Cut-away tools have a strong tendency to produce a complicated plastic flow ahead of the restricted tool-chip contact area as far as the restriction is in effect. The paper stresses the importance of the plastic field for better understanding the machining characteristics with cut-away tools. By applying the theory of ideal plasticity, the plastic field is found to be composed of one centered fan and two straight slip-line fields. The stress and velocity fields are then calculated based upon the construction of slip lines. The plastic deformation during chip-forming process is obtained analytically as a deformed pattern of originally square grids printed on the side of workpiece. The analytical pattern is verified to be in good agreement with the pattern actually produced in experiments. A quantitative analysis for variation of coefficient of friction on rake face with the artificial reduction of tool-chip contact area is proposed, in comparison with experimental results. The experimental results appear to support the analysis.