行政院國家科學委員會補助專題研究計畫
行政院國家科學委員會補助專題研究計畫
行政院國家科學委員會補助專題研究計畫
行政院國家科學委員會補助專題研究計畫
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計畫名稱:微量潤滑式微切削之研究
計畫類別：█ 個別型計畫 □ 整合型計畫
計畫編號：NSC96－2218－E－110－007
執行期間： 96 年 08 月 01 日至 97 年 07 月 31 日
計畫主持人：李貫銘
共同主持人：
計畫參與人員： 周士嚴、林乘鵬
成果報告類型(依經費核定清單規定繳交)：█精簡報告 □完整報告
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處理方式：除產學合作研究計畫、提升產業技術及人才培育研究計畫、
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執行單位：中山大學機械與機電工程學系
Abstract
Micro-milling is a suitable technique for manufacturing of microstructures characterized by high aspect ratios and complex geometries. The application of the micro-milling process in cutting hardened tool steel is particularly challenging. The low strength of the miniaturized end mills implies reduction and accurate control of the chip load, which requires high positioning accuracy. Application of cutting fluids can improve the performance of machining operations because of their lubrication, cooling, and chip flushing functions. However, the conventional cutting fluids are not suitable for miniature machine tools due to the abundant electronic components used to construct micro-scale machine tools. Minimum quantity lubrication (MQL) presents itself as a viable alternative for micro-cutting with respect to the minimum impact on the electronic components as well as low tool wear, better heat dissipation, and machined surface quality in metal cutting. This study compares the mechanical performance of MQL to completely dry lubrication for the micro-milling of SKD 61 steel based on experimental measurement of tool wears and surface finish. The effect of MQL on the burr formation is also observed. Results indicated that the use of MQL leads to reduced tool wears, surface roughness, and burr formation.
Introduction
In metal cutting processes, the use of cutting fluids is the most common strategy to improve the tool life, the product surface finish and the size accuracy. Cutting fluids also make chip-breaking and chip-transport easier. However, the introduction of cutting fluids often produces airborne mist, smoke and other particulates in the shop floor air quality. These products bring the environmental, health and safety concerns. In addition, the cost of using cutting fluids is several times higher than tool costs [1]. The economical and environmental concerns on the use of cutting fluids lead to the research of minimum quantity lubrication (MQL) several years ago [1, 2].
In order to alleviate the economical and environmental impacts, minimum quantity lubrication was addressed as an alternative to the traditional flood cooling application a decade ago [1, 2]. Minimum quantity lubrication refers to the use of a small amount of cutting fluid, typically in the order of 100 ml/hr or less, which is about ten-thousandth of the amount of cutting fluid used in flood–cooled machining [3, 4]. The concept of minimum quantity lubrication is based on the principle of loss lubrication with dry surface after the machining process. This is the minimum quantity lubrication required in the machining process. Therefore, minimum quantity lubrication is also recognized as near dry machining (NDM).
At the same time, the commercial machine tools are in the scale of meters. To obtain high accurate product dimensions, ultra-precision machine tools are put into service. Ultra-precision macro-machine tools have several advantages including high rigidity, damping and the ability to actuate precisely based on precision sensors and actuators [5]. However, the large scale and precisely controlled machining environment may add very high costs for the fabrication of miniature components. On the other hand, micro-machine tools do not necessarily have to be large to achieve the required precision [6]; and, several benefits from this miniaturization include reduction in energy, space, materials and cost. The overall size of machine tools is an underlying
factor to the success of the micro-scale machining processes. The miniaturization of machine tools has a number of inherent and unparalleled advantages including: (a) the increase of thermal resistance due to the reduced heat source intensity and the machine tool volume involved in thermal deformation; (b) the increase of static stiffness resulting from the shortening of structural overhangs; (c) the improvement of dynamic stability by virtue of the higher structural resonance frequency for smaller machine size/weight; and (d) the lowering of machine cost thereby allowing the use of higher end construction materials. [7]
While showing possible benefits for both air quality control and tool performance improvement [1-4, 8-11], minimum quantity lubrication has only limited applications so far. This is because of the lack of scientific and quantitative studies on the appropriate near dry lubrication parameters, such as oil properties, oil flow rate and air pressure for different machining processes. Moreover, the application of MQL in micro-cutting has not been study yet. Most of the micro-scale machine tools are developed in the laboratories and the electronic devices are widely adopted for the construction of machine tool components. Therefore, the MQL is a reasonable solution to supply the cutting fluid with minimum impact on the machine tool components in micro-cutting. The objective of this research is to develop a systematic and scientific methodology to study the tool wear and surface quality in the micro-milling process on a meso-scale machine tool developed at National Sun Yat-Sen University.
