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Development of a micro-graphite impregnated grinding wheel
International Journal of Machine Tools & Manufacture 56 (2012) 94–101
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International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Development of a micro-graphite impregnated grinding wheel Ming-Yi Tsai n, Shi-Xing Jian Department of Mechanical Engineering, National Chin-Yi University of Technology, No.57, Section 2, Zhongshan Road, Taiping District, Taichung 41170, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 November 2011 Received in revised form 14 January 2012 Accepted 16 January 2012 Available online 25 January 2012
This paper introduces a newly designed grinding wheel where micro-graphite particles are impregnated in an aluminum oxide matrix to form a grinding wheel to lubricate the grinding site; these are known as graphite-impregnated grinding wheels. The graphite particles were heat-treated under a layer of hydrogen ions at a temperature of about 500 1C for 30 min to disperse them uniformly in the aluminum oxide matrix. In this study, grinding wheels with five different graphite contents (0.1, 0.5, 1, 3, and 5 wt%) were investigated. Different aspects of the grinding performance (e.g., surface roughness, morphology, wheel wear ratio, grinding temperature, and grinding forces) using these grinding wheels under two different coolant strategies (dry and minimum quantity lubrication with pure water) were compared with the corresponding values for a conventional grinding wheel. The experimental results indicate that using graphite-impregnated grinding wheels considerably improves the grinding process performance compared to using a conventional grinding wheel. A graphite content of below 0.5 wt% is recommended because this provides not only better surface roughness and topography, and lower grinding temperature and force but also less grinding wheel consumption; hence, the wheel life was extended. In summary, combining graphite-impregnated grinding wheels with minimum-quantity lubrication technology has the potential to effectively eliminate the use of any oils or toxic organic lubricants in the grinding process. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Grinding wheel Minimal quantity lubrication Grinding
1. Introduction Grinding, which is an abrasive material removal technique, is a chip-removal process that uses an individual abrasive grain as the cutting tool. Abrasive material removal processes can be very challenging due to the high power requirements and high temperature, especially at the workpiece–wheel interface; this is because the abrasive grits in contact with the workpiece do not cut due to the high negative rake angle but instead generate heat by rubbing and plowing the workpiece surface in the contact zone [1]. The most common strategy to control the cutting temperature in the contact zone is flooding it with cutting fluid. Kalpakjian et al. [2] showed that flooding with cutting fluid is extensively used in machining operations to achieve the following results: (1) reduce friction and wear to thus improve the tool life and surface finish of the workpiece; (2) cool the cutting zone to thus improve tool life and reduce the temperature and thermal distortion of the workpiece; (3) reduce forces and energy consumption; and (4) flush chips from the cutting zone to thus prevent the chips from interfering with the cutting process. Brinksmeier et al. [3] pointed out that in grinding, cooling by flooding is necessary to protect the workpiece and wheel from
n
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damage such as thermal burn, residual stresses, phase transformation, and microcracks. However, many of these fluids are health hazards with major environmental concerns and difficult to recycle and manage, which significantly increases the total manufacturing cost [4]. Metalworking fluids have been estimated to constitute about 7–17% of the total machining cost, whereas depreciation and waste disposal contributes to 54% of the cooling cost [5]. All of these factors have prompted investigation into the development of new coolant strategies with minimum quantity lubrication (MQL) or that eliminate flooding with cutting fluids. MQL is based on the principle that a drop of liquid is split by an airflow, distributed in streaks, and transported in the direction of the flow of air. In MQL machining, the oil consumption in industrial applications is in the range of approximately 10–100 ml/h [6]. Choi et al. [7] investigated the cooling effect of compressed cold air and compared it with that of normal coolants. They found that the compressed cold air efficiently minimizes the thermal defects of the workpiece and can also help solve the problem of environmental pollution. The mechanics of machining under cryogenic cooling was proposed as a new concept for dissipating the large amount of heat generated in the machining zone without polluting the environment. Experiments were conducted to study the effect of liquid nitrogen on the grinding forces and check the validity of the proposition. The results indicated a substantial reduction in the grinding forces under cryogenic cooling [8]. Cryogenic machining by liquid nitrogen with the help of a
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modified tool holder provides longer tool life and more wear resistance compared to dry machining [9]. Tawakoli et al. [10] investigated the influence of workpiece hardness and grinding parameters, including wheel speed, feed rate, and depth of cut on MQL grinding. They pointed out that compared to dry grinding, MQL grinding substantially enhances the cutting performance in terms of increasing the wheel life and improving the quality of the ground parts. Silva et al. [11] analyzed the behavior of the MQL technique and compared it with the conventional cooling method. They concluded that the Ra values are substantially reduced with the use of the MQL technique. In addition, MQL does not negatively affect the surface integrity. Barczak et al. [5] compared three cooling methods: conventional flood cooling, dry grinding, and grinding with MQL. They pointed out that the low grinding forces indicate that MQL can be used as a low-temperature process. Workpiece quality under MQL is comparable to and can be better than that achieved under conventional flood cooling. Tawakoli et al. [12,13] investigated the effect of ultrasonic vibrations on dry grinding of soft steel. They showed that using ultrasonic-assisted dry grinding, the normal and tangential grinding forces can be reduced by up to 70% and 50%, respectively. Although the abovementioned coolant strategies have their respective merits, they also have some drawbacks. For example, dry grinding results in workpiece damage, poor surface finish, and accelerated wheel wear. The MQL coolant can evaporate due to the grinding heat before it reaches the workpiece–wheel interface. In addition, the small amount of oil or synthetic ester used in MQL grinding still affects health and the environment. Recently, many studies have focused on cutting fluids containing nanoparticles or solid lubricant particles. For example, Liao et al. [14] investigated the use of a cutting fluid containing nanoparticles in wet grinding; they studied MQL grinding of Ti– 6Al–4V. The use of cutting fluids containing nanoparticles was found to result in less loading of the wheel and a better ground surface compared to using general-purpose flood cutting fluids. Reddy and Rao [15] presented the effects of solid lubricantassisted machining with graphite and molybdenum disulphide lubricants on the surface quality, cutting force, and specific energy when AISI 1045 steel is machined with cutting tools having different tool geometries. Rao and Krishna studied the specific application of solid lubricants in turning [16]. The process performance was judged in terms of cutting force, tool temperature, tool wear, and the surface finish of the workpiece as the cutting condition was kept constant. The experimental results showed that using the solid lubricant was more effective than dry and wet machining. Alberts et al. [17] evaluated the performance of graphite nanoplatelets coating the workpiece surface as a lubricant in grinding. They also compared their results with those obtained from conventional MQL grinding. A review of the literature shows the lack of studies on grinding wheels embedded with solid lubricants. In this study, a newly
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designed grinding wheel was fabricated by impregnating submicron graphite particles in an aluminum oxide matrix. In this study, grinding wheels with five different submicron graphite contents – 0.1, 0.5, 1, 3, and 5 wt% – were investigated. Different aspects of the grinding performance, such as the surface roughness, morphology, and grinding forces, using these grinding wheels under two different coolant strategies – dry and MQL with pure water – were compared with the corresponding values for a conventional grinding wheel.
