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Design and manufacture of composite high speed machinetool structures
Dai Gil Lee *, Jung Do Suh, Hak Sung Kim, Jong Min Kim
Abstract
The high transfer speed as well as the high cutting speed of machine tools is important for the productivity improvement in thefabrication of molds/dies because non-machining time, called the air-cutting-time, amounts to 70% of total machining time withcomplex shape products. One of the primary reasons for low productivity is large mass of the moving parts of machine tools, whichcannot afford high acceleration and deceleration encountered during operation. Moreover, the vibrations of the machine toolstructure are among the other causes that restrict high speed operations.In this paper, the slides of high speed CNC milling machines were designed with fiber reinforced composite materials toovercome this limitation. The vertical and horizontal slides of a large CNC machine were manufactured by joining high-moduluscarbon-fiber epoxy composite sandwiches to welded steel structures using adhesives and bolts. These composite structures reducedthe weight of the vertical and horizontal slides by 34% and 26%, respectively, and increased damping by 1.5–5.7 times withoutsacrificing the stiffness. Without much tuning, this machine had a positional accuracy of 5 lm per 300 mm of the slidedisplacement.
1. Introduction
CNC milling machines and machining centers areemployed in the fabrication of various molds/dies thatare used for electrical appliances, automobile interiors,
stamping and injection molding. During normal machiningwith machine tools, their cutting tools aremoved with nominal feed rates, while the feed rates are
switched to a rapid traverse mode during the transfer ofcutting tools without contacting workpieces: The timespent to transfer a cutting tool without contactingworkpieces is called air-cutting-time. Generally, onlyabout 30% of the total machining time is spent in theactual cutting or making chips, while the remaining 70%is spent in the air-cutting-time [1,2]. Therefore, not onlyhigh cutting speeds but also high transfer speeds arerequired to obtain the enhanced productivity of machiningwhich is essential to survive in the global competitionof machine tool markets. Although the cuttingspeed has been increased due to newly developed cuttingtool materials such as ceramic, CBN, diamond and soon, productivity is still restricted by the low transferspeed of massive moving frames which are usually madeof steel. Conventional steel moving frames of machinetools operate with maximum speeds of 0.2–0.8 m/s, andmaximum acceleration of 0.2–2.1 m/s2 (ConventionalMachining Center, Mynx400/ACE-TC320D, DaewooHeavy Industries & Machinery Ltd., Korea). However,modern high speed milling machines are required tohave the maximum acceleration of 14 m/s2 and the speedof 2 m/s. These high transfer speeds are hard to be realizedif massive steel moving frames are employed.Furthermore, machine tool structures vibrate creatingproblems during manufacturing at these high speeds,which may result in poor quality products by the relativepositional error between the cutting tools and workpieces[3–5]: Recently machine tools are required to havebeen kept the positional accuracy within 10 lm, whichis closely related to the precision of products [6]. For thehigh speed operation with accuracy, machine toolstructures should be designed with light moving frameswithout sacrificing stiffness and damping properties,which are contradictory requirements if conventionalmetallic materials are employed because conventionalmetals have almost same low specific stiffness (E=q) withlow damping characteristics. Machine tool structureswith high specific stiffness and high damping are requiredto increase their fundamental natural frequenciesand decrease the vibration induced. The requirement ofhigh specific stiffness with high damping for high speedmachine tool structures can be satisfied by employingfiber reinforced polymer composite materials [7,8]. Sincethe fiber reinforced composite materials consist of reinforcingfibers with very high specific stiffness and matrixwith high damping, the resulting material characteristicsof composite materials reflect the best characteristics ofeach material, i.e., high specific stiffness with highdamping. Moreover, sandwich structures whose facestructures are made of fiberreinforced composite materialsand whose core materials are made of honeycombor foam structures maximize their advantages when theyare applied to the structures resisting bending moment.Consequently, sandwich structures and composite materialshave been employed increasingly in spacecrafts,airplanes, automobile parts [9], robot arms [8,10], andeven machine tools [11,12].The deformation of machine tool structures undercutting forces and structural inertia loads during startand stop motions produces not only poor qualityproducts but also noise and vibration. A simple way toreduce the deformation is to employ structures withlarge cross-sections. However, it increases the mass ofmachine tool structures and consequently requires largemotors, bearings and motion guide systems. Therefore,the best way to enhance the stiffness of machine toolstructures without much increase of mass is to employhigh specific stiffness structures such as compositesandwich structures.In this study, the vertical and horizontal machine toolslides of a high speed CNC milling machine were designedand manufactured with sandwich compositestructures that are adhesively bonded to welded steelstructures – a hybrid machine tool structure. The verticalcolumn of the horizontal slide (X-slide) was manufacturedwith composite sandwich structures while thehorizontal column of the vertical slide (Y-slide) wasreinforced with high modulus composite plates. Thehybrid structures were designed to have the equivalentstructural stiffness of conventional steel structures,which was calculated by the classical beam theory andFEM analysis. Then, the natural frequency and dampingcapacity as well as weight savings of the compositehybrid machine tool structures were measured and
compared with those of comparable conventional steelmachine tool structures.
