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This paper iticludes the findings of an experitnental study on instabilities of the chip formation process during end milling ofTi6Al4V alloy and the infiuence of these instabilities on chatter formation. It has been identified that the chip formation process has a discrete nature, associated with the periodic shearing process during machining. The chip formed during machining of titanium alloy TÍ6A14V is found to be mainly with primary serrated teeth appearing in the main body of the chip. Secondary .serrated teeth resulting from the coagulation of a certctin number of primary serrated teeth also happen to appear at the free or constrained edge of the chip, especially when the .nstetn enters into chatter. In order to identify the interaction of these chip instabilities with the prominent natural vibration of the machine tools system components, the different mode frequencies of the vibrating components of the system have been identified using experimental atid finite eletnent modal atialyses, and vibration responses during actual cutting have also been recorded using an online vibration tnonitoring system. The vibration signals in frequency domain (fast Fourier transform) have been atiahzed to identify the chatter frequencies and the peak amplitude values. Chatter was found to occur at two domitiant mode frequencies of the spindle. These mode frequencies at which chatter occurred have been compared with the chip serration frequencies in a wide cutting speed range for different conditions of cutting. It has been concluded from these findings that chatter occurs during end milling due to the resonance of the machine tools system component when the frequeticy of primary .serrated teeth formation is approximately equal to the "prominent natural frequency" modes of the system components, which are the two mode frequencies of the VMC machine spitidle in this particular case.

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In the present work a review of the existing theories of chatter formation has been conducted and the weaknesses of the most widely accepted ‘Regenerative Chatter theory’ in explaining various phenomena related to chatter formation have been identified. An attempt has been made in this work to determine the common causes of chatter formation in different metal cutting operations, namely, turning, thread cutting and end milling conducted on plain carbon steel AISI 1040. Experimental investigations have been conducted during the above types of machining processes to identify the marks of instability and chatter on the formed chips. It has been identified that in all the three machining operations the chips formed show a common type of discreteness in the form of secondary saw teeth, which appear at the free edge of the chip. Mechanism of formation of these teeth has been studied and the frequencies of their formation have been determined for different cutting conditions. Apart from the secondary saw teeth primary saw teeth have also been identified at the main section of the chip and their frequencies were also determined. At the same time the natural vibrations of the main system components have been identified and the acceleration amplitudes at the prominent natural frequencies during actual machining were recoded using a dedicated vibration monitoring system. The frequencies of secondary chip serration and the natural frequencies of the system components were plotted against cutting speed. Acceleration amplitudes at the prominent natural frequencies were also plotted separately against cutting force. Based on comparison and analysis of these two frequency and amplitude graphs it was concluded that chatter (vibration with relatively high amplitude) appears in the system when the frequency of secondary saw teeth approaches values equal to half or integer multiple of a prominent natural frequency of the system resulting in resonance. In the case of thread cutting and turning the main vibrating component was identified as the tool holder and in the case of end milling the main vibrating components were the tool holder and the spindle. Severe vibration/chatter was found to appear during end milling when the tool and the spindle simultaneously entered into resonance. This occurred when the chip serration frequency got close to the spindle’s natural frequency, which was approximately twice the value of the natural frequency of the tool holder in the conducted experiments.

The paper presents a systematic procedure and details of the use of experimental and analytical modal analysis technique for structural dynamic evaluation processes of a vertical machining centre. The main results deal with assessment of the mode shape of the different components of the vertical machining centre. The simplified experimental modal analysis of different components of milling machine was carried out. This model of the different machine tool's structure is made by design software and analyzed by finite element simulation using ABAQUS software to extract the different theoretical mode shape of the components. The model is evaluated and corrected with experimental results by modal testing of the machine components in which the natural frequencies and the shape of vibration modes are analyzed. The analysis resulted in determination of the direction of the maximal compliance of a particular machine component.