Literature review
Machado and Wallbank [3] applied 200-300 ml/hr of lubricant when turning steel bars. The lubricant was delivered in a flowing air stream at a pressure of 29-34 psi. The experimental results showed that surface roughness, chip thickness and cutting forces variations were improved compared to the conventional flood cooling situation. The authors found the following phenomena. (1) Cutting and feed forces were reduced with the use of cutting fluids when turning medium carbon steel bars under low cutting speeds and high feed rates. In some cases, cutting with near dry lubrication had better results than conventional flood cooling. (2) Minimum quantity lubrication reduced variation in cutting forces and extended the tool life. (3) The effect of near dry lubrication on surface finish and chip thickness was only noticeable at low cutting speeds and high feed rates. (4) Application of near dry lubrication reduced the cost of cutting fluids and related equipments. However, the aerosol concentration increased compared with traditional flood cooling case.
Varadarajan et. al. [10] performed experiments in the area of hard turning AISI 4340 with 2 ml/hr oil in a flow of high pressure air at 20 MPa. It was found that cutting under near dry lubrication had better performance than that in dry or wet cutting in terms of cutting forces, cutting temperatures, surface roughness, tool life, cutting ratio and tool-chip contact length. Lower cutting forces, lower cutting temperatures, better surface finish, shorter tool-chip contact length, larger cutting ratio and longer tool life were observed in near dry turning compared with those in dry or wet cutting. The method to estimate the cutting temperature was also provided but there was not any comparison between predicted cutting temperatures and measurements.
steel with the use of 17 ml/hr oil and 150 ml/hr water mixture. The use of oil-water combined mist could prevent the production of built-up edge (BUE) while BUE was observed when cutting dry or with oil mist. BUE is an important factor of workpiece surface roughness. Therefore the workpiece surface finish under oil-water combined mist was better than that under dry, oil mist or water soluble oil applications. Lower cutting temperatures were also observed with the use of oil-water combined mist compared to cutting dry or with oil mist.
Diniz et. al. [13] applied 10 ml/hr oil in turning AISI 52100 steel with CBN tools. The supplied air pressure was 4.5 bar. According to the experimental data, the following conclusions were drawn. (1) Dry and minimum quantity lubrication had similar performance in terms of CBN tool flank wear, always better than the tool life under flood cooling. (2) The workpiece surface roughness measured in near dry cutting was close to that obtained from dry cutting.
Rahman et. al. [4, 14] performed experiments in end milling with the use of lubricant at 8.5 ml/hr oil flow rate. The oil was supplied by the compressed air at 0.52 MPa. The workpiece material was ASSAB 718HH steel. The experimental results showed that: (1) tool wear under near dry lubrication was comparable to that under flood cooling when cutting at low feed rates, low speeds and low depth of cuts; (2) the surface finish generated by minimum quantity lubrication was comparable to that under flood cooling; (3) cutting forces were close in both minimum quantity lubrication and flood cooling; (4) fewer burrs formed during minimum quantity lubrication compared to dry cutting and flood cooling application; (5) the tool-chip interface temperature under near dry lubrication was lower than in dry cutting but higher than that in flood cooling.
