**Abstract:** In response to the limitations of traditional CA6140 lathes, this paper presents a numerical control (NC) retrofitting scheme and a single-chip microcomputer system design aimed at enhancing machining accuracy, broadening the machine's application scope, and increasing productivity. The study details the process of converting an ordinary lathe into a CNC machine, focusing on both the mechanical modifications of the CA6140 and the design of its CNC control system. The 8031 microcontroller is employed as the central processing unit, responsible for signal processing. The I/O interface generates stepping motor pulses, which are then transmitted through a gear reduction system to drive the ball screw, enabling precise vertical and horizontal feed movements. **Keywords:** CNC machine tool, single-chip CNC system, retrofit design **I. Introduction** Machine tools play a crucial role in the manufacturing industry, and their development has long been a focus of attention. With the advancement of computer technology, a significant breakthrough has occurred in the control systems of machine tools, leading to the emergence of CNC (Computer Numerical Control) machines that use digital technology for flexible automation. These machines have revolutionized the way mechanical processes are automated, offering greater precision, efficiency, and adaptability. Over time, CNC machines have become widely adopted, fundamentally changing traditional concepts of machine tool transmission and structure. Their superior performance, flexibility, and integration of mechanical and electrical functions have made them a global standard in modern manufacturing. Compared to conventional machine tools, CNC machines offer numerous advantages: 1. High adaptability for small-batch or complex part production. 2. Excellent machining accuracy. 3. Improved production efficiency. 4. Reduced labor intensity and better working conditions. 5. Strong economic benefits. 6. Facilitates modern production management. CNC machines have become the mainstream in China’s market, with increasing demand year after year. Although progress has been made in developing high-speed, multi-axis, and precision CNC machines, there is still a gap compared to advanced countries. China has over three million conventional machine tools, but many are outdated, inefficient, and costly. Retrofitting these old machines with economical CNC systems offers a cost-effective solution, allowing for increased automation without requiring large investments. Since 1984, China has begun producing such systems, and today, many manufacturers are involved in this field. It is expected that the retrofitting of conventional machine tools will continue to grow rapidly in the future. This graduation project serves as a practical example of such a transformation. **II. Overall System Design** **2.1 Design Objectives** The primary goal of this project is to retrofit the CA6140 lathe with a CNC system. The design uses a microcomputer-based open-loop control system for vertical and horizontal feed motions, with pulse equivalents of 0.01 mm/pulse for the Z-axis and 0.005 mm/pulse for the X-axis. A stepper motor drives the system, using a ball screw mechanism, and an automatic indexing tool holder is integrated for improved functionality. **2.2 Plan Justification** When planning the retrofit, it was essential to minimize changes to the original machine while meeting all functional requirements. The chosen NC system employs two-axis continuous control to handle complex contours. An open-loop control system was selected for its simplicity, low cost, and ease of maintenance. The 8031 microcontroller was chosen as the core due to its affordability, availability, and compatibility with peripheral components. **2.3 Final Design Plan** The redesigned system retains the original lathe’s spindle speed control, allowing manual gear shifting. The longitudinal and transverse feeds are now driven by stepper motors controlled by the 8031-based CNC system. A power amplifier, I/O interface, and ring distributor manage the motor rotation, while the ball screw converts rotational motion into linear movement. A spindle pulse generator ensures accurate thread cutting, and the tool holder is replaced with an automatically indexed, motor-driven version. **III. Hardware Circuit Design of the CNC System** **3.1 Overview of the Microcontroller-Based CNC Hardware** The system uses the 8031 microcontroller as the main processor. An external 2764 EPROM stores the monitoring and processing programs, while a 6264 RAM holds variable data and parameters. Address decoding is handled by a 74LS138 decoder, and input/output expansion is managed via 8279 and 8255 chips. The system also includes optical isolation and power amplification circuits for safety and reliability. **3.2 Off-Chip Three-Bus Architecture** The single-chip system supports a three-bus architecture: address bus (AB), data bus (DB), and control bus (CB). The 8031 provides 16-bit address lines through P0 and P2 ports, while the 8-bit data bus is managed by P0. Control signals include RST, EA, ALE, and PSEN. The 8255A programmable I/O chip enables efficient communication between the CPU and peripherals, supporting various transfer modes like direct, query, and interrupt. This detailed hardware setup ensures the system operates efficiently, providing reliable control for the CNC lathe.

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