Abstract : This paper introduces the modification of the CNC system of a second-hand large steam turbine generator base machining center and the software repair of the C-axis positioning oscillation problem. The paper analyzes the time delay caused by the large inertia of large CNC equipment, clamping and loosening, coupling of NC and PLC programs, and the selection range of certain parameters. The modification system replaced the original A-B7320CNC system with an A-B9/260CNC. The paper analyzes and compares the functions of the A-B7300 series and the 9 series software. Keywords: base machining center, modification, software, repair, CNC system. In 1997, at the invitation of Shanghai Steam Turbine Generator Co., Ltd., the author conducted a feasibility study on the modification of the company's large steam turbine generator base machining center and performed software repair on the C-axis positioning oscillation problem. This provided valuable experience in the repair and modification of large second-hand equipment. This base machining center was a second-hand piece of equipment imported from Westinghouse in 1992 and manufactured by Freer Corporation of the United States. It consists of three main parts: an end boring machine, a side boring machine, and a rotary table. The cantilever extends 7.5m and has a diameter of 1.5m. The rotary table is 12m long and 4.2m wide, with a load capacity of 140t. It can be used to process 300, 600, and 900MW steam turbine generator bases, making it a super-large specialized piece of equipment in my country's steam turbine generator manufacturing industry. The control system of this machining center consists of two A-B7320 CNC systems, controlling the end boring machine and the side boring machine respectively. The control software was developed by Company A-B and includes PAL (i.e., PLC software). Due to the special nature of large CNC equipment, its PAL and NC software are developed together. Therefore, the PAL interface software is unreadable and writable, only executable, which brings great difficulties to the modification of CNC equipment. To thoroughly understand the PAL interface software, we deciphered and analyzed it, clarifying some special issues in the design of the large equipment's PAL interface software, and simultaneously mastering a large number of machine tool parameters. [b][align=center]For details, please click: Maintenance and Modification of Large Machine Base Machining Centers[/align][/b]