R R
R R R
R R R
R R R
R R R
R R R
R R R
Note: R: no or slight burr formation; W: wavy burr formation observed; S: both wavy and needle-like burr formation observed.
Fig. 9. Surface profile for the machined surface under dry condition (spindle speed = 30,000 rpm, feed = 2 m/rev and cutting length = 168 mm).
in the figure that the machined surface is U-shape after slotting 168 mm long material. This means that the tool wear at the perimeter is more severe when the tool sees larger cutting speeds. However, the tool flank wear is limited in MQL milling so the surface roughness is acceptable along the feed direction. Fig. 9 shows the surface profile under the same cutting conditions as in Fig. 8 but without any lubricant applied. It is found that the center of the slot is damaged by the worn tool and a rough surface is left along the feed direction. Therefore the values of the surface roughness along the feed direction in dry cutting are larger than those observed in MQL machining. The bumpy surface is caused by the tool tip rubbing on the workpiece. Because the edge of the tool wears faster during cutting under higher cutting speeds, the tip of the end mill has a cone shape. However, the tip of the micro-tool does not have much capacity to remove the material. The material removing near the tool tip is a rubbing process rather than a milling process. When the cone shape is considerable compared with that of a new tool, a rough machined surface is observed. 3.3. Burr formation Fig. 10 shows the burr formation in dry cutting in this study. The burr formation is observed in all cutting tests. In the slot milling
tests, it is found that the down-milling burrs (on the right of the slot in the figure) are larger and up-milling burrs are smaller as identified by Lee and Dornfeld (2002) in their work. The first 24mm slots under both dry and MQL conditions only create slight burrs on both down-milling and up-milling sides. Nevertheless, the burr heights in dry cutting increase drastically since slotting 24-mm long material. On the other hand, the burr sizes do not change much with increasing cutting length under MQL conditions. The burr formation under different feeds for dry and MQL machining is shown in Table 2. When wavy burrs are observed on the down-milling side and needle-like burrs are observed on the up-milling side, it is noted as “S” in Table 2. When slight wavy burrs are noticed on the down-milling side and very small burrs are observed on the up-milling side, it is noted as “W” in Table 2. When only very small burrs observed on both side of the slots as shown, it is noted as “R” in Table 2. For dry cutting, it is found that larger burrs are observed for small feed of 1 m/rev, followed by the feed of 1.5 m/rev and 2 m/rev. The same results are also seen in Table 2 in which wavy burrs and needle-like burrs are seen for lower feeds with less material removed. In contrast, the burr formation is not strongly affected by the increasing cutting lengths under MQL slotting. In all cutting tests under MQL conditions, only small burrs are observed. The use of MQL in near micro-milling could reduce the burr formation. It is deduced from the relationships among tool wear, surface roughness and burr formation that the diminished burr formation under MQL conditions is partly attributed to the low tool wear in MQL machining. 3.4. Different tool materials It is known that for tungsten carbide tools, the quantity of cobalt contained in the tool has an effect on its hardness and tool life (Trent and Wright, 2000). Tool hardness decreases with respect to the increased Co content. In this section, the effect of the content of Co in carbide tools on the tool life under both dry and MQL conditions are discussed. The cutting tests are performed under the spindle speed of 30,000 rpm and the feed of 1 m/rev. Two different Co-content tools are listed in Table 3. Tool A and Tool B have the same tool geometry but different Co content, i.e. different tool material. In fact, Tool A and Tool B are under the same specifications by the tool manufacturer. Nevertheless, the tool manufacturer does Table 3 Different tool materials for cutting tests. Tool A
Fig. 10. Burr formation under dry cutting for spindle rotati0nal speed = 30,000 rpm, feed = 2 m/rev, and depth of cut = 300 m.
Tool materials (EDS quantitative results)
Tool B
Elememt
wt%
Element
wt%
C O W Co
9.20 1.39 69.23 20.18
C O W Co
8.80 1.06 76.78 13.35
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Fig. 11. Tool wear progression for speed = 30,000 rpm and feed = 1 m/rev).
