Investigation and Analysis of Chatter Vibration in Centerless Bar Turning Machine

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1 Investigation and Analysis of Chatter Vibration in Centerless Bar Turning Machine M. Girish Kumar* Prakash Vinod 1 P V Shashikumar 2 Central Manufacturing Technology Institute, Bangalore , *girishkumarm@cmti-india.net, 1 prakash.vinod@cmti-india.net, 2 pvs@cmti-india.net Abstract Chatter vibrations are present in almost all cutting operations and they are the major obstacles in achieving desired surface finish. Chatter vibrations are the self excited vibrations, which has an adverse effect on surface roughness, dimensional accuracy of the machined components. This paper investigates the occurrence of regenerative chatter vibrations in centerless bar turning operation. The root cause for the chatter marks on component was analysed as beat vibrations. The beat vibrations are minimized in centerless bar turning operation to overcome the chatter marks on the machined components. Keywords:centerless turning, chatter vibrations, beat vibration 1.Introduction The turning operation is the very basic machining operation in the manufacturing industry. In a turning process, three different types of mechanical vibrations are present due to a lack of dynamic stiffness/rigidity of the machine tool system comprising of the tool, tool holder, work piece and the machine tool itself. These are free, forced and self excited vibrations. Free vibrations are generated due to shock and forced vibrations are due to unbalance, misalignment, mechanical looseness and gear defects in the machine tools. The free and forced vibrations can be easily rectified. But self excited vibrations are complex in nature and difficult to understand. The self excited vibrations are of primary and secondary chatter. The primary chatter is caused by friction between the tool and work piece, thermomechanical effects or by mode coupling. Whereas, secondary chatter is caused by the regeneration of a wavy surface on the work piece. Regenerative vibration is the most destructive among all other vibrations [1]. Chatter vibrations will be the limitation of machining productivity, in which extensive studies were carried out on metal cutting processes as early as in the 1800s [2]. The mechanisms generating chatter were studied and the proposed cutting forces as a function of speed were explained [3]. The studies shown that the most important characteristic property of chatter vibration is that, it is not induced by external periodic forces, but rather that the forces which bring it into being and maintain it, are generated in the vibratory process (dynamic cutting process) itself. Chatter vibrations are due to instability in the cutting processes. It was observed that modulated chip thickness due to vibration, affects cutting forces dynamically, which in turn, increases vibration amplitudes yielding a process known as regenerative chatter [1]. In this paper, the root cause for the occurrence of chatter marks on the machined component in a centerless bar turning was eliminated with the aid of vibration analysis. The centreless bar turning machine is of twin spindle cutter head configuration and both the cutter head spindles are rotating in opposite direction with the speed difference of 100rpm. The bar is fed through the cutter head and the material is removed from the bar. The bar is not held between the centre as in conventional turning operation. The bar of continuous length can be turned in this machine. 2.Experimental Setup The experiments were carried out on the centreless twin spindle turning machine on SS306 work material. The bars of 15mm and 35mm diameter and length of 4 meter were used for the experiments. The turning experiments were conducted at one constant speed and feed rate for a particular diameter work piece. The CSI Machinery Analyzer 2130 and CSI Accelerometer A0760GP are used for the vibration measurement. The vibration was measured in both horizontal and vertical direction in radial direction on the both the cutter head. The accelerometer output was connected to the two channel machinery health analyser to obtain vibration spectrum. The vibration measurement and 707-1

Investigation and Analysis of Chatter Vibration in Centerless Bar Turning Machine dynamic balancing setup on centre less bar turning machine is shown in Figure-1.

Vibration Measurement and Analysis The vibration measurement was carried out when roughing and finishing cutter heads are rotating at 350rpm and 450rpm respectively in opposite direction.

2 Investigation and Analysis of Chatter Vibration in Centerless Bar Turning Machine dynamic balancing setup on centre less bar turning machine is shown in Figure-1. Figure 1 Vibration measurement and dynamic balancing setup 3. Vibration Measurement and Analysis The vibration measurement was carried out when roughing and finishing cutter heads are rotating at 350rpm and 450rpm respectively in opposite direction. The accelerometers are mounted on the spindle cutter head bearing housing in both vertical and horizontal direction. The accelerometer outputs were connected to the two channel of the machinery health analyzer. The time domain vibration analysis reveals the beat vibration pattern due to two cutter heads are rotating at closely spaced frequencies. The beat vibration is due to, as the two source of vibration amplitudes come in phase and go out of phase with one another. The frequency analysis shows predominant vibration amplitude at 1xRPM. In case of beat vibration phenomenon as shown in Figure-2, it is best to remove or reduce the one or both of the exciting forces by balancing. The high precision balancing was carried out for both the cutter heads (roughing and finishing) using the CSI Machinery Analyzer Figure 2 Beat Vibration Phenomenon 4. Results and Discussion The high precision dynamic balancing was carried out for the 15mm and 35mm diameter work piece twin cutter heads. Initially roughing cutter head was balanced independently without running the finishing cutter head in order to overcome the hunting phenomenon due to beat vibration. After balancing the roughing cutter head, the finishing cutter head was balanced independently without running the roughing cutter head. The twin cutter heads were balanced to a very minimum residual unbalance level. Before balancing, the unbalance induced vibration level was mm/sec and after balancing, the unbalance induced vibration level was reduced to mm/sec. The comparative vibration spectra of before and after balancing is depicted in Figure-3 Figure3 Comparative Vibration Spectra of Cutter Head Before and After Balancing The Figure-4 shows the comparison of surface roughness, cylindricity and roundness for 15mm diameter work piece before and after balancing of the cutter heads. Before balancing, the form accuracies like cylindricity of 102µm, roundness of 45.55µm and surface roughness values (Ra) of µm was measured. After balancing, the unbalance induced vibration level has reduced, which has resulted in improving the form accuracies like cylindricity 59.41µm, roundness of 14.93µm and surface roughness (Ra) of 0.34µm. The surface roughness of the component before and after balancing of the cutter head was measured using Form Talysurf roughness tester. The roughness of the component has reduced significantly after balancing of the cutter heads and hence, the elimination of the chatter marks on the machined surface

