Abstract: The algorithm of cutter position in 5-axis side milling of ruled surface based on surface feature is proposed and the scallop height estimating is introduced as well．The torsion degree is used to express the ruled surface feature according to differential geometrical structure．The ruled surfaces to be machined are divided into extensible surface and non-extensible surface．For the former，by means of adjusting the changing rate of straight generatrix，the surface can be reformed into an extensible ruled surface；for the later，tool axis bias angle is used to avoid interference，and the principle of the least bias being adopted to reduce the scallop height across the surface．And the interference-free cutter position can be obtained．

Key words: Cutter position, Side milling, Ruled surface