18.1 Investigation of the upgrading potentials of out-of-date cutting machine tools to promote sustainable and global value creation

Transcription

1 18.1 Investigation of the upgrading potentials of out-of-date cutting machine tools to promote sustainable and global value creation E. Uhlmann 1, K. Kianinejad 1 1 Department for Machine Tools and Factory Management, Technical University Berlin, Germany Abstract Cutting machine tools play a major role in global value creation. During last decades machine tools have been developed regarding production efficiency, working accuracy and flexibility. Nevertheless there exist a large number of conventional milling machines particularly in developing countries, which cannot be applied effectively in global creation networks due to the low processing efficiency and low flexibility. Upgrading existing machines can save raw material and energy resources and supports sustainability. In this paper the improvement potentials of out-of-date machine tools without exchanging system relevant components have been investigated and necessary tests and measurements have been implemented. Two machine tools, a newer and an older milling machine as representative example were compared and some possible improvement solutions of the older machine have been proposed concerning working accuracy and flexibility. Keywords: Machine tool, Optimization, Working accuracy, Performance comparison, Sustainability 1 INTRODUCTION 1.1 Present Situation The global production of machine tools has almost tripled in last two decades. In 2011 the production of machine tools has increased by 30% compared with 2010 and exceeds significantly previous record of 54.3 billion Euros in 2008 [1]. With 73% share of total global production volume in 2011 in Germany, cutting machine tools have a leading position among other machine tools [1]. Ascending production of machine tool can be an indicator of rapidly growing demand for machine tools in near future. To meet these demands, by means of production of new machine merely, huge amount of material and energy resources are required. At the same time there already exist a remarkable number of machine tools which still can be considered as useable. According to a VDMA report the average lifetime of NC-controlled metal working machine tools estimated by manufacturers is about 9.5 year [2]. Despite of estimated lifetime, a survey of the German machine tool builders association [3] shows that about only 43% of used machine tools has maximum life span of 25 year or more. According to the survey over 80% of the machine tools are retrofitted or refurbished between 5 years and 15 years. Change of guidance and bearings, change of core components and overhaul are the most dominating measures of retrofitting and refurbishment [3]. The above mentioned statistics clarifies that a large proportion of machine tools are not going to be used longer than 25 years and will be replaced at the end of life span. The main idea in this paper is to investigate the improvement potentials of outof-date machine tools without retrofitting or refurbishing. This approach can on one hand extend the lifetime of machine tools. On the other hand this approach can be seen as an alternative method to retrofitting and refurbishment. These two aspects highly support the sustainability since a partial or complete renewal of machine tools or machine tool parts can be prevented. In order to be able to study the improvement potentials of out-of-date machines, it should be understood why these machines are outdated. To answer this question it should be clear, which developing trends the machine tools have experienced within last 3 decades. 1.2 Developing Trends Considering current topics in research and development communities and recent publications on technological trends of machine tools four main fields of developing can be identified [4] and [5]: High-speed and high-efficiency machine tools Multifunctional and flexible machine tools High-Precision and Ultra-Precision machine tools Advanced and Intelligent controlled machine tools These trends can also be seen as an indicator of demands and needs on present and future production processes. The machine tools should be able to meet high requirements concerning efficiency, working accuracy, flexibility and control systems. The machine tools which cannot meet some or all of above requirements are considered as outdated. In order to find a quantified measure for increased requirements two machine tools, an outdated machine and a new machine, are compared. In this way the weak spots of outdated machine under operational conditions could be identified. Beside sustainability the economic aspects of outdated machine tools are becoming increasingly important. Second-hand machinery and equipment to the value of more than 75 billion Euros are traded yearly, mostly to developing countries and emerging markets [6]. Almost unnoticed the transfer of second-hand machinery and equipment has now become an important business sector. Second-hand machinery and equipment from industrialized countries are recognized by enterprises in G. Seliger (Ed.), Proceedings of the 11 th Global Conference on Sustainable Manufacturing - Innovative Solutions ISBN Universitätsverlag der TU Berlin

