DYNAMIC ANALYSIS OF THE STRUCTURE OF A MACHINE TOOL FOR WOODWORKING

Transcription

1 IOMAC'13 5 th International Operational Modal Analysis Conference 2013 May Guimarães - Portugal DYNAMIC ANALYSIS OF THE STRUCTURE OF A MACHINE TOOL FOR WOODWORKING José Gomes 1, José Meireles 2 ABSTRACT The aim of this work is the dynamic analysis of a woodworking machine tool structure. It is intended to seek solutions for its improvement through the application of a unified framework for multidisciplinary virtual simulation on open architecture, for the problem customized simulation, involving structural motion dynamic. The solutions are developed to improve the quality and reliability of the structure from dynamic point of view, using applications to modeling dynamic analysis, based on CATIA V5. The methodologies employed in the study of structural dynamics are characterized and their limitations are identified. Models are applied to study dynamic structures using the dynamic analysis program LMS Virtual Lab in the study of machine tools for wood, and is developed their suitability to the role played, by improving their behavior. It is thus analyzed, in order to improve the existing solution, the process of manufacture and assembly of the structure in particular as regards to the influential in the dynamic behavior. In a second phase are performed theoretical models, representative of solutions, using a method to obtain the static and dynamic behavior of the structure. The theoretical results obtained are compared with the numerical results. Afterwards identical studies are performed for the new proposed solutions and are compared with the existing solutions. Finally, there is obtained improvement in the dynamic properties of the structural components of the machine tool, by varying several variables. Keywords: machine tool, dynamic analysis, virtual simulation 1. INTRODUCTION This study focuses in the behavior of a part from a machine tool to work wood, that have as main characteristic the planning, an operation that consists in obtaining plane surface in horizontal, vertical or sloping position. The operations of planning are accomplished with tools that have a cutting edge that removes the material with his movement [1]. The planning is a rough-hewing operation. Therefore, and depending on the piece type that is to be manufactured, it can be necessary the use other machines for the subsequent finish. In the planning operations, the cut is made in a single direction. On the other hand, the planning uses cut tools with one or more cutting edges than are cheaper, easier to sharpen and with faster assembly than other machining processes [1]. 1 M. Eng., Minho University, a27475@alunos.uminho.pt 2 PhD, Minho University, meireles@dem.uminho.pt

2 J. Gomes, J. Meireles 2. INFLUENCE OF CUTTING PARAMETERS IN MACHINING WOOD For better understanding the behavior of wood in different machining processes of cutting, one of the basic parameters to consider are the forces generated during the cutting process. The parallel shear force has great importance in determining the tool geometry and in dimensioning the power requirement to the machine. The normal cutting force is intimately related to the quality of the machined surface. Figure 1 shows the typical behavior of the normal force measured by a load cell and an oscilloscope. By convention, the negative normal forces are defined as those that deviate the tip of the cutting blade from the timber. Positive forces are pushing the cutting blade on the wood [2] [3]. Figure 1 Typical behavior of the normal force [2] Several studies were conducted for the variation of the forces generated by the angles in orthogonal cutting of woods [2] [3] [4]. These show that in orthogonal cutting, the parallel force component can be vital to the appearance of defects at the edges that delimit the timber when the tool leaves the work piece in the final machining operation. A study demonstrates that the factors which significantly influence the parallel force component are, in order of importance: the average thickness of the chip, the rake angle (angle of inclination in the blade witch attacks the wood) and direction of feed [3]. The compression results in the negative part of the normal force are significantly affected by the following parameters: the feed direction, the rake angle and the average thickness of the chip [3].The effect of the rake angle was much more pronounced than that related to the length of the cut marks (Figure 2). Figure 2 Maximum normal forces for both negative (pushing) and positive (pulling) components in function of rake angle, depth of cut, and wavelength [2]. Is Perceive that the effect of the rake angle on the maximum value from the negative part of the normal force depends little on the length of the cut marks considered (Figure 2) [2]. Decreasing the rake angle, the cutting edge tends to push more perpendicular to the fibers, which results in a higher value from the negative part of the normal force. In addition, the contributions from the friction between the rake face of the tool and the timber, as well the effect of pushing the face caused by the 2

