Investigation of damping effect of magnetorheological fluid damper on internal turning operation

Transcription

1 Investigation of damping effect of magnetorheological fluid damper on internal turning operation E.Mohan 1, U.Natarajan 2, C Selva Prasanth 3 1&2 Department of Mechanical Engineering, Mount Zion College of Engineering and Technology, Pudukkottai, Tamilnadu, India. 3 Department of Mechanical Engineering Alagappa Chettiar College of Engineering and Technology, Karaikudi. Tamilnadu, India Abstract Vibration in the metal cutting operation is a very familiar to each one in manufacturing industry. This occurrence is predictable in operations for instance internal turning, threading, grooving, milling, and drilling etc. These problems are also related to the machine tool itself, the clamping condition of the tool, length and diameter (L/D) ratio of the machine tool holder. Chatter is a one of the type of vibrations, it produce more effects on the surface of the work piece and tool life. This research deals with reduction of chatter on boring tool with Magnetorheological (MR) fluid damper. MR damper contains the MR fluid, which changes stiffness and undergoes a phase transformation when subjected to an external magnetic field, is applied to adjust the stiffness of the boring bar and suppress the chatter. The stiffness and energy dissipation possessions of the MR fluid controlled boring tool can be adjusted by varying the strength of the functional magnetic field. The analysis was made to study the damping effect of MR fluid damper for control the chatter vibration during machining operation with the help of Analysis software ANSYS workbench R15.0. The analysis results show the significant improvement in the suppression of chatter and it is observed that from model, harmonic and transient analysis by comparing both conditions with and without damping. Keywords internal turning; boring tool; chatter supression; magnetorheological damper I. INTRODUCTION Machine tool vibration is a frequent and main problem in the manufacturing industry where metal cutting operations take place. It mainly affects the surface finish of the work piece, tool life and creates frustrating noise. In order to restrict tool vibration, it is essential to develop and analyze suitable methods which increases stability and improves the cutting performance. In turning operations the cutting tool is subjected to a dynamic excitation due to the deformation of work material during the cutting operation. In particular the surface finish is affected by the relative dynamic motion between the cutting tool and the work piece during machining[1]. When machining high precision mechanical parts, selfexcited chatter vibrations must be absolutely avoided since they cause unacceptable surface finish and dimensional errors. Such unstable vibrational phenomena typically arise when the overall machining system stiffness is relatively low, as in the case of internal turning operations performed with slender boring bars. In general, it is not easy to determine stable tooling system configurations for a given machining operation, since data available in literature are often incomplete or inaccurate[2]. MR fluid is composed of oil and varying percentages of iron particles that have been coated with an anticoagulant material. This new class of device known as Magneto Rheologycal (MR) dampers will match well with the requirements and constraints of applications, including the necessity of having very low power requirements. A simple MR damper model is created for the work. The influence of MR damper was checked by measuring various parameters such as tool vibration, cutting force, surface roughness and tool wear[3]. Vibrations in the machining operations are intricate phenomena. During machining, work pieces are being cut and remove the unwanted portions of the components. Each time the cutting tool takes a bite, it applies a force on the work piece. The work piece responds to this force by deflecting or by molecules compressing closer together, and generate mechanical stress. This mechanical stress travels through the work piece as a whole and the work piece acts like a spring to deflect and then return into shape. This explains vibration phenomenon during machining process[4]. Vibration is a continual and severe trouble in the metal cutting operation. These vibrations normally decrease the surface finishing of the machining operation and additionally in an internal turning operation. It is owing to its slenderness and high length-to-diameter ratio of the cutting tool. The cutting speed, feed rate, depth of cut and overhang length of the tool are the most influencing parameters in the machining operation is observed by various research articles. Characteristics of boring tool with and without damper were investigated and results are compared in terms of surface roughness and tool vibration test[5] ISSN: Page 23

