Mirrors with large aperture, high asphericity and tight shape tolerances are indispensable for modern astronomical telescope. This pressing demand provides new challenges on optical fabrication. Paper here presents a 2.5m CNC machine which has been designed and constructed in Nanjing Institute of Astronomical Optics & Technology (NIAOT). Mechanical layout of 2.5m OFM is first studied basing on analyzing fabrication requirements. Equipped with six motion axes, it can process parts up to 2500mm in diameter. Rotary symmetrical component can be finished under cylindrical coordinates and Cartesian coordinates are applied for off-axis workpiece. Then control system using CAN field bus is discussed. This control architecture offers many characteristics, such as high openness, expansibility and flexibility. Since 2.5m OFM is developed to combine faculties of grinding, lapping and polishing, we arrange a series of tests for each function. Now a 1.1m hexagonal mirror is carried for grinding. Polishing test using sub-aperture tool and active stressed lap will be programmed later.