2 K. Kianinejad, E. Uhlmann developing countries as a low-cost and fast solution to the problem of replacing outdated machinery and/or building up new capacities [6]. Upgrading outdated machine tools will enable enterprises in developing countries to manufacture products with higher quality und lower costs, so that new capacities for participation in global value creation can be created. 2 APPROACH Based on the comparison of two milling machine tools, weak points of the outdated machine tool were identified at first. The compared machine tools are frequently in use, so that quantitative measures for weak points of outdated machine under operating conditions could be identified. At the second step, based on determined weak points, sustainable improvement solutions of outdated machines could have been proposed, which can be realized in context of sustainability. This means an upgrading of machine tools without changing core components like feed drives and spindle unit. Furthermore, the possible solutions have been considered to be generally applicable to other comparable milling machine types. Milling process play an important role among other manufacturing processes. Milling machine tools and machining centers made up of 30% of German machine tool production in 2011 [7]. At the same time the metal working machine tools experience a continuous shift from non-cnc machine tools to CNC-machine tools [8]. Therefore the results of comparison between CNC milling machines can affect a large number of exciting milling machine tools. The older machine tool, FP4NC, DECKEL GmbH, Germany, is a vertical knee-type CNC milling machine with 4 simultaneously controlled axis built in year 1986 (Figure 1). Figure 2. DMU-50 universal milling machine, DECKEL MAHO GmbH Both compared machine tools have common working space for stand-alone solutions of between 450 mm and 560 mm per linear axis and are suitable for machining of single parts and small series. Concerning age (27 years) and technology, the FP4 milling machine has been considered as a representative example for an outdated machine tool. Further technical information available for these machine tools including main drive, feed drives, working space, control system, power of main drive and maximum velocity of feed drives are shown in Table 1. Machine type Manufacturing year Manufacturer CNC Milling Machine Deckel GmbH, Germany Designation FP4NC DMU 50 Z X Y CNC Universal Milling Machine Deckel Maho Seebach GmbH, Germany Base area [m] 2 x x 1.4 Z Number of machine axes CNC control system Grundig/Dialog 4 Heidenhain/ ITNC 530 Y X Travels X/ Y/ Z 560/ 500/ / 450/ 400 Feed speed X/ Y/ Z [mm/min] Up to 3600 Up to Maximum spindle power [kw] Spindle revolution [1/min] Up to 3150 Up to Tool holder system SK 40 SK 40 Figure 1. FP4 milling machine, DECKEL GmbH. The newer machine tool, DMU-50, DECKEL MAHO GmbH, Germany is a vertical universal 3+2 axis CNC milling machine manufactured in year 2008 (Figure 2). Table 1. Technical information of compared milling machines. The comparisons are carried out in following main fields: working accuracy, dynamic behavior and productivity. The above fields represent some of the important developing trends by machine tools and are more relevant for standalone milling machines 575

3 Investigation of the upgrading potentials of out-of-date cutting machine tools to promote sustainable and global value creation 3 EXPERIMENTAL AND ANALYTICAL PROCEDURE At the first step, aspects of the geometrical, kinematic and dynamic behavior of the machine tools have been studied. It is important to notice that a comprehensive comparison in each of above fields is extremely time consuming, so that some representative and more significant aspects are focused. To find a measure for geometric behavior positioning accuracy and straightness of two machines tools are measured along all linear axes using laser interferometer. Due to high precision and high measuring repeatability laser interferometer is particularly adequate for above measurements. Furthermore, it is possible to carry out the positioning and straightness tests dynamically by means of laser interferometry which aims at the analysis of machine tool kinematics. To analyze dynamic behavior, the dynamic characteristic of the machine structure and cutting tool is deduced at first from measured frequency-response functions at the tool center point (TCP). To measure frequency-response of TCP, the structure is excited at the TCP in x- and y-direction of the machine tool coordinate system using an impact hammer type Kistler 9722A500 with a mass of m = 0.2 kg. The impact hammer is equipped with a steel tip. The response in x- and y-direction of the machine tool coordinate system is measured at the TCP with use of a triaxial acceleration sensor type Kistler 8692C10M1. The used bandwidth is 4000 Hz. It is important to notice that the described measurements have been carried out under in idle state. A comprehensive modal analysis of FP4 machine has been carried out by [9]. Finally, based on analytical equation of Kienzle, the theoretical accessible removal rate of both machine tools has been estimated for several milling strategies. In this way the milling process efficiency of both machines can be compared. However, due to stability of milling process, the estimated removal rate should be considered merely as a measure of real accessible removal rate under machining conditions. Furthermore, the results may vary for different cutting materials and different conditions of cutting tools. The following calculations are applied for estimation of mechanical power of milling processes resulting from cutting force. To calculate the cutting force, the cutting velocity (v c ) has to be determined at first (equation (1)). v c = D π n 1000 In equation (1) D represents tool diameter and n spindle speed in revelations per min. To calculate cutting forces, equation of Kienzle is used. Therefore, the value for specific cutting forces ( k c ) is needed. k c can be calculated using equations: (1) In equation (2) and (3) k c1 represent principal value of specific cutting force [N/mm²], a e contact width [ ], κ contact angle, φ s cutting arc angle [ ], f z feed per tooth, m slope and h m average chip thickness. With k c it is possible to estimate the average force in milling process (F cmz ) and calculate the mechanical power which is needed for milling (P c ): F cmz a p = 1 sin κ h m k c k γ k V. k ver (4) P c = F cmz v c z iu a p In equations (4) and (5) is a p the cutting depth. k γ, k V and k ver represent correction factors for processing material, tools material and tool conditions and z iu stand for number of tooth in use. By taking the efficiency of spindle motor η into account, an approximation of the needed spindle power (P A ) can be evaluated: P A = P c η Considering above assumption the removal rate of milling machines has been calculated using accessible spindle power, maximum feed velocity of linear axis and maximum revolution of spindle. 4 RESULTS 4.1 Working accuracy Figure 3 and Figure 4 illustrates the positioning accuracy along x- and y-axis of each machine s coordinate system. Table 2 and Table 3 clarify a static analysis of measured deviations according to ISO The travel distance of each axis is divided into 11 measuring points. The measurements are carried out at measuring points using pilgering step method with 3 and 5 steps. DP4 Mean backlash [µm] Mean deviation [µm] Repeatability [µm] 17 2 Accuracy [µm] DMU-50 Table 2. Static analysis of measured deviations of positioning according to ISO along x-axis. (5) (6) and k c = k c1 h m m (2) h m = f φ z sin κ a e s D (3) 576