2 A. Chatter Noise in Turning Operation Chatter noise in machining is complex phenomena too similar to the vibration in machining. Chatter is an abnormal tool behaviour which it is one of the most critical problems in machining process and must be avoided to improve the dimensional accuracy and surface quality of the product. Chatter is a harmonic imbalance that occurs between the tool and the work piece because they are bouncing against each other. Chatter can be caused by the tool bouncing in or out of the work piece or the work piece bouncing against the tool, or both[7]. Fig. 1 chatter mechanism on the cut surface finish It is not always easy to determine why chatter is happening. Chatter needs to be taken into account during machining as it causes serious problems in machining instability. One of the most detrimental phenomena to productivity in machining is unstable cutting or chatter. To ensure stable cutting operations, cutting parameters must be chosen in such a way that they lie within the stable regions. Ideally, cutting conditions are chosen such that material removal is performed in stable manner. However, sometimes chatter is unavoidable because of the geometry of the cutting tool and work piece. Fig. 2 Chatter mark on turned metal work piece Such phenomena of chatter occurs during machining is due to material removal process in turning operation, both cutting tool and work piece are in contact with each other. Vibration and chatter noise are suppressed under certain conditions by this dynamic interaction between a rotating work piece and moving cutting forces from the tool. The cutting tool is subjected to a dynamic excitation due to the deformation of the work piece during cutting. The relative dynamic motion between the cutting tool and the work piece produce vibration and chatter thus affect the surface finish. Poor surfacefinish and dimensional accuracy of the work piece, possible damage to the cutting tool and irritating noise from excessive vibration are the results of uncontrolled vibration and chatter. Thus vibration related problems are of great interest in turning operations. B. Mode Coupling Mode coupling is recognized as one of the causes of chatter which is often called primary chatter. Mode coupling is a mechanism of self excitation that can only be associated with situations where the relative vibration between the tool and the work piece can exist simultaneously in at least two directions in the plane of the cut. Usually mode coupling occurs without any interaction between the vibration of the system and undulated surface of work piece. It acts only within vibratory systems with at least two degrees of freedom, which is due to the fact that the system mass vibrates simultaneously in the directions of the degrees of freedom of the system, with different amplitudes and phase[6]. C. Regenerative Chatter Regenerative chatter is renowned as a secondary chatter and it is a self excited vibration. It is caused by the regeneration of waviness of the surface of the work piece or by the oscillating cutter running over the wavy surface produced from the previous cut. It occurs whenever cuts overlap and the cut produced at time leaves small waves in the material that are regenerated with each subsequent pass of the tool on the previous cut surface. The tool in the next pass encounters a wavy surface and removes a chip periodically. The chip thickness produced varies after each successive cut. This will produce vibration and depending on conditions derived further on, these vibrations may be at least as large as in the preceding pass. Thus, the cutting force, which is a function of the chip thickness, depends not only on the current position of the tool and work piece but also on the delayed value of the work piece displacement. The newly created surface is again wavy in this way the waviness is continually regenerated. Regenerative chatter is considered to be the dominant mechanism of chatter in turning operations. If regenerative tool vibrations become large enough that the tool looses contact with the work piece, then a type of chatter known as multiple regenerative chatter occurs. This mechanism has been the subject of study by Shi and Tobias (1984). D. Chatter Suppression in Turning Operation A great deal of research has been carried out since the late 1950s to solve the chatter problems. Researchers have studied how to detect, identify, ISSN: Page 24

3 avoid, prevent, reduce, control or suppress regenerative chatter. Analysis and suppression of chatter has received great attention during the past two decades. The aim is to suppress chatter instability by reducing the relative displacements between the tool and work piece. Methods can involve active, semiactive or passive control. Active control systems do not require external assistance. They depend essentially upon a source of power to drive active device which may be electro mechanical, electro hydraulic or electro pneumatic actuators. In contrast, passive vibration control involves modification of the stiffness, mass and damping of vibrating system to make the system less responsive to its vibratory environment. Passive control, compared to active control, exhibit the advantages of easy implementation, low cost and no need for external energy. II. MACHINE TOOL CHATTER A. Machine Tool Chatter Machine tool can vibrate due to the cutting process itself under some particular conditions. In these cases, exciting force is not coming from an outside element; it is supplied by the cutting process itself.these types of vibration are self-induced and commonly known as machine tool chatter. Basically, instability or chatter may arise through the cutting fundamental principles: 1. Introduction of the negative damping coefficient either through the cutting process itself or through interaction with system parameters. 2. Instability through directional effects of model reacceptance. Den Hartog defined the characteristics of the selfexcited vibration as: in a self-excited vibration the alternating forces that sustains the motion is created or controlled by the motion itself and when the motion stops, the alternating force exists independently of the motion and persists even when the vibratory motion is stopped. Timoshenko defines self-sustaining, drawing itself-excited vibrations as a free vibration with negative damping. The distinction between a self-excited and a forced vibration system is, the former is self-sustaining, drawing its energy from an extraneous agency by its own periodic motion while the latter is controlled by the characteristics of an external alternating energy source. Self-excited vibrations therefore occur at a frequency close to the natural frequency of the un damped system while steady-state forced vibrations occurs at the frequency of the alternating external energy sources. II. ELIMINATION OF VIBRATIONS A. Vibration Control There are different methods to control vibration. If the system is not designed for vibration, problems may be faced once vibration is observed during operation. In this case, a vibration control strategy needs to be employed to reduce the levels of vibration and enable reliable and comfortable operation. Vibration control can be divided into three main categories, namely, Passive, Active, and Hybrid vibration control. Active vibration control is typically achieved by incorporating sensor and actuator pairs in the structural design to modify the response via feedback control. Passive vibration control methods, which are usually simpler compared to active methods, involves modifying the existing design, or adding extra components to the system. B. Damping When design a new machine tool, the design should be such that unwanted vibrations are absent or least reduced. During the design of the machine tool should make it a point to consider all the possibilities of vibration elimination. The overall stiffness of the machine tools is very important. To reduce intensity of vibration, it is also necessary that damping should be introduced from the conceptual stage of the design stage. Active vibration control is achieved by Embedding the actuator and accelerometer into the boring bar. In the passive damping, the basic principle of vibration absorber depends on the principle of transferring vibrational energy to an auxiliary system. III. SMART MATERIALS AND MAGNETORHEOLOGICAL TECHNOLOGY Smart materials and their use in semi-active vibration control are reviewed. These smart materials are considered as possible candidates for use in vibration isolation of a 6 DOF parallel mechanism. In choosing which technology to use for the vibration damper, important considerations are the cost of the damper, size, weight, repeatability, and power requirements. Magnetorheology stands out from the rest of the smart material options, as a more practical and feasible answer to the vibration isolation problem. It represents a novel technology which promises new design solutions. A. Smart Fluids These smart fluids exploit the rheological effect, which causes the solid particles in the fluid to align when the appropriate energy field is applied. This alignment creates a reduction in the ability of the fluid ISSN: Page 25

