Active Tailstock for Precise Alignment of Precision Forged Crankshafts during Grinding

Within the Collaborative Research Centre 489 at the Leibniz Universitaet Hannover a new and innovative process chain for the manufacture of crankshafts is being investigated. By burr-free and near-net-shaped precision forging the process chain can be significantly shortened. However, this new production process requires a precise workpiece alignment before the grinding process due to the characteristics of the new process chain. In this paper a new machine-integrated positioning system consisting of an optical measurement system (sensor) and an active tailstock (actuator) is presented. For the detection of positioning errors, the geometric elements of the crankshaft are measured by the machine integrated optical measurement system. An algorithm evaluates the geometry data and calculates an adjustment vector. This vector contains the correction of the eccentric and tilt error. The degree of freedom (DOF) of the pendulum stroke of the grinding machine will be used to correct the eccentric error. The tilt error of the crankshaft is corrected by a new active tailstock. This tailstock produces a counter-tilt during the grinding process. For this purpose, a dynamic drive of the tailstock center in two DOF as a function of the angular position has been realized by two new developed piezo-hydraulic linear drives (stroke 4 mm). The dynamics and positioning accuracy of the active tailstock were verified. Up to 10 Hz a positioning accuracy in the range of ±1.5 µm can be achieved by using an iterative learning control. Furthermore, active alignment tests during grinding were performed.