2. Design of hybrid machine tool structures
2.1. Characteristics of hybrid beam
The bending stiffness D of a simply supported sandwichbeam as shown in Fig. 1 is expressed as followswhen Ef >> Ec and d >> t [13–15]:
( 1)
where Ef and Ec represent the Youngs moduli of faceand core, respectively. The deflection D of the simplysupported sandwich beam under a concentrated load P
based on the simple beam theory is the sum of D1 due tobending deformation and D2 due to shear deformation[15,16]:
where A and Gc represent equivalent cross-section areaand the shear modulus of core material, respectively.Since the sandwich structure has low core shear stiffness,the simple beam theory neglecting shear deformationmay not give an accurate result. Therefore, the calculatedresults of stiffness of sandwich beam specimen werecompared with the measured results obtained by thethree-point bending test shown in Fig. 1 as well as theresults by FEM analysis. The three-point bending testwas performed using Instron 4206 under 1 mm/mindisplacement rate and the FEM analysis was performedwith a commercial software ANSYS 5.5 (USA) usingshell 99 and solid 95 elements. Table 1 shows the dimensionsof sandwich specimens. The sandwich beamspecimens were made of composite faces and honeycombcore. To join the faces and the core, both an adhesivefilm (AF126, 3M, USA) and an epoxy adhesive
Fig. 1. Dimensions of the simply supported sandwich beam used forthree-point bending test: (a) longitudinal direction; (b) cross-section ofA–A1.
Table 1
Dimensions (mm) of the simply supported sandwich beam under threepointbending test
(2216, 3M, USA) was used to prevent delaminationfailure of sandwich structures [17,18]. Unidirectionalcarbon-epoxy composite (USN150, SK Chemical, Korea)and glass fabric composite (GEP215, SK Chemical,Korea) were used for the face material while aramid fiberhoneycomb (HRH-10-1/8-4.0, Hexcel, UK) wasused for the core material. Tables 2 and 3 list theproperties of these materials. The composite faces forthe sandwich specimens were laid up with a stackingsequence of [02;G/010;C/01;G/05;C]S where the subscriptsG and C represent glass-fabric and carbon-epoxy, respectively.Fig. 2 shows the measured deflection as wellas the calculated ones by the beam theory and FEManalysis. Both the beam theory and the FEM analysispredicted the experimental deflection within 8% error.From the above results, it was found that the deflectionof 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 surfacesreinforced with steel plates as shown in Fig. 3 wereadopted for the hybrid moving frames to reduce theshear deformation of the sandwich beam. For the boxtype beams reinforced with steel plates neglectingwarping, the shear stress sxz;h in the honeycomb and sxz;sin the side steel are related from the geometric compatibilityas follows:
where R is the ratio of the shear moduli between the steel(Gxz;s) and honeycomb (Gxz;h). Then, the shear stress inthe honeycomb in Fig.
2.2. Design of light weight composite reinforced machinetool frames
Fig. 4 shows the photograph of a high speed CNCmilling machine of 15 kW equipped with 35,000 rpm
Fig. 4. Photograph of a high speed milling machine tool structure(F500, Daewoo heavy industries & Machinery Ltd., Korea).
In order to develop a lighter hybrid frame, the X-slidesteel base, made of thinner steel plates of 16 mmthickness compared to 20 mm thick steel plates for
conventional one, was reinforced with composite sandwichstructure as shown in Figs. 5 and 8. Since the sheardeformation of a simple sandwich structure is usuallylarge, in this study, the hybrid structure was designed asa box type structure as shown in Figs. 8 and 9 whosesides were reinforced with steel plates. The calculatedvalues of RBS from Eq. (7) for the designed box typehybrid structure was larger than 10.4, which meant thatthe deflection due to shear was less than 8.8% of thetotal deflection. Therefore, during the design of thestructure, the flexural rigidity D was used as the objectiveparameter, where
Since the reinforcement of the outer face of the movingframes is most effective to increase the flexural rigidityD, the inner face thickness of the sandwich was determinedto be 5 mm considering the joining of the innerfaces of the sandwich beams to the steel base with bolts.