Abstract: This paper introduces the working principle of a dedicated four-axis linkage CNC gear planer and the hardware and software structure of its dedicated CNC system. The gear planer can efficiently and accurately complete the machining of special-tooth bevel gears designed according to special requirements. Keywords: Gear planer; CNC system; Modular structure 0 Introduction For bevel gears with special tooth profiles designed according to special requirements (such as variable transmission ratio differential gears), it is impossible to complete the machining of their tooth profile using equipment that generally processes spur gears or ordinary bevel gears. If manual indexing is used on a profile milling machine, it is time-consuming, labor-intensive, and has a high labor intensity. Our developed four-axis linkage CNC gear planer can automatically complete the entire process of indexing, feeding, and generating special-tooth bevel gears. 1 Structure and Working Principle of the Four-Axis Linkage Gear Planer The four-axis linkage gear planer consists of four main parts: the machine tool body, two-axis tool holders, two-axis worktables, control hardware, and control software, as shown in Figure 1. The machine tool body adopts the bed of a traditional ordinary shaper (BA6050). The main motor drives the ram, and the ram drives the tool post to make continuous reciprocating linear motion along the horizontal axis, i.e., the Z-axis, to achieve cutting of the workpiece. Its main motion principle of planing is not much different from that of an ordinary shaper. The two-axis worktable is one of the key mechanical components for machining special tooth profiles. A horizontally rotating sector-shaped turntable is mounted on the worktable base. A servo motor controls the turntable to rotate around the vertical axis (i.e., the Y-axis), called the rotary axis B. An adjustable pitch bracket is fixed on the sector-shaped turntable, and an adjustable tilt workpiece head is fixed on the bracket. The workpiece is clamped on the workpiece spindle, and the servo motor controls the workpiece spindle to drive the workpiece to rotate around the workpiece's central axis, called the rotary axis C. A simplified diagram of the worktable structure is shown in Figure 2. The two-axis tool post is mounted at the end of the shaper ram and consists of a cross structure formed by the linear motion of the X-axis and Y-axis. The tool holder consists of two slides. The rear slide is fixed to the end of the planer ram. The middle slide can move along the guide rail on the rear slide in the Y-axis direction, and the front slide can move along the guide rail on the middle slide in the X-axis direction. The X-axis feed distance is shorter, therefore, the X-axis is in front and the Y-axis is behind. The planer tool is mounted on the front slide and has a tool retraction and lifting mechanism. Its simplified structure is shown in Figure 3. During operation, the X and Y axes mainly perform horizontal movement and depth and indexing retraction feed movements. They have relatively small mass, and their movement direction is orthogonal to the main cutting force, requiring relatively small driving force. However, since they are mounted at the sliding end of the planer, the structure must be as simple and compact as possible, and the weight as light as possible. Therefore, a small SGMPIEA31 servo motor and servo system from Yaskawa Electric Corporation of Japan are used. To improve transmission accuracy and efficiency, reduce friction, eliminate axial backlash, and improve system rigidity, a rolling guide pair and a rolling lead screw pair structure are adopted. The connection between the motor shaft and the lead screw shaft uses a semi-rigid coupling. When machining a workpiece, firstly, the pitch angle of the workpiece is manually adjusted according to the root cone angle of the workpiece to be machined; secondly, the planer tool mounted on the tool holder is precisely set using a tool setting fixture mounted on the workpiece fixture axis. Then, after starting the CNC gear planer, no manual intervention is required. The CNC system controls the linkage of the tool holder and the worktable axes to perform envelope machining on the bevel gear tooth profile until the bevel gear is machined. 2. Hardware Structure of the CNC System Due to the abundant software resources of PCs, general-purpose components can be used, fully utilizing the technology of general-purpose microcomputers, and the development of CNC equipment can be completed at a lower cost and in a shorter time. The CNC system developed by our institute adopts a modular structure scheme with direct card insertion on the 16-bit ISA industrial bus of the PC. The position servo controller shares a set of computer hardware resources with the CNC system hardware. The position control algorithm is embedded as an interrupt service routine into the main program of the CNC system, resulting in a simple and compact structure. Figure 4 shows the hardware block diagram of the CNC gear planer control system. The hardware system mainly consists of a main control computer (containing a position controller), a position servo control board, a servo drive module, a