Method of retracting and returning a tool from and to a workpiece during the machining of this workpiece by a numerically controlled machining device, and numerically controlled machining device for performing the method

A numerically controlled machining device comprising:

• (a) drive means for driving a tool along a tool machining path during the execution of a program for machining a surface of a workpiece, said tool machining path being determined from a calculated tool path wherein for each reference position on the calculated tool path there is an associated machining position of the tool machining path;
• (b) interrupting means for interrupting the tool progression on the tool machining path along the workpiece during said machining;
• (c) storage means connected to said interrupting means for storing a first set of data which identify the progress reached in the interrupted machining program and for storing a second set of data which identify the reference position reached at the beginning of the interruption;
• (d) first means connected to said drive means and being provided for determining a tool retraction position which is situated remote from the workpiece;
• (e) second means connected to said storage means and being provided for determining on the basis of said reference position reached at the beginning of the interruption, a restart reference position which has its associated restart machining position situated on the tool machining path on a location already reached by the tool before said interruption occurred;
• (f) path determining means connected to said first and said second means being provided for determining a calculated return path which extends between the retraction position and the restart reference position, and wherein at least part of said calculated return path describes a curved portion which, near the restart reference position, diverges towards a line interconnecting successive reference positions of the calculated tool path, and wherein said calculated return path extends substantially in the same direction as the calculated tool path near the restart reference position, said path determining means being further provided for determining a tool return path on the basis of said calculated return path;
• (g) control means connected to said drive means, said first and second means and to said path determining means and being provided for controlling the retraction of the tool from the workpiece to the retraction position and for controlling the return of the tool to the workpiece along the tool return path.