3 5 th International Operational Modal Analysis Conference, Guimarães May 2013 rake angle on the chip increases. These effects diminish with the increasing of the rake angle (Figure 2). Within the cutting parameters studied, the effect caused by the rake angle at the maximum value of the parallel force was the highest, followed by the effect of cut marks in the wood, and finally the depth of cut. The maximum parallel force decreased almost linearly increasing the rake angle [2]. The normal force obtained experimentally decreases from 20 N with an rake angle of 0, to 19 N with an rake angle of 10, decreases to 12N for a rake angle of 20, and finally to about 8 N with a rake angle of 30º [2]. 3. PROBLEM DESCRIPTION The dynamic analysis of the machine tool elaborated in LMS Virtual Lab simulates a mechanism constituted by a shaft that will be requested dynamically, to different rotation speeds ( rpm), with loads applied in the straight knife head that simulate the wood cut and originate normal tension and shear tension due to the involved torsion. Coupled in the spindle are two ball bearings, supports of the system in which the dynamic reaction is obtain motivated by the loads, the planer head, where the straight knifes are fixed, the pulley, to which is transmitted the power of the motor and the bushing that serves as support and allow the best fixation of the components. The first step is the accomplishment of modeled drawing of each one of the component parts. In this case all the components of the mechanism in subject were modeled using Catia V5, which Virtual LMS Lab incorporates, and the material is specified. After all the components are designed, occurs the assembly of the machine tool with all their components as can verify in the Figure 3. Figure 3 Vertical planer and applied forces There are other components involved in this system, however is considered that they are little relevant for the objective of the analysis and would increase difficulties of modulation in the system. In Figure 3 is represented the maximum force to which the blade is subjected, which simulates a limit shear load for cutting wood, 5000 N. The load for the simulation with the experimental analysis will be only 20 N. 4. SOFTWARE VALIDATION LMS Virtual Lab is used to study the dynamic behavior of a machine tool to cut wood. 3

4 J. Gomes, J. Meireles To validate the software, the results obtained are compared to the ones obtained analytically for applying loads on the shaft ends, 5000 N at the top of the shaft, which simulates a limit shear load for cutting wood obtained from experimental data and 1150 N that simulates the resultant force from the tension introduced by the belts. After running the numerical calculation, are obtained the reactions from the bearings and compared with the ones obtained analytically. The software analysis shows that the results obtained (5436 and 721 N) are similar to the results calculated analytically (5422 and 729 N). The small difference is because you cannot introduce a completely static behavior through the LMS Virtual Lab, having to be inserted a minimum speed, 1 rpm, which influences the final results. Consulting the maximum values obtained for reactions on the bearings, either analytically or numerically, the results are close (Table 1). Table 1 Comparison of analytical and numerical results Reactions from the bearings Top Ball Bearing [N] Lower Ball Bearing [N] Analytic data Software data Thus, due to the low difference found between the analytical results and the numerical ones, we can conclude that the program will have a high degree of reliability. 5. DYNAMIC ANALYSIS WITH APPROXIMATION TO A REAL CASE The study data about the forces produced by the peripheral cut are still scarce, thus, in order to study these forces was used a method that facilitates the interpretation of its dynamic behavior during the rotation of the blade when the piece is being cut and the results are compared with those obtained experimentally. Based in different works [2] [3] [4] is carried out a test that simulates the behavior of forces in the router over a complete cycle of cutting. Thus the study starts from a maximum force studied in the behavior of shear forces in discordant planning, a force of 20 N per aproximation of the maximum value at study for a rake angle of 10, a thickness of 0.25 mm and cut marks of 1.75 mm. Because of the parameters of thickness and cut marks cannot be studied numerically by LMS Virtual Lab, we can only have security in the behavior of the movement of the forces rather than on their absolute values obtained. Nevertheless the study provides insight into their behavior with different rake angles, as shown in Figure 4. Figure 4 Behavior of parallel cutting forces with an rake angle of zero degrees to an applied force 20 N Analyzing the data obtained with the LMS Virtual lab to a shearing force exceeding 20 N for t = 0.02 s, which represents a cutting cycle, it was found that with the increasing rake angle from 0 to 10, 20º and 30º, respectively, the maximum cutting parallel force decreases with increasing this angle. It is noted that, until reaching the maximum parallel force with the gradual increase of the rake angle, and analyzing an intermediate value preceding the maximum parallel force for t =0.005 s, the shear forces tend to withdraw each time more with increasing rotational speed as shown in Figure 4. 4

