The Compensation Technique For The Deformation Error Of NC Machine Tool Ram

In the machine tool with a ram structure, when the ram reaches out it becomes a cantilever beam. Under the influence of the gravity of the ram and the spindle or some other factors such as cutting force and so on, the ram will bend down, especially at the most front end, deviating from the ideal horizontal position badly and in the end it can cause displacement error and angle error of spindle and cutting tool. Also, the sagging deflection increases nonlinearly with the extending of the ram. In addition, the increasing fit clearance and asymmetrical wear of the ram's slide rail and slide block can aggravate the ram's bending deflection in different extent. In this paper, a method of real-time compensation for ram's downward bending deflection is designed, to decrease the process error of the machine tool by the ram's extending and sagging and improve the machining accuracy of machine tool.Based on the material mechanics and finite element method, the model of ram's cantilever structure is built, and the most sagging deflection is calculated used this model. The calculated results coincide with the practical situation of machining process very well, which can provide a good basis for the design of compensation method in the latter chapters.The stress characteristic of slide rail and slide block is studied when ram reaches out, and the influence of shifting the ram's center of gravity upon stress variation and slide rail's wear is also analyzed.A real-time compensation method for ram's bending deflection is designed. The ram's bending deflection is detected by laser emitter and one-dimensional PSD position sensor, and the flow of hydraulic valve is controlled by the pulsing signal emitted by PLC systems. By regulating the drawing force of hydraulic cylinder and rod, the ram can have some upward bending deformations, the downward deflection of ram will be compensated as a result.With finite-element analysis software, ram's sagging deflection and stress distribution are simulated and analyzed, and the deformation amount before and after compensation is compared. The simulation results of this method demonstrate that the designed compensation method gains a significant effect. In additional, the modal analysis of the before and after compensated ram are accomplished, and the according to the results from these analysis, the compensated system installed in the ram can not destroy the strength of the ram.