4 to flow, or shear. Each of the two main types of rheological fluids that have been researched is based on different applied energy fields. Electrorheological (ER) fluids are responsive to a voltage field and magnetorheological (MR) fluids are responsive to a magnetic field. ER fluids have been extensively investigated. Some of the disadvantages of these fluids are that they require thousands of volts for operation, and yield low shear stresses. The fact that these fluids are sensitive to contaminants and the high voltage needed for operation clearly make safety and packaging significant design problems. On the other hand, MR fluids operate with minimal voltage and generate high fluid shear stress (Sims et al., 1999). Commercially produced ER fluids are now available, but despite the design, construction, and testing of numerous prototype devices, the mass-production of an ER device is still awaited. However, the more recent MR devices are commercially available. This is because of the performance characteristics of MR fluids. They generate much higher dynamic yield stress, they have a wider temperature range, and they are insensitive to temperature variations and contaminants. B. Magnetorheological Fluids MR fluids were first discovered and developed by Jacob Rabinow at the US National Bureau of Standards in 1948 (Rabinow, 1948). MR fluids are suspensions of small iron particles in a base fluid. They are able to reversibly change from free-flowing, linear viscous liquids to semi-solids having controllable yield strength under a magnetic field. When the fluid is exposed to a magnetic field, the particles form linear chains parallel to the applied field as shown in Figure water or glycol is used as a suspension. In order to prevent gravitational settling, promote particle suspension, enhance lubricity, change viscosity, and inhibit wear, a variety of additives are used. The strength of a MR fluid is directly proportional to the amount of saturation magnetization. Alloys of iron and cobalt have large saturation magnetization and are therefore ideal, but they are very expensive. Therefore, simple iron particles which have a moderately high saturation magnetization are commonly used (Carlson and Spencer Jr., 1996). IV. TOOL GEOMETRY A. Tool Holder With and Without Damper Fig.4 Tool holders without damper Fig.5 Tool holders with damper B. Solid Model of Tool Holder without and With Damper Fig.3 Magnetorheological fluid: a) No magnetic field, b) with magnetic field. Fig.6 Solid model of tool holder without damper This phenomenon develops a yield stress which increases as the magnitude of the applied magnetic field increases (Jolly et al., 1998). Typically, 20-40% of the MR fluid consists of soft iron particles by volume (e.g., carbonyl iron). Mineral oil, synthetic oil, ISSN: Page 26

