立式高速銑削加工中心圓盤刀庫機構(gòu)設(shè)計,立式高速銑削加工中心圓盤刀庫機構(gòu)設(shè)計,立式,高速,銑削,加工,中心,圓盤,機構(gòu),設(shè)計 A STUDY ON THE HIGH-SPEED MILLING USING SMALL-DIAMETER END MILL TOOL KATO Hideharu 1 , SHINTANI Kazuhiro 1 , IWATA Kazuo 1 , SUGITA Hiroaki 2 1 Department of Mechanical Engineering, Kanazawa Institute of Technology, 7-1 Ohgigaoka, Nonoichi, Ishikawa, 921-8501, Japan 2 OSG Corporation, 1-15 Honnogahara Toyokawa, Aichi, 442-8544, Japan Abstract Recently, the demand for small parts has increased with the popularization of portable products. It is necessary to develop small machine tools in which high-speed machining is possible. Highly efficient processing using high-speed machining is useful for milling small parts with a small- diameter tool. In this study, the design and production of a desk-top-type small machine tool were carried out and the cutting performance of a small-end mill tool at high-speed condition is investigated. The size of the developed machine tool is 300 400 340mm, and the weight is 50kg. This machine could reach a cutting speed of 25.0m/s using a small-diameter tool of 1.7mm. As a result of high-speed milling with this machine, a surface roughness of 0.6mRz without tear parts was obtained at cutting speed of 25.0m/s. In addition, the thickness of deformed layers produced by cutting at a speed of 25.0m/s decreased in comparison with that of the layers produced at 2.5m/s. It was confirmed that the thickness of deformed layers at 25.0m/s was 0.6m. Key words desk-top-type miniature machine tool, small end mill tool, high-speed milling, thickness of deformed layer, surface roughness 1 INTRODUCTION Recently, with the advance of the diversification of consumer needs, an agile manufacturing system for production involving frequent changes in product type and quantity has been applied 1,2. Therefore, not only more efficient production is required, but also an increase in machining speed is needed. On the other hand, the demand for small parts on the millimeter scale increases with the popularization of portable products. In addition, the miniaturization, lightening and performance enhancement of such products have been required 3,4. It is important to improve processing efficiency and accuracy in order to attempt the miniaturization of the product. It is also necessary that higher precision and miniaturization of the machine tool are realized. However, an increase in cutting speed is difficult to achieve in a traditional-model high-speed machine tool at a spindle rotational number of approximately 40000min -1 , when machining a small product using a small-diameter tool. Therefore, an increase in cutting speed seems to be necessary by mounting an ultrahigh rotation spindle in order to improve productivity when using a small-diameter tool. On the other hand, its large size and very low energy efficiency are disadvantages of the traditional-model machine tool. The traditional-model high-speed machine tool is larger than small product. In addition, machine elements such as stage and spindle are also large and the energy consumed during the operation is considerable. Therefore, the development of a desk-top-type miniature machine tool that can realize high-speed machining and save energy is expected 5. In this study, the design and production of a desk-top-type miniature machine tool that can realize high speed machining and save energy are carried out. The cutting performance of a small end mill tool at a high-speed is investigated. 2 SPECIFICATIONS OF MINIATURE MACHINE TOOL Table1 shows the specifications of the desk-top-type miniature machine tool. Silicon nitride- based ceramics were adopted in the frame material in order to reduce its weight. The material used has half the specific gravity of cast iron. In addition, it has advantages in following; the tensile strength is about the double, its young modulus is about 3 times and linear expansion coefficient is low (1/3) 6. The machine weight is 50kg and the machine dimensions are 300 400 342mm. Two types of air turbine spindle (Type A: rotational number 320,000min -1 and Type B: 200,000min -1 ) were selected in order to achieve high-speed cutting using the small-diameter tool. These spindles are light and small, and can be used under oil-free conditions. Thus, a spindle cooling system is not necessary in this machine. The air turbine spindle is supplied with compressed air by a compressor with a 100V power supply. A linear motor drive stage with 0.1m positioning accuracy was used for the X-axis and Y-axis in order to carry out high-precision