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Abstract
Machine tool manufacturers, aiming to develop their machines. and machine tool users, trying to choose the best machine tool for their work, both need a measure to check the performance of newly developed machine tool.
During cutting processes, dynamic instability of the cutting process, affects mainly the appearance of chatter. Chatter can lead to unacceptable surface finish of the machined parts, and in extreme cases, chatter can cause failure of the cutting tool or work-piece. Also, chatter represents the most detrimental vibrations to the machine tool.
Within the past decade, there has been a marked progress in the development of machining centres capable of spindle and slide speeds that are an order of magnitude higher than those available on conventional machining centres. While these new machining centres offer the possibility of much higher material removal rates, markedly improved surface finish and higher work-piece quality. Chatter is one of the most difficult phenomena to master in the milling process. which is a promising machining technology with high industrial relevance. high material removal rates, which are expected from modem machining centres.
The most obvious way to assess chatter performance is by actual cutting in order to record the working conditions at which the machine starts to chatter. When comparing machine tools. it is necessary to cover the whole range of cutting conditions. Consequently. this requires an excessive number of tests which will be further complicated by the variability of cutting conditions. This gives the way to the modal analysis to be used for testing machine tools through which the variables associated with the cutting process are eliminated.
This thesis represents a step towards the main goal of predicting and preventing chatter occurrence in CNC vertical machining centre by means of experimental modal analysis. The main objective of this thesis was to correlate the modal parameters of the Chen Ho CNC Vertical Machining Centre (Type MCV - 600) to chatter occurrence at different positions of the machine tool table and spindle. This objective was achieved by applying a model correlating chatter occurrence to the machine tool modal parameters.
Experimental modal analysis of the system was conducted at different positions of the machine tool table and spindle in order to get the modal parameters of the machine tool. The analysis of the experimental results was held using several techniques, and a model correlating the modal parameters of the machine tool at different positions of the machine tool table and spindle. was determined. With the aid of artificial neural networks (ANN). the modal parameters of the machine tool were predicted at various positions of the machine tool table and spindle other than the positions measured during the experimental work, and then chatter occurrence was predicted when cutting a specific material.
As chatter represents the most detrimental vibrations to the machine tool. in addition to its unacceptable effect on machined parts’ quality, chatter has received considerable attention from both academia and industry. Two suggested procedures to predict chatter occurrence were presented; they are ”The Asymptotic Stability Margin” and ”The Stability Limit”.
Both procedures were presented when simulating the cutting conditions of milling: ”St 50” and ”St 70”. The two suggested procedures of predicting chatter occurrence are applied when testing a new machine tool.