5 5 th International Operational Modal Analysis Conference, Guimarães May 2013 The maximum parallel force behavior is analyzed in Figure 5, where one can easily infer their performance. For this analysis is extracted the maximum force parallel to each rake angle, whereas the speed variation does not influence their behavior and shows that with increasing rake angle parallel force gradually decreases. Thus, for an initial force of 20 N with a rake angle of 0, the maximum deviation of rake angle is obtained for the angle of 30 degrees, with a parallel force of approximately 17.3 N. Figure 5 Maximum parallel force in function of the rake angle Observing changes in the rake angle on the normal component of the cutting force, is observed that these rise to a different behavior than the parallel cutting forces. Further it is found that in the normal force the direction changes during the cutting cycle. The negative part of the normal force occurs in the first phase of the cutting cycle and the maximum value of the negative part of the normal force is significantly affected by the rake angle. With the increase of the rake angle appears that the smaller value (absolute value) of the negative normal force is obtained, in Figure 6, when the rake angle is 30. The positive normal force maximum increases with the increasing in the rake angle, as can be readily observed, thus are also affected by this. Figure 6 shows the normal forces on the cutting in function of the rake angle on the cutting tool. Figure 6 Behavior of normal cutting forces with an rake angle of zero degrees to an applied force 20 N The normal force has a wave cutting behavior, as can be seen in Figure 6, and the negative component tends to form a parabola, with the point of reverse at t = 0,006 s to an rake angle of 0, but for a rake angle of 30 is observed the inversion point at t= s. Therefore the maximum negative component of the cutting force is reached sooner. It is further understood that negative component of the normal force becomes increasingly smaller with the increasing rake angle, and with lower rotation speeds of the tool, this component tends to disappear for the negative portion of the normal force. The positive component of the normal force for cutting wood, tends to increase with the increase rake angle from 0º to a maximum angle of 30º. The point at which this maximum occurs happens earlier with increasing the rake angle. It is observed that the peak positive portion of the cutting force is reached at t = s of cut cycle, whereas for a rake 5

6 J. Gomes, J. Meireles angle of 30 degrees this happens for t = s. The behavior of the normal force to the maximum negative and positive components is examined in Figure 7. Figure 7 Maximum positive and negative normal forces in function of rake angle For this analysis are obtained the maximum values from positive and negative forces of the normal force, for each rake angle with the speed of 1000 rpm, because this is the intermediate rotation of the tests performed. It is found that increasing the rake angle for the negative component of the force gradually decreases, and the positive component of the normal force increases with increasing rake angle. Thus for a positive initial strength of 4.8 N with an rake angle of 0, varying the angles, are obtained to 30 a maximum normal force of approximately 10.7 N. The negative component of the force decreases from a negative force of -4.8 N with zero rake angle, to a force of -1.2 N for an rake angle of 30º. 6. SIMULATION WITH EXPERIMENTAL DATA In the analysis made previously, the load applied simulates a peak strength, now for this analysis is tried to reproduce the behavior of the maximum force as close as possible to the cutting force obtained experimental [2]. Is then introduced the load [2] and is analyzed their behavior at rake angles of the blade between 10º and 30º, verifying the results of the approximation to the data obtained experimentally. The load is introduced from the maximum values given, for the different rake angles and reproduces the shear forces arising. The analysis is performed in the LMS Virtual Lab to a rotational speed of 1000 rpm. The results for the forces with an rake angle of the blade with 10º are those that are closer to the behavior of the experimental forces obtained as shown in Figure 8, while the maximum load is only 10 N, less than the obtained with experimental data for the same rake angle. Figure 8 Cutting forces with an rake angle of 10 degrees For Figure 9 it is found that the behavior of the forces analyzed are close to those obtained experimentally [2], showing that varying the rake angle the maximum cutting forces tend to decrease. 6

