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1 Influence of Cutting Speed and type of Damping Insert on the Roughness of Surface Produced by Damper Inserted End Mill Cutter G.KRISHNA MOHANA RAO * Associate Professor in Mechanical Engineering, JNTUH College of Engineering, Hyderabad, AP, INDIA * Corresponding author: kmrgurram@gmail.com P.RAVI KUMAR Associate Professor in Mechanical Engineering, CMR College of Engg., Hyderabad, AP, INDIA rpanthangi@gmail.com SOUMYA, J. Post Graduate student, Department of Mech Engg., JNTUH College of Engineering, Hyderabad, AP, INDIA Jeripotulasoumya88@gmail.com Abstract:-Long end-mills, the most widely used tool in high-speed machining operations, undergo a bending vibration similar to a cantilevered structure during machining. High-speed milling productivity may be improved if the damping typically seen in cutting tools can be increased significantly. In the present study an attempt is made to understand the influence of cutting speed of an end mill cutter on the roughness of the surface produced on the workpiece. End mill cutters are inserted with different types of dampers in a centrally drilled hole in the body of the tool. The influence of cutting speed and nature of inserts on the surface roughness is studied. It is observed that the surface roughness is decreasing with increasing cutting speed. The use of end mill cutter with three fingered damper insert has resulted in better surface finish among all the cases of tools considered. Key Words: End milling; Vibration; Surface roughness; Cutting speed; Damper Insert; Number of Fingers 1. Introduction The demand for increased productivity, with good surface finish and better tool life, continually pushes manufacturing industries to find optimal combinations of machines and cutting tools. In order to reduce production time, it is expected that the machine tools are required to have a high-speed and feed rate. Long end-mills, the most widely used tool in high-speed machining operations, undergo a bending vibration similar to a cantilevered structure during machining. Beyond a certain depth, the vibrations may become unstable. This characteristic of the tool known as chatter creates an uneven surface, reducing the tool life and hence results in low material removal rate. Due to the internal and external forces, the machine or structure moves. This motion is the subject of dynamics and vibration. Authors still refer to vibrations as a limiting factor, one of the most important machining challenges and, of course, an aspect to be improved. If a tool can be designed with increased damping, it should offer enhanced stability against chatter. Increasing the damping will have the same effect on chatter stability as increasing the stiffness. Thus, high-speed milling productivity may be improved if the damping typically seen in cutting tools can be increased significantly. 2. Literature Study Researchers in the area of high-speed milling have implemented various chatter recognition techniques. ISBN:

2 Professor Jiri Tlusty[1] developed a method that detects chatter during machining, and in turn, suggests a new speed for the same depth. Cobb [2] found after testing, that impact dampers served better in controlling the vibrations. The types of impact dampers used were a spring/mass liquid impact damper and a tapered impact damper. He developed two types of dampers, one with end caps and mass sandwiched between them and the other, where a mass is slit half and bolted together. Smith[3], Keyvanmanesh[4], and Cheng[5] did an extensive research in understanding the dynamic characteristics of the tool and spindle to control chatter during machining. Keyvanmanesh[4] developed an algorithm to detect the chatter region based on the information obtained from the cutting tests on different end mills. Similar to the work done by Smith, he did a time domain simulation using the values obtained from the dynamic response of the tool to plot the stability lobes. By using the control system developed from the algorithm, he was able to locate the stable speed more clearly. Cook et al. [6] developed damping mechanisms to control vibrations on traffic signal structures. Traffic signal structures that are subjected to cyclic loading due to the wind and fast moving vehicles, sometimes, result in premature fatigue failures. They investigated this problem and proposed devices to provide damping to the structures. The research damper model was based on the work done by