machining of the small workpiece. The strokes of the X-axis, Y-axis and Z-axis are 50, 50 and 16mm respectively. The miniature machine tool was installed in a box in order to control the atmosphere during machining. Figure 1 shows schematic of the miniature machine tool produced with the above specifications. The main body and axis control unit were designed using a personal computer. Figure 2 shows the structural elements of the main body. The air turbine spindle has been installed on the manual Z-axis stage with a 0.5m positioning accuracy. The arrow in the figure shows the transfer direction of each axis. Figure 3 shows the control system of the X-axis and Y-axis. A closed-loop system has been adopted for the control unit. A high positioning accuracy has been realized using the feedback mechanisms of position, speed and electric current. Positioning is carried out using a visual basic program on a personal computer. The comparison between the electric power consumption of this equipment and that of existing high-speed machining center (M/C) is shown in Fig. 4. In this figure, the electric power consumption of this equipment is less than 1/10 that of the M/C. It can be confirmed that the miniature machine tool is superior in energy efficiency. Table 1 Specification of the miniature machine tool Material Si 3 N 4 ceramics Machine size (X Y Z) 300 400 342mm Frame Machine Weight 50 kg Driving method Linear motor drive Full stroke X:50mm, Y:50mm, Z:16mm Stage Resolving power 0.1 m Main spindle Air turbine spindle Spindle Rotational number 320,000 min -1 Fig. 1 Photograph of the miniature machine tool Main body Personal Computer Stage control unit X Y Z Linear motor stage Ceramics body Vise Fig. 2 Photograph of the main body Air turbine spindle V=25.0m/s V=16.7m/s Using area Rotational number min -1 Load N Type B spindle (200,000min -1 ) Tool diameter: 1.7mm Type A spindle (320,000min -1 ) 3 BASIC EV ALUATION OF PRODUCED MINIATURE MACHINE TOOL 3.1 Characteristics of Air Turbine Spindle The air turbine spindle can achieve a high rotational number, but it has a low torque. In this section, the effect of force for the air turbine spindle on the rotational number was investigated. The test bar was installed in the spindle and the load was added to the tip of the test bar (overhang length: 9mm and diameter: 1.7mm) from the horizontal direction during spindle rotation. The behavior of the rotational number was determined using a laser-type tachometer. Figure 5 shows the measurement results for the A and B spindles. In this figure, both spindle rotational numbers decrease rectilinearly with increasing load. It is confirmed that a 320,000min -1 -type spindle can be used at a high speed of 25.0m/s under a horizontal load of 0.3N. On the other hand, a 200,000min -1 -type spindle can be used at a speed of 16.7m/s under a horizontal load of 0.6N. 3.2 Characteristics of Positioning of X-axis and Y-axis In this machine, the X-axis and Y-axis are moved using the linear motor drive. The positioning accuracies of both axes were examined. The examination was carried out by inputting a step positioning of 5m for the plus and negative sides for 3 times at a feed speed of 40 mm/s. The positioning accuracy for the command was measured using a laser-type displacement sensor with a 10nm resolution. The results for both axes are shown in Fig. 6. In this figure, it is clear that both axes follow for the input value accurately. However, it is confirmed that a transient characteristic is present in the X-axis. The inertial force is caused the weight (4.5kg) of the Y-axis stage, since the Y-axis stage is on the X-axis stage. 3.3 Characteristic of Machine Frame Natural frequency and damping ratio were measured, and the attenuation characteristic of the frame was examined 7. Figure 7 shows a schematic of the vibration measurement equipment and experimental method. The vibration in the frame is detected by a piezoelectric acceleration sensor in Position control section Speed control section Electric current control section Linear motor Speed detection Position detection Electric current detection Linear scale F/V change Pals command Air-compressor Main body Stage control unite Computer Control method Fig. 3 Schematic illustration of the miniature machine tool control system 0 5 10 15 20 25 30 MC Small machine tool ：Spindle consumption electric power ：Stage consumption electric power Total consumption electric power of machine tool kW Spindle consumption electric power Stage consumption electric power Miniature machine tool Machining