INCREASING THE EFFICIENCY OF EXTERNAL TURNING BY USING THE MULTIPLE LAYER CONSTRUCTION OF THE CUTTING TOOL HOLDER

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1 8th International DAAAM Baltic Conference "INDUSTRIAL ENGINEERING April 2012, Tallinn, Estonia INCREASING THE EFFICIENCY OF EXTERNAL TURNING BY USING THE MULTIPLE LAYER CONSTRUCTION OF THE CUTTING TOOL HOLDER Madissoo, M.; Maksarov V.; Olt, J. Abstract: The treatment of complex products from hard materials is accompanied by loss of stability of the cutting process, which leads to oscillations, causing an increase in the wear of cutting tools, reducing the longevity of the machine actuators, which ultimately affects the quality and accuracy of processing responsible products. The aim of the work is to investigate the effect of the multiple layers cutting tool holder construction. The different layers are made from plastically deformed steel. The different plates have a anisotropic structure and have different mechanical properties. This construction is considered as a method to improve the effectivity of the cutting tool and helps to reduce the oscillations in the finishing turning. We hereby propose a method for the construction of the multilayer damper instrument holder together with a mathematical model suitable for processing hard materials. Key words: chatter vibrations, metal cutting, turning tool holder, multiple layer construction. 1. INTRODUCTION Today tool holders are available in a large variety of forms to suit the specific machining requirements. The material properties of the tool holder have a large influence on both the surface quality and dimensional accuracy of the machined workpiece, and on the life of the cutting tool. The material properties of the workpiece and various other factors, can lead to excessive vibrations in the tool 181 shaft, which in turn causes undesirable chattering. By the use of passive damping elements, integrated on the tool shaft, the dynamic behavior of the tool can be optimized. From the standpoint of dynamic stability we assume that the tool is the weakest subsystem of the turning process. The impact of periodical external forces causing an oscillatory process with a frequency equal to the exciting forces or complex periodic processes caused by nonlinear properties of the system, having its own damped and forced oscillations of parametrically excited oscillations and selfoscillation system can violate the stability of the technological system. The intensity of the forced vibrations is particularly great in the resonant region, which is not allowed in metal cutting machines as an operating method for finishing. One aspect of improving the dynamic stability of the subsystem "tool" is the creation of damped tools with increased resistance, with elastic and damping elements that do not change their appearance It is important for the damped instruments to be characterized by its adaptation to the variable tolerances, load balancing between the cutting edges, as well as to prevent breakage of the cutting edge. [1] One of the most effective means available to ensure the stability of the partial subsystem "tool" for finishing turned parts is the creation of damped instrument equipped with a multilayer cutting tool holder construction with anisotropic properties.

2 2. METHODS The chatter vibration system can be represented by the block diagram shown in (Fig. 1.), where the parameters of the dynamic cutting process are shown in the Laplace domain. Fig. 1. Block diagram of chatter dynamics. [2] Input to the system is the desired chip thickness h, and the output of the feedback system is the current vibration y(t) left on the inner surface. In the Laplace domain, y(s) = Ly(t), and the vibration imprinted on the outer surface during the previous revolution is e st y(s) = Ly(t T), where T is the spindle period. The dynamic chip thickness in the Laplace domain is h(s) = h 0 y(s) + e st y(s) = h 0 + (e st 1)y(s), (1) which produces a dynamic cutting force of F f (s) = K f ah(s). (2) The cutting force excites the structure and produces the current vibrations y(s) = F f (s)φ(s) = K f ah(s)φ(s), (3) Where the transfer function of the single degree of workpiece structure is Φ(s) = y(s) = ω2 n (4) F f (s) k y (s 2 +2ζω n s+ω 2 n ) Substituting y(s) into h(s) yields h(s) = h 0 + (e st 1)K f ah(s)φ(s), (5) and the resulting transfer function between the dynamic and reference chip loads becomes h(s) = 1. (6) h 0 (s) 1+(1 e st )K f aφ(s) The stability of this closed-loop transfer function is determined by the roots (s) of its characteristic equation, that is, 1 + (1 e st )K f aφ(s) = 0. (7) Let the root of the characteristic equation be s = σ + jω c. If the real part of the root is positive (σ > 0), the time domain solution will have an exponential term with positive power (i. e., e + σ t ). The