2. Experimental procedure 2.1. Development of the graphite-impregnated grinding wheels The graphite-impregnated grinding wheel was manufactured using an aluminum oxide base with the addition of various graphite contents (0.1, 0.5, 1, 3, and 5 wt%). The graphite particles were heattreated under hydrogen flowing at 2 L/min at a temperature of about 500 1C for 30 min to uniformly disperse the graphite in the grinding wheel. After hydrogenation, the graphite particles on the surface were covered by a layer of hydrogen ions (H þ ) due to the graphite particles repelling each other. Images of the graphite and aluminum oxide particles are shown in Fig. 1. The graphite and aluminum oxide particle were about 50 and 150 mm in size, respectively. Excluding the graphite, these wheels consisted of 80% abrasive, 12.5% liquid bonding material, 5.2% powder resins, and 2.3% additives by weight. This study used grinding wheels made with the resin bond method. An A100K8B1A (180 mm 13 mm 31.75 mm size) wheel was taken as the standard wheel. Fig. 2 shows the graphite-impregnated grinding wheel manufacturing process. (a) First, the abrasive grains, additives, bonding, and hydrogenated graphite materials are prepared. (b) The abrasive grains and liquid phenolic resins are uniformly mixed together. Also, the hydrogenated graphite materials, powdered phenolic resins, and additives are mixed. (c) The mold is filled with the mixture. (d) The mixture (50 kg) is pressed into the shape of a grinding wheel. (e) Finally, the mixture is cured at a temperature of about 180 1C before the graphite-impregnated grinding wheel is tested and finished. These wheels have a soft structure and are used for high-speed grinding, rough grinding, and cutting. 2.2. Experimental apparatus The experiments were conducted with traverse grinding using a horizontal-spindle high-speed surface grinder. The workpiece was secured on a magnetic chuck attached to the worktable of the grinder, as shown in Fig. 3(a). SKD 11 structural steel with dimensions of 60 mm 11 mm 25 was used. Before machining, the materials were pre-machined with a 1 mm cut to remove any
Fig. 1. Appearance of (a) graphite and (b) aluminum oxide particles.
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Fig. 2. Manufacturing stages of graphite-impregnated grinding wheel: (a) preparing related materials, (b) mixing abrasive and graphite materials, (c) filling mixture in mold, (d) forming a cylindrical wheel by pressing, and (e) curing and testing.
Fig. 3. (a) Experimental system configuration: (1) grinding wheel, (2) nozzle, (3) dynamometer, (4) magnetic bases, (5) thermocouple, (6) dust catcher, (7) signal transmission line, and (8) workpiece. (b) Details on the MQL equipment fixed to the grinder: (1) export of pure water mist, (2) filler, (3) knob for lubricant flow rate adjustment, (4) droplet transparent window, (5) manometer (0–150 psi or 0–10.2 kgf/cm2), (6) water meter, and (7) air inlet. Table 1 Grinding conditions. Grinding wheel
Grinding mode Grinding conditions Environments
Workpiece material Dressing conditions
1. Graphite impregnated grinding wheels with 0.1, 0.5, 1, 3, and 5 wt% graphite 2. conventional grinding wheel: A100K8B1A 3. grinding wheel size: 180 mm 13 mm 31.75 mm Up-grinding Wheel speed: 2400 rpm, work speed: 20 m/min, depth of cut: 15 mm 1. Dry grinding 2. MQL with pure water grinding, flow rate: 90 ml/h, air pressure: 25 psi SKD 11, HRc¼ 607 2 Workpiece size: 60 mm 11 mm 25 mm With single diamond dresser, dressing depth: 50 mm, dressing speed: 1800 rpm
possible surface irregularities and ensure similar surface properties for all of the specimens. The tests evaluated the performance of five different graphite grinding wheels using a surface grinding machine. All of the experiments were conducted three times, and the average value was taken as the response value. The dressing operation was kept constant using a single-point diamond dresser so that it would not influence the output variable of the process. The machining parameter settings used in this study are summarized in Table 1. Two coolant strategies were also employed to evaluate the wheels and workpiece performances in the experiments: dry grinding (DG) without cooling and lubrication, and minimum quantity lubrication (MQL) grinding where pure water is fed into the grinding contact zone. The control unit of the MQL equipment was fixed to the grinding machine, where the lubricant dosage
and airflow rate were adjusted. Fig. 3(b) shows details of the MQL equipment, and its parts are numbered to clearly show the description and functioning. The MQL system consists of a compressor, pressure regulator, rotameter, doser, and spray nozzle. The nozzle was placed at a distance of about 30 mm from the grinding wheel–workpiece interface. After the workpiece was ground, its surface roughness was measured by the Hommel Tester T-1000 with a cutoff length of 0.8 mm. At the end of each test, Ra across the grinding direction was measured at five different points of the ground surface. A scanning electron microscope (SEM) was used to analyze the surface morphology for possible damage caused by the thermal and mechanical forces acting on the material’s surface. The normal and horizontal grinding forces were recorded using a piezo-electric transducer based dynamometer (type Kistler 9257B) positioned under the workpiece clamping device, which was connected to an amplifier equipped on a computer. The workpiece temperature was measured by the thermocouple with a minimum sensitivity of 1 1C over a range of 20–950 1C. Each sample was measured three times to determine the reproducibility of the results.
3. Results and discussion 3.1. Surface roughness and surface topography It is well known that the surface finish can significantly affect the mechanical strength of components when they are subjected to fatigue cycles. The surface roughness also plays a wider role with regard to the performance of a product. A poor surface can make the performance vulnerable and the product to fail well before the expected lifespan. Fig. 4 compares the mean values of Ra parameters with graphite-impregnated grinding wheels
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0.4
0.4 Dry grinding
0.3 0.25 0.2
MQL grinding
0.35 Ra (µm)
Ra (µm)
0.35
97
0.3 0.25
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%)
0.2
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%)
Grinding wheels
Grinding wheels
Fig. 4. Variation in surface roughness for different grinding wheels under (a) dry grinding and (b) MQL conditions.
Fig. 5. Schematic illustration of (a) dry grinding and (b) MQL conditions.