center Total consumption Power of machine tool kW Fig. 5 Relation between load and rotational number 0 5 10 15 20 0 5 10 15 20 Moving distance m axis axis Moving distance m Fig.6 Micro step response of the X,Y axis 100ms 100ms Fig. 4 Comparison of the consumption electric power between miniature machine tool and Machining center the upper part of the Z-axis frame, after the A point of the frame is shaken using an impulse hammer. The output value was input into the Fast Fourier Transform (FFT) analyzer through the amplifier, and natural frequency and damping ratio were obtained. Table 2 shows the measurement results of natural frequency and damping ratio when shaking in the X-axis and Y-axis directions. In this table, it was confirmed that the natural frequency of the X-axis is 525Hz and that the natural frequency of the Y-axis is 532Hz. From this result, it seems that the natural frequency of the miniature machine tool is approximately 530Hz. The damping ratio of this machine is 0.119. The frequency of the both air turbine spindle are 5333 and 3333 Hz, and it does not seem to resonate from the measurement result of the natural frequency of the miniature machine tool. 4 SUPERIORITY OF HIGH-SPEED MACHINING USING SMALL END MILL TOOL 4.1 Experimental Procedure The workpiece material is carbon steel (JIS-S45C). The workpiece material was annealed. Table 3 shows the chemical compositions and brinell hardness of the workpiece material. The microstructure of this material is shown in Fig.8. The shape and geometry of the workpiece material is a rectangular parallel-pipe shape of 15 10 20mm. A (Ti,Al)N coated two-flute square small-end mill tool was used. The substrate material is cemented carbide and the coating thickness is about 3.0m. Figure 9 shows tool geometry. Figure 10 shows the tip cutting edge of the small end mill tool. The actual helix angle of used end mill tool is approximately zero degrees in order to add an end gash about 50m. In milling experiments, the fine cutting conditions were as follows: feed rate (Sz): 4.3m /tooth, axial depth of cut (Aa): 50m, radial depth of cut (Ar): 30m and cutting speeds (V): 2.5, 16.7 and Table 3 Chemical composition and brinell hardness of S45C Chemical composition mass% C Si Mn P S Fe Brinell hardness 0.45 0.29 0.71 0.027 0.018 Bal. 180 Length of cut 2.6mm 1.6 Radial rake angle 3 End cutting edge concavity angle 6 Helix angle 30 Axial relief angle 8 Fig. 9 Tool geometry 10m Pearlite Ferrite Table 2 Measurement results of damping Impact direction Measurement item Measurement result Natural frequency n 525 Hz X axis Phase 120 deg Natural frequency n 532 Hz Y axis Phase 49 deg Damping ratio 0.119 Fig. 8 Microstracture of the workpiece material (3% nitric acid alcohol solution, 60 sec.) FFT analyzer Measurement point Amplifier Impulse hammer Impact points Y X A Y X Flame Detail of A Fig. 7 Experimental method and device of vibration measurement 25.0m/s. In the experiment, the effect of cutting speed on cutting properties was examined using three types of spindle, since the rotational number of each spindle cannot be freely controlled. A 320,000min -1 type air turbine spindle (Type A), 200,000min -1 type air turbine spindle (Type B) and 30,000 min -1 type direct current motor spindle (Type C) were used. The cutting method was up cutting, and the cutting point was supplied with dry air at a pressure of 0.2MPa. The observation of the machined surface was performed using an electron microscope and a blue laser microscope. 4.2 Experimental Results and Discussion The superiority of the high-speed machining of S45C using a small end mill tool was investigated. Figure 11 shows the variation in the maximum height of the cutting surface with increasing cutting length at several cutting speeds. The mark in this figure shows dispersion and average of surface roughness. The maximum height of 2 m is maintained with increasing cutting length at a high speed, and the dispersion of the value is also small. On the other hand, it is clear that fluctuation and dispersion are very large at low cutting speed. Figure 12 shows a comparison of machined surfaces for several cutting speeds. Tear parts can be observed in the machined surface at a low cutting speed (2.5m/s). At high cutting speed (16.7m/s and 25.0m/s), the tear parts are not observed even if cutting distance increases. From the measurement result of the spindle deflection, the deflection circumference of the direct-current motor spindle used at a low cutting speed was approximately 9m, which is large in comparison with