Lopez et. al. [15] studied the effects of cutting fluid on tool wear in high speed milling. Both near dry lubrication and flood cooling were applied when cutting aluminum alloys. In addition to experiments, they also performed computational fluid dynamics (CFD) simulations for estimating the penetration of the cutting fluid to the cutting zone. The oil flow rates of 0.04 and 0.06 ml/min were studied. The pressurized air was applied at 10 bar. The results showed that (1) with the help of compressed air, the oil mist could penetrated the cutting zone and provide cooling and lubricating while the CFD simulation showed that the flood coolant was not able to reach the tool teeth; (2) the nozzle position relative to feed direction was very important for oil flow penetration optimization.
Sasahara et al. [11] reported that in the case of helical feed milling for boring aluminum alloy, cutting forces, cutting temperature and dimension accuracy under near dry lubrication were close to those under flood cooling condition.
Kelly and Cotterell [16] employed near dry lubrication to optimum drilling cast aluminum alloys. A flow of 20 ml/hr oil was delivered with the compressed air at the gauge pressure of 6 bar. The authors observed that the feed force, drill torque and surface roughness under near dry lubrication were the lowest compared with those in flood cooling, compressed air or dry cutting. However, the experimental results also showed that the hole accuracy for near dry drilling was worse than that for flood cooling situation.
Heinemann et. al. [17] investigated the effect of minimum quantity lubricant on tool life when drilling carbon steels with high speed steel twist drills. The cutting fluid flow rate was 18
ml/hr. 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. A low-viscous and high cooling-capable lubricant provided a longer tool life when different lubricants were used for an external MQL-supply in the tests.
Brinksmeier et. al. [18] applied minimum quantity lubrication in grinding. Two different work materials were used: hardened steel (16MnCr5) and tempered steel (42CrMo4V). The minimum quantity lubrication was implemented under 0.5 ml/min oil flow rate and 6 bar pressurized air. With reference to the grinding tests, the following results were observed: (1) both dry and near dry grinding would cause thermal damage on the hardened material with the creep feed grinding operation; (2) acceptable surface finish was obtained under minimum quantity lubrication if the material removal rate was low; (3) the type of lubricant used in minimum lubrication had a significant influence on the surface finish. The analysis of the cooling effect of cutting fluid for both minimum quantity lubrication and flood cooling was also presented. However, there was not a comparison between predicted and measured cutting temperatures.
Cutting fluids offer several important mechanical benefits in machining processes such as cooling, lubricating and chip flushing. A recent survey indicated that the cost of cutting fluids and the auxiliary equipments compromise nearly 7-17% of the total machining costs [1]. Compared with the cost of the cutting tools (2-4%), the cutting fluid cost is significantly high. As a result, there is a need to reduce the use of the cutting fluids. Furthermore, machining processes often produce small lubricant droplets in the form of mist, smoke and gases that are harmful to the environment and human health. In this section, the effects of the cutting fluids on human health will be discussed.
During machining operations, workers could be exposed to cutting fluids by skin contact and inhalation [19]. Skin contact usually occurs in the following situations: (1) the worker directly touches cutting fluid without any protective equipment; (2) the worker handles work, machine, and equipment that are covered with cutting fluid; (3) the worker is exposed to cutting fluid splashes from the machine tool or workpiece. Skin exposure to cutting fluid can cause various skin diseases [20]. In general, skin contact with straight cutting oils cause folliculitis, oil acne, and keratoses while skin exposure to soluble, semi-synthetic and synthetic cutting fluid would result in irritant contact dermatitis and allergic contact dermatitis. Another source of exposure to cutting fluids is by inhalation of mists or aerosols. Airborne inhalation diseases have been occurring with cutting fluid aerosols exposed workers for many years. These diseases include lipid pneumonia, hypersensitivity pneumonitis, asthma, acute airways irritation, chronic bronchitis, and impaired lung function [20]. In response to these health effects through skin contact or inhalation, the National Institute for Occupational Safety and Health (NIOSH) has recommended that the permissible exposure level (PEL) is 0.5 mg/m3 as the metalworking fluid concentration on the shop floor [20, 21].