(wheels B–F) and a conventional grinding wheel (wheel A) under dry and MQL environments, respectively. As shown in Fig. 4(a), the mean Ra values for wheels B–F were markedly lower than that of wheel A. This is because graphite is a good lubricant due to its layered structure. In addition, impregnating graphite particles into grinding wheels directly results in more effective lubrication of the workpiece–tool contact interface (see Fig. 5(a)). Efficient lubrication allows the chips to slide more easily over the tool’s surface, resulting in a better surface finish. An interesting observation was that the mean values of Ra were unchanged when the graphite content was increased from 0.1 wt% (wheel A) to 5 wt% (wheel F). The same trend in the results is shown in Fig. 4(b). A comparison of wheel A with wheels B–F in an MQL environment showed that Ra (0.325 mm) for wheel A was larger than those for wheels B, C, D, E, and F (0.224, 0.223, 0.228, 0.21, and 0.234 mm, respectively). An analysis of the results obtained with graphiteimpregnated grinding wheels under dry grinding and MQL conditions indicates that the latter leads to superior performance than the former due to more efficient penetration of the fluid into the cutting region. The MQL technique leads to lower roughness values; this is probably because of the more effective lubrication and cooling of abrasive grains at the workpiece–tool interface (see Fig. 5(b)) [18]. Fig. 4(b) again indicates that the mean values of Ra were altered when the graphite content was increased from 0.1 wt% to 5 wt%. To understand the variable results of the surface roughness measurements, the surface topographies of workpieces after grinding by the various grinding wheels in dry and MQL environments were also studied. Fig. 6(a) shows the cross-sectional SEM images of the workpiece surface after dry grinding with wheel A. As shown in the figure, some surface burning and surface damage as well as side ridges were formed due to a lack of lubrication leading to a high temperature for the workpiece surface. Fig. 6(b) and (c) show some scratches on the workpiece surface, but no thermal cracks formed with wheels B and C; this confirms that the graphite particles functioned effectively in the lubricant contact zone. Fig. 6(d) and (f) show slight scratches with no thermal damage after grinding with wheels D and F; this again proves that more graphite particles can provide better lubrication at the tool–chip
and tool–workpiece interfaces. However, a careful observation of these figures (see Fig. 6(d) and (f)) reveals some fine particles adhering to the workpiece surface and contributing to a poor surface roughness. This may be why the surface roughness did not change when the graphite content was increased from 0.5 wt% to 5 wt%. Fig. 7 shows the cross-sectional SEM images of the workpiece surface under the MQL condition. The surface scratches caused by wheel A were still more than those caused by wheels B–F. Compared to dry grinding, less thermal damage and material side flow was observed when MQL was used. Again, some particles were found to remain on the work surface when high graphite content (3 and 5 wt%) was used. This result may be attributed to the higher grinding wheel wear ratio, which is discussed later. 3.2. Grinding wheel wear ratio Grinding wheel wear is generally correlated with the amount of workpiece material that is ground; it is captured by a parameter called the wear ratio (Wr), which is defined as Wr ¼
Vw Vm
where Vm is the volume of material removed and Vw is the volume of wheel wear. The wear ratio has been found to be higher when softer bonds are used. In general, softer bonds are recommended for harder materials and to reduce residual stresses and thermal damage to the workpiece. Fig. 8 shows the variation in the wheel wear ratio with different grinding wheels. As shown in Fig. 8, the wear ratio increased with increasing graphite content. The highest wear ratio of 1.8 was obtained with wheel F, which has a graphite content of 5 wt%. This may be because the graphite particles cluster easily in the grind wheel to become block structures. When subjected to force, the grains with clusters of graphite particles are easily dislodged to weaken the bonding, leading to higher wheel wear (see Fig. 9). The observations discussed previously were confirmed. One interesting observation was that the wear ratio was lower than that of a conventional grinding wheel when the graphite content was below 0.5 wt%;
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Fig. 6. Cross-sectional SEM images of workpiece surface after dry grinding with a (a) conventional grinding wheel, (b) wheel B, (c) wheel C, (d) wheel D, (e) wheel E, and (f) wheel F.
this implies that if consumption of the grinding wheel is the chief concern, then a graphite content of below 0.5 wt% is recommended to extend the wheel life. 3.3. Temperature and cutting force The grinding contact temperature was measured during the tests. This allowed the thermal performance of the graphiteimpregnated grinding wheels to be compared with the conventional grinding wheels and also gave insight into the strength of the lubrication effect on the graphite-impregnated grinding wheels. The graphite-impregnated grinding wheels were observed to reduce the average cutting temperature by about 7–17% compared to the conventional grinding wheel, as shown in Fig. 10. This is because graphite particles are directly applied to the heat generation zones in the grinding process. A comparison of wheel A with wheels B–F under MQL conditions shows that wheel A had a considerably higher temperature (88.6 1C) than wheels B, C, D, E, and F (80, 81.3, 78.2, 79.3, and 77.1 1C, respectively). The results seem to be due to the low coefficient of friction, as solid lubricants soften at elevated temperatures. The combined effects of the embedded graphite particles and MQL
technique is why the grinding temperature was reduced. Yuan et al. [19] described the decrease in grinding temperature as leading to less adhesion and better surface roughness. Vamsi and Rao [20] proposed that the thermal conductivity of a solid lubricant mixture allows the tool temperature to decrease. Grinding forces are important parameters by which the performance of any grinding process can be evaluated. The relative contributions of different parameters on the cutting force depend on the type of work material, the tool material, and the presence of a cutting fluid or solid lubricants. The cutting force component determines the power requirement of the process. The intensity of heat generation depends upon this force, and it is of primary importance as far as the machining temperature and surface quality of the products are concerned [15]. Fig. 11 shows the variation in the tangential forces (Ft) and normal forces (Fn) with different grinding wheels in a dry grinding environment. The results showed that the cutting force (Ft and Fn) was considerably less with solid lubrication–embedded grinding wheels compared to the conventional grinding wheel. The lubricant effectiveness at minimizing the frictional effects of the tool and workpiece interaction for wheels B–F is evident from the reduced grinding forces compared to wheel A. This performance of the graphite particles is due to their lattice layer
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Fig. 7. Cross-sectional SEM images of workpiece surface after grinding in an MQL environment with (a) a conventional grinding wheel, (b) wheel B, (c) wheel C, (d) wheel D, (e) wheel E, and (f) wheel F. Abrasive grains
2
Bond
Wear ratio
1.8 1.6 Bond fracture
Porosity
1.4 Graphite particles
1.2 Workpiece surface
1
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%) Grinding wheels Fig. 8. Variation in wear ratios for different grinding wheels.
structure, which allows them to act as an effective solid lubricant film [20]. The normal force was found to be more than twice the tangential force, unlike in other machining processes such as turning and milling. This is due to grinding process characteristics such as very large negative rake angles of the grit, excessive rubbing action,
Fig. 9. Schematic illustration of a physical model for the structure of a graphiteimpregnated grinding wheel.
and adverse chip accommodation space that leads to wheel loading. Fig. 12 also shows the variation in the grinding forces for different grinding wheels in an MQL environment. Similar results are shown in the figure. Again, the tangential and normal forces of graphiteimpregnated grinding wheels were confirmed to eventually become lower than those of a conventional grinding wheel. In addition, as shown in the figure, MQL grinding considerably reduces the tangential and normal forces in comparison to dry grinding due to the
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106 104 102 100 98 96 94 92 90 88
Dry grinding
90 88 86 84 82 80 78 76
Temperature(°C)
Temperature(°C)
100
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%) Grinding wheels
MQL grinding
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%) Grinding wheels
60
26 24 22 20 18 16 14 12
55
Dry grinding
Dry grinding
50
Fn (N)
Ft (N)
Fig. 10. Variation in temperature for different grinding wheels under (a) dry grinding and (b) MQL conditions.