different
tool
materials
(spindle
not provide detailed tool geometry and chemical composition. It is assumed that the tool geometry variation is limited and its effect on the tool life is ignored in this study. The element contents of new tools are examined by EDS. It is seen from Table 3 that Tool A has a higher content of Co compared to that of Tool B. The tool wear progression for different lubrication conditions and tools are shown in Fig. 11. It is not surprising to observe that the tool wear rates for Tool A are higher than those of Tool B. The supplies of MQL reduce the tool wear rates for both Tool A and Tool B. Example pictures of worn tools for Tool A and Tool B are shown in Fig. 12. The effect of MQL in near micro-milling for different Co-content carbides is compared by dividing the tool flank wear under MQL by the tool flank wear under dry cutting, as shown in Fig. 13. It is shown in the figure that the tool wear ratio is trending down with a decreasing rate. Moreover, the tool wear ratios are close for Tool A and Tool B. The significant difference of tool wears for cutting length of 24 mm is attributed to the fast initial tool wear in cutting. After that, tool wear behaviors for Tool A and Tool B are similar. It is presumed that the quantity of Co in tungsten carbide tools is not involved in the performance of MQL for tool wear progressions. In the following sections, Tool B is used for discussing the effect of the lubrication conditions of MQL on tool life. 3.5. Different oil flow rates Two parameters of the MQL, oil flow rate and air flow rate, are discussed as following. The effect of the oil flow rates on tool life
Fig. 13. Normalized tool wear progression for different tool materials (spindle speed = 30,000 rpm and feed = 1 m/rev).
Fig. 14. Tool wear progression for different oil flow speed = 30,000 rpm, feed = 1 m/rev and air flow rate = 40 l/min).
rates
(spindle
is presented in this section while the effect of the air flow rates is discussed later. Three different oil flow rates are used in the cutting tests. They are 1.88, 3.75 and 7.5 ml/h. In addition to the dry cutting test, a milling operation with the supply of air without any oil is also performed for comparison. The results are shown
Fig. 12. Pictures of worn tools (spindle speed = 30,000 rpm, feed = 1 m/rev and cutting length = 120 mm under MQL).
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Fig. 15. Tool wear progression for different air flow speed = 30,000 rpm, feed = 1 m/rev and oil flow rate = 7.5 ml/h).
rates
(spindle
in Fig. 14. The tool wear progressions are similar when the oil flow rate decreases from 7.5 to 1.88 ml/h. This is attributed to the small tool used in near micro-milling. The small amount oil supplied in cutting, such as 1.88 ml/h, is sufficient for lubrication in this machining operation. However, the use of pure air in near micro-milling does not improve the tool life compared with that of dry cutting. The supply of small amount of oil is necessary to extend the tool life in near micro-cutting. It is inferred that the cooling effect of air does not improve the tool life while the lubricating effect of oil under MQL is important for long tool life of micro-tools.
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sible tool flank wear of 100 m in length before tool breakage is observed in dry cutting. Surface roughness measurements when milling SKD 61 steels show that minimum quantity lubrication is better than that of dry cutting in terms of surface roughness. The values of surface roughness (Ra ) under MQL are smaller than 0.2 m and do not change much with respect to the cutting speeds or the feeds. In dry cutting, the values of surface roughness increase abruptly when the tool flank wears reach 80 m. It is conservative to presume that the maximum tool flank wear of the micro-tool is 80 m while the surface finish suddenly gets worse after the tool flank wear reaches 80 m. Burr formation can be alleviated by the use of minimum quantity lubrication. It is found that from the relationship between tool wear and burr formation, the diminished burr formation under MQL conditions is attributed to the low tool wear in near micro-cutting. The effect of MQL in near micro-milling on different Co-content tungsten carbides is negligible in this study. It is observed that the quantity of Co in tungsten carbide tools is not involved in the performance of MQL for tool wear progressions. The small amount oil of 1.88 ml/h supplied in cutting tests is sufficient for lubrication in near micro-cutting while the use of pure air in near micro-milling does not improve the tool life. It is inferred that the lubricating effect of oil under MQL is important for long tool life of micro-tools. It is observed that use of 40 l/min air flow in MQL is sufficient in near micro-slotting tests. The effect of air flow rate in near micromilling under MQL is to increase the oil mist velocity and to reduce the oil drop size. Acknowledgements
