The 1/32 subsector of China Fusion Engineering Test Reactor vacuum vessel mockup is fully penetration welded together with four poloidal segments (PSs) which are made by forming and welding, so that each PS needs to machine the welding grooves. However, the measurement results show that there are dimensional errors between the design model and each PS which has been made. This means that the welding grooves cannot be machined directly according to the design model. So, the method of reverse engineering is used to achieve the 3-D model of each PS which has been made. Based on this 3-D model, the model for CNC machining can be designed. At the same time, the machining equipment for PS is a three-coordinate CNC floor-type milling and boring machine which needs a datum plane when it is milling. Considering each PS has a complex surface profile, 3-D datum transformation is used for solving the problem of lacking the machining benchmark. At present, based on these two methods, the machining work of welding grooves for eight PSs has been completed which has met the requirements of assembly and welding.