45 40 35 30
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%)
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%)
Grinding wheels
Grinding wheels
Fig. 11. Variation in (a) tangential forces and (b) normal forces for different grinding wheels.
50
20 MQL grinding
MQL grinding
45 Fn (N)
Ft (N)
18 16
40 35
14
30
12 Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%)
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%)
Grinding wheels
Grinding wheels
14.8
Grinding efficiency (%)
Actual depth of cut (µm)
Fig. 12. Variation in (a) tangential forces and (b) normal forces for different grinding wheels.
14.6 14.4 14.2 14
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%) Grinding wheels
98 97.5 97 96.5 96 95.5 95
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%) Grinding wheels
Fig. 13. Variations in (a) actual depth of cut and (b) grinding efficiency for different grinding wheels.
presence of lubricant around the grinding wheel, which provides better slippage of the grain at the workpiece–tool interface. An interesting observation was that at higher graphite contents (3 and 5 wt%), the grinding force increased compared to lower graphite contents (0.1 and 0.5 wt%). This may be attributed to excessive wheel wear for higher graphite content (3 and 5 wt%) (see Fig. 8), which leads the bonding to fracture easily; these abrasive grains adhere to the workpiece (see Fig. 6(f)) to affect the grinding force at the tool– workpiece interface. Shaji and Radhakrishnan [21] pointed out that
increasing the grinding force results in more material removal. This should be taken into account for the design of grinding wheels. 3.4. Grinding efficiency Fig. 13(a) shows the variation in the actual depth of cut for different grinding wheels in a dry grinding environment. The experimental results indicate that the set depth of cut of 15 mm was slightly larger than the actual depth of cut for wheels A–F
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(14.37, 14.58, 14.56, 14.62, 14.5 and 14.28 mm, respectively), as expected. However, the high graphite content (5 % wt) for wheel F caused the actual depth of cut to be markedly lower than the set depth of cut. When a graphite content of below 1 wt% was used, the actual depth of cut was found to be close to the set depth of cut. Fig. 13(b) shows the variation in the accumulated depth of cut for different grinding wheels. In this experiment, the set depth of cut was 15 mm per iteration, and the accumulated depth of cut was 75 mm. An interesting observation was that a graphite content of below 1 wt% provided superior grinding efficiency to that of a conventional grinding wheel, but a high graphite content (5 wt%) provided poor grinding efficiency. This implies that if the grinding efficiency is the chief concern, then using a graphite content of below 1 wt% is recommended.
4. Conclusion This paper presents a newly designed grinding wheel where micro-graphite particles are impregnated in an aluminum oxide matrix to lubricate the grinding site; this is called a graphiteimpregnated grinding wheel. Grinding performance parameters such as the surface roughness, morphology, wheel wear ratio, grinding temperature, and grinding forces of grinding wheels with five different graphite contents (0.1, 0.5, 1, 3, and 5 wt%) under dry and MQL conditions were compared with the corresponding values for a conventional grinding wheel. Graphiteimpregnated grinding wheels in an MQL environment were observed to reduce the cutting temperature by 8%–14%, the cutting force by 24%–36%, and the surface roughness by a maximum of 25%–32% compared to a conventional grinding wheel in an MQL environment. A graphite content of below 0.5 wt% is recommended because this not only produces better surface roughness and lower grinding temperature and grinding forces but also lower wheel consumption, so the wheel lifetime is prolonged. This study helps advance the understanding of lubricant techniques and design of grinding wheels as well as further developing an application method that eliminates the use of any oils or toxic organic lubricants in the grinding process.
Acknowledgment The author thanks the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract no. NSC-100-2221-E-167-009.
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References [1] H.E. Hofy, Fundamental of machining process, Taylor & Francis Group, 2007. [2] S. Kalpakjian, S.R. Schmid, H. Musa, Manufacturing engineering and technology, Pearson Education, Inc., 2006. [3] E. Brinksmeier, C. Heinzel, M. Wittmann, Friction, cooling and lubrication in grinding, Annnals of the CIRP 48 (2) (1999) 581–598. [4] M.C. Shaw, Principles of abrasive processing, Oxford University Press Inc., New York, 1996. [5] L.M. Barczak, A.D.L. Batako, M.N. Morgan, A study of plane surface grinding under minimum quantity lubrication (MQL) conditions, International Journal of Machine Tools and Manufacture 50 (2010) 977–985. [6] S. Thamizhmanii, Rosli, S. Hasan, A study of minimum quantity lubrication on Inconel 718 steel, Archives of Materials Science and Engineering 39 (2009) 38–44. [7] H.Z. Choi, S.W. Lee, H.D. Jeong, The cooling effects of compared cold air in cylindrical grinding with alumina and CBN wheels, Journal of Material Process Technology 127 (2) (2002) 155–158. [8] S. Paul, A.B. Chattopadhyay, Effects of cryogenic cooling on grinding forces, International Journal of Machine Tool and Manufacture 36 (1996) 63–72. [9] M.I. Ahmed, A.F. Ismail, Y.A. Abaker, A.K.M. Nurul Amin, Effectiveness of cryogenic machining with modified tool holder, Journal of Material Process Technology 185 (2007) 91–96. [10] T. Tawakoli, M.J. Hadad, M.H. Sadeghi, A. Daneshi, S.S. Tockert, A. Rasifard, An experimental investigation of the effects of workpiece and grinding parameters on minimum quantity lubrication—MQL grinding, International Journal of Machine Tools and Manufacture 49 (2009) 924–932. [11] L.R. da Silva, E.C. Bianchi, R.Y. Fusse, R.E. Catai, T.V. Franca, P.R. Aguiar, Analysis of surface integrity for minimum quantity lubricant—MQL in grinding, International Journal of Machine Tools and Manufacture 47 (2007) 412–418. [12] T. Tawakoli, B. Azarhoushang, Influence of ultrasonic vibrations on dry grinding of soft steel, International Journal of Machine Tools and Manufacture 48 (2008) 1585–1591. [13] T. Tawakoli, B. Azarhoushang, M. Rabiey, Ultrasonic assisted dry grinding of 42CrMo4, International Journal Advance Manufacture Technology 42 (2009) 883–891. [14] Y.S. Liao, Y.P. Yu, C.H. Chang, Effects of cutting fluid with nano-particles on the grinding of Titanium alloys, Advanced Materials Research 126–128 (2010) 353–358. [15] N.S.K. Reddy, P.V. Rao, Experimental investigation to study the effect of solid lubricants on cutting forces and surface in end milling, International Journal of Machine Tools and Manufacture 46 (2006) 189–198. [16] D.N. Rao, P.V. Krishna, The influence of solid lubricant particle size on machining parameters in turning, International Journal of Machine Tool and Manufacture 48 (2008) 107–111. [17] M. Alberts, K. Kalaitzidou, S. Melkote, An investigation of graphite nanoplatelets as lubricant in grinding, International Journal of Machine Tools and Manufacture 49 (2009) 966–970. [18] L.R. Silva, E.C. Bianchi, R.E. Catai, R.Y. Fusse, T.V. Franca, P.R. Agular, Analysis of surface integrity for minimum quantity lubricant-MQL in grinding, International Journal of Machine Tools and Manufacture 47 (2007) 412–418. [19] S.M. Yuan, L.T. Yan, W.D. Liu, Q. Liu, Effects of cooling air temperature on cryogenic machining of Ti–6AL–4V alloy, Journal of Material Process Technology 211 (2011). 356–263. [20] P.V. Krishna, D.N. Rao, Performance evaluation of solid lubricants in terms of machining parameters in turning, International Journal of Machine Tools and Manufacture 48 (2008) 1131–1137. [21] S. Shaji, V. Radhakrishnan, An investigation on solid lubricant moulded grinding wheels, International Journal of Machine Tools and Manufacture 43 (2003) 965–972.