3.6. Different air flow rates The effect of air flow rate on the tool life in MQL cutting is shown in Fig. 15. It is shown that the tool wear rate for dry cutting is maximum followed by the use of MQL with 25 l/min air flow and subsequently 40 l/min air flow. The tool wear for the use of 40 l/min air flow is not significant after slotting 168 mm material. It is clear that use of 40 l/min air flow in MQL is sufficient in this study. The outcome of air flow rate to the effect of MQL in near micro-milling is to increase the oil mist velocity and to reduce the oil drop size. Both could provide better penetration of the oil mist to the cutting zone (Lopez de Lacalle et al., 2006). Smaller oil drops also bring more heat away the cutting zone by evaporation. It is deserved to be mentioned that the cutting heat in near micro-milling is only a small amount. The better penetration of small oil drops providing better lubrication is assumed to be the major function of higher air flow rate for reducing tool wear. Comparing the oil flow rates and air flow rates used in previous and this section, it is found that the air flow rate has a more considerable effect on tool life than the oil flow rate in this study. 4. Conclusions This paper compares the mechanical performance of MQL to completely dry condition for the near micro-milling of SKD 61 steels based on experimental investigations in terms of tool wear, surface finish and burr formation. Micro-tools with 600 m diameter are used to slotting the hardened steels. The following conclusions can be drawn from the findings of this study. Minimum quantity lubrication in near micro-cutting helps to reduce flank wear for all cutting conditions chosen in this study and hence is expected to improve tool life. The reductions of tool flank wear lengths in MQL cutting compared to dry cutting are about 60% under all cutting conditions in this paper. The maximum permis-
The authors would like to express their appreciation to National Science Council in Taiwan for their financial support of this research. References Chen, D.C., Suzuki, Y., Sakai, K., 2001. A study of turning operation by oil–water combined mist lubrication machining method. Key Engineering Materials 202–203, 47–52. Diniz, A.E., Ferreira, J.R., Filho, F.T., 2003. Influence of refrigeration/lubrication condition on SAE 52100 hardened steel turning at several cutting speeds. International Journal of Machine Tools and Manufacture 43 (3), 317–326. Filiz, S., Conley, C.M., Wasserman, M.B., Ozdoganlar, O.B., 2007. An experimental investigation of micro-machinability of copper 101 using tungsten carbide micro-endmills. International Journal of Machine Tools and Manufacture 47 (7–8), 1088–1100. Friedrich, C.R., 2000. Near-cryogenic machining of polymethyl methacrylate for micromilling tool development. Materials and Manufacturing Processes 15 (5), 667–678. Heinemann, R., Hinduja, S., Barrow, G., Petuelli, G., 2006. Effect of MQL on the tool life of small twist drills in deep-hole drilling. International Journal of Machine Tools and Manufacture 46 (1), 1–6. Heisel, U., Lutz, M., Spath, D., Wassmer, R., 1994. Application of minimum quantity cooling lubrication technology in cutting process. Production Engineering II (1), 49–54. Jun, M.B.G., Joshi, S.S., DeVor, R.E., Kapoor, S.G., 2008. An experimental evaluation of an atomization-based cutting fluid application system for micromachining. Journal of Manufacturing Science and Engineering, Transactions of the ASME 130 (3), 0311181–0311188. Klocke, F., Eisenblaetter, G., 1997. Dry cutting. CIRP Annals: Manufacturing Technology 46 (2), 519–526. Lee, K., Dornfeld, D.A., 2002. An experimental study on burr formation in micro milling aluminum and copper. United states, Society of Manufacturing Engineers, West Lafayette, ID, 1–8. Liang, S.Y., 2006. Mechanical machining and metrology at micro/nano scale, Xinjiang, China. International Society for Optical Engineering, Bellingham, WA, United States, 628002. Lopez de Lacalle, L.N., Angulo, C., Lamikiz, A., Sanchez, J.A., 2006. Experimental and numerical investigation of the effect of spray cutting fluids in high speed milling. Journal of Materials Processing Technology 172 (1), 11–15. Machado, A.R., Wallbank, J., 1997. Effect of extremely low lubricant volumes in machining. Wear 210 (1–2), 76–82.
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Okazaki, Y., Mishima, N., Ashida, K., 2004. Microfactory—concept, history, and developments. Journal of Manufacturing Science and Engineering, Transactions of the ASME 126 (4), 837–844. Rahman, M, Senthil Kumar, A., Salam, M.U., 2002. Experimental evaluation on the effect of minimal quantities of lubricant in milling. International Journal of Machine Tools and Manufacture 42 (5), 539–547. Trent, E.M., Wright, P.K., 2000. Metal Cutting. Butterworth-Heinemann, Boston. Varadarajan, A.S., Philip, P.K., Ramamoorthy, B., 2002. Investigations on hard turning with minimal cutting fluid application (HTMF) and its comparison with dry and
wet turning. International Journal of Machine Tools and Manufacture 42 (2), 193–200. Weinert, K., Inasaki, I., Sutherland, J.W., Wakabayashi, T., 2004. Dry machining and minimum quantity lubrication. CIRP Annals: Manufacturing Technology 53 (2), 511–537. Woon, K.S., Rahman, M., Fang, F.Z., Neo, K.S., Liu, K., 2008. Investigations of tool edge radius effect in micromachining: a FEM simulation approach. Journal of Materials Processing Technology 195 (1–3), 204–211.