servo motor, and an encoder. The main control computer is a 586 general-purpose microcomputer, and the actual digital controller is implemented by position control software. The worktable has two rotary axes, B-axis and C-axis, each driven by two servo motors. The B-axis motor drives the entire worktable rotation, with a large load inertia, while the C-axis motor only drives the workpiece axis, with a lighter load; therefore, two motors with specific brushes were selected. The tool holder has two moving axes, X-axis and Y-axis, and the loads on both axes are relatively small and not significantly different. Therefore, two small AC motors with the same characteristics were selected to fully utilize the characteristics of each motor and to well meet the mechanical performance requirements. During system operation, the timer requests an interrupt from the main control computer every 5ms. The DOS platform boots and enters the interrupt service routine. After processing by the position control module, the speed setpoint values ​​of each control axis are output and sent to the position servo control board via the SIA bus. After being converted into control voltages corresponding to the motor speeds by a high-precision D/A converter, the control signals drive the motors after passing through the servo drive. The rotation of each axis motor is then transmitted to the worktable and tool holder through their respective transmission chains, causing the workpiece and the planer tool to move along a predetermined trajectory. The photoelectric encoders used for position detection are installed on the output shafts of each motor and obtain position and speed feedback through the computer position control module. Due to the actual working conditions of the CNC gear planer, there are often many high-power equipment, which will generate strong power interference and electromagnetic noise during startup and operation. In order to ensure that the system can work reliably, a series of measures to improve reliability have been taken. Among them, the main ones are: (1) In order to prevent interference from the power supply system, in addition to AC voltage regulators, surge protectors, super isolation transformers, low-pass filters, etc. are also used in the power supply line. In addition, the power supply of each functional module of the servo system is distributed and independent to reduce the mutual coupling of common impedance and the mutual coupling of common power supply. (2) The weak current part of the entire servo CNC system is covered with metal wire mesh and installed in an iron chassis to shield electromagnetic radiation. (3) Differential method is used to suppress common mode noise. (4) The analog ground and digital ground of all devices in the system are separated, the ground wires of power circuits and signal circuits are separated, and the area of ​​the ground wire of linear circuits is increased as much as possible. The adoption of these measures has greatly improved the reliability of the system. 3. Software Structure of the Control System Based on the machining characteristics of the CNC gear planer, DOS is used as the working platform, and a modular structure is adopted. The working time of a single CPU is shared, and a certain time slice is allocated to each task, allowing each task to complete its work within its own time slice. In this system, time slice allocation is completed by interrupts, and all programming is done in Turbo C++ language. The system control software structure is shown in Figure 5. The main loop program calls different module subroutines according to the instructions from the operating console. The two most important subroutines are the manual operation subroutine and the program machining subroutine, which respectively realize the manual operation and automatic operation of the machining process. Manual operation is essential in operations such as tool setting, fine adjustment, and transmission clearance measurement. Automatic operation (program-controlled machining) completes the machining of the workpiece after clamping and tool setting. Considering the real-time problem, the position control algorithm applies the neural network adaptive complex table control algorithm. In other words, after a period of operation, the neurons will determine the best weights through self-learning. Before each machine tool operation is completed, the latest weights are stored in the disk. The next time the machine is started, the previous weights are used as the initial values. If the CNC system parameters do not change much between two starts, such weight values ​​can always achieve good results. 4 Conclusion The four-axis linkage CNC gear planer developed has been tested and put into production. A batch of bevel gears with special tooth profiles used on variable transmission ratio differentials have been processed. The precision of the parts is satisfactory. References [1] Jiang Ruiquan, Wang Xiaochun • Research on two-axis linkage position servo system • Combined machine tool and automated machining technology, 1998 (6): 33-35 [2] Liao Xiaohuang, Zhu Qiqiu • Digital control machine tool • Wuhan: Huazhong University of Science and Technology Press, 1996