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International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Development of a micro-graphite impregnated grinding wheel Ming-Yi Tsai n, Shi-Xing Jian Department of Mechanical Engineering, National Chin-Yi University of Technology, No.57, Section 2, Zhongshan Road, Taiping District, Taichung 41170, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 November 2011 Received in revised form 14 January 2012 Accepted 16 January 2012 Available online 25 January 2012
This paper introduces a newly designed grinding wheel where micro-graphite particles are impregnated in an aluminum oxide matrix to form a grinding wheel to lubricate the grinding site; these are known as graphite-impregnated grinding wheels. The graphite particles were heat-treated under a layer of hydrogen ions at a temperature of about 500 1C for 30 min to disperse them uniformly in the aluminum oxide matrix. In this study, grinding wheels with five different graphite contents (0.1, 0.5, 1, 3, and 5 wt%) were investigated. Different aspects of the grinding performance (e.g., surface roughness, morphology, wheel wear ratio, grinding temperature, and grinding forces) using these grinding wheels under two different coolant strategies (dry and minimum quantity lubrication with pure water) were compared with the corresponding values for a conventional grinding wheel. The experimental results indicate that using graphite-impregnated grinding wheels considerably improves the grinding process performance compared to using a conventional grinding wheel. A graphite content of below 0.5 wt% is recommended because this provides not only better surface roughness and topography, and lower grinding temperature and force but also less grinding wheel consumption; hence, the wheel life was extended. In summary, combining graphite-impregnated grinding wheels with minimum-quantity lubrication technology has the potential to effectively eliminate the use of any oils or toxic organic lubricants in the grinding process. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Grinding wheel Minimal quantity lubrication Grinding
1. Introduction Grinding, which is an abrasive material removal technique, is a chip-removal process that uses an individual abrasive grain as the cutting tool. Abrasive material removal processes can be very challenging due to the high power requirements and high temperature, especially at the workpiece–wheel interface; this is because the abrasive grits in contact with the workpiece do not cut due to the high negative rake angle but instead generate heat by rubbing and plowing the workpiece surface in the contact zone [1]. The most common strategy to control the cutting temperature in the contact zone is flooding it with cutting fluid. Kalpakjian et al. [2] showed that flooding with cutting fluid is extensively used in machining operations to achieve the following results: (1) reduce friction and wear to thus improve the tool life and surface finish of the workpiece; (2) cool the cutting zone to thus improve tool life and reduce the temperature and thermal distortion of the workpiece; (3) reduce forces and energy consumption; and (4) flush chips from the cutting zone to thus prevent the chips from interfering with the cutting process. Brinksmeier et al. [3] pointed out that in grinding, cooling by flooding is necessary to protect the workpiece and wheel from
n
Corresponding author. Tel.: þ8660423924505 7195; fax: þ 8860423920681. E-mail address: [email protected] (M.-Y. Tsai).