chatter vibrations will grow indefinitely, and the system will be unstable. A negative real root (σ < 0) will suppress the vibrations with time (i. e., e σ t ), and the system is stable with chatter vibration-free cutting. When the real part is zero (s = jω c ), the system is critically stable, and the workpiece oscillates with constant vibration amplitude at chatter frequency ω c. The chatter vibration frequency does not equal the natural frequency of the structure, since the characteristic equation of the dynamic cutting process has additional terms beyond the structure s transfer function. The chatter vibration frequency is still close to the natural mode of the structure. For critical borderline stability analysis (s = jω c ), the characteristic function becomes e jωct K f a lim Φ(jω c ) = 0, (8) Where a lim is the maximum axial depth of cut for chatter vibration-free machining. The transfer function can be partitioned into real and imaginary parts (i. e., Φ(jω c ) = G + jh). Rearranging the characteristic equation with real and complex parts yields 1 + K f a lim [G(1 cosω c T) Hsinω c T] + j K f a lim [Gsinω c T + H(1 cosω c T] = 0 (9) Both real and imaginary parts of the characteristic equation must be zero. If the imaginary part is considered first, then Gsinω c T + H(1 cosω c T) = 0 (10) and tanψ = H(ω c) sinω ct (11) = G(ω c) cosω c T 1 where ψ is the phase shift of the structure s transfer function. Using the trigonometric identity cosω c T = cos 2 (ω c T 2) sin 2 (ω c T 2) and sinω c T = 2sin(ω c T 2) cos(ω c T 2) we have tanψ = cos (ω ct 2) = sin (ω c T 2) tan[(ω c T 2 (3π) 2] and 182

3 ω c T = 3π + 2ψ ψ = tan 1 H. (12) G The spindle speed (n[rev s]) and the chatter vibration frequency (ω c ) have a relationship that affects the dynamic chip thickness. Let us assume that the chatter vibration frequency is ω c [rad s] or f c [Hz]. The number of vibration waves left on the surface of the workpiece is f c [Hz] T[s] = f c ε = k +, (13) n 2π where k is the integer number of waves and ε 2π is the fractional wave generated. The angle ε represents the phase difference between the inner and outer modulations. If the spindle and vibration frequencies have an integer ratio, the phase difference between the inner and outer waves on the chip surface will be zero or 2π; hence the chip thickness will be constant despite the presence of vibrations (Fig. 2.). In this case, the inner (y(t)) and outer (y(t T)) waves are parallel to each other. T = 2kπ+ε n 60. (15) 2πf c T The critical axial depth of cut can be found by equating the real part of the characteristic equation to zero: a lim = 1 2K f G(ω c ). (16) Since the depth of cut is a physical quantity, the solution is valid only for negative values of the real part of the transfer function (G(ω c )). The chatter vibrations may occur at any frequency where G(ω c ) is negative. If a lim is selected using the minimum value of G(ω c ), the avoidance of chatter is guaranteed at any spindle speed. The expression indicates that the axial depth of cut is inversely proportional to the flexibility of the structure and to the cutting constant of the workpiece material. The harder the work material is, the larger the cutting constant K f will be, thus reducing the axial depth of cut. Similarly, flexible machine tool or workpiece structures will also reduce the axial depth of cut or the productivity. [2, 3, 4] 3. THE MULTIPLE LAYER CUTTING TOOL HOLDER CONSTRUCTION Fig. 2. Wave generation. [2] If the phase angle is not zero, the chip thickness changes continuously. Consider k integer number of full vibration cycles and phase shift 2πf c T = 2kπ + ε, (14) Where the phase shift between the inner and outer waves is ε = 3π + 2ψ. The corresponding spindle period (T[s]) and speed (n[rev min)]is found to be The peculiarity of the process of turning tool fitted with the proposed method is to reduce the level of self-oscillations arising in the process of cutting. It is achieved through the orderly disorientation of alternate plates with anisotropic material textures that form the multilayer holder. This construction enables effectively dissipate the energy of the vibrational waves at the boundaries of the transition points between the holder different plates. This method allows to significantly increase the resistance of the tools cutting edge and helps to expand technology opportunities for effective choice of cutting conditions to ensure compliance with the requirements for the dimensional and geometric accuracy, which will in turn contribute the quality of the workpiece. To achieve this goal we have resolved following tasks: 183

