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A STUDY ON THE HIGH SPEED MILLING USING SMALL DIAMETER END MILL TOOL KATO Hideharu1 SHINTANI Kazuhiro1 IWATA Kazuo1 SUGITA Hiroaki2 1Department of Mechanical Engineering Kanazawa Institute of Technology 7 1 Ohgigaoka Nonoichi Ishikawa 921 8501 Japan 2OSG 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 6 mRz 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 6 m 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 1 m 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 5 m 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 Si3N4 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 X Y Y Z Z Linear motor stage Ceramics body Vise Fig 2 Photograph of the main body Air turbine spindle 負荷 N 回 転数 min 1 32万回転仕様主軸 20万回転仕様主軸 00 20 40 60 8 100 150 200 250 300 350 400 103 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 EVALUATION 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 5 m 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 MCSmall 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 0 m 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 50 m In milling experiments the fine cutting conditions were as follows feed rate Sz 4 3 m tooth axial depth of cut Aa 50 m radial depth of cut Ar 30 m 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 10 m 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 9 m 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 0 m 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 3 m at the cutting speed of 2 5m s and approximately 0 6 m 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 050100150200250300 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 5 m5 m 5 m5 m 5 m 5 m Tear part Tear part Fig 10 Tip cutting edge of small end mill tool 10 m 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 1 m 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 unde