Autonomous form measurement on machining centers for free-form surfaces

This research aims at developing a measurement technique on machining centers for 3D free-form contours. An autonomous measuring principle is proposed and a prototype measuring device applicable to a machining center has been produced. In the measuring device, a laser displacement detector in a narrow range, which directly detects the distance from a point on the measured surface to the reference position of the detector output, is put together with the movable part of a linear encoder on the nut of a ball screw. A stepping motor controls the laser detector position to keep the output at the central value of the detector measuring range by driving the ball screw. Both the motor and the fixed part of the linear encoder are placed on the device base. The linear encoder detects the moving displacement of the screw nut, i.e. the position change of the laser detector. By installing the base on the spindle of a machining center and moving the table along a plane perpendicular to the spindle, the laser detector can automatically follow the contour of a work piece set on the table and measure its form along a scanning line, simultaneously. The displacement of a measured point relative to the reference position of the linear encoder output on the spindle side is just equal to the sum of the outputs of the two sensors, i.e. the laser detector and the linear encoder. Moreover, a simple experimental approach to identifying the sensing direction errors for an assembled measuring device is developed. The results of some experiments are also shown, which sufficiently demonstrate the effectiveness of the proposed inspection method and error identification approach.