A new approach for modelling and simulation of the cutting forces in helical end milling processes is presented. In this approach, the cutting forces in helical end milling are modelled based on a predictive machining theory, in which the machining characteristic factors are predicted from input data of fundamental workpiece material properties, tool geometry and cutting conditions. In the model, each tooth of a helical end milling cutter is discretised into a number of slices along the cutter axis to account for the helix angle e ect on the cutting forces. The cutting action of each of the slices is modelled as an oblique cutting process. For the ΓΏrst slice of each tooth, it is modelled as oblique cutting with end cutting edge e ect and tool nose radius e ect, whereas the cutting actions of other slices are modelled as oblique cutting without end cutting edge e ect and tool nose radius e ect. The cutting forces in the oblique cutting processes are predicted using a predictive machining theory. The total cutting forces acting on the cutter is obtained as the sum of the forces at all the cutting slices of all the teeth. A Windows-based simulation system for the cutting forces in helical end milling is developed using the model. Experimental milling tests have been conducted to verify the simulation system.