Prediction of Geometrical Error Due to Gas and Plasma Cuttings and Its Control

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To achieve automation or mechanization of a ship structure assembling line, it is necessary to maintain much higher precision in each manufacturing stage, such as cutting and welding, compared to that required for a system which depends on skills of workers. Since flame cutting is the first stage of assembly and it determines a large part of the accuracy problems, the precision of flame cutting is the receiving more attention. Although it has long been considered that residual plastic strain and the motion of the plate due to the transient thermal deformation during cutting are the main causes of cutting error, hardly any studies have been conducted to investigate these factors theoretically and quantitatively. In this research, a computational method, based on thermal-elastic-plastic FEM,has been developed to simulate the cutting process. The proposed method is verified by comparing the computations with the experimental results obtained for plasma cutting.Using the FEM model, the residual plastic strain and the motion of the plate due to the transient thermal expansion are computed. Their effects on cutting error are quantitatively discussed. The results show that these factors have great influences on one-side cutting and small effects on two-side simultaneous cutting. The influences of the residual stress existing in the plate before cutting are also investigated.