Studies on Design of Spindle-tool System and Their Effects on Overall Milling Process Stability

Abstract

High speed machining using vertical CNC milling centres continues to be a popular approach in a variety of industries including aerospace,automobile,mould and die casting etc.Chatter oscillations have significant influence in restricting the metal removal rates of the machining process.The cutting process instability or chatter is assessed by prediction of frequency response at the tool tip.Present work aims at evaluating the combined effect of a spindle-housing and tool holder on the dynamics of cutting tool by considering the flexibility of spindle unit supported on bearings.The spindle-tool is analysed by using finite element modeling using Timoshenko beam theory.The dynamic characteristics and tool-tip frequency responses are obtained without considering the cutting forces.The results are compared with receptance coupling approach and using 3D modeling in ANSYS.Further experimental modal analysis on the machining spindle of same dimensions has revealed the same dynamic modes.Using the validated FE model of the system,the effects of nonlinear bearing contact forces,spindle-tool holder interface stiffness,bearing span and axial preload, tool overhang and diameter on the frequency response and cutting process stability are studied.Optimal spindle-tool system is designed for achieving maximum dynamic stiffness.The analytically stability lobe diagrams are obtained from the real and imaginary terms of these frequency responses at the tool tip.Dynamic stability issues in helical end-milling using the two and three dimensional cutting force models are considered for the analysis.The stability boundaries are experimentally verified using the cutting tests on both CNC milling spindle and modified drilling tool spindle systems while machining Al-alloy work pieces.Vibration and sound pressure levels are also employed to assure the stability of cutting operations,while surface images are used to identify the chatter marks at various combinations of cutting parameters.Dynamic milling model is employed with the flexible spindle-tool system by considering several effects including variable tool pitch, tool run-out,nonlinear feed forces and process damping. Design and stability studies on the modified drill spindle with a custom-designed work table for milling operations allowed in understanding several interesting facts about spindle-tool systems. Some control strategies including semi-active and active methods are implemented using finite element model of the spindle-tool system to minimize the chatter vibration levels/maximize the stable depth of cut during cutting operations.