4 1. Developed a method to construct an instrument equipped with a multilayer anisotropic structure; 2. Investigated the anisotropic properties of the holder plates material; 3. Developed a dynamic model of the technological systems in accordance with the rheological characteristics of chip forming to assess the stability of cutting process using dampening tool. The cutting tool holder replacement plates are made of steel 45, which consists of many crystalline grains oriented randomly and the material structure is generally isotropic. The anisotropic structure of the material is achieved with pressure treatment. The plastic deformation caused by cold rolling has been made without creating an uneven surface, the resulting micro-structural changes in the material will lead to a change in the direction of macro-fibers which form the material texture. [5] The proposed cutting tool holder is made from a package of layers assembled together parallel in different planes (Fig. 3). Fig. 3. The assembly of the cutting tool holder To the bearing holders surface plates are cut from sheet metal with a longitudinal, transverse and vertical orientation of the planes which are relative to the direction of rolling and placed into the holder so that the plates of different structural orientation form the desired texture of the holder. When oscillations arising in the process of machining characteristics the behavior of the tool holder for small deformations are considered in the form of Hooke's law, taking into account the internal friction in the holder material and that the resistances friction in the joints between the plates is fixed. Caused by the presence of inelastic effects in the texture of the material the internal friction is associated with the movement of dislocations witch cause irreversible hysteresis energy losses within the metal cutter holder during the mechanical vibrations. Line charts of stress - strain do not match for loading and unloading of the tool holder as a result of incomplete elasticity of the metals, but they form a hysteresis loop. It covers an area characterizes the energy dissipated per cycle of loading. Internal friction associated with the static hysteresis, does not depend on the frequency of oscillation when the shape and area of its loop is not connected with the temporary relaxation processes but strongly depend on the amplitude and properties of the material holder. [6] To achieve the maximum damping effect of the deformed plates texture the disorientation of two adjacent plates must be maximized. Then the vibration waves, when passing the interface changes their direction, resulting in the dissipation of energy fluctuations. For small values of disorientated deformed plate textures the energy dissipation is negligible. Therefore in the proposed technical solution the plates in the holder are oriented so that the tangential component of the cutting force transition from one plate to another is carried out so that the deformation texture of the plates was varied by 90 ± 10 relative to the original holder s core. Under the action of cutting forces in the upper holder having predominantly the maximum tensile stress, and in the lower, supporting, compressive stresses. Therefore, in order to stabilize the strength of the various zones and increase the durability and reliability of the holder, 184

5 additional conditions on the orientation of the texture of the deformed plates in the assembly of the holder is needed with respect to the cutting forces. It is known that the maximum tensile stress resistance of the rolled metal is in the longitudinal direction, and the minimum - in a vertical direction relative to the rolling direction. On the contrary the maximum compressive stress resistance has rolled metal in the vertical direction and the minimum in the longitudinal direction. The resistance of the metal in the transverse direction and the tensile and compressive stresses have intermediate values. In this regard the rolling direction in the base plate is oriented parallel to the tangential component of the cutting force. The top plate is oriented parallel to the rolling direction of the radial component of the cutting force, and the plane perpendicular to the rolling mill is the tangential component of the cutting force. The direction of rolling in the middle of the plate is oriented parallel to the action of the axial component of the cutting force. The so called "elastic anisotropy", can greatly change the state of stress in the elastic region and also affects beyond the elastic limit. It has a strong influence on the stress concentration. In general, the larger the anisotropy of the characteristics of elasticity, the greater the stress concentration factor varies depending on the orientation of the force relative to the axes of symmetry, the greater the maximum concentration ratio is the greater is its value for the same isotropic sample. [7] 4. RESULTS AND DISCUSSIONS The experimental studies to determine the effect of reducing the level of oscillation when using the tool holder