Incremental Sheet Forming (ISF) is a versatile manufacturing method in which a