Composites in Production Machines - Science Direct

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ScienceDirect Procedia CIRP 66 (2017) 2 – 9

1st Cirp Conference on Composite Materials Parts Manufacturing, cirp-ccmpm2017

Composites in production machines H.-C. Möhringa,* a Institute of Manufacturing Technology and Quality Management (IFQ), Otto-von-Guericke-University Magdeburg, Universitätsplatz 2, 39106 Magdeburg, Germany

* Corresponding author. Tel.: +49-391-67-58552 ; fax: +49-391-67-42370. E-mail address: [email protected]

Abstract This paper gives an overview of the design and application of composite materials in machine tools. Some exemplary machine elements, tool structures and components are introduced and the technical potentials, requirements and challenges are discussed. The presented prototypes and industrial products show that besides the lightweight construction aspect furthermore the specific mass related stiffness characteristics and the advantageous damping properties as well as the thermal behavior of composite structures provide outstanding chances regarding the improvement of the machine performance and accuracy. New degrees of freedom in machine component design and layout can be exploited especially if material combinations are implemented. © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2017 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite Materials Parts Manufacturing. Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite Materials Parts Manufacturing Keywords: Machine tool; Design; Material

1. Introduction The design, manufacturing and application of composite parts is mainly driven by the aerospace and automotive industry. Mass reduction in order to save fuel is the predominant goal and reason for the use of materials like carbon fiber reinforced polymers (CFRP) or similar composites with glass, aramid or basalt fibers. These materials can also be combined with cellular, foam or honeycomb cores as well as sheet metal elements. Various composite parts manufacturing processes have been and are still investigated and developed in order to achieve a cost efficient realization of the composite components. During the last two decades, a significantly increasing level of automation can be observed that is still improving thanks to the development work in industry and research institutions. Against this background, applications of composite parts become more and more interesting and relevant also in general machine construction and especially in the machine tool sector. Whereas for a long time, CFRP and other composite materials have been used predominantly in scientific works and prototypes of machine tool structural

elements, tool components and spindles, nowadays a rising number of industrial implementations can be found [1] (Fig. 1 and Fig. 2).

Fig. 1. High speed lightweight machine tool by company EEW Protec [2]

2212-8271 © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite Materials Parts Manufacturing doi:10.1016/j.procir.2017.04.013

H.-C. Möhring / Procedia CIRP 66 (2017) 2 – 9

machine component with improved properties and performance can only be answered by an appropriate value benefit analysis. Another aspect of composite material application in machine tool structures is the ability of functional integration. Especially sensor systems can be embedded during the composite part manufacturing processes. Furthermore, additive manufacturing and conductive material printing technologies allow an automated and cost efficient generation of the necessary wiring or realization of circuitry which can be integrated in the part production chain. By this functional integration, the performance of composite machine tool elements can be enhanced beyond the limits of conventional component design. 2. The use of composites in machine structures

Fig. 2. High speed machining center by company MAP [3]

The lightweight construction potentials provide new possibilities to reduce moved masses (e.g. of slides, columns, rams), to lower inertia moments (e.g. of spindle or tool shafts), to relieve the drive systems of the machines, to enhance the feed motion dynamics and path accuracy, and to improve the energy efficiency of the machine tools during machining operations [2]. Furthermore, due to the higher material damping properties of composite materials, increased structural damping ratios can be achieved. In addition, the very low or even negative thermal expansion coefficients of CFRP parts can be utilized for a higher thermal stability of the machines [3, 4]. As a consequence of the buildup and internal structure of composite parts, including fibers and matrix, by exploiting the design degrees of freedom regarding the types of fibers, fiber orientation and layer composition, quasiisotropic or targeted anisotropic mechanical and thermal characteristics can be created. This, on the other hand, requires sophisticated design, layout and optimization methods as well as related modelling and simulation approaches in order to gain the full potential of the material application but to implement an efficient design process. As an example, Lasova et al. investigated the use of Pareto optimization in order to find an optimal lay-up of a wound square tube including CFRP with high modulus fibers and cork composition damping layers [5]. Three dimensional numerical simulations were conducted considering the static bending stiffness, the natural frequency and the damping ratio as optimization criterions. Considering the huge bandwidth of mechanical fiber properties but also of the associated price of the different types of fibers, comprehensive design optimization strategies must, in addition to the physical parameters, also take the related costs of design variants into account. The crucial question of the permitted price of a newly designed composite

A fundamental aim when using composite materials in production machinery – of course – is to reduce the mass and inertia of moved machine elements. This can exemplarily be seen in pick-and-place robot systems as depicted in figure 3. In these applications, the functional requirements regarding positioning and path accuracy are relatively low compared to machine tools.