5 Fig.7 Solid model of tool holder with damper Fig.8 First mode shape of boring tool without damper V. FINITE ELEMENT ANALYSIS A. Modal Analysis Modal analysis is used to determine the natural frequencies and mode shapes of a structure. The natural frequencies and mode shapes are very important parameters in the design of a structure for dynamic loading conditions. They are also required for to do a spectrum analysis or a mode superposition harmonic or transient analysis. Modal analysis in the ANSYS program is a linear analysis any non-linear ties such as plasticity and contact element, will be ignored even if are defined. B. Harmonic Response Analysis Harmonic response analysis is a technique used to determine the steady-state response of a linear structure s response at several loads that vary sinusoidal with time. The idea is to calculate the structure response at several frequencies and obtain a graph of some response quantity verse frequency. Peak response is then identified on the graph and stressed reviewed at those peak frequencies. C. Transient Dynamic Analysis Transient dynamic analysis is a technique used to determine the dynamic response of structure under the action of any general time-dependent loads. We can use this type analysis to determine the time-varying displacement, strain, stresses and forces in a structure as it s response to any combination of static, transient, harmonic load. The time scale of the loading is such that the inertia or damping effect is considered important. D. Results of Modal Analysis Results of modal analysis are tabulated in the Table I. The sample results are presented in Fig. 8 and 9. Fig.9 First mode shape of boring tool with damper E. Discussion on Modal Analysis The results were obtained from the modal analysis of the boring tool with and without dampers. The natural frequencies of the boring tool for overhang length of 100mm are given in Table I and it indicates the frequencies at various modes ranging from the first mode to fifth mode. TABLE I NATURAL FREQUENCIES OF BORING TOOL Mode Without damper With damper e e It is observed that the natural frequency of boring tool has been reduced with the damped tool. F. Results of Harmonic Analysis Fig. 10 Harmonic analysis of boring tool without damper Fig. 11 Harmonic analysis of boring tool with damper ISSN: Page 27

6 G. Discussion on Harmonic Analysis The results obtained from the harmonic analysis of the boring tool with and without dampers tabulated in Table II. The peak value of the amplitude has been reduced with respect to the frequencies in this analysis. TABLE II COMPARISONS OF PEAK VALUE S.No Damper Peak value (mm) 1 Without e-6 2 With e-3 It is observed that the amplitude of the vibration has been reduced with damped tool. In that analysis copper damped boring tool amplitude of vibration has been reduced when compared with undamped boring tool. H. Results of Transient Analysis Results of transient analysis are tabulated in the Table III. The sample graphical results are presented in Fig. 12 and 13. Fig. 12 Transient analysis of boring tool without damper Fig. 13 Transient analysis of boring tool with damper I. Discussion on Transient Analysis The results obtained from the transient analysis of boring tool with and without dampers tabulated in Table III. Amplitude of the boring tool without damper is reduced with Copper damper. Time taken to decay the amplitude of boring tool without damper 4 seconds is reduced to 2.5 seconds with Copper damper. TABLE III TIME TO DECAY THE AMPLITUDE OF BORING TOOL VI. CONCLUSION Finite Element Analysis (modal, harmonic and transient) was carried out in order to investigate improvements in the damping capability of boring tools and suppression of chatter vibration using MR fluid damper. The damping capability of boring tool is considerably improved using this damper. It is observed that damped boring tool shows a significant improvement in the dynamic behavior of the structural analysis, when compared without damper. Acknowledgment The authors would like to express thanks to all the staff of Mount Zion College of Engineering & Technology, Pudukkottai, Tamilnadu, India to carry out this investigations as successful. References [1] Suraj Suresh,S. Radhakrishnan "Investigation and analysis of magneto rheological fluid damper for suppressing tool vibration during hard turning" International Journal for Scientific Research & Development Vol. 3, Issue 09, [2] Giovanni Totis, Marco Sortino robust analysis of stability in internal turning 24th DAAAM International Symposium on Intelligent Manufacturing and Automation, [3] Experimental investigation of chatter vibrations in facing and turning processes M. Siddhpura and R. Paurobally, International Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering Vol:7, No:6, [4] Mohan.E, Natarajan.U Experimental investigation of chatter control in an internal turning operation using MR fluid damper, International conference on smart design and sustainable manufacturing, Alagappa Chettiar College of Engineering and Technology, Karaikudi. Tamilnadu. [5] P. Sam Paul, A.S. Varadarajan Effect of magnetic field on damping ability of magnetorheological damper during hard turning Archives of Civil and Mechanical Engineering (2013). [6] Mohan.E, Natarajan.U Investigation of chatter suppression in internal turning operation by impact damper International Journal of Applied Engineering Research, ISSN Vol. 10 No.67 (2015). [7] Mohan.E, Natarajan.U Experimental investigation on boring tool vibration control using MR fluid damper Journal of Advanced Manufacturing Systems, Vol. 15, No. 1 (2016) [8] D. Sathianarayanan, L. Karunamoorthy Chatter Suppression in Boring Operation Using Magnetorheological Fluid Damper Materials and Manufacturing Processes, 23(2008) pp [9] D. Sathianarayanan, L. Karunamoorthy Chatter Suppression in Boring Operation Using Magnetorheological Fluid Damper Materials and Manufacturing Processes, 23(2008) pp S.No Damper Time taken to decay (sec) 1 Without 4 2 With 2.5 It is observed that there is a decrease in time taken to decay the amplitude of the boring tool with dampers. ISSN: Page 28