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Mechanics and dynamics of serrated end mills Merdol, Sukru Doruk

Abstract

Roughing, semi-finishing and finish milling operations constitute most significant portion in manufacturing aerospace parts. Thermally resistant materials, such as Titanium and Nickel alloys are rough milled using end mills with serrated or wavy profiles on their cutting edges. The serrations must be designed to have a phase shift from one flute to another in order to prevent unstable, self excited chatter vibrations during machining. Finish milling is usually required in peripheral milling of thin aircraft webs with long end mills where the structures are flexible and radial depth of cuts are small. The spindle speed and depth of cut must be selected optimally to avoid both forced and chatter vibrations in order to produce the parts within specified tolerances. The thesis presents mathematical model of serrated cutter profiles and their influence on chip load, force, torque, power and vibrations. The waves ground on the flutes are represented mathematically by cubic splines. The chip load distribution is predicted by integrating spline representation of flutes and previously developed kinematic model of dynamic milling which considers the structural vibrations. The cutting forces, torque, power, vibrations and surface form errors are predicted using time domain solution of the process. The chatter vibration stability is studied both in time and frequency domain when the radial immersion is small in milling operations. The physics behind the use of average and multiple harmonics of directional coefficients are explained in frequency domain solution of stability lobes. It is shown that multiple harmonics of the directional factors predict extra stability lobes at odd multiples of half tooth passing frequencies. The analytical solution favorably is verified by exact, time domain solution of the dynamic problem. The solution is also compared against specially designed milling experiments.

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