复合高速机刀具结构的设计与制造.pdf

Design and manufacture of composite high speed machine tool structures Dai Gil Lee *, Jung Do Suh, Hak Sung Kim, Jong Min Kim Mechanical Design Laboratory with Advanced Materials, Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, ME3261, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea Received 28 October 2002; received in revised form 29 September 2003; accepted 30 October 2003 Available online 12 February 2004 Abstract The high transfer speed as well as the high cutting speed of machine tools is important for the productivity improvement in the fabrication of molds/dies because non-machining time, called the air-cutting-time, amounts to 70% of total machining time with complex shape products. One of the primary reasons for low productivity is large mass of the moving parts of machine tools, which cannot aff ord high acceleration and deceleration encountered during operation. Moreover, the vibrations of the machine tool structure are among the other causes that restrict high speed operations. In this paper, the slides of high speed CNC milling machines were designed with fi ber reinforced composite materials to overcome this limitation. The vertical and horizontal slides of a large CNC machine were manufactured by joining high-modulus carbon-fi ber epoxy composite sandwiches to welded steel structures using adhesives and bolts. These composite structures reduced the weight of the vertical and horizontal slides by 34% and 26%, respectively, and increased damping by 1.55.7 times without sacrifi cing the stiff ness. Without much tuning, this machine had a positional accuracy of ?5 lm per 300 mm of the slide displacement. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Carbon fi ber epoxy composite; B. Vibration; C. Sandwich; E. Welding/joining 1. Introduction CNC milling machines and machining centers are employed in the fabrication of various molds/dies that are used for electrical appliances, automobile interiors, stamping and injection molding. During normal ma- chining with machine tools, their cutting tools are moved with nominal feed rates, while the feed rates are switched to a rapid traverse mode during the transfer of cutting tools without contacting workpieces: The time spent to transfer a cutting tool without contacting workpieces is called air-cutting-time. Generally, only about 30% of the total machining time is spent in the actual cutting or making chips, while the remaining 70% is spent in the air-cutting-time 1,2. Therefore, not only high cutting speeds but also high transfer speeds are required to obtain the enhanced productivity of ma- chining which is essential to survive in the global com- petition of machine tool markets. Although the cutting speed has been increased due to newly developed cutting tool materials such as ceramic, CBN, diamond and so on, productivity is still restricted by the low transfer speed of massive moving frames which are usually made of steel. Conventional steel moving frames of machine tools operate with maximum speeds of 0.20.8 m/s, and maximum acceleration of 0.22.1 m/s2(Conventional Machining Center, Mynx400/ACE-TC320D, Daewoo Heavy Industries fax: +82-42-869- 5221. E-mail address: dgleekaist.ac.kr (D.G. Lee). 0266-3538/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/pscitech.2003.10.021 Composites Science and Technology 64 (2004) 15231530 COMPOSITES SCIENCE AND TECHNOLOGY which may result in poor quality products by the relative positional error between the cutting tools and work- pieces 35: Recently machine tools are required to have been kept the positional accuracy within ?10 lm, which is closely related to the precision of products 6. For the high speed operation with accuracy, machine tool structures should be designed with light moving frames without sacrifi cing stiff ness and damping properties, which are contradictory requirements if conventional metallic materials are employed because conventional metals have almost same low specifi c stiff ness (E=q) with low damping characteristics. Machine tool structures with high specifi c stiff ness and high damping are re- quired to increase their fundamental natural frequencies and decrease the vibration induced. The requirement of high specifi c stiff ness with high damping for high speed machine tool structures can be satisfi ed by employing fi ber reinforced polymer composite materials 7,8. Since the fi ber reinforced composite