Figure5B Cylindricity of shaft (Ø15mm) After Balancing The roundness of the component was

256mm/sec) and after balancing (0.0034mm/sec) for 15mm diameter work piece.

heads was measured using ultra precision roundness tester (Talyrond 290) measuring instrument

3 Figure5B Cylindricity of shaft (Ø15mm) After Balancing The roundness of the component was measured at before and after balancing, the Figure-6A & 6B shows the roundness of the component before and after balancing. Figure4 Comparison of surface roughness, Cylindricity and Roundness before (0.256mm/sec) and after balancing (0.0034mm/sec) for 15mm diameter work piece. The cylindricity of the machined component (Ø15mm) before and after balancing of the cutter heads was measured using ultra precision roundness tester (Talyrond 290) measuring instrument and the obtained plot is depicted in Figure-5A & 5B. Figure6A Roundness of shaft (Ø 15mm) before balancing Figure5A Cylindricity of shaft (Ø15mm) Before Balancing Figure6BRoundness of shaft (Ø 15mm) after balancing 707-3

Investigation and Analysis of Chatter Vibration in Centerless Bar Turning Machine The Figure-7 shows the comparison of surface roughness, cylindricity and roundness for 35mm diameter work piece.

59µm was measured. After balancing, the unbalance induced vibration level has been reduced to 0.010mm/sec, which has resulted in improving the form accuracies like cylindricity 63.

4 Investigation and Analysis of Chatter Vibration in Centerless Bar Turning Machine The Figure-7 shows the comparison of surface roughness, cylindricity and roundness for 35mm diameter work piece. Before balancing, the unbalance induced vibration level was 0.147mm/sec and the form accuracies like cylindricity of µm, roundness of µm and surface roughness value (Ra) of 0.59µm was measured. After balancing, the unbalance induced vibration level has been reduced to 0.010mm/sec, which has resulted in improving the form accuracies like cylindricity 63.32µm, roundness of 10.71µm and surface roughness value (Ra) of 0.073µm. The roughness of the component has reduced significantly after balancing of the cutter heads. This is due to reduction in unbalance induced excitation forces resulting in elimination of beat vibration amplitude and hence, the elimination of the chatter marks on the machined surface. and the component surface finish (Ra) was 2.33µm. The chatter marks are not visible on the components after elimination of beat vibration phenomenon by high precision balancing of the cutter heads and the surface finish was improved to 0.34µm. Figure8 Surface Finish of the machined components Before and After balancing of 15mm diameter Bar Figure 7 Comparison of surface roughness, Cylindricity and Roundness before (0.147mm/sec) and after balancing (0.010mm/sec) for 35mm diameter work piece. The Figure-8 depicts machined components before and after balancing of 15mm diameter bar. The chatter marks on the components are visible with beat vibration Figure9 Surface Finish of the machined components Before and After balancing of 35mm diameter Bar The Figure-9 depicts machined components before and after balancing of 35mm diameter bar. The chatter marks on the components are visible with beat vibration and surface finish on the component was 0.596µm. The chatter marks are not visible on the components after elimination of beat vibration phenomenon by high precision balancing of the cutter heads and the surface finish was improved to 0.073µm. 5. Conclusion The chatter marks produced on the machined component in centerless bar turning operation was investigated through vibration analysis. The beat vibration was generated due to unbalance in the two 707-4

5 cutter heads rotating at closely spaced frequencies. This beat phenomenon was the root cause of chatter marks on the components. The beat vibration amplitudes are reduced by reducing the excitation force by means of high precision dynamic balancing of the cutter heads resulting in elimination of chatter marks on the machined components in centreless bar turning operation. References 1. M Siddhpura and R Paurobally (2013), Experimental Investigation of Chatter Vibrations in Facing and Turning Processes, World of academy of science, Engineering and Technology. Volume: 7 2. F Taylor (1907),On the art of cutting metalstransactions of ASME, R N Arnold (1946), The mechanism of tool vibration in the cutting of steel, Proceedings of the Institution of Mechanical Engineers, 154 (1946), pp Ivana Konacic (1998), The chatter vibrations in metal cutting-theoretical Approach, The scientific journal of FACTA Universitatis, Mechanical Engineering, Vol.1, No.5. pp Heisaburo Nakagawa, Yutaka Kurita, Keiji Ogawa, Yuji Sugiyama and Hideyasu Hasegawa (2008), Experimental Analysis of Chatter Vibration in End- Milling Using Laser Doppler Vibrometers, International Journal of Automation Technology, Vol.2, No

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