Criteria of Ball Screws Selection for CNC Machines

Ball screws are used as elements of feed systems in CNC machine tools working with constantly higher cutting speed. Work concerns designing ball screws with regard of dynamic conditions of feed system. The method of calculation depends on the positioning system existing in the machine tool. There are two positioning system possible - indirect (semi-closed loop) and direct (closed loop). For the indirect system, the main criterion of the ball screws of choice is a suitable drive rigidity and for direct positioning system the diameter of the screw is determined by the axial resonance of the table-screw ensemble. To improve the positioning accuracy it requires a good balance in axial rigidities of composing parts of feed system. The geometric and dimensional errors must be also considered in calculation process especially in the hertzian deflection of ball/balltrack area. A coefficient known a the geometric correction factor was introduced in order to estimate the influence of machining errors on the rigidity lowering. This coefficient was compared to the correction factor of accuracy applied in ISO standard for estimating ball screws rigidity.