4 Deviation from positioning Compliance G Deviation from positioning K. Kianinejad, E. Uhlmann 60 m FP4 m + 2σ FP4 m - 2σ FP4 m DMU m + 2σ DMU m - 2σ DMU m: mean measured deviation σ: standard deviation DP4 Mean backlash [µm] Mean deviation [µm] 6 15 Repeatability [µm] DMU-50 µm Accuracy [µm] Table 3. Static analysis of measured deviations of positioning according to ISO along y-axis. Furthermore the straightness accuracy of FP4 machine is measured along x- and y- axis of machine coordinate system and in x-y and x-z coordinate plains (Table 4). In this way, it could be concluded that the guides of FP4 machine cannot be responsible for inaccuracy along x-axis mm 300 Position along x axis Figure 3. Deviation from positioning along x-axis Figure 3 and Table 2 point out that the positioning accuracy of FP4 machine is significantly lower than DMU-50 machine along x-axis. The magnitude of positioning accuracy along y- axis for both machines is comparable (Figure 4). 15 µm 7 3 m FP4 m + 2σ FP4 m - 2σ FP4 m DMU m + 2σ DMU m - 2σ DMU m: mean measured deviation σ: standard deviation x-z-plain Bidirectional accuracy [µm] Bidirectional repeatability [µm] Mean backlash [µm] Mean deviation [µm] 5 5 x-y-plain Table 4. Static analysis of measured deviations of straightness according to BS 3800 along x-axis for FP4 milling machine. Figure 5 illustrates the comparison of compliance-frequency G response of both machines in direction of x- and y- axis of machines coordinate system. 1E-5 1E m 1E-7 N FRF G xx FP4 FRF G yy FP4 FRF G xx DMU FRF G yy DMU -1 1E1E mm 300 Position along y axis Figure 4. Deviation from positioning along y-axis However, Table 2 and Table 3 show that DMU-50 machine is significantly more accurate concerning backlash and repeatability of measured results for both x- and y-axis. The inaccuracy of FP4 machine can be a result of defects in guides and feed drive system or inaccuracy of CNC system. It can also be seen that the deviation of both machines has diverse characteristics and properly have different origins. 1E1E-11 1E1E Hz 3600 Frequency f Figure 5. Comparison of frequency response function in x- and y-direction. Figure 5 clarifies that the DP4 milling machine tool has considerable higher stiffness comparing to DMU-50 machine. The reason for this is an oversizing of dp4 machine by construction. The DMU-50 machine is constructed based on light lightweight design and efficiency. 577