that of the air turbine spindle. Therefore, a similar supporting experiment was attempted using an existing high-speed machining center (the deflection circumference: 1.0m). The results obtained were the same as those obtained using the direct-current motor spindle. Next, the deformed layer above the machined surface for both cutting speeds (2.5m/s and 25.0m/s) was observed. Figure 13 shows a comparison of the cross section after etching with 3% nitric acid alcohol solution for both cutting speeds. As seen this in figure, it is confirmed that the deform layer thickness are approximately 1.3m at the cutting speed of 2.5m/s and approximately 0.6m at the cutting speed of 25.0m/s. It is considered that the decrease in the thickness of the deformed layer in the case of high-speed cutting is better superiority in terms of product quality. Figure 14 shows that the chip thickness decreases with increasing cutting speed. It becomes approximately half that obtained at a low cutting speed. With increasing cutting speed, it is confirmed that the angle of the first plastic 05 01 0 01 5 0 200 250 300 ：V =2.5m/s ：V=16.7m/s ：V=25.0m/s 切削距離 Ln m 最大高粗 Rz m 5.0 10.0 Surface roughness Rz m Cutting length Ln m :V=2.5m/s :V=16.7m/s :V=25.0m/s Ar: 30 m Aa: 50 m Sz: 4.3 m /tooth Fig. 12 Comparison of machined surface at several cutting speeds Ln 25m Ln 150 V=2.5m/s V=16.7m/s V=25.0m/s 5m 5m 5m 5m 5m 5m Tear part Tear part Fig. 10 Tip cutting edge of small end mill tool 10m Peripheral cutting edge Radial relief face Gash length End cutting Edge End mill tool Fig. 11 Relation of between cutting length and surface roughness flow of the chip decreases. It is shown that shear angle increases. Therefore, the cutting force was small in high-speed cutting using the small-diameter tool, and the improvement in the machined surface accuracy was confirmed. Hence, the superiority of high-speed machining using a small end mill tool can be expected. 5. CONCLUSIONS The design and production of a desk-top-type miniature machine tool that can realize high-speed machining and save energy were carried out and cutting performance of a small-end mill tool at a high speed is investigated. The results obtained are summarized as follows. 1) The desk-top-type miniature machine tool was designed. This machine tool was composed of an air turbine spindle (320,000min -1 -type or 200,000min -1 -type), a silicon nitride based ceramic frame and a linear motor drive stage with 0.1m positioning accuracy. 2) The power consumption of this machine tool was below 1/10 that of existing high-speed machining centers. 3) It was confirmed that a 320,000min -1 -type spindle can be used at a high speed of 25.0m/s under a horizontal load of 0.3N. 4) The damping ratio was 0.119 and the natural frequency was 530Hz of this miniature machine tool. 5) The surface roughness of 2.0mRz is maintained with increasing cutting length at a high speed (25.0m/s), and the dispersion of the value is also small. Under these conditions, a machined surface without tear parts was obtained. 6) It was clarified that the deformed layer thickness are approximately 1.3m at a cutting speed of 2.5m/s and approximately 0.6m at a cutting speed of 25.0m/s. 7) The first plasticity flow angle of the chip decreased at a high cutting speed (25.0m/s). REFERENCES 1. KAKINO Y, IHARA Y, MORIGUCHI H, et al, High-speed Machining Technologies for Agile Manufacturing, IMS, 2001, 89 92 (in Japanese) 2. IDETA M, FUKUSHIGE M, et al., Toyota Tech. Rev., 1995,45(1), 92 97(in Japanese) 3. JAMES M, Miniaturization., Tech. Pap. Soc. Manuf. Eng., 1999, 1 6 4. REDFORD A, Small Parts Feeding, Assem. Autom., 1991,11(4), 8 11 5. ASADA N, KURIHARA G, MORITA N, et al., Development of Numerical Controlled Micro Milling Machine (Micro Cube), J. of JSAT, 2003,47(7), 373 378(in Japanese) 6. MISHIMA N, Characteristics of Materials for Machine Tool Structure. (1st report)., Purpose of the Study and Damping Characteristics, Report of Mechanical Engineering Laboratory, 1995, 49(5), 192 200 (in Japanese) 7. ISO 10791-2 Test conditions for machining centers - Part 2: Geometric tests for machines with vertical spindle or universal heads with vertical primary rotary axis (vertical Z-axis) (2001) V=2.5m/s 10m 7.9m V=25.0m/s 4.3m 3m 10m 3m Pearlit Ferrite Pearlit Ferrite 1m 1m V=25.0m/s V=2.5m/s 1.3m 0.6m Fig. 14 Comparison of cross section chip between 2.5 m/s and 25.0 m/s of Ln=10m (3% nitric acid alcohol solution, 60 sec.) Fig. 13 Comparison of the deformed layer between 2.5m/s and 25.0m/s (3% nitric acid alcohol solution, 60 sec.)