precision control for CNC machining

With the development of cnc cnc machine tool processing products, the contour precision control function for high-speed machining of cnc CNC machine tools is getting higher and higher; the representative two CNC systems are used in China, Siemens cnc machining center 840D sl and FANUC machining center 31 i, in order to reduce the contour error of high-speed machining, adds acceleration control and pre-interpolation acceleration control, speed feedforward channel control, contour precision control and RTCP five-axis control.
1. Pre-interpolation acceleration controlThe speed feedforward channel control is earlier than the pre-interpolation acceleration function. In high-speed feed, the speed feedforward channel control function is usually used for first-order network correction. In the case of linear interpolation, each feed axis follows the error (FollowingError) The difference is significantly reduced, but in surface machining, especially when the curvature changes greatly, the contour error is not significantly reduced. Although the feedforward channel is used for second-order network correction (acceleration feed channel Acceleration)Forward, NUM1060CNC system has this function), it will significantly reduce the contour error, but because the adjustment and optimization parameters are more difficult, the system is prone to vibration, so it is less used.The curvature of the machining path is large and a large contour error occurs. The key reason is the control characteristics of the acceleration of each feed axis and interpolator. Adjust the acceleration value of each feed axis and the path feed acceleration value or time constant.Significant control of the contour error; the former is called Acceleration afterinterpolation;The latter is called the pre-interpolation acceleration function. The post-interpolation acceleration function is generated by the interpolator according to the acceleration set value of each feed axis; otherwise, the higher the contour accuracy of the workpiece is required, the larger the acceleration value of each feed axis or the smaller the time constant. Higher acceleration requires high mechanical rigidity, and the inertia of each feed axis is small, which is difficult to achieve, and high-value acceleration can cause mechanical shock and vibration, especially when performing small line approach curve processing at high speed. Contrary to the fact that the contour machining accuracy under the control of the pre-interpolation acceleration function is inversely proportional to the path acceleration; the path acceleration can be reduced to obtain a higher contour machining accuracy; the machining time is longer, and the small line segment approach is performed at high speed. When the curve machining program is used, the set speed and the actual feed speed differ greatly. The combination of the two is currently recommended by most CNC systems. The acceleration values before and after interpolation are set according to the specific processing conditions, and the path corner speed is set reasonably.At corner), will get better processing results.Set the acceleration value and acceleration type before and after interpolation (Bell or Linear –ShapedAcceleration and path corner speed depend on three factors: contour machining accuracy, low vibration (Small Shock) and surface quality of the workpiece (Surface)Quality) and processing time. In high-speed machining, three elements are evaluated according to the requirements of the workpiece. Only the pre-interpolation bell acceleration function and the path corner transition contour error analysis are performed.Condition: X-Y plane machining program: N1 G01 G91X100 F10000; N2 Y100; acceleration type: bell path corner speed:F400 (400mm/min) setting feedrate: F10000 (10000mm/min) has the following three conditions:(1) Contour processing precision and processing time are preferred. Acceleration time constant before interpolation: 100ms, acceleration time constant after interpolation: 30ms, contour error of corner point: 0.015mm, block conversion time: 60ms.(2) Preferred processing time and workpiece surface quality. Acceleration time constant before interpolation: 100ms, acceleration time constant after interpolation: 60ms, contour error of corner point: 0.08mm, block conversion time: 70ms.(3) The contour processing accuracy and the surface quality of the workpiece are preferred. The acceleration time constant before interpolation is significantly increased, and the path corner velocity is significantly reduced. Path corner speed:F100 (400mm/min), acceleration time constant before interpolation: 400ms, acceleration time constant after interpolation: 30ms, contour error of corner point: 0.005mm, block conversion time: 200ms. According to the above analysis, the acceleration time constant after interpolation is minimized to increase the acceleration of each feed axis; then the acceleration time constant and path corner speed before interpolation are set according to the processing technical requirements.
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