Abstract : This paper presents an economical CNC retrofit design for a conventional lathe. The main focus is on the hardware design of the CNC system, using an 8031 ​​microcontroller as the core, expanding the memory, and controlling the stepper motor rotation via input/output interfaces, opto-isolation circuits, and power amplifier circuits. After gear reduction, the stepper motor drives the ball screw, thus achieving the longitudinal and transverse feed movements of the lathe. Keywords : Pulse; Expansion; Stepper motor 0 Introduction Economical CNC systems typically use open-loop stepper control systems with power stepper motors as the driving element. Without a detection feedback mechanism, the system's positioning accuracy can generally reach ±0.01 to 0.02 mm, meeting the accuracy requirements for machining parts on a retrofitted CW6140 lathe. However, after multiple major overhauls, the interconnected dimensions of the conventional lathe's components change significantly, and the main transmission parts have undergone several replacements and adjustments, yet the failure rate remains high. Traditional repair methods are difficult to meet the overhaul acceptance standards and are costly. Therefore, the rational selection of the CNC system is crucial for successful retrofitting. 1. Design Description Since the 8031 ​​chip lacks dedicated external address and data buses, expanding external memory or I/O interfaces requires first expanding the external bus. This expansion can be achieved using the 8031 ​​pin ALE. During the active high-level period of ALE, port PO outputs A7 to A0. Therefore, only an external address latch needs to be added to the CPU, using the active high-level edge of ALE as the latch signal to latch the address information on port PO until ALE becomes active again. (Full text download of the economical CNC retrofit of a standard CA6140 lathe is available.)