0890-6955/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2012.01.007
damage such as thermal burn, residual stresses, phase transformation, and microcracks. However, many of these fluids are health hazards with major environmental concerns and difficult to recycle and manage, which significantly increases the total manufacturing cost [4]. Metalworking fluids have been estimated to constitute about 7–17% of the total machining cost, whereas depreciation and waste disposal contributes to 54% of the cooling cost [5]. All of these factors have prompted investigation into the development of new coolant strategies with minimum quantity lubrication (MQL) or that eliminate flooding with cutting fluids. MQL is based on the principle that a drop of liquid is split by an airflow, distributed in streaks, and transported in the direction of the flow of air. In MQL machining, the oil consumption in industrial applications is in the range of approximately 10–100 ml/h [6]. Choi et al. [7] investigated the cooling effect of compressed cold air and compared it with that of normal coolants. They found that the compressed cold air efficiently minimizes the thermal defects of the workpiece and can also help solve the problem of environmental pollution. The mechanics of machining under cryogenic cooling was proposed as a new concept for dissipating the large amount of heat generated in the machining zone without polluting the environment. Experiments were conducted to study the effect of liquid nitrogen on the grinding forces and check the validity of the proposition. The results indicated a substantial reduction in the grinding forces under cryogenic cooling [8]. Cryogenic machining by liquid nitrogen with the help of a
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modified tool holder provides longer tool life and more wear resistance compared to dry machining [9]. Tawakoli et al. [10] investigated the influence of workpiece hardness and grinding parameters, including wheel speed, feed rate, and depth of cut on MQL grinding. They pointed out that compared to dry grinding, MQL grinding substantially enhances the cutting performance in terms of increasing the wheel life and improving the quality of the ground parts. Silva et al. [11] analyzed the behavior of the MQL technique and compared it with the conventional cooling method. They concluded that the Ra values are substantially reduced with the use of the MQL technique. In addition, MQL does not negatively affect the surface integrity. Barczak et al. [5] compared three cooling methods: conventional flood cooling, dry grinding, and grinding with MQL. They pointed out that the low grinding forces indicate that MQL can be used as a low-temperature process. Workpiece quality under MQL is comparable to and can be better than that achieved under conventional flood cooling. Tawakoli et al. [12,13] investigated the effect of ultrasonic vibrations on dry grinding of soft steel. They showed that using ultrasonic-assisted dry grinding, the normal and tangential grinding forces can be reduced by up to 70% and 50%, respectively. Although the abovementioned coolant strategies have their respective merits, they also have some drawbacks. For example, dry grinding results in workpiece damage, poor surface finish, and accelerated wheel wear. The MQL coolant can evaporate due to the grinding heat before it reaches the workpiece–wheel interface. In addition, the small amount of oil or synthetic ester used in MQL grinding still affects health and the environment. Recently, many studies have focused on cutting fluids containing nanoparticles or solid lubricant particles. For example, Liao et al. [14] investigated the use of a cutting fluid containing nanoparticles in wet grinding; they studied MQL grinding of Ti– 6Al–4V. The use of cutting fluids containing nanoparticles was found to result in less loading of the wheel and a better ground surface compared to using general-purpose flood cutting fluids. Reddy and Rao [15] presented the effects of solid lubricantassisted machining with graphite and molybdenum disulphide lubricants on the surface quality, cutting force, and specific energy when AISI 1045 steel is machined with cutting tools having different tool geometries. Rao and Krishna studied the specific application of solid lubricants in turning [16]. The process performance was judged in terms of cutting force, tool temperature, tool wear, and the surface finish of the workpiece as the cutting condition was kept constant. The experimental results showed that using the solid lubricant was more effective than dry and wet machining. Alberts et al. [17] evaluated the performance of graphite nanoplatelets coating the workpiece surface as a lubricant in grinding. They also compared their results with those obtained from conventional MQL grinding. A review of the literature shows the lack of studies on grinding wheels embedded with solid lubricants. In this study, a newly
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designed grinding wheel was fabricated by impregnating submicron graphite particles in an aluminum oxide matrix. In this study, grinding wheels with five different submicron graphite contents – 0.1, 0.5, 1, 3, and 5 wt% – were investigated. Different aspects of the grinding performance, such as the surface roughness, morphology, and grinding forces, using these grinding wheels under two different coolant strategies – dry and MQL with pure water – were compared with the corresponding values for a conventional grinding wheel.
2. Experimental procedure 2.1. Development of the graphite-impregnated grinding wheels The graphite-impregnated grinding wheel was manufactured using an aluminum oxide base with the addition of various graphite contents (0.1, 0.5, 1, 3, and 5 wt%). The graphite particles were heattreated under hydrogen flowing at 2 L/min at a temperature of about 500 1C for 30 min to uniformly disperse the graphite in the grinding wheel. After hydrogenation, the graphite particles on the surface were covered by a layer of hydrogen ions (H þ ) due to the graphite particles repelling each other. Images of the graphite and aluminum oxide particles are shown in Fig. 1. The graphite and aluminum oxide particle were about 50 and 150 mm in size, respectively. Excluding the graphite, these wheels consisted of 80% abrasive, 12.5% liquid bonding material, 5.2% powder resins, and 2.3% additives by weight. This study used grinding wheels made with the resin bond method. An A100K8B1A (180 mm 13 mm 31.75 mm size) wheel was taken as the standard wheel. Fig. 2 shows the graphite-impregnated grinding wheel manufacturing process. (a) First, the abrasive grains, additives, bonding, and hydrogenated graphite materials are prepared. (b) The abrasive grains and liquid phenolic resins are uniformly mixed together. Also, the hydrogenated graphite materials, powdered phenolic resins, and additives are mixed. (c) The mold is filled with the mixture. (d) The mixture (50 kg) is pressed into the shape of a grinding wheel. (e) Finally, the mixture is cured at a temperature of about 180 1C before the graphite-impregnated grinding wheel is tested and finished. These wheels have a soft structure and are used for high-speed grinding, rough grinding, and cutting. 2.2. Experimental apparatus The experiments were conducted with traverse grinding using a horizontal-spindle high-speed surface grinder. The workpiece was secured on a magnetic chuck attached to the worktable of the grinder, as shown in Fig. 3(a). SKD 11 structural steel with dimensions of 60 mm 11 mm 25 was used. Before machining, the materials were pre-machined with a 1 mm cut to remove any
Fig. 1. Appearance of (a) graphite and (b) aluminum oxide particles.
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Fig. 2. Manufacturing stages of graphite-impregnated grinding wheel: (a) preparing related materials, (b) mixing abrasive and graphite materials, (c) filling mixture in mold, (d) forming a cylindrical wheel by pressing, and (e) curing and testing.
Fig. 3. (a) Experimental system configuration: (1) grinding wheel, (2) nozzle, (3) dynamometer, (4) magnetic bases, (5) thermocouple, (6) dust catcher, (7) signal transmission line, and (8) workpiece. (b) Details on the MQL equipment fixed to the grinder: (1) export of pure water mist, (2) filler, (3) knob for lubricant flow rate adjustment, (4) droplet transparent window, (5) manometer (0–150 psi or 0–10.2 kgf/cm2), (6) water meter, and (7) air inlet. Table 1 Grinding conditions. Grinding wheel
Grinding mode Grinding conditions Environments
Workpiece material Dressing conditions
1. Graphite impregnated grinding wheels with 0.1, 0.5, 1, 3, and 5 wt% graphite 2. conventional grinding wheel: A100K8B1A 3. grinding wheel size: 180 mm 13 mm 31.75 mm Up-grinding Wheel speed: 2400 rpm, work speed: 20 m/min, depth of cut: 15 mm 1. Dry grinding 2. MQL with pure water grinding, flow rate: 90 ml/h, air pressure: 25 psi SKD 11, HRc¼ 607 2 Workpiece size: 60 mm 11 mm 25 mm With single diamond dresser, dressing depth: 50 mm, dressing speed: 1800 rpm
possible surface irregularities and ensure similar surface properties for all of the specimens. The tests evaluated the performance of five different graphite grinding wheels using a surface grinding machine. All of the experiments were conducted three times, and the average value was taken as the response value. The dressing operation was kept constant using a single-point diamond dresser so that it would not influence the output variable of the process. The machining parameter settings used in this study are summarized in Table 1. Two coolant strategies were also employed to evaluate the wheels and workpiece performances in the experiments: dry grinding (DG) without cooling and lubrication, and minimum quantity lubrication (MQL) grinding where pure water is fed into the grinding contact zone. The control unit of the MQL equipment was fixed to the grinding machine, where the lubricant dosage
and airflow rate were adjusted. Fig. 3(b) shows details of the MQL equipment, and its parts are numbered to clearly show the description and functioning. The MQL system consists of a compressor, pressure regulator, rotameter, doser, and spray nozzle. The nozzle was placed at a distance of about 30 mm from the grinding wheel–workpiece interface. After the workpiece was ground, its surface roughness was measured by the Hommel Tester T-1000 with a cutoff length of 0.8 mm. At the end of each test, Ra across the grinding direction was measured at five different points of the ground surface. A scanning electron microscope (SEM) was used to analyze the surface morphology for possible damage caused by the thermal and mechanical forces acting on the material’s surface. The normal and horizontal grinding forces were recorded using a piezo-electric transducer based dynamometer (type Kistler 9257B) positioned under the workpiece clamping device, which was connected to an amplifier equipped on a computer. The workpiece temperature was measured by the thermocouple with a minimum sensitivity of 1 1C over a range of 20–950 1C. Each sample was measured three times to determine the reproducibility of the results.
