Development Of An Active Changing Cutting Tool Angle Servo Apparatus For Free-form Surface Turning

Optical free-form surface components can improve the performance of optical systems andare attracting increasing attention, which have been widely used in LED lighting, green energy,medical, aerospace and many other fields. The diamond turning technology based on fast toolservo (FTS) is generally considered to be the most promising method of optical free-formsurface generation. Its application fields have been extended to optical free-form surfaceturning, non-circular surface machining and surface generation with micro/nanostructures.Actively changing the transient angles of cutting tool can improve machining performance,reduce the tool wear, extend tool life, reduce the cutting force fluctuations and enhance theprocessing stability and the surface quality of the workpiece, eliminating the flank interventionand over cutting. The tool transient angle change in the machining of the typical opticalfree-form surface that can be expressed analytically is calculated, the effect of tool pathtrajectory on which is considered.To reduce the dependence of the transient tool angles on surface profile characteristics, az-θxvariable tool angle FTS system with two degrees of freedom is developed, each motionaxis series, no crosstalk in theory, and the integration machining for the FTS apparatus canreduce assembly errors. The quasi-ring flexure hinge mechanisms driven by piezoelectric stackactuator is designed as the flexible rotating mechanism, which can enable the tool rotate alongthe axis which bypass the tool tip and parallel to machine tool x-axis with a range of±0.02°(about350μrad), and the resolutions of the angular displacements are1.2μrad (θx+) and0.6μrad (θx-) respectively. The z-direction working stroke is10μm with a resolution of10nm.Each motional axis has corresponding capacitive displacement sensor to detect thedisplacements as tracking feedbacks.The design of critical dimensions of the apparatus structure is optimized by the orthogonalexperiment method using the finite element software and the static and modal harmonic response characteristics are analyzed. Through data fitting, the motional axis stiffness can beobtained as: θx+rotational stiffness of0.02865N m/μrad, θx-rotational stiffness of0.1011N m/μrad, and z-axis stiffness of35.587N/μm. The first order resonance frequency is analyzedas1234Hz, and the measured one from the apparatus is240Hz. The rigid-flexible couplingbody dynamic simulation is conducted using Adams combining with Patran.On the basis, the strategy of actively changing tool transient angles and the implementationmethod of tool path are proposed, and the matching relationship between the tool path and thedrive output displacements is established. The performance of the developed variable angleservo apparatus is tested. Using the motion control card PMAC, by adjusting its built-in PIDparameters, the performance test of servo tracking is conducted.