<p>Ruled surfaces are a special kind of free form surfaces which plays an important role in aeronautics and automotive manufacturing. They are particularly well suited for five axis machining by the flank milling method. In the case of non-developable ruled surfaces, however, the flank milling method of having the cutter touch these ruling lines leads to interference problems. A wide range of positioning strategies had been studied to reduce interference between the cutting tool and the designed surface. We need to calculate and minimize interference that may arise. Aiming at the defects of the previous equiangular error algorithm, an analytical equal error algorithm is proposed. Simulation calculation results and the machining experiments on the main blade with non-developable ruled surface of an impeller show that the analytical equal error method has obvious advantages over previous methods in machining accuracy. The work provides an analytical approach for machining error control in flank milling non-developable ruled surface.</p>