introduction

With the continuous development of mechanical manufacturing technology, the machine tool industry has also transitioned from the traditional machine tools to the replacement of CNC machine tools and has developed rapidly. The popularity of CNC machine tools has increased year by year, mainly due to the superiority of CNC technology. CNC technology is developed for the production of defense industry such as aviation, shipbuilding, Ningzhou flight, weapon production, etc. It is especially suitable for the production of small and medium-sized mechanical parts with high processing precision, complex geometry, large size and frequent modification. Research abroad from the late 1940s, with the development of transistor integrated circuits and computing technology, began to be used in production in the late 1950s and early 1960s, and more and more popularized and applied. As far as the technical level and economic development of China's current manufacturing industry are concerned, economical CNC machine tools are more suitable for use in Chinese enterprises and related industries. Currently, the occupancy rate of such machine tools is relatively high, and most of them belong to open-loop or semi-closed loop control systems. The machining accuracy is greatly affected by the mechanical precision of the machine tool, so solving the machining error caused by the mechanical clearance is an important part of ensuring the processing quality.

Analysis of clearance error of CNC machine tools

Gap error

The mechanical clearance error of CNC machine tools refers to the comprehensive error caused by the mechanical clearance from the head end of the kinematic chain of the machine tool to the whole process of the actuator, as shown in Figure 1. The feed chain of the machine tool is derived from the gap between the motor shaft and the pinion due to the keying, the clearance of the gear pair, the gap caused by the key connection between the gear and the screw, the gap caused by the key connection in the coupling, and the lead screw. Nut clearance, etc. The backlash error of the machine tool refers to the error of the movement amount of the actuator and the theoretical value (programmed value) when the machine tool actuator changes from the forward motion to the reverse motion due to the existence of the mechanical clearance in the machine tool transmission chain. Finally, it is reflected as the error of the machining accuracy superimposed on the workpiece. When the CNC machine table is reversing in its direction of motion, the servo motor is idling due to the existence of the backlash and the table is not physically moved. This is called a loss. For example, in g01 cutting motion, the reverse deviation will affect the accuracy of the interpolation motion. If the deviation is too large, it will cause the situation that the circle is not round enough and not square enough. In the rapid positioning motion of goo, the reverse deviation affects the machine tool. The positioning accuracy is such that the positional accuracy between the holes during drilling, drilling and the like is reduced. If the backlash has a small value and does not affect the machining accuracy, no measures are required. If the value is large, the stability of the system is significantly reduced, and the machining accuracy is significantly reduced. Especially the curve processing will affect the size. Tolerance and curve consistency, the backlash must be measured and compensated. Especially for CNC machine tools with semi-closed loop control, the backlash will affect the positioning accuracy and repeat positioning accuracy. This requires us to pay attention to and study the factors, influences and compensation functions of the backlash when using CNC machine tools. In the study and practice, it is concluded that some regular errors in the backlash automatic compensation process are found, and appropriate processing measures are taken to improve the machining accuracy of the parts.

Figure 1 Principle of the formation of the backlash

Figure 2 Backlash measurement interface

Gap error measurement

In order to study the influence of backlash error on machining, we use a small three-dimensional coordinate teaching and training platform. This platform integrates multi-axis motion controllers, motors and their drives, electrical control boxes, motion platforms and more. The mechanical device is a modular cross working platform that uses a ball screw drive to achieve the target trajectory and motion. The actuator adopts a stepping motor, and the control device is composed of a PC, a dsp closed-loop motion control card and a corresponding driver. The motion control card accepts the position and track command issued by the PC, performs planning processing, converts it into an instruction format acceptable to the servo drive, sends it to the servo driver, processes and amplifies it by the servo driver, and outputs it to the executing device.

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