Fig. 3. Pick-and-place robots by Convitech (left) and KUKA (right)

In contrast, very high precision and the requirement of a maximum dynamic stiffness and damping capacity is demanded in metrology devices and inspection machines. Jung et al. investigated the design of a hybrid compositealuminum beam structure with high modulus (HM) carbon/epoxy composites with respect to the design of a LCD glass panel inspection machine [6]. The layout was optimized in terms of the cross section shape of the beam, the stacking sequence and the thickness of the composite reinforcement with respect to the fundamental natural frequency and bending deformation (Fig. 4). A composite robot end effector for the handling of LCD glass panels is introduced in [7]. The beneficial dynamic properties of composites were exploited by Lee et al. with respect to the design of a guiding arm of an electrical discharge wire cutting machine [8]. With the support of Finite Element (FE) simulations, the detailed design regarding bonding length and number of reinforcing plies was conducted. Compared to the conventional arm, the mass was reduced to less than 50%, the static stiffness was maintained and the fundamental natural frequency as well as the damping ratio were significantly improved.

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A hybrid steel-composite headstock for high-precision grinding machines was analyzed by Chang et al. [9]. Especially in the higher frequency range of 100-500 Hz, the composite reinforcement led to an improvement of the dynamic stiffness and damping.

Fig. 5. Steel-composite hybrid headstock [9]

Suh and Lee presented the design of a hybrid material slide structure with composite reinforcements [10, 11] (Fig. 6). The fundamental natural frequency was increased from 64 Hz to 92 Hz and the damping factors for the first 5 modes were enhanced by up to more than 100%.

Fig. 4. LCD inspection machine and beam cross sections [6]

Regarding the design of machine tools for cutting and grinding machining operations, various investigations were carried out and prototypes were built in order to achieve the best compromise between mass reduction, static stiffness, fundamental natural frequencies, damping ratios as well as thermal stability. Besides more or less pure composite structures, the majority of approaches considers hybrid approaches in which different materials are combined.

Fig. 6. Milling machine with hybrid material slide [10]


The strength of the adhesively bonded sandwich structure of the horizontal moving body was analyzed in [12]. Furthermore, the thermal properties of the composite sandwich were investigated. In [13], carbon/epoxy composite-aluminum hybrid structures with friction layers were applied with the aim to achieve a high structural damping. The static deflection due to deadweight and the first natural frequency were analyzed with respect to the stacking angle and thickness of the composite. Between the aluminum and composite interface, a friction damping layer was inserted. Composite reinforcements were also investigated in [14]. The use of composite sandwich materials in machine tools was discussed in [15]. Compositefoam-resin concrete sandwich structures were studied in [16]. Kulisek et al. performed case studies on rams with, on the one hand, a thick-walled composite body and minimal amount of steel as well as, on the other hand, a hybrid structure with fiber composites and cork layers [17] (Fig. 7).

A serious aspect especially for hybrid combinations of materials with different thermal expansion coefficient concerns thermally induced mechanical stresses in the interfaces and joints which can lead to de-bonding and a loss of structural stiffness. Residual stresses in material interfaces can already occur during the curing of the composite parts. Therefore, co-curing strategies were investigated in order to reduce these stresses [23-26]. In [27] a thermal expansion clamp for the reduction of residual stresses during curing is presented. The characteristics of composite structures are influenced by the joints, e.g. towards metal parts as interfaces for guides, drives or machine components. The layout of mechanically fastened joints has to be carried out carefully in order to avoid structural damage of the composite elements [28]. Kolesnikov et al. provide an overview of the aspects of bolted joints in composites and propose a hybrid material reinforcement in order to improve the strength of the joints [29]. 3. The use of composites in spindles and tools In order to reduce the moment of inertia of spindle rotors and tool bodies, to reduce their thermal growth and to improve the dynamic behavior, the use of composites in spindles and cutting tools was investigated and demonstrators as well as first products are available (Fig. 9).

