these parameters are sent into the computer 16 for point by point computation of the rounding arc rk the radius circuit 22 is activated by a rounding signal s r which is supplied through operation of a rounding key 23 in the keyboard 20 the rounding radius r is transmitted from the input units 18 into the memory 17 where it is temporarily stored the radius circuit 22 is preferably a microcomputer of high operating speed programmed to produce the rounding arc parameters computer 16 is further capable of providing straight line interpolation as well as tool radius compensation straight line interpolation is the provision of desired positions along a straight line by the computer 16 tool radius compensation is where for a given contour k the computer calculates a compensated contour k 39 the compensated contour k 39 is delivered through control 8 to the directional control loops for the x and y directions and to the control loop for the z direction which is not represented in fig2 thus the center point m of the tool 7 moves along the compensated contour k 39 equidistant to the prescribed contour k the operation of the invention can be explained more fully by referring to fig3 a prescribed contour k composed of two intersecting straight lines g 2 and g 3 is to be milled in a workpiece 2 the.