On the studies of miniature machine tools, around the world, a number of government-level laboratories such as NIST (U.S.), AIST (Japan), RIKEN (Japan), and academic groups [22, 23], among others, have actively engaged in the research of micro-scale machine tools [7]. However, the application of MQL in micro-cutting has not been study yet. Most of the
abovementioned micro-scale machine tools are developed in the laboratories where the electronic devices are widely adopted for the construction of machine tool components such as high-speed spindles, linear stages and controllers. In that environment, the traditional flood cooling is not an appropriate method to supply the cutting fluid. Therefore, the MQL is an alternative choice to provide both cooling and lubricating effects in micro-cutting.
Minimum quantity lubrication is of great interest in both air quality control and tool life extension as discussed in the literature review. Although previous researches showed this technology could be an alternative machining process other than dry machining or flood cooling machining, those studies are restricted to qualitative experimental results on macro-scale machine tools. It is necessary to develop a systematic methodology for minimum quantity lubrication study on miniature machine tools. To address the tool performance and product surface quality of near dry milling on a micro-scale machine tool as a result of cooling and lubricating effects, this study presents the application of minimum quantity lubrication to the micro-milling process applied to the cutting hardened steels on a desktop machine tool. The comparison between minimum quantity lubrication and completely dry cutting process is also presented.
Experimental setup
Figure 1 shows a system developed by National Sun Yat-Sen University (NSYSU) based on a 3-axis machining table (linear AC motor with 0.6nm resolution and 25mm2travel footprint), knee-column frame, and spindle (125W, 0.5-50Krpm) along with multiflute tools (solid carbide, 2 flute, 25~800µm dia). In this research, a 600µm diameter 2-flute flat end mill is used. This represents a machine tool 5,000~10,000 times smaller than traditional machines and 1,000 times higher in resolution and accuracy than commodity machine tools. The milling is done on the square groove and the tool wear and surface roughness are measured.
3-axis table Spindle
Nozzle for MQL
Figure 1: A miniaturized vertical machining center developed by NSYSU.
The cutting fluid is delivered to the tool flank face by a cutting fluid applicator (Bluebe FK type). The Blube system is used to supply the air-fluid mixture of 7.5 ml/hr at a pressure of 0.45 MPa. Bluebe (Accu-lube) lubricant LB-1, a vegetable oil, is chosen as the cutting fluid.
The flank wear land length is recorded with an optical microscope when cutting SK61 (with hardness of HRC38) with uncoated carbide end mills on the micro-milling system at NSYSU. The workpiece is 40 mm by 90 mm by 5 mm. Each cut takes away 25 mm long material. Experimental setup is shown in Figure 2. Pretests are performed in order to observe the tool wear in the limited cutting length (the linear stages only provide 25mm2travel footprint). 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.