5 Investigation of the upgrading potentials of out-of-date cutting machine tools to promote sustainable and global value creation 4.2 Productivity The theoretical removal rate for different processing parameter has been compared using equations (1) to (6). For each processing strategy the combination of processing parameters have been chosen in such a way that a maximum power of spindle can be utilized for respective strategy. The comparison has been carried out for two materials: steel and aluminum alloy 2017A. Table 5 and Table 6 show the comparison of different processing strategies for aluminum alloy. Revolutions of main spindle [Rev/min] Cutting velocity [m/min] Feed velocity [mm/min] Str. 1 Str. 2 Str. 3 Str Cutting width Feed per tooth [m/min] Feed velocity [mm/min] cutting width Feed per Tooth Average force [N] cutting Mechanical power of cutting [kw] spindle per Cutting depth [kw/mm] Cutting depth spindle [kw] Removal [mm^3/min] rate 24,000 24,000 24,000 2, , , , ,000 Average cutting force [N] Mechanical power of cutting [kw] Table 6. Comparison of influence for different milling strategies for milling machine DMU-50, cutting tools width of 20 mm and processing material aluminum alloy 2017A. spindle per Cutting depth [kw/mm] Cutting depth spindle [kw] Removal rate [mm^3/min] ,000 72,000 95, ,500 Table 5. Comparison of influence for different milling strategies for milling machine FP4, cutting tools width of 20 mm and processing material aluminum alloy 2017A. As expected, the accessible theoretical removal rate by DMU- 50 machine is significantly higher than FP4 machine. Furthermore it can be seen that a higher removal rate can be achieved for both machines by using greater feed per tooth. The results for processing of steel have shown similar tendencies and have not been discussed here. A more detailed consideration of processing parameters in Table 5 and Table 6 shows that the main restrictive factor, concerning removal rate, is maximum spindle power. Beside this, the maximum feed velocity is important. Revolutions of main spindle [Rev/min] Str. 1 Str. 2 Str. 3 Str. 4 10,000 10,000 10,000 10,000 Cutting velocity SUMMARY AND OUTLOOK Considering the above results the outdated machine exhibits at least in two fields considerable improvement potentials: increasing of working accuracy and increasing of processing efficiency. In case of working accuracy deviations from set points can be divided into repeatable and stochastic parts. In this context by repeatable is meant a spatial and temporal reproducibility of measured deviation. One possible solution for improvement of working accuracy is to compensate the repeatable part of deviations by means of modification of program in CNC system. The repeatable deviations can be determined by systemic measuring of machine tools. Due to spatial nonlinearity of deviation and low repeatability accuracy, a compensation CNC system may not be appropriate for investigated outdated machine. Stochastic deviation cannot be reproduced under the same initial and boundary conditions. In this context the stochastic errors can be caused by geometric, kinematic and dynamic effects. The stochastic accuracy deviations can be compensated by means of add-on mechatronic components, as an example. These components can be installed additionally on the tool center point or on machine table. By using appropriate actuators and sensors the relative position of tool and work piece can be determined and controlled. In this way a share of deviations can be compensated. An implementation of above approaches can increase static and kinematic accuracy of an outdated machine tool with a minimal modification of machine. On one hand, this can extend the lifespan of outdated machine and prevent a complete replacement of machine tool. On the other hand, the 578

6 K. Kianinejad, E. Uhlmann part of inaccuracy resulting from worn or partially defect components can be compensated so that replacing or repairing of such components is not necessary. In this way material and energy resources, which are required for manufacturing and transporting of new machine or spare parts, can be saved. Concerning the enormous growing demands for machine tools on one side and scarcity of resources on the other side the above approach can facilitate the suitable value creation in global context. From other point of view, the machine tools which are updated with above method are able to manufacture parts with higher quality or more complexity with minimal investments. This affects the efficiency and lifespan of the final product positively and has a secondary supporting effect on suitability. In case of productivity add-on mechanical components like an external gear system can be used to increase maximum revolution of main spindle or maximum feed velocity. However due to restricted power of spindle or feed drives no significant improvement of removal rate by roughing can be achieved. This has been presented by means of above calculations. Instead an improvement of processing efficiency may be achieved by reducing of secondary processing time. In order to determine the improvement potential of outdated machine tools comprehensively, more detailed analysis and measurements should be carried out. Particularly the machine behavior has to be studied under load and machinery conditions. 7 REFERENCES [1] G.M.T.B. Association, The German Machine Tool Industry, Frankfurt am Main, [2] VDMA-Kennzahlen Fertigung und Montage, [3] V. D. Werkzeugmaschinenfabriken, Art und Umfang von Retrofit-/Refurbishing-Maßnahmen (R/R) an Werkzeugmaschinen, survey., February [4] T. Moriwaki, Multi-functional machine tool, CIRP Annals - Manufacturing Technology, [5] T. MORIWAKI, Trends in Recent Machine Tool Technologies, NTN TECHNICAL REVIEW, [6] J. Janischweski, "The Export of Second-Hand Goods and the Transfer of Tec hnology," Berlin, [7] G.M.T.B. Association, The German Machine Tool Industry, Frankfurt am Main, [8] Machine tools and related machinery, Fraunhofer IZM/IPK, Berlin, [9] E. Uhlmann, J. Mewis, P. Rasper, Microsystem and Adaptronic Machine Tool Concepts for Global Value Creation, The 10th Global Conference on Sustainable Manufacturing, ACKNOWLEDGMENTS This work was funded by the German Research Foundation (Deutsche Forschungsgemeinschaft), within the Collaborative Research Centre 1026 (SFB). 579