Fig. 7. Composite and hybrid spindle ram [17]

Fleischer, et al. and Koch et al. filled a composite machine slide with different amounts of fluids in order to control the structural dynamics during machine utilization [18, 19]. Examples of downscaled “desktop” machines can be seen in [20, 21, 22] (Fig. 8).

Fig. 9. Composites in metal cutting and grinding tools by Xperion

Fig. 8. Modular machine frame for desktop machines [20]

A graphite epoxy composite boring bar for chatter reduction was introduced and further developed by Lee [30, 31]. Heisel developed a composite reamer shaft with specific interfaces for the metal segments of the tool [32] (Fig. 10). A mass reduction from 28.5 kg of the conventional steel tool down to 10.5 kg of the newly designed tool, an increase of stiffness and a significant improvement of the fundamental natural frequency were achieved. Kim presented a hybrid material circular plate structure for cutting tools in [33] (Fig. 11). The constrained damping characteristics were investigated with respect to the adhesive bonding thickness and the use of composite or PVC foam. Since the clamping of composite tool bars could lead to damages and a loss of mechanical performance and strength,

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Hwang et al. proposed a metal core or sleeve inserted in the composite body of a tool bar [34] (Fig. 12).

order to improve the magnetic flux. The reinforcement of a spindle cover with carbon fiber epoxy composite material was analyzed in [38]. 4. The use of composites in clamping systems

Fig. 10. Modular construction of composite reamer tool [32]

In case that high accelerations are applied to fixtures and clamping systems during machining operations, the goal of mass reduction and lowered moments of inertia is also valid for these core elements of machine tools. With respect to accuracy issues, the low thermal expansion of composites can be exploited within fixtures. Furthermore, regarding process stability in cutting and grinding processes, the dynamic stiffness and structural damping properties of the workpiece holding systems has a significant influence. Thus, the use of composite materials in fixtures reveals a high potential in order to improve the overall performance of the machining system. In [39] the use of CFRP components as damping elements in fixture frames was investigated (Fig. 13).

Fig. 11. Hybrid material circular plate tool body [33]

Fig. 12. Composite tool bars with new design of the clamping part: (a) the sleeve type clamping part; (b) the core type clamping part [34]

In 1985, Lee et al. presented the idea and a design approach of a composite rotor for machine tool spindles [35]. Compared to a steel spindle bearing system, the maximum width of cut could be increased by 20%. Furthermore, the pre-loading characteristics were more stable with the composite system. Bang and Lee introduced a composite shaft of a high speed air spindle in [36]. The stacking sequence was determined considering the bending stiffness of the carbon composite shaft and the static stiffness of the air bearing. In [37], the rotor of an AC induction motor was manufactured using magnetic powder containing epoxy composite and the motor shaft was made of high modulus carbon fiber epoxy composite material. A steel core was inserted into the composite rotor in

Fig. 13. Test fixtures with CFRP frame component [39]

The pure CFRP frame element shown in Fig. 13d provided much higher natural frequencies and damping ratios than the comparable steel element (Fig. 13c). However, the joints in the test fixture (Fig. 13b) led to a lower dynamic stiffness of the assembled system. Therefore, for the final prototype fixture design, improved joints were developed and integrated (Fig. 14).


Fig. 14. Final fixture prototype with CFRP damping frame [39]

5. Functional integration Composite material structures are suitable for inherent sensor and actuator integration. On the one hand, integrated sensors can be used for structural health monitoring [40]. In machine tool applications, structural health monitoring of composite parts is highly relevant. In case of overloading or slight crashes, internal damages of CFRP parts can hardly be detected by external measuring devices in a workshop environment. Consequently, an inherent state observation is required. Here it must be considered, that inacceptable changes of structural properties are already reached, if the static or dynamic stiffness characteristics differ from the initial values so that the machining processes are affected in terms of accuracy, workpiece surface defects or instable cutting conditions.

Integrated sensors also provide relevant information for machine and process state monitoring. Meo et al. integrated fiber optic Bragg strain sensors into a ram of a vertical milling center in order to gather bending deformations and tool displacements during the process [41] (Fig. 15). In [42] and [43], piezo ceramic sensors are embedded in composite machine components. A printed circuit is integrated in [42] in order to realize the wiring of distributed sensors within the analyzed test specimen. Brecher et al. exploit the thermal stability of CFRP rods for a direct integrated measuring device for thermal state monitoring and compensation of thermally induced machine deflections [44]. The application of piezo sensor integrated CFRP structures in a workpiece clamping intelligent chuck system is introduced in [45] (Fig. 16). The sensory piezo patch transducers are embedded in CFRP fingers which are pre-stressed against the workpiece during the clamping setup. Due to their high sensitivity, the sensors are capable to measure workpiece vibrations during the milling operations. By this, process monitoring regarding chatter occurrence becomes possible as well as an adaptive control of countermeasures (e.g. targeted counter excitation). Figure 17 shows exemplary signal frequency spectra for the three integrated sensors (PZT1…PZT3) which were obtained during a milling test process.