materials consist of rein- forcing fi bers with very high specifi c stiff ness and matrix with high damping, the resulting material characteristics of composite materials refl ect the best characteristics of each material, i.e., high specifi c stiff ness with high damping. Moreover, sandwich structures whose face structures are made of fi ber reinforced composite ma- terials and whose core materials are made of honeycomb or foam structures maximize their advantages when they are applied to the structures resisting bending moment. Consequently, sandwich structures and composite ma- terials have been employed increasingly in spacecrafts, airplanes, automobile parts 9, robot arms 8,10, and even machine tools 11,12. The deformation of machine tool structures under cutting forces and structural inertia loads during start and stop motions produces not only poor quality products but also noise and vibration. A simple way to reduce the deformation is to employ structures with large cross-sections. However, it increases the mass of machine tool structures and consequently requires large motors, bearings and motion guide systems. Therefore, the best way to enhance the stiff ness of machine tool structures without much increase of mass is to employ high specifi c stiff ness structures such as composite sandwich structures. In this study, the vertical and horizontal machine tool slides of a high speed CNC milling machine were de- signed and manufactured with sandwich composite structures that are adhesively bonded to welded steel structures a hybrid machine tool structure. The verti- cal column of the horizontal slide (X-slide) was manu- factured with composite sandwich structures while the horizontal column of the vertical slide (Y-slide) was reinforced with high modulus composite plates. The hybrid structures were designed to have the equivalent structural stiff ness of conventional steel structures, which was calculated by the classical beam theory and FEM analysis. Then, the natural frequency and damp- ing capacity as well as weight savings of the composite hybrid machine tool structures were measured and compared with those of comparable conventional steel machine tool structures. 2. Design of hybrid machine tool structures 2.1. Characteristics of hybrid beam The bending stiff ness D of a simply supported sand- wich beam as shown in Fig. 1 is expressed as follows when Ef? Ecand d ? t 1315: D Ef? bt3 6 Ef? btd2 2 Ec? bc3 12 ? Ef? btd2 2 1 where Efand Ecrepresent the Young?s moduli of face and core, respectively. The defl ection D of the simply supported sandwich beam under a concentrated load P based on the simple beam theory is the sum of D1due to bending deformation and D2due to shear deformation 15,16: D D1 D2 P3 48D P 4AGc 2 where A and Gcrepresent equivalent cross-section area and the shear modulus of core material, respectively. Since the sandwich structure has low core shear stiff ness, the simple beam theory neglecting shear deformation may not give an accurate result. Therefore, the calcu- lated results of stiff ness of sandwich beam specimen were compared with the measured results obtained by the three-point bending test shown in Fig. 1 as well as the results by FEM analysis. The three-point bending test was performed using Instron 4206 under 1 mm/min displacement rate and the FEM analysis was performed with a commercial software ANSYS 5.5 (USA) using shell 99 and solid 95 elements. Table 1 shows the di- mensions of sandwich specimens. The sandwich beam specimens were made of composite faces and honey- comb core. To join the faces and the core, both an ad- hesive fi lm (AF126, 3M, USA) and an epoxy adhesive Fig. 1. Dimensions of the simply supported sandwich beam used for three-point bending test: (a) longitudinal direction; (b) cross-section of AA1. 