Abstract: Based on the analysis of the requirements for steel pipe cutting and end milling, this paper presents a system for automatic control of the steel pipe processing using a PLC, completing the hardware and software design of the system. According to the characteristics of the workpiece processing requirements, a step-by-step sequence program and a main control program are adopted to facilitate the implementation of three control modes for the machine tool. A distributed control system is formed by networking the host computer with the PLC controlling the machine tool. The host computer manages and controls the machine tool, achieving production automation and improving the modernization level of the factory. Keywords: PLC; cutting; milling; stepping sequence control; master control program Abstract: This article analyzes the problem of steel tube cutting and milling end face processing, using PLC for steel tube processing to carry out automatic control. We have completed the systematic hardware and software design. According to the processing requirements and characteristics of the workpiece, with stepping sequence control and master control program, we make the three kinds of machine tool control methods to facilitate realization. Uniting the last position computer and the control of the machine tool with PLC to network, forming a distribution type control system, through the last position computer for machine tool management and control, realizes production automation and improves the factory's modern level. Key words: PLC, cut, mill, stepping sequence control, master control program 0 Introduction This article introduces a high-efficiency and specialized automated steel tube cutting and milling end face machine tool that can complete the steel tube blanking work, improve the labor intensity of workers, realize automatic control, and improve labor productivity. This machine tool can accurately cut long steel pipes to the required length and mill both end faces, completing the steel pipe cutting process in one operation. 1. Working Process of the Milling Machine This machine tool has one cutting power head and two milling power heads, each driven by a motor. Workpiece feeding is achieved by the motor driving friction wheels. Workpiece clamping, the movement of the cutting power head, and the movement of the worktable are all accomplished by pneumatic piston cylinders. A schematic diagram of its working process is shown in Figure 1, and the work cycle is shown in Figure 2. The machine tool's working process is as follows: The long steel pipe to be processed is manually installed and fixed, the power head is started, and the machine tool is in a ready state. 1) Press the start button SB7 to start the feed motor, which drives the long steel pipe to feed through the friction wheel. When the steel pipe reaches the specified length, it triggers the limit switch SQ1, stopping the feed motor. 2) The workpiece is clamped, and the pressure relay KP activates. 3) The cutting head feeds to cut the steel pipe, triggering the limit switch SQ2 after cutting. 4) The cutting head retracts, triggering the limit switch SQ3 after retraction. 5) The worktable feeds with the clamped and cut workpiece to mill both ends. After milling, it triggers the limit switch SQ4, stopping the worktable feed. 6) The workpiece is released, triggering the limit switch SQ5 after release, and the workpiece slides down the ramp under its own weight. 7) The worktable retracts, triggering the limit switch SQ6 after retraction, stopping the retraction. This completes the current cycle. Afterward, the machine tool enters standby mode or restarts the next cycle depending on the operating mode. 2. Electrical Circuit Design of the Milling Machine The three power heads of this machine tool use a conventional start/stop holding relay-contactor control circuit. The feed motor and three cylinders are controlled by a PLC. According to the work requirements, there are a total of 7 signals on site, including pressure relays and limit switches. In addition, there are 6 manual control buttons, 4 automatic control buttons, and 3 function selection switches, so the PLC needs 20 input signals. The controlled objects of the system are the feed motor and the three cylinders. The feed motor operates in one direction and only requires one output point. The clamping cylinder and cutting cylinder are controlled by two-position four-way directional valves, each controlled by one solenoid valve. The table movement cylinder is controlled by a three-position four-way directional valve, requiring two solenoid valves. Therefore, there are 5 control signals on site. In addition, there are 4 system status indicator lights, such as "Ready," "Cycle," "Pause," and "Fault," so the system needs 9 output signals. Based on the above analysis, we selected the Mitsubishi FX2N-48MR-001 PLC controller, which has 24 input points and 24 output points, using relay outputs, which can meet the system requirements. The electrical circuit schematic of the PLC control section is shown in Figure 3 (the relay-contactor control circuit is omitted). 3. Software Design of the Milling Machine According to the machine tool's operational requirements, it is a typical sequential program control; therefore, we selected stepper program control. To achieve manual, single-cycle, and automatic cycle working modes, the main block diagram of our ladder diagram is shown in Figure 4. The main block diagram uses two main control programs. When the system reset button is pressed or power is applied, the first main control program is executed, clearing the relevant components and putting all parts of the system in a de-energized state, used for initializing the machine tool. The second main control program is executed when power is applied or the continue button is pressed and can maintain its execution state; it is not executed when paused. That is, when paused, the output of the second main control program should be paused, while the sequential function state is maintained, so that when resuming work, execution can continue from the pause point. The sequential control function diagram of the automatic program is shown in Figure 5. In single-cycle and automatic-cycle modes, S0 is set to enter the initial step of the stepping program. To ensure safety, the clamping mechanism is designed for power-off clamping; therefore, a Y1 set instruction is used in the S0 state to loosen the clamping mechanism. After loosening, X5 activates. If the start button is pressed at this time, X15 is energized, and the machine tool can enter the cycle state S520. Afterward, the machine tool can automatically complete the transitions between states according to the work requirements. After entering state S526, depending on the working mode, it can automatically return to the S0 state to standby (single-cycle) or directly enter the S520 state to automatically enter the next cycle (automatic-cycle). The ladder diagram program design of this machine tool is not described further due to space limitations. 4. Communication between PLC and Host Computer In automated factories, various machine tools can be networked with a host computer to form a distributed control system to achieve mutual communication and production management. Managers can adjust, manage, and schedule the PLCs on-site in real time via a host computer without interrupting production, ensuring that various production machines operate at their optimal configuration and maximizing the production efficiency of various specialized and general-purpose equipment. In production practice, a 1:N host computer-to-server communication method is generally adopted, with one system computer managing multiple slave PLCs. RS232C/RS485 communication standards are typically used, with additional adapters added to increase communication distance. 5. Conclusion The innovative aspect of this design is that it completes steel pipe cutting and milling in one operation, using a main control program and stepper program to control and coordinate the three working modes required by the machine tool. By networking the PLC communication system with the host computer to form a distributed control system, real-time control and management of the site can be achieved, ensuring that factory equipment operates at its optimal configuration. This system is reliable, technologically advanced, and highly automated, improving production efficiency, realizing the integration and intelligentization of mechanics, electricity, and pneumatics in the factory, improving workers' working conditions, and ensuring product quality. References: [1] Liao Jingsheng, Design and application of PLC in automatic cargo hoist. Microcomputer Information. 2004. Vol.20(9):1-3 [2] Jin Guangliang et al., Application of programmable controller in machine tool control system transformation. Water Conservancy and Electric Machinery, 1991.2:21-24 [3] Wang Liwei, Liu Xuan, Automatic control of press using PLC. Journal of Luoyang University, 2006. Vol.21(2):72-76