Method of operation for a numerical controller, - wherein the numerical controller has access to a parts program which can be executed by the numerical controller, - wherein a production machine can be controlled by the numerical controller by execution of the parts program, - wherein an operator issues to the numerical controller, via an operator interface, a selection command which effects the selection in the parts program of a program instruction to which is assigned at least one parameter (PAR1 to PARN), - wherein, when the parts program is executed by the numerical controller, the selected program instruction is converted into a processing movement, of a tool which processes the workpiece, which depends on the at least one parameter (PAR1 to PARN), - wherein the numerical controller determines, by reference to data (D1, D2) which specifies at least the three-dimensional geometry of the workpiece, at least one image of the workpiece, - wherein the numerical controller outputs to the operator of the numerical controller via the operator interface the at least one parameter (PAR1 to PARN) of the parameterised program instruction, in a form which can be changed by the operator, and the image, - wherein, in the case when a parameter (PARn) of the selected program instruction is changed, the numerical controller determines by reference to changed data (D1, D2), which specifies at least the changed three-dimensional geometry of the workpiece, at least one amended image of the workpiece and outputs to the operator of the numerical controller via the operator interface the amended image, - wherein the numerical controller, as soon as it has accepted from the operator an appropriate storage command, stores a modified parts program which corresponds to the changed program instruction, characterised in that, - the numerical controller communicates directly or indirectly to a CAM system, via a computer-to-computer link, information (ident) which identifies the selected program instruction, - the numerical controller accepts in return from the CAM system the data (D1, D2) which specifies at least the three-dimensional geometry of the workpiece, - in the case when a parameter (PARn) of the selected program instruction is changed, the numerical controller communicates the change to the CAM system, and accepts in return from the CAM system the changed data (D1, D2), and - the data (D1, D2) and the changed data (D1, D2) specify the three-dimensional geometry of the workpiece, as it would be if the parts program were executed respectively up to and including the selected program instruction or up to and including the changed program instruction. - wherein the numerical controller has access to a parts program which can be executed by the numerical controller, - wherein a production machine can be controlled by the numerical controller by execution of the parts program, - wherein an operator issues to the numerical controller, via an operator interface, a selection command which effects the selection in the parts program of a program instruction to which is assigned at least one parameter (PAR1 to PARN), - wherein, when the parts program is executed by the numerical controller, the selected program instruction is converted into a processing movement, of a tool which processes the workpiece, which depends on the at least one parameter (PAR1 to PARN), - wherein the numerical controller determines, by reference to data (D1, D2) which specifies at least the three-dimensional geometry of the workpiece, at least one image of the workpiece, - wherein the numerical controller outputs to the operator of the numerical controller via the operator interface the at least one parameter (PAR1 to PARN) of the parameterised program instruction, in a form which can be changed by the operator, and the image, - wherein, in the case when a parameter (PARn) of the selected program instruction is changed, the numerical controller determines by reference to changed data (D1, D2), which specifies at least the changed three-dimensional geometry of the workpiece, at least one amended image of the workpiece and outputs to the operator of the numerical controller via the operator interface the amended image, - wherein the numerical controller, as soon as it has accepted from the operator an appropriate storage command, stores a modified parts program which corresponds to the changed program instruction, - the numerical controller communicates directly or indirectly to a CAM system, via a computer-to-computer link, information (ident) which identifies the selected program instruction, - the numerical controller accepts in return from the CAM system the data (D1, D2) which specifies at least the three-dimensional geometry of the workpiece, - in the case when a parameter (PARn) of the selected program instruction is changed, the numerical controller communicates the change to the CAM system, and accepts in return from the CAM system the changed data (D1, D2), and - the data (D1, D2) and the changed data (D1, D2) specify the three-dimensional geometry of the workpiece, as it would be if the parts program were executed respectively up to and including the selected program instruction or up to and including the changed program instruction.