3. Results and discussion 3.1. Surface roughness and surface topography It is well known that the surface finish can significantly affect the mechanical strength of components when they are subjected to fatigue cycles. The surface roughness also plays a wider role with regard to the performance of a product. A poor surface can make the performance vulnerable and the product to fail well before the expected lifespan. Fig. 4 compares the mean values of Ra parameters with graphite-impregnated grinding wheels
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0.4
0.4 Dry grinding
0.3 0.25 0.2
MQL grinding
0.35 Ra (µm)
Ra (µm)
0.35
97
0.3 0.25
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%)
0.2
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%)
Grinding wheels
Grinding wheels
Fig. 4. Variation in surface roughness for different grinding wheels under (a) dry grinding and (b) MQL conditions.
Fig. 5. Schematic illustration of (a) dry grinding and (b) MQL conditions.
(wheels B–F) and a conventional grinding wheel (wheel A) under dry and MQL environments, respectively. As shown in Fig. 4(a), the mean Ra values for wheels B–F were markedly lower than that of wheel A. This is because graphite is a good lubricant due to its layered structure. In addition, impregnating graphite particles into grinding wheels directly results in more effective lubrication of the workpiece–tool contact interface (see Fig. 5(a)). Efficient lubrication allows the chips to slide more easily over the tool’s surface, resulting in a better surface finish. An interesting observation was that the mean values of Ra were unchanged when the graphite content was increased from 0.1 wt% (wheel A) to 5 wt% (wheel F). The same trend in the results is shown in Fig. 4(b). A comparison of wheel A with wheels B–F in an MQL environment showed that Ra (0.325 mm) for wheel A was larger than those for wheels B, C, D, E, and F (0.224, 0.223, 0.228, 0.21, and 0.234 mm, respectively). An analysis of the results obtained with graphiteimpregnated grinding wheels under dry grinding and MQL conditions indicates that the latter leads to superior performance than the former due to more efficient penetration of the fluid into the cutting region. The MQL technique leads to lower roughness values; this is probably because of the more effective lubrication and cooling of abrasive grains at the workpiece–tool interface (see Fig. 5(b)) [18]. Fig. 4(b) again indicates that the mean values of Ra were altered when the graphite content was increased from 0.1 wt% to 5 wt%. To understand the variable results of the surface roughness measurements, the surface topographies of workpieces after grinding by the various grinding wheels in dry and MQL environments were also studied. Fig. 6(a) shows the cross-sectional SEM images of the workpiece surface after dry grinding with wheel A. As shown in the figure, some surface burning and surface damage as well as side ridges were formed due to a lack of lubrication leading to a high temperature for the workpiece surface. Fig. 6(b) and (c) show some scratches on the workpiece surface, but no thermal cracks formed with wheels B and C; this confirms that the graphite particles functioned effectively in the lubricant contact zone. Fig. 6(d) and (f) show slight scratches with no thermal damage after grinding with wheels D and F; this again proves that more graphite particles can provide better lubrication at the tool–chip
and tool–workpiece interfaces. However, a careful observation of these figures (see Fig. 6(d) and (f)) reveals some fine particles adhering to the workpiece surface and contributing to a poor surface roughness. This may be why the surface roughness did not change when the graphite content was increased from 0.5 wt% to 5 wt%. Fig. 7 shows the cross-sectional SEM images of the workpiece surface under the MQL condition. The surface scratches caused by wheel A were still more than those caused by wheels B–F. Compared to dry grinding, less thermal damage and material side flow was observed when MQL was used. Again, some particles were found to remain on the work surface when high graphite content (3 and 5 wt%) was used. This result may be attributed to the higher grinding wheel wear ratio, which is discussed later. 3.2. Grinding wheel wear ratio Grinding wheel wear is generally correlated with the amount of workpiece material that is ground; it is captured by a parameter called the wear ratio (Wr), which is defined as Wr ¼
Vw Vm
where Vm is the volume of material removed and Vw is the volume of wheel wear. The wear ratio has been found to be higher when softer bonds are used. In general, softer bonds are recommended for harder materials and to reduce residual stresses and thermal damage to the workpiece. Fig. 8 shows the variation in the wheel wear ratio with different grinding wheels. As shown in Fig. 8, the wear ratio increased with increasing graphite content. The highest wear ratio of 1.8 was obtained with wheel F, which has a graphite content of 5 wt%. This may be because the graphite particles cluster easily in the grind wheel to become block structures. When subjected to force, the grains with clusters of graphite particles are easily dislodged to weaken the bonding, leading to higher wheel wear (see Fig. 9). The observations discussed previously were confirmed. One interesting observation was that the wear ratio was lower than that of a conventional grinding wheel when the graphite content was below 0.5 wt%;
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Fig. 6. Cross-sectional SEM images of workpiece surface after dry grinding with a (a) conventional grinding wheel, (b) wheel B, (c) wheel C, (d) wheel D, (e) wheel E, and (f) wheel F.