equipped with a multi-layer construction and studies on the wear of the inserted cutting tools and the quality improvements of the treated surface showed that the wear of the cutting part of the cutters top plate, by a multi-layer holder could be less than the equivalent tool. According to test results, that the tool is equipped with multi-layer holder as compared to an analog cutter improves the quality of treatment. The most intense alteration is observed when changing the cutting speed. Practical issues related to the control selection of the cutting process, are depending on the requirements of dimensional and geometric accuracy of machined surfaces [8] which can be solved using an instrument equipped with a multilayer holder. According to the results of experimental studies using a fractional experiment obtained the functional dependence of the actual margin of accuracy of the machined surface of the cutting. The functional dependence of the actual margin of accuracy in roundness φ 06 (17), ovality φ Nk (18) and cone φ k (19) are obtained as the ratio of tolerance to process parts to the field of geometrical deviations of the form of dispersion as a function of cutting speed, depth of cut and feed by using an instrument equipped with multilayer construction. φ 06 = 6, t0.29 s 0.22 (17) ν 2.14 φ Nk = 83, t φ k = 61,31 s ν2.14 (18) 1 (19) t s ν0.74 The calculated values φ = f(v, s, t) of the functions allow you to predict the actual value of the stock on the accuracy of the deviations of geometric shapes, machined surfaces, depending on the set of the cutting tool equipped with the use of multilayer holder, as well as provide an opportunity to make a comparative analysis of the dependence of the actual margin of accuracy of the cutting conditions for finish turning cutting tools equipped with a multi-layer holder and tools counterpart. The actual margin of accuracy in the processing with the proposed cutter holder design is higher than when operating with an analog cutter on the same cutting conditions. 185

6 CONCLUSION The developed method for increasing the stability of the cutting process by finishing turning is based on the use of anisotropic properties of plastically deformed structural steel and the effects of dissipation of vibrational waves in the transition section between the disorientated plates which form texture of the tool holder. Based on the studied physical properties of anisotropic rolled steel the tool holder with a multi-layer construction was created. First experimental research examining the influence of the damping properties of the holder to the tool wear, the parameters of cutting and quality of the machined surface were carried out. Future work is planned to be made on experimental studies to investigate the influence of different parameters to the dimensional and geometric accuracy of the machined surfaces. It is important to obtain the functional dependence of the efficiency of selected cutting parameters, depending on the requirements of dimensional and geometric accuracy of machined surface using an instrument equipped on multi layer holder. REFERENCES [1] Stephenson, David A. John S. Metal cutting theory and practice 2 nd. ed. Taylor & Francis Group, LLC, [2] Yusuf A. Metal cutting mechanics. Machine tool vibrations, and cnc. Cambridge University Press, New York, [3] Elyasberg M. E. Self-oscillations of machine tools. Theory and practice. St. Petersbourg. Univ. OKBS, [4] Murashkin S.L. Applied Nonlinear mechanics tools. L. Mashinostroenije, (in Russian) [5] Callister W. D. Fundamentals of Materials Science and Engineering. United States, Wiley, [6] Miklyaev P. G. Friedman J. B. The anisotropy of mechanical properties of metals. Moscow, Metallurgiya, [7] Vlado A. Lubarda. Elastoplasticity theory. CRC Press, p [8] Maksarov, V.V., Olt, J., Leemet, T. Increase in the stability of technological system with control of the process of cutting. INTERNATIONAL SCIENCE CONFERENCE OF MATERIAL SCIENCE AND MANUFACTURING TECHNOLOGY, June 2008, Prague, Czech Republic, pp ADDITIONAL DATA ABOUT AUTHORS 1) Marten Madissoo, MSc. Ph. D. Student. Jüri Olt, Professor Viacheslav Maksarov, Professor 2) Increasing the efficiency of external turning by using the multiple layer construction of the tool holder. 3) Marten Madissoo, MSc. Ph. D. Student Postal Address: Estonian University of Life Sciences Kreutzwaldi 1 Tartu 51014, Estonia. marten.madissoomu.ee Phone Jüri Olt, Professor and head of the department for agriculture and production techniques. Postal Address: Estonian University of Life Sciences Kreutzwaldi 1 Tartu 51014, Estonia. jyri.olt@emu.ee Phone Viacheslav Maksarov, Professor and head of the department of mechanical engineering of the National Mineral Resources University "Mining University". Sankt Peterburg, Russia 4) Corresponding Author: Marten Madissoo 186