Determining an operating path of a tool in relation to a three-dimensional surface of a workpiece

In a method of determining an operating path of a tool in relation to a surface of a workpiece wherein a plurality of points of contact between a tool and the workpiece surface are determined and information in the form of digitised co-ordinate axis values relating to each determined point of contact is stored in a computer memory device, from which set of co-ordinate axis values an operating path for the tool can be derived, the improvement consisting in a method for controlling the spacing between adjacent points, said method comprising the steps of:

• (i). starting from a point of contact, constituted by a last preceding stored point, of the tool and the workpiece surface, effecting relative movement between the tool and the workpiece surface out of such contact through a predetermined distance along a first axis, and thereafter
• (ii). maintaining the tool at said predetermined distance from said starting point of contact while returning the tool and the workpiece surface into contact, which return movement is effected selectively (a) along said first axis, whereupon compensating relative movement is caused to take place automatically along a second axis and/or a third axis such that the tool describes a great circle of a sphere of which the starting point of contact forms the center, or (b) along said third axis, whereupon compensating relative movement is caused to take place automatically along said second axis such that the tool describes a small circle of such sphere, such return movement being terminated when the next-to-be-stored point of contact of the tool and the workpiece surface is reached, which point of contact is thus spaced at the predetermined distance from the starting point of contact, regardless of the contour of the workpiece surface.