7 5 th International Operational Modal Analysis Conference, Guimarães May 2013 Figure 9 Cutting forces with an rake angle of 10 degrees The analysis of the dynamic behavior of the forces that simulate a load obtained experimentally verified that this is within the expected to obtain [2]. The absolute values obtained are close to the experimental mainly for rake angles of 20º and 30º. Both the parallel force, the negative and positive components of the normal force cutting behave in exactly the same way as the data obtained experimentally. 7. RESULTS AND CONCLUSIONS In the analysis of the forces obtained it appears that the parallel force component gradually decreases as it increases the rake angle, as happens in the experimental analysis. In data obtained by LMS Virtual Lab, the maximum component of parallel force varies between 11 and 6 N, with values very close to those obtained experimentally. The dynamic behavior between the experimental data and the performed analysis is identical. Thus it appears that the LMS Virtual Lab can be used with great confidence to simulate the behavior of shear forces on the blade of the tool. By analyzing the dynamic behavior of the forces when trying to approach to a real case, from a value obtained experimentally, 20 N, is attested that this is within expectations and in line with what expected to obtain [2]. The difference focuses on the absolute values obtained, but both the parallel force, the negative and positive components of the cutting normal force behave exactly in the same way as studied [2] [3]. In the analysis of the forces obtained, in the approximation to a real case, it is found that the parallel force component gradually decreases as it increases the rake angle, as happens in the experimental analyses [2] [3]. The difference found is in the values, whereas the experimental values of the maximum parallel force component varies between 19 N and 7.5 N for rake angles of 10 and 30º respectively. In the data obtained by the LMS Virtual Lab, the maximum component of parallel force varies between 7.2 and 3.2 N. It is then observed that the slope of numerical analysis performed is far less pronounced than the value of experimental analysis, despite the dynamic behavior being similar and behaving like what would be expected. In the analysis of the normal forces, again, the dynamic behavior is in line with the expected, but the absolute values obtained experimentally and numerically differ again. The positive component of the normal force, ranges between approximately 0.5 and 1.5 N [2] (Figure 2), and numerically to a cutting speed of 1000 rpm, the values varies between 5 and 11 N approximately. In the negative part of the normal force is obtained, experimentally, a variation for rake angles between 10 and 30 degrees respectively, values between -1.7 and -1 N, and numerically for the same rake angles, values ranging between -5 and -1 N. This discrepancy in absolute values of the normal and parallel forces is due to the effect of other parameters in the behavior of the cutting forces, as already mentioned. Other parameters that could be introduced simultaneously, which affect the amount and force behavior are the cutting depth, the wear of the blade, wood density, moisture content, and temperature. However the parameters with more influence are the section thickness and length of cut marks, which are studied simultaneously with the variation of the rake angle. Numerically it was not possible to introduce these parameters which may be the cause of the differences found. 7

8 J. Gomes, J. Meireles ACKNOWLEDGEMENTS The authors gratefully acknowledge the Centre for Mechanical and Materials Technologies (Centro de Tecnologias Mecânicas e de Materiais CT2M). REFERENCES [1] Carvalho, A. (1996) Madeiras portuguesas vol. 1. In : Instituto florestal. [2] Iskra, P. and Hernandez, R. (2012) Analysis of cutting forces in straight-knife peripheral cutting of wood. In: Wood and Fiber Science, 44(2), pp [3] Palmqvist, J. (2003) Parallel and normal cutting forces in peripheral milling of wood. In: Holz als Roh- und Werkst 61. [4] Palmqvist, J., Lenner, M. and Gustafsson, S. (2003) Cutter head forces and load cell scanning. In: Wood Sci Technology. 8