Figure 2: Experimental setup
Table 1: Cutting conditions
Tool material Tungsten carbide
Work material SKD 61 steel (Hardness: HRC38)
Spindle rotational speed 30000, 40000 and 50000 rpm
Feed rate 0.5, 0.75 and 1.0 mm/sec
Depth of cut 300 µm
MQL supply Air: 0.45Mpa, Lubricant: 7.5ml/hr (through external nozzle)
Results and discussion
Tool flank wear
Cutting tools may fail by brittle fracture, plastic deformation or gradual wear. With the progress of machining, the tools achieve crater wear at the rake surface and flank wear at the flank surfaces due to continuous interaction with the chips and the work surfaces, correspondingly. Among the aforementioned wears, the principal flank wear is the most
Workpiece Micro-tool
MQL nozzle
important because it raises the cutting forces and the related problems. In gradual wear, the tool life is assessed by the actual cutting lengths or machining time after which the average value of its principal flank wear reaches a limiting value, such as 0.3 mm. Therefore, attempts should be made to reduce the rate of growth of flank wear in all possible ways without much sacrifice in material removal rate. This study was conducted according to the cutting conditions illustrated in Table 1, while the performance of the lubrication modes were evaluated in terms of tool wear, surface finish, and burr formation. The last two will be depicted and discussed in the following sections
Figure 3 shows the progressive tool wears of the micro tools under different feed rates for both dry and MQL conditions. Figure 3 clearly shows that flank wear, especially the wear rate, decreased by the use of MQL. The reduction of tool flank wear may be attributed to the lubrication and cooling effects of MQL. However, inadequate lubrication may cause opposite effect on the tool wear [4], such as the more serious tool wear under feed rate of 0.75 mm/sec in MQL than that of dry cutting, as shown in the figure. Although the use of MQL can improve the tool life, the difference between tool wear under dry cutting and MQL does not show significant dissimilarity under the selected feed rates. The maximum reduction of tool flank wear of dry cutting to MQL is only 11.6% at the end of the 6th cut.
Tool wear (Dry cutting)
0 10 20 30 40 50 60 70 80 90 1 2 3 4 5 6 Number of cuts μ m 0.5mm/sec0.75mm/sec 1.0mm/sec Tool wear (MQL) 0 10 20 30 40 50 60 70 80 90 1 2 3 4 5 6 Number of cuts μ m 0.5mm/sec0.75mm/sec 1.0mm/sec
Figure 3: Tool wear progressions for different feed rate under dry and MQL conditions (spindle rotational speed = 30000 rpm)
Figure 4 shows the progressive tool wears of the micro tools under different spindle rotational speed for both dry and MQL conditions. It is shown in the figure that the use of MQL in micro-milling does not always lead to lower tool wear growth. For the first three cuts, where the tool wear are not serious, the use of MQL does reduce the tool wear compared with that of dry cutting. However at the end of 6th cut, it is only effective for the low cutting speed (spindle rotational speed = 30000 rpm), but having the negative influence on the tool wear for higher cutting speeds. The interaction between tool wear and lubrication should be taken into consideration for micro-cutting processes.
Tool wear (Dry cutting) 0 20 40 60 80 100 120 1 2 3 4 5 6 Number of cuts μ m 30000rpm40000rpm 50000rpm Tool wear (MQL) 0 20 40 60 80 100 120 1 2 3 4 5 6 Number of cuts μ m 30000rpm40000rpm 50000rpm
Figure 4: Tool wear progressions for different spindle rotational speed under dry and MQL conditions (feed rate = 0.5 mm/sec)
Surface roughness
Surface finish is also an important index of machinability because performance of the machined component are often affected by its surface finish, residual stresses and presence of surface or subsurface microcracks, particularly when that component is to be used under dynamic loading. Generally, good surface finish is achieved by finishing processes like grinding but sometimes it is left to machining. In micro-machining, it is expected that the surface finish is good because of the low feed rate in this process.
Figure 5 shows the surface roughness of the machined surface under different feed rates for both dry and MQL conditions. It is shown that the surface roughness for MQL is better compared to dry cutting for all feed rates. The maximum difference of the surface roughness between MQL and dry cutting happens when the feed rate is 1.0 mm/sec, which has 59.7% difference in magnitude. Therefore, the application of MQL can improve the surface finish in micro-milling.
Rmax 0.0 0.5 1.0 1.5 2.0 2.5 0.5 0.75 1
Feed rate (mm/sec)
µ
m Dry cutting
MQL
Figure 5: Surface roughness for different feed rate under dry and MQL conditions (spindle rotational speed = 30000 rpm)
Figure 6 shows the effect of spindle rotational speeds on surface roughness of the machined surface under both dry and MQL cutting. It can be seen that at 30000 rpm, the surface roughness obtained in MQL is much better than that in dry milling. However, at spindle rotational speed of
40000 and 50000 rpm, the surface finish in both cutting speeds are close for MQL and dry cutting. The reason for the different experimental results may be due to the inefficient lubrication for higher cutting speeds.
