the invention relates to a method for determining a geometrical object for modeling the geometry of a metal sheet forming stage in a cad system . according to said method , an operator is defined which links a first geometrical model with a second geometrical model . the link is associated with a method for physically modeling a treating process which transfers a forming stage from a corresponding first state into a second state . when the first geometrical model is modified , the second geometrical model is automatically updated in accordance with the physical modeling concept . the physical modeling concept for forming stages is thereby integrated into the static geometrical model environment of a cad system . the physical modeling method is a method for calculating a border line of a forming stage in an initial state prior to a forming process from a geometry of the border line in a resulting state after the forming process .