1524D.G. Lee et al. / Composites Science and Technology 64 (2004) 15231530 (2216, 3M, USA) was used to prevent delamination failure of sandwich structures 17,18. Unidirectional carbon-epoxy composite (USN150, SK Chemical, Ko- rea) and glass fabric composite (GEP215, SK Chemical, Korea) were used for the face material while aramid fi - ber honeycomb (HRH-10-1/8-4.0, Hexcel, UK) was used for the core material. Tables 2 and 3 list the properties of these materials. The composite faces for the sandwich specimens were laid up with a stacking sequence of 0?2;G/0?10;C/0?1;G/0?5;CSwhere the subscripts G and C represent glass-fabric and carbon-epoxy, re- spectively. Fig. 2 shows the measured defl ection as well as the calculated ones by the beam theory and FEM analysis. Both the beam theory and the FEM analysis predicted the experimental defl ection within 8% error. From the above results, it was found that the defl ection of the sandwich beam due to shear was not negligible (three times larger than that due to bending in this case). Therefore, box type hybrid beams with side surfaces reinforced with steel plates as shown in Fig. 3 were adopted for the hybrid moving frames to reduce the shear deformation of the sandwich beam. For the box type beams reinforced with steel plates neglecting warping, the shear stress sxz;hin the honeycomb and sxz;s in the side steel are related from the geometric com- patibility as follows: sxz;s Gxz;scxz Gxz;s Gxz;h sxz;h Rsxz;h3 where R is the ratio of the shear moduli between the steel (Gxz;s) and honeycomb (Gxz;h). Then, the shear stress in the honeycomb in Fig. 3 is expressed as 16,19 sxz;h dM dx R AcEzdA DRbs bh V R AcEzdA DRbs bh 4 where M, V , Acrepresent bending moment, shear force and the integrated area from the neutral axis to the top boundary, respectively. Hence the defl ection due to shear is expressed as follows: D2 cxz;h 2 P R AcEzdA 4DRbs bhGxz;h 5 From Eqs. (2) and (5), the total defl ection D at the center of the hybrid beam due to a concentrated load P with Table 1 Dimensions (mm) of the simply supported sandwich beam under three- point bending test bcdh 55182328600 Table 2 Properties of composite materials E1(Gpa)E2(GPa)G12(GPa)m12Ply thickness (mm)Density (kg/m3) USN150130.010.05.060.280.151550 GEP21535.517.23.70.220.152050 HYEJ34M45D270.05.94.50.300.31730 0 200 400 600 800 1000 00.00050.0010.00150.002 Deflection m Force N Beam theory (shear only) Beam theory (bending only) FE-analysis Experiment Beam theory (bending + shear) Fig. 2. Defl ection of the sandwich beam obtained by the beam theory, experiment and FEM analysis. B-B x z B B1 honeycomb composite steel 2 bs 2 bs h b Fig. 3. Box type hybrid beam to reduce shear defl ection. Table 3 Properties of HRH-10-1/8-4.0 honeycomb Coordinate directionStrength (MPa)Modulus (MPa)Density (kg/m3) 3 (compressive)13 (shear)23 (shear)3 (compressive)13 (shear)23 (shear) 1 3 2 3.961.750.97193.0659.3032.4048.05 D.G. Lee et al. / Composites Science and Technology 64 (2004) 152315301525 simply supported boundary condition is the sum of bending defl ection D1 and shear defl ection D2: D D1 D2 P3 48D P R Ac EzdA 4DRbs bhGxz;h 6 If RBS, the ratio of the bending defl ection to the shear defl ection, is large, the shear defl ection can be neglected, which condition can be expressed as following: RBS D1 D2 2Rbs bhGxz;h 12 R Ac EzdA ? 17 2.2. Design of light weight composite reinforced machine tool frames Fig. 4 shows the photograph of a high speed CNC milling machine of 15 kW equipped with 35,000 rpm spindle and the hybrid moving frames, the horizontal slide (X-slide) and the vertical slide (Y-slide), whose vertical columns and horizontal columns were rein- forced with composite sandwich structures and com- positeplates(F500,DaewooHeavyIndustries (b) Y-slide under spindle weight of 4 kN. 1526D.G. Lee et al. / Composites Science and Technology 64 (2004) 15231530 they were the weakest parts of the moving frames of the milling machine considered. In order to develop a lighter hybrid frame, the X-slide steel base, made of thinner steel plates of 16 mm thickness compared to 20 mm thick steel plates for conventional one, was reinforced with composite sand- wich structure as shown in Figs. 5 and 8. Since the shear deformation of a simple sandwich structure is usually large, in this study, the hybrid structure was designed as a box type structure as shown in Figs. 8 and 9 whose sides were reinforced with steel plates. The calculated values of RBS from Eq. (7) for the designed box type hybrid structure was larger than 10.4, which meant that the defl ection due to shear was less than 8.8% of the total defl ection. Therefore, during the design of the structure, the fl exural rigidity D was used as the objec- tive parameter, where D X n i1 EiIi8 Since the reinforcement of the outer face of the moving frames is most eff ective to increase the fl exural rigidity D, the inner face thickness of the sandwich was deter- mined to be 5 mm considering the joining of the inner faces of the sandwich beams to the steel base with bolts. The thickness