Fig. 16. CFRP integrated piezo sensors in an intelligent chuck [45]

Fig. 17. Frequency spectra obtained by CFRP integrated sensors Fig. 15. Integration of fiber optic Bragg sensors in composite ram [41]

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6. Conclusions This paper gives an overview of the application of composite materials in production machines. As a summary, especially carbon fiber reinforced polymer materials can be found in robot and machine tool structural components, spindles and tools as well as in fixtures. The advantageous material properties of low specific weight, high mass related specific stiffness, high material damping, low thermal expansion and functional integration capability have already been utilized in various exemplary implementations, prototypes and products. This shows that new degrees of freedom in production machinery design are provided by composite materials. However, the material costs as well as the requirement of automated and reproducible manufacturing processes for precision parts in the machinery sector are challenges that have to be solved. So far, predominantly individual machine components were realized in single piece or small batch production environments. In order to implement the material technology in machine tool industry, medium batch scenarios up to mass production strategies are necessary. Furthermore, structural health monitoring systems are required in order to enable an observation of the material and structural properties of machine components during their long term usage. Especially if overload or collision situations occur, the reliability of the structural performance has to be assessed. Finally, influences of chips and coolant lubricant have to be considered carefully in component design. References [1] Möhring, H.-C.; Brecher, C.; Abele, E.; Fleischer, J.; Bleicher, F. (2015) Materials in machine tool structures, CIRP Annals – Manufacturing Technology 64(2): 725-748 [2] www.eew-protec.de [3] www.map-wzm.de [4] Kroll, L.; Blau, P.; Wabner, M.; Frieß, U.; Eulitz, J.; Klärner, M. (2011) Lightweight components for energy-efficient machine tools, CIRP Journal of Manufacturing Science and Technology 4: 148-160 [5] Mayr, J.; Jedrzejewski, J.; Uhlmann, E.; Donmez, M.A.; Knapp, W.; Härtig, F.; Wendt, K.; Moriwaki, T.; Shore, P.; Schmitt, R.; Brecher, C.; Würz, T.; Wegener, K. (2012) Thermal Issues in Machine Tools, CIRP Annals—Manufacturing Technology 61(2): 771–791 [6] Liebetrau, M. (1997) Thermische Stabilisierung von Werkzeugmaschinen-Spindelkästen durch Carbonfaserverbundkunststoff, Dr.-Ing. Dissertation, Technische Universität Berlin [7] Lasova, V.; Vacik, J.; Kottner, R. (2012) Investigation of Dynamic Properties of Hybrid Laminate Structure, Procedia Engineering 48: 358366 [8] Jung, S.C.; Lee, J.E.; Chang, S.H. (2004) Design of inspecting machine for next generation LCD glass panel with high modulus carbon/epoxy composites, Composite Structures 66: 439-447 [9] Oh, J.H.; Lee, D.G.; Kim, H.S. (1999) Composite robot end effector for manipulating large LCD glass panels, Composite Structures 47: 497-506 [10] Lee, C.S.; Oh, J.H.; Lee, D.G.; Choi, J.H. (2001) A composite cantilever arm for guiding a moving wire in an electrical discharge wire cutting machine, Journal of Materials Processing Technology 113: 172-177 [11] Chang, S.H.; Kim, P.J.; Lee, D.G.; Choi, J.K. (2001) Steel-composite hybrid headstock for high-precision grinding machines, Composite Structures 53: 1-8 [12] Suh, J.D.; Lee, D.G. (2002) Composite Machine Tool Structures for High Speed Milling Machines, Annals of the CIRP 51(1): 285-288 [13] Lee, D.G.; Suh, J.D.; Kim, H.S.; Kim, J.M. (2004) Design and manufacture of composite high speed machine tool structures, Composite Science and Technology 64: 1523-1530

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