Operating method for a numerical controller, - wherein the numerical controller has access to a part program which can be executed by the numerical controller, - wherein a machine tool can be controlled by the numerical controller by executing the part program, - wherein a selection command is predetermined by way of a user interface by a user to the numerical controller, by means of which selection command a program instruction of the part program is selected, to which at least one parameter (PAR1 to PARN)is assigned, - wherein when the part program is executed by the numerical controller, the program instruction is converted into a movement of a tool processing the workpiece as a function of the at least one parameter (PAR1 to PARN), - wherein the numerical controller outputs the at least one parameter (PAR1 to PARN) of the parameterized program instruction in a form which can be changed by the user and an original image which corresponds to a processing of the workpiece effected by executing the part program up to and including the selected program instruction by way of the user interface to the user of the numerical controller, - wherein in the event of a change to a parameter (PARn) of the selected program instruction, the numerical controller outputs a changed image which corresponds to a processing of the workpiece effected by executing the part program up to and including the changed program instruction by way of the user interface to the user of the numerical controller, - wherein once the numerical controller has received a corresponding confirmation command from the user, it stores a part program (TP') changed according to the changed program instruction, characterised in that, - the numerical controller transfers an item of information (ident) identifying the selected program instruction via a computer-computer link directly or indirectly to a CAM system, by means of which the part program was generated, and receives the original image from the CAM system, - in the event of a change to a parameter (PARn) of the selected program instruction, the numerical controller transfers the change to the CAM system and receives the changed image from the CAM system, and - the numerical controller automatically determines the CAM system, by means of which the part program was generated, on the basis of a header of the part program. - wherein the numerical controller has access to a part program which can be executed by the numerical controller, - wherein a machine tool can be controlled by the numerical controller by executing the part program, - wherein a selection command is predetermined by way of a user interface by a user to the numerical controller, by means of which selection command a program instruction of the part program is selected, to which at least one parameter (PAR1 to PARN)is assigned, - wherein when the part program is executed by the numerical controller, the program instruction is converted into a movement of a tool processing the workpiece as a function of the at least one parameter (PAR1 to PARN), - wherein the numerical controller outputs the at least one parameter (PAR1 to PARN) of the parameterized program instruction in a form which can be changed by the user and an original image which corresponds to a processing of the workpiece effected by executing the part program up to and including the selected program instruction by way of the user interface to the user of the numerical controller, - wherein in the event of a change to a parameter (PARn) of the selected program instruction, the numerical controller outputs a changed image which corresponds to a processing of the workpiece effected by executing the part program up to and including the changed program instruction by way of the user interface to the user of the numerical controller, - wherein once the numerical controller has received a corresponding confirmation command from the user, it stores a part program (TP') changed according to the changed program instruction, - the numerical controller transfers an item of information (ident) identifying the selected program instruction via a computer-computer link directly or indirectly to a CAM system, by means of which the part program was generated, and receives the original image from the CAM system, - in the event of a change to a parameter (PARn) of the selected program instruction, the numerical controller transfers the change to the CAM system and receives the changed image from the CAM system, and - the numerical controller automatically determines the CAM system, by means of which the part program was generated, on the basis of a header of the part program.