of the outer face of the sandwich beam was tentatively determined to give the equivalent D of the conventional one and then the more specifi c calcu- lations were performed to determine the suitable di- mensions of the reinforcements using FEM considering local warping or twisting. From the analysis, it was found that a larger defl ection occurred in the hybrid beams when both the beams had the same fl exural ri- gidity D because the hybrid beam did not have the transverse reinforcing steel plates, while the conven- tional steel beam was designed as a lattice type structure with reinforcing plates, Fig. 9. Therefore, the outer face thickness of the sandwich beam was increased to 13 mm. Furthermore, the dimensions of other reinforcements were calculated through a procedure starting with equivalent fl exural rigidity D and then calculating with FEM considering warping of the structure. 2.3. Manufacture of hybrid machine tool structure High strength carbon epoxy composite (USN150, SK Chemical, Korea) and glass fi ber epoxy (GEP215, SK Chemical, Korea) were mainly used for the faces of sandwich beams and reinforcing plates for the X and Y-slides. The vertical column of the X-slide was rein- forced with the two sandwich beams of 1462 mm and 1223 mm long, respectively, while the top and bottom parts were reinforced with the four composite plates and six small sandwich beams as shown in Fig. 5. The Y-slide on which the spindle unit of the milling machine Fig. 8. Section views of vertical columns of the X-slide: (a) reference of section view; (b) hybrid; (c) conventional. Fig. 9. Confi gurations of the vertical column of X-slide: (a) conven- tional one with transverse reinforcing plate; (b) hybrid one without transverse reinforcing plate. D.G. Lee et al. / Composites Science and Technology 64 (2004) 152315301527 mounted should resist the bending moment produced by the spindle weight, cutting force and the inertia force due to fast acceleration and deceleration of the slide. The horizontal column of Y-slide with strict dimen- sional constraint was reinforced with very high modulus carbon fi ber epoxy composite whose properties are given in Table 2 (HYEJ34M45D, Mitsubishi, Japan), to avoid interference with other parts. Moreover, the left and right vertical columns of the Y-slide were reinforced with sandwich beams with holes for lubrication and electrical wiring as shown in Fig. 6. Additionally, four triangular shape plates were used to reinforce the twisting rigidity of the rectangular frames. The com- posite reinforcements were bonded to the steel base structures with an epoxy adhesive (2216, 3M, USA) combined with the mechanical joining with bolts to enhance the reliability and manufacturing effi ciency. 3. Characteristics of the hybrid slides 3.1. Dynamic and static characteristics The impulse response tests for both the conventional steel slides and the hybrid slides were performed with free-free boundary conditions to investigate the eff ects of composite reinforcement on the dynamic characteristics such as natural frequencies and damping characteristics. In addition, the dynamic characteristics of the CNC milling machine tool structure equipped with the hybrid X-slide were measured. During the test, a 4-channel FFT analyzer (B (b) second mode. Fig. 11. Mode shapes of the hybrid Y-slide with free-free boundary condition: (a) fi rst mode; (b) second mode. 1528D.G. Lee et al. / Composites Science and Technology 64 (2004) 15231530 speed of 2 m/s and acceleration of 14 m/s2owing to the light weight moving frames made of composite sandwich structures, which will enable to reduce the total ma- chining time signifi cantly and to obtain the enhanced productivity. 3.2. Performances and economic estimation During machine tool operation, the errors in the po- sitional accuracy mainly come from inaccuracies in the geometry, fi nite stiff ness, and thermal deformation of the machine components. Recently CNC controllers are equipped with the error compensation algorithm, which enables machine tools to obtain the positional accuracy of about ?2 lm 22,23. The positional accuracy of modern machine tools is determined not only by struc- tural stiff ness but also by adjustment or tuning of their controller to compensate the structural defl ection or the thermal deformation. The developed machine tool had a structural stiff ness comparable with conventional high precision machine tools and w