Rmax 0 0.5 1 1.5 2 2.5 3 30000 40000 50000 Spindle speed (rpm) µ m Dry cutting MQL
Figure 6: Surface roughness for different spindle rotational speed under dry and MQL conditions (feed rate = 0.5 mm/sec)
Burr formation
Figure 7 shows the burr formation in this study. The burr is observed in all cutting conditions. It is clear that the presence of burrs constitutes one of the main limitations of micro-milling towards its application in industrial production, as the part needs to be further processed for burr removal, which is particularly complicated due to the small size of the machined parts or features. In the figure below, it is found that the burr height is higher on the side where the down milling occurs.
Figure 7: Burr formation for the 4th cut under the dry cutting for spindle rotational speed = 50000 rpm, feed rate = 0.5 mm/sec and depth of cut = 0.3 mm.
The burr formation under different feed rates for both dry and MQL conditions is shown in Tables 2 and 3. The serious burr formation is observed in both dry and MQL cuttings at the end of 6th cut. Nevertheless, the use of MQL in micro-milling does defer the serious burr formation. As the feed rate increases, the effect of MQL on the burr formation is more helpful, as shown in
Workpiece moving direction
Tool rotational direction
Burr formation
Tables 2 and 3.
Table 2: Burr formation for different feed rate under dry conditions (spindle rotational speed = 30000 rpm)
Dry cutting Number of cuts
Feed rate 1 2 3 4 5 6
0.5 mm/s N Y S S S S
0.75 mm/s N S S S S S
1.0 mm/s N S S S S S
Note: N: no burr formation, Y: burr formation observed, S: serious burr formation
Table 3: Burr formation for different feed rate under MQL conditions (spindle rotational speed = 30000 rpm) MQL Number of cuts Feed rate 1 2 3 4 5 6 0.5 mm/s N N N S S S 0.75 mm/s N N Y Y Y S 1.0 mm/s N N N N Y S
Note: N: no burr formation, Y: burr formation observed, S: serious burr formation
The burr formation under different spindle rotational speed for both dry and MQL conditions is shown in Tables 4 and 5. Similar to results for different feed rate, the serious burr formation is also observed in both dry and MQL cuttings at the end of 6th cut. The use of MQL in micro-milling also defers the serious burr formation. However, the effect of MQL only applies for low cutting speed of 30000 rpm spindle speed.
Table 4: Burr formation for different spindle rotational speed under dry conditions (feed rate = 0.5 mm/sec)
Dry cutting Number of cuts
Spindle speed 1 2 3 4 5 6
30000 rpm N Y S S S S
40000 rpm N S S S S S
50000 rpm N N S S S S
Note: N: no burr formation, Y: burr formation observed, S: serious burr formation
Table 5: Burr formation for different spindle rotational speed under MQL conditions (feed rate = 0.5 mm/sec)
MQL Number of cuts
Spindle speed 1 2 3 4 5 6
30000 rpm N N N S S S
50000 rpm N N S S S S
Note: N: no burr formation, Y: burr formation observed, S: serious burr formation
Conclusion
The performance of MQL technique in micro-milling with respect to dry cutting was tested. Results clearly indicate that MQL can definitely be regarded as a superior method to dry cutting in micro-cutting. The following conclusions can be deduced from the findings of this study.
MQL presents reduced flank wear for all feed rates and low cutting speed and hence is expected to improve tool life. However, at high cutting speeds, MQL is not effective due to insufficient lubrication.
Surface roughness obtained in MQL is better than that of dry cutting although at high cutting speeds, surface roughness under both lubricating conditions is comparable.
Burr formation can be alleviated by the use of MQL. Less burr formation indicates better surface quality and no further process is required to remove the burr.
Acknowledgement
The authors would like to express their appreciation to National Science Council in Taiwan for their financial support of this research.
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