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12ENGINEERING SCIENCES
THE DEVICE FOR SELF-EXCITED VIBRATIONS INVESTIGATION IN TURNING OPERATIONS
Dr. of Sc. Tech., Prof. Vnukov Y. M., Kuchuhurov M. V., Chernovol N. M.
Ukraine, Zaporizhzhya, Zaporizhzhya National Technical University
Abstract. The construction of the device for self-excited vibrations researching in turning is presented. It allows to conduct the experimental investigations of all types of vibrations in turning. The original construction solution gives the opportunity to study the main reasons of self-excited vibrations occurring: regenerative effect and mode coupling. The tuning opportunities of the device significantly extend the abilities of it and provide wide range of changing the cutting conditions, static and dynamic characteristics of tool oscillation system, tool geometry and etc.
Keywords: Turning, self-excited vibrations, elastic element, regeneration, mode coupling.
The nature of self-excited vibration excitation and self-organizing in machining by cutting is extremely complex. There is still no clear understanding of mechanism of this phenomenon. The difficult of its researching is cutting process represents the complex of multiplicity of physical processes, which are realized in elastic motion conditions of complete machine - tool - detail system. Interactions of them and mutual influence under specific conditions can lead to excitation of strong vibrations, which have negative impact on machining basically: quick equipment wearing, tool life reducing and tool breakage, machined surface quality reducing, high level of noise and vibrations in working zone.
Self-excited vibrations - type of vibrations that occur in conditions of absence of external periodical perturbing forces. There is the set of theories and experimental investigations, which explain the reasons of its occuring. All of them can be summarized as follows:
- Initial excitation - leads to initial tool displacement from equilibrium position in absence of wavy track on cutting surface. The appearance of the initial self-excited vibrations may cause next reasons: decreasing characteristic of cutting force of cutting speed V [1], time lag between cutting force and uncut chip thickness changing and time lag between friction force on tool rake face and cutting force variation [2], variable hardness of machined material during tool infeed and outfeed in detail [3], random jolts due to imperfection of detail material structure and others;
- regenerative excitation - effect, that represents cutting process on wavy track, that is left on cutting surface at previous tool path, that leads to constant changing of actual uncut chip thickness and respectively leads to oscillations of cutting force, which acts on tool and detail [4];
- mode coupling in different directions of tool or (and) detail, that leads to the last ones make vibration displacement within close-loop trajectory. The area of it is proportional to energy of self-excited vibrations excitation. In this case the connection of cutting force direction and main stiffness axes of partial detail or (and) tool elastic systems are considered [5].
Regenerative effect and mode coupling have proved experimental confirmation, but the others earlier theories generally base on hypothetical authors imagination. Current researches [6] of vibration processes in cutting basically are based on modeling, where static and dynamic characteristics of dominant oscillating system (tool and detail) are determined and used in mathematical models without conducting real cutting process. These algorithms allow to predict the level of vibrations in current modes. The main result of modeling is building of stability lobe diagram [7], according to which the cutting speed zones of intolerable vibrations are determined at specified depth of cut. The main lack of this approach is that the developing mathematical models don't take into account all complexity and versatility of cutting process in dynamic. Therefore obtained results have good confirmation with experiment only in narrow range of modeling cutting conditions and require corrections all the time.
Mathematical modeling allows to reduce the time of optimal combination of cutting condition determination, however, the accuracy of obtained results depends on proper representation of self-excited vibrations excitation and elimination mechanisms, that are basis of computational model.
The new knowledge about technological processes can be obtained exclusively by experimental way. Modern evolution of sensors equipment, the ability of analog-digital signal
conversion of high-speed processes, and also the technologies of data acquisition and recording of unlimited amount of data, allow to conduct the experimental researches of vibration processes in cutting processes on high level.
Fig. 1 presents the scheme of dynamic cutting processes in main section plane Ft. The authors of this paper believe, that the reason of tool displacement from the condition of force equilibrium state is cutting force F, playing the role of exciting force, and tool elastic system (ES) reaction R is as restoring force. Therefore, two joint zones should be considered at the same time: cutting zone, in which the chipformation processes and friction on contact tool surfaces occur; and zone of elastic properties manifestation of tool ES. If the conditions of stiffness of detail ES is many times higher (making the assumption that it is rigid) of tool ES can be created, the appearance of any tool vibrations in X axis direction will lead to appearance of waviness on cutting surface, and as the consequence to regenerative effect appearance. Thus the appearance of waviness on cutting surface should be considered as the result of self-excited vibrations occurring and as the source of energy supply of them. The height and period of waves on cutting surface in all modern experimental investigations are the main source of information about self-excited vibrations occurring. Cutting surface is formed by cutting edge and next two motions: detail rotation and feed displacement. The appearance of waviness on cutting surface is connected to tool edge displacement within the height of wave - 2A in direction, that is perpendicular to cutting surface. That is why it is usual practice to consider tool ES only with one degree of freedom (along X or Z axes). In our opinion such approach methodically is wrong, since it does not allow to consider the physical phenomenon in cutting zone with taking into account different orientation of main stiffness axes and different values of static and dynamic characteristics of tool ES.
Cutting surface at current detail revolution
Fig. L Fhe scheme of cutting process in main section plane Pt: KX, KZ, CX, CZ - stiffness and damping parameters of tool along X and Z axes; m - mass of tool structure system; So - feed movement; V - cutting speed; ho - nominal uncut chip thickness; Al - phase shift of waves within two successive revolutions on cutting surface; li, li+1, li+2 - the length of waves on cutting surface; F - cutting force; R - reaction of tool elastic system
Therefore, the next requirements to developing device for self-excited vibrations researching in turning can be formulated as:
1) The device should model tool ES in wide range of static and dynamic characteristics changing.
2) The device should allow to change the orientation of main stiffness axes of tool ES for establishing the connections between mode coupling and regeneration.
3) The device should have the displacement sensors, which allow to measure the actual value of cutting edge displacement from the equilibrium position in Z and X axes directions.
On fig. 2 the construction of developed by authors device for self-excited vibrations researching in turning [9] is presented. The approach, technique of vibrations measuring and phase shift of regenerative excitation calculation are described in authors paper [9].
J f, 5
Fig. 2. The construction of device for self-excited vibration researching in turning: 1 - body; 2 - elastic element; 3 - cutter head; 4 - prisms; 5 - cover, 6 - main screws;7 - insert; 8 - shim; 9 - holder; 10 - horizontal tool displacement sensor; 11 - vertical tool displacement sensor; 12 - damper
The device consists on body 1, that has special box-like shape. It allows to mount it on tool holder of any turning machine. Tool elastic element 2 is mounted in body 1 and is based on prisms 4. Tool clamping is carried by cover 5 and main screws 6. Measuring of tool displacement in horizontal (along X axis) and vertical (along Z axis) directions is carried with contactless eddy-current sensors 10 and 11 of Schneider Electric production, model XS4-P12AB110.
Cutting tool represents the assembly (fig. 3), which consists of cutter header 1, where the insert 3 is mounted, and elastic element 2 that plays the role of spring in tool ES. Insert 3 is based on cutter header 1 through shim 4 and holder 5. The insert is mounted in such way that the cutting edge is situated on the axis of elastic element 2. In combination with round cross-section of the last one, it allows to completely eliminate the influence of mode coupling and to study the regenerative excitation separately.
Fig. 3. Cutting tool with elastic element: 1 - cutter header; 2 - elastic element; 3 - insert; 4 - shim; 5 -
holder
The developed device is used in longitude turning of short detail, that has the dimension's ratio l / d < 1.5. It allows to neglect its elastic deformations and to present the cutting tool as dominant oscillation system, assuming that detail ES is rigid.
The machining is conducted by longitude turning according to the scheme, which is maximally closed to orthogonal cutting (fig. 4).
Fig. 4. Principal scheme of turning: Ds - feed displacement; Dr - detail rotation
It can be achieved by using the inserts with lead angle y = 90° and big end cutting edge angle. In this case the direction of horizontal tool vibrations coincides with longitude feed direction perpendicularly to the cutting surface. The last ones are measured by displacement sensors. Hence, the turning process can be considered as plane scheme in main cutting plane Pt (fig. 1). According to the presented scheme (fig. 4) longitudinal turning allows: - to neglect the vibrations along Y axis direction, since the part of cutting force Py has minimum value, and the tool has maximal stiffness in radial direction; so the dominant oscillation system is confined to the system with two degrees of freedom;
- to measure directly the regenerative part of tool vibrations in horizontal direction perpendicularly to the cutting surface;
- to provide the constant cutting speed V in longitudinal tool feed So comparing with radial turning that is widely spread in vibration researches according to the scheme of orthogonal rectangular cutting where the cutting speed is variable;
- to form the machined surface, which can be used for researching the influence of vibrations on the its roughness and topography;
- to conduct the tool life-time researches in different conditions of vibrations excitation.
The developed device has a number of features, which significantly extend its possibilities.
1. Since the dominant tool oscillation system has two degrees of freedom, there are two sensors are set up (fig. 2). Each of them measures the displacement of tool along X and Z axis through measuring the corresponding gaps AX and AZ between sensor and tool (fig. 1). The presence of two sensors allows to study full trajectory of tool tip motion, considering it as summary displacement, and as separate components along X and Z axes.
2. The insert is situated on the tool header in such way, that the tool tip and cutting edge are on the axis of elastic element, which has round cross-section (fig. 3). It allows to eliminate the appearance of oblique bending at unbalanced load of elastic element by cutting force P relatively to its cross-section, and also to eliminate cutting of machined surface by the flank face of tool. Furthermore, it completely eliminates the influence of mode coupling, so the experimental investigation of pure regeneration phenomenon can be conducted.
3. The tuning of tool elastic element stiffness is made through changing the length of it (fig. 5). There are two prisms, which can be moved in elastic element axis direction, changing their position and fixing it by screws. The actual tool length lact can be tuned in range lmin - lmax.
4. The developed device has the ability to change the elastic element. Standard configuration includes the last one with round cross-section (fig. 6a). It has equal stiffness in all direction of cutting force acting and, as it was said, it allows to eliminate the influence of mode coupling and to study the regenerative effect separately.
However, the elastic element with non-round cross-section (fig. 6b) also can be mounted. It has axes of minimum (X;) and maximum (X2) stiffness, so the elastic properties of it are similar to cutters with rectangular shank. It significantly extends the possibilities of the device, since it gives the opportunities to study the mode coupling effect, changing the parameters of stiffness K1, K2 and the direction of main stiffness axes through changing a angle.
Fig. 5. Tool elastic element length tuning: lmin, lmax, lact - minimum, maximum and actual length of tool ES
a) b)
Fig. 6 - Scheme of tool elastic system at different types of elastic elements: a) with round cross-section; b) with non-round cross-section: X1, X2 - main stiffness axes directions, K1, K2, C1, C2 - stiffness and damping along X1 and X2 axes, m - lumped mass of cutter, a - rotation angle between elastic element cross-section and global coordinate system, P - cutting force, ft - angle of cutting force action
5. The box-like and demountable construction of device allows to fill the space between the tool elastic element and inner walls of device with different damping substances, changinig the dapming propertices of tool ES.
REFERENCES
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4. Kai Cheng. Machining Dynamics. Fundamental, applications and practies / Cheng Kai. -Springer Series in Advanced Manufacturing. Advanced Manufacturing & Enterprice Engineering Department School of Engineering and Design. Brunel University. Middlesex UB8 3PH. UK, 2009.
5. Tlusty J. Self-excited vibrations in machine-tools / J. Tlusty // Translation from Czech. Mashgiz, Moscow, 1956.
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7. Altintas. Y. Identification of dynamic cutting force coefficients and chatter stability with process damping / Y. Altintas, M. Eynian, H. Onozuka - CIRP Annals - Manufacturing Technology 57 (2008) / 371-374
8. Vnukov Y.M., Djadja S.I., Kuchuhurov M.V., Kondratyuk E.V. The device for self-excited vibrations researching in turning, Ukrainian Patent UA101906. (2015).
9. Vnukov Y.M., Kuchugurov M.V, Djadja S.I. and others: Method and device for researching the regenerative self-excited vibrations when turning / Cutting & Tool in technological system, International Scientific-Technical Collection, Kharkov, 2013. - Vol. 83, p.42-54.