Slocum [7] on damping bending in beams. In his book, Slocum introduced the concept of friction damping between layered elements. The book explains that, when two cantilevered beams stacked on top of each other undergo bending there occurs a relative shear motion between the inner surfaces of the layered elements causing friction energy to be produced at the interface, which in turn, used to reduce the deflection of the layered beam. One of his patented works [8] implements this idea. He developed a method to damp bending vibrations in beams and similar structures. J.Tlusty, S.Smith and W.R.Winfough [9] studied two techniques using end mills in high-speed milling. Proportional Derivative fuzzy logic controller is designed and implemented by E. Soliman and F. Ismail [10] to suppress chatter in peripheral milling. The controller was successful in suppressing the chatter at certain operating points. F. Ismail and E.G. Kubica [11] proposed the maximum quantity of material that can be removed by the milling operation which is often limited by the stability of the cutting process, and not by the power available on the machine. Smith and Dilio[12] have described a control strategy for chatter suppression by adjusting the spindle speed to operate in high stability lobe. Experimentally, they achieved a remarkable increase in metal removal rates. Weck et al [13] attempted to assess the merits of using the spindle speed modulation and for that matter any other technique for chatter suppression, one needs to detect the onset chatter reliably. M. Liang, T.Yeap, A. Hermansyah [14] reported a fuzzy logic approach for chatter suppression in end milling processes. Vibration energy and the peak value of vibration frequency spectrum are jointly used as chatter indicators and inputs to the proposed fuzzy controller. Tlusty and Smith [15] suggested the use of stability lobes to select chatter free spindle speeds. Lin and Engelhardt et al[16] have been demonstrated the technique of spindle speed modulation to be very effective in suppressing chatter in milling at regular cutting speeds. Ralston and Ward [17] proposed the use of spindle speed modulation by adjusting the parameters on-line via feedback control. Zhu [18] developed a fuzzy logic control to control the surface finish by adjusting the feed rate in plunge grinding. Kosuke Nagaya, Jyoji Kobayasi, Katuhito Ima i [19] gave a method of micro-vibration control of milling machine heads by use of vibration absorber. An auto-tuning vibration absorber is presented in which the absorber creates anti-resonance state. Ziegert John C. Stanislaus Charles, Schmitz Tony L. Streling [20] Robert found that the limiting chatterfree depth of cut in milling is dependent on dynamic stiffness of the tool or spindle system. N.H.Kim, K.K.Choi, J.S.Chen and Y.H.Park [21] proposed a continuum-based shape design sensitivity formulation for a frictional contact problem with a rigid body using mesh less method. Tony L. Schmitz, John C. Ziegert, [22] Charles Stanislaus predicted that the stable cutting regions are a critical requirement for high-speed milling operations. Sridhar et al [23] presented the first detailed mathematical model with time varying cutting force coefficients. Budak and Altintas [24] derived the finite order characteristic equation for the stability analysis in milling. Recent investigation performed by Alauddin [25] has revealed that when the cutting speed is increased, productivity can be maximized, and surface quality can be improved. According to Hasegawa [26] surface finish can be characterized by various parameters such as average roughness (Ra), smoothening depth (Rp), root mean square (Rq), and maximum peak-to-valley height (R t). EI-Baradie [27] and Bandyopad [28] have shown that by increasing cutting speed, the productivity can be maximized, and the surface quality can be improved simultaneously. According to Gorlenko ISBN:

3 [29] and Thomas [30], surface finish can be characterized by various parameters. tools had an internal blind hole of 9.5mm diameter with a length of 105 mm. 3. Methodology In detecting, identifying,avoiding, preventing, reducing,controlling, or suppressing chatter a review of the great deal of literature regarding the chatter problem leads to a existing method,in which modifying certain machine tool elements to passively change the behaviour of the system composed of the machine tool, the cutting tool and tool holder. In this method, the design of the machine tool is changed to improve its performance against vibration or extra devices like friction dampers are used which can absorb extra energy or disrupt the regenerative effect. This is focused on ensuring chatter-free operations by using passive strategies to damp, reduce and control the phenomenon. To reduce the excessive vibrations of an end-mill cutter, a mechanical damper is introduced into a cylindrical hole in the centre of a standard end-milling cutter to dissipate chatter energy in the form of friction. An investigation is made to test surface roughness of workpiece with solid end milling cuter, hollow milling cutter and damper inserted milling cutters at various speeds ranging from 40 rpm to 960rpm at a feed of 29mm/rev and a depth of cut 0.5mm. The increased damping in end milling is achieved by hollowing the tool body and inserting a multi fingered damper into the center opening. The fingers on the damper insert are created by cutting axial slits along most of the length of a cylinder whose outer diameter matches the inner diameter of the tool body, thus forming multiple fingers. The cross sections of the dampers used in the present study are shown in figure 1. Fig.2: Hollow and solid milling cutter The blind hole allowed cutting edges to be ground on the end of the tool. Testing was performed on the solid and hollow end mills, and later the damper was inserted into the hollow tool, which was measured again. The damper insert had fingers and was constructed from a 9.5 mm diameter tungsten carbide blank. Hollow and solid Milling cutters used in the present study are shown in fig 2. The damper insert was slit down 76mm of its 105mm length by wire electro discharge machining in order to form the separate fingers. Figure 3 and 4 shows a photograph of an end view of a hollow tool blank with the damper insert inside. Fig.3: Hollow tool with one dampers Figure 1: Cross Sections of the Dampers 4. Experimental details To experimentally test the performance of the damper insert, two end mills are constructed. The tool is made of high-speed steel with 19.05mm outer diameter, 125mm length, and has 3 cutting flutes. Their external geometry was identical. One of the Fig.4: Hollow milling cutters with three dampers. ISBN:

4 As shown in fig.3 and fig.4 tools with different numbers of dampers are chosen, so that different interactions between independent variables could be effectively investigated. The diameter of the damper insert was such that the solid portion provided a light press fit into the tool body.the independent variables in the study are cutting speed, feed rate, depth of cut and the number of fingers of dampers. The last variable is introduced in to the experiment since it is suspected that the vibrations generated by varying the number of dampers could affect the resulting surface finish since the natural frequencies are modified. The dependent variable is the resulting first cut surface finish. The experiments were carried out in two stages. Initially the Aluminum material parts are machined using milling machine with end mill cutter as per the table 1. Then the surface roughness profile was investigated. The surface roughness values were recorded for at different cutting speeds using surface roughness tester SURFTEST SJ301 shown in figure5. Table 1: Experimental Details for Surface Roughness Analysis S.NO Speed(rp m) Feed (mm) Depth of cut(mm) Type insert of Solid Hollo w One Damper 31 0 Two finger Three finger Each experiment was repeated using a new cutting edge every time to obtain accurate reading of surface roughness. The work piece is 50mm in length, 12mm in width. The end milling cutter is of High Speed Steel (HSS). 5. Results and discussions The R a values obtained for different conditions of testing are given in table 2. It is observed that with increase in cutting speed surface roughness is getting reduced. Also, beyond 310 rpm the roughness is almost unchanged for all types of inserts. Cutting speed PRM Table.2: Ra values at different cutting speeds and different inserts SOLID HOLLO W HOLLO WITH ONE DAMPE R HOLLO W WITH TWO DAMPE RS HOLLOW WITH THREE DAMPER S Fig 6. Depicts the variation of surface roughness Ra with cutting speed. It shows the influence of number of damper inserts also. Among all the tools used, cutter with three fingered input resulted in better surface finish. This may be due to the increased area of friction surface which increases the amount of energy dissipation and hence reduced chatter and vibration.. Fig.5: Taly Surf SJ301 Fig.6. variation of Surface roughness Ra with cutting speed for different damping inserts. When the tool bends, the neutral surface of the outer cylindrical tool body is located on the tool centerline. ISBN:

5 However, the fingers will bend with their neutral surfaces passing through their own centroids. The net result is that the axial strain experienced on the outer surface of the fingers will be different than the axial strain experienced on the inner surface of the tool body, causing a relative sliding between them. When the tool rotates at high speed, large centrifugal forces press the fingers against the inner surface of the tool body. Press fitting causes an enhanced pressure between the parts that makes the effect of damper more sensible. This is because of increasing the amount of frictional force between the surfaces. The effect of press fit pressure seems to be much more than centrifugal effect. Wire electro discharge machining is used to form fingers of the damper. When the tool experiences bending vibrations, the fingers slide over the inner surface of the tool body. The relative sliding is proportional to the distance from the neutral axis of the tool to the neutral axis of the individual fingers. The frictional forces, which arise during this sliding of the fingers over the inner surface of the tool body, dissipate energy and produce damping. Of course, when the center section of a tool is removed, the stiffness will decrease. This stiffness loss must, at minimum, be compensated by increases in the damping ratio provided by the centrifugal damper. However, the stiffness loss is minimal for holes of reasonable size. For example, in the tools developed for this work, the diameter of the central hole is one-half of the outer diameters of the tool body, which reduces the bending stiffness of the tool by only 7%. 6. Conclusions The machinability of Aluminium when subjected to milling operation using high speed cutting steel tools are studied. Machining parameters, surface finish were investigated. The results indicated that the surface roughness decreases with increasing cutting speed. The damped tool outperformed both the solid and the hollow tool. Hence the overall performance of the damped tool with three damper inserts was exceptional compared to solid milling cutter, hollow milling cutter, hollow milling cutter with one and two damper inserts. The surface roughness for damper with 3 fingers is the lowest among all the cases of tools considered. REFERENCES [1] S. Smith, J. Tlusty, Efficient simulation programs for chatter in milling, CIRP Annals Manufacturing Technology 42 (1) (1993) [2] Cobb, W. T., 1989, Design of Dampers for Boring Bars and Spindle Extensions, Master s Thesis, Mechanical Engineering, University of Florida, Gainesville, FL. [3] Smith, Kevin Scott, 1985, Chatter, Forced Vibrations, And Accuracy In High- Speed Milling, Master s Thesis, Mechanical Engineering, University of Florida, Gainesville. [4] Keyvanmanesh, Amir, 1990, Evaluation of Chatter Detection and Control System, Master s Thesis, Mechanical Engineering, University of Florida, Gainesville, FL. [5] Cheng, E., 1992, A Chatter-Free Pocketing Routine For Any Two-And-A Half- Dimensional Pocket With Islands, Master s Thesis, Mechanical Engineering, University of Florida, Gainesville, FL. [6] Cook, R. A., Bloomquist, D., Richard, D. S., and Kalajian, M.A., 2001, Damping Of Cantilevered Traffic Signal Structures, Journal of Structural Engineering, Vol. 25, pp [7] Slocum, H. Alexander, 1992, Precision Machine Design, Society Of Manufacturing Engineers, Dearborn, MI. [8] Slocum, H. Alexander, Concord, N. H., Marsh, R. Eric, Smith, H. Douglas, 1998, Replicated In-Place Internal Viscous Shear Damper For Machine Structures And Components, U. S. Patent No. 5, 799, 924 (September 1, 1998). [9] J. Tlusty, K.S. Smith, W. Winfough, Techniques for the use of long slender end mills in high-speed machining, Annals of the CIRP 25 (1) (1996) [10] E. Soliman, F. Ismail, Chatter suppression by adaptive speed modulation, International Journal of Machine Tools & Manufacture 37 (3) (1997) [11] E.G. Kubica, F. Ismail, Active suppression of chatter in peripheral milling. 2. application of fuzzy control, International Journal of Advanced Manufacturing Technology 12 (4) (1996) [12] Delio T., Tlusty J., Smith S. (1992) Use of audio signals for chatter detection and control. ISBN:

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