this implies that if consumption of the grinding wheel is the chief concern, then a graphite content of below 0.5 wt% is recommended to extend the wheel life. 3.3. Temperature and cutting force The grinding contact temperature was measured during the tests. This allowed the thermal performance of the graphiteimpregnated grinding wheels to be compared with the conventional grinding wheels and also gave insight into the strength of the lubrication effect on the graphite-impregnated grinding wheels. The graphite-impregnated grinding wheels were observed to reduce the average cutting temperature by about 7–17% compared to the conventional grinding wheel, as shown in Fig. 10. This is because graphite particles are directly applied to the heat generation zones in the grinding process. A comparison of wheel A with wheels B–F under MQL conditions shows that wheel A had a considerably higher temperature (88.6 1C) than wheels B, C, D, E, and F (80, 81.3, 78.2, 79.3, and 77.1 1C, respectively). The results seem to be due to the low coefficient of friction, as solid lubricants soften at elevated temperatures. The combined effects of the embedded graphite particles and MQL
technique is why the grinding temperature was reduced. Yuan et al. [19] described the decrease in grinding temperature as leading to less adhesion and better surface roughness. Vamsi and Rao [20] proposed that the thermal conductivity of a solid lubricant mixture allows the tool temperature to decrease. Grinding forces are important parameters by which the performance of any grinding process can be evaluated. The relative contributions of different parameters on the cutting force depend on the type of work material, the tool material, and the presence of a cutting fluid or solid lubricants. The cutting force component determines the power requirement of the process. The intensity of heat generation depends upon this force, and it is of primary importance as far as the machining temperature and surface quality of the products are concerned [15]. Fig. 11 shows the variation in the tangential forces (Ft) and normal forces (Fn) with different grinding wheels in a dry grinding environment. The results showed that the cutting force (Ft and Fn) was considerably less with solid lubrication–embedded grinding wheels compared to the conventional grinding wheel. The lubricant effectiveness at minimizing the frictional effects of the tool and workpiece interaction for wheels B–F is evident from the reduced grinding forces compared to wheel A. This performance of the graphite particles is due to their lattice layer
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Fig. 7. Cross-sectional SEM images of workpiece surface after grinding in an MQL environment with (a) a conventional grinding wheel, (b) wheel B, (c) wheel C, (d) wheel D, (e) wheel E, and (f) wheel F. Abrasive grains
2
Bond
Wear ratio
1.8 1.6 Bond fracture
Porosity
1.4 Graphite particles
1.2 Workpiece surface
1
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%) Grinding wheels Fig. 8. Variation in wear ratios for different grinding wheels.
structure, which allows them to act as an effective solid lubricant film [20]. The normal force was found to be more than twice the tangential force, unlike in other machining processes such as turning and milling. This is due to grinding process characteristics such as very large negative rake angles of the grit, excessive rubbing action,
Fig. 9. Schematic illustration of a physical model for the structure of a graphiteimpregnated grinding wheel.
and adverse chip accommodation space that leads to wheel loading. Fig. 12 also shows the variation in the grinding forces for different grinding wheels in an MQL environment. Similar results are shown in the figure. Again, the tangential and normal forces of graphiteimpregnated grinding wheels were confirmed to eventually become lower than those of a conventional grinding wheel. In addition, as shown in the figure, MQL grinding considerably reduces the tangential and normal forces in comparison to dry grinding due to the
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106 104 102 100 98 96 94 92 90 88
Dry grinding
90 88 86 84 82 80 78 76
Temperature(°C)
Temperature(°C)
100
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%) Grinding wheels
MQL grinding
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%) Grinding wheels
60
26 24 22 20 18 16 14 12
55
Dry grinding
Dry grinding
50
Fn (N)
Ft (N)
Fig. 10. Variation in temperature for different grinding wheels under (a) dry grinding and (b) MQL conditions.
45 40 35 30
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%)
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%)
Grinding wheels
Grinding wheels
Fig. 11. Variation in (a) tangential forces and (b) normal forces for different grinding wheels.
50
20 MQL grinding
MQL grinding
45 Fn (N)
Ft (N)
18 16
40 35
14
30
12 Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%)
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%)
Grinding wheels
Grinding wheels
14.8
Grinding efficiency (%)
Actual depth of cut (µm)
Fig. 12. Variation in (a) tangential forces and (b) normal forces for different grinding wheels.
14.6 14.4 14.2 14
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%) Grinding wheels
98 97.5 97 96.5 96 95.5 95
Wheel-A Wheel-D Wheel-C Wheel-D Wheel-E Wheel-F (0 wt%) (0.1 wt%) (0.5 wt%) (1 wt%) (3 wt%) (5 wt%) Grinding wheels
Fig. 13. Variations in (a) actual depth of cut and (b) grinding efficiency for different grinding wheels.
presence of lubricant around the grinding wheel, which provides better slippage of the grain at the workpiece–tool interface. An interesting observation was that at higher graphite contents (3 and 5 wt%), the grinding force increased compared to lower graphite contents (0.1 and 0.5 wt%). This may be attributed to excessive wheel wear for higher graphite content (3 and 5 wt%) (see Fig. 8), which leads the bonding to fracture easily; these abrasive grains adhere to the workpiece (see Fig. 6(f)) to affect the grinding force at the tool– workpiece interface. Shaji and Radhakrishnan [21] pointed out that
increasing the grinding force results in more material removal. This should be taken into account for the design of grinding wheels. 3.4. Grinding efficiency Fig. 13(a) shows the variation in the actual depth of cut for different grinding wheels in a dry grinding environment. The experimental results indicate that the set depth of cut of 15 mm was slightly larger than the actual depth of cut for wheels A–F
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(14.37, 14.58, 14.56, 14.62, 14.5 and 14.28 mm, respectively), as expected. However, the high graphite content (5 % wt) for wheel F caused the actual depth of cut to be markedly lower than the set depth of cut. When a graphite content of below 1 wt% was used, the actual depth of cut was found to be close to the set depth of cut. Fig. 13(b) shows the variation in the accumulated depth of cut for different grinding wheels. In this experiment, the set depth of cut was 15 mm per iteration, and the accumulated depth of cut was 75 mm. An interesting observation was that a graphite content of below 1 wt% provided superior grinding efficiency to that of a conventional grinding wheel, but a high graphite content (5 wt%) provided poor grinding efficiency. This implies that if the grinding efficiency is the chief concern, then using a graphite content of below 1 wt% is recommended.
4. Conclusion This paper presents a newly designed grinding wheel where micro-graphite particles are impregnated in an aluminum oxide matrix to lubricate the grinding site; this is called a graphiteimpregnated grinding wheel. Grinding performance parameters such as the surface roughness, morphology, wheel wear ratio, grinding temperature, and grinding forces of grinding wheels with five different graphite contents (0.1, 0.5, 1, 3, and 5 wt%) under dry and MQL conditions were compared with the corresponding values for a conventional grinding wheel. Graphiteimpregnated grinding wheels in an MQL environment were observed to reduce the cutting temperature by 8%–14%, the cutting force by 24%–36%, and the surface roughness by a maximum of 25%–32% compared to a conventional grinding wheel in an MQL environment. A graphite content of below 0.5 wt% is recommended because this not only produces better surface roughness and lower grinding temperature and grinding forces but also lower wheel consumption, so the wheel lifetime is prolonged. This study helps advance the understanding of lubricant techniques and design of grinding wheels as well as further developing an application method that eliminates the use of any oils or toxic organic lubricants in the grinding process.
Acknowledgment The author thanks the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract no. NSC-100-2221-E-167-009.
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