Solutions for avoiding thermally induced dimensional deviations of workpieces have become more crucial than ever for the machine tool industry.
click here for more images The successful fulfi llment of manufacturing orders requires machine tools with high thermal stability. Machine accuracy must be maintained even under strongly varying load conditions. As a consequence, feed axes must achieve the required accuracy over the complete traverse range even with strongly varying speeds and machining forces. Thermal expansion in the recirculating ball screws of the linear feed axes adversely affects accuracy and varies depending on the velocity and load. Position errors of 100 Ám and more may result within 20 minutes during a machining operation if the slide position is only determined from the spindle pitch and a rotary encoder on the motor side. Because essential drive errors are not compensated in the control loop when this method is used, this is referred to as semiclosed-loop operation of the feed drive. These errors can be completely eliminated by using linear encoders. Feed drives with linear encoders are operated in closed-loop mode because errors in the recirculating ball screw are considered in position measurement and compensated in the position control loop. Angle encoders used on rotary axes provide similar benefits since the mechanical drive components are also subject to thermal expansion. Linear and angle encoders therefore ensure high precision of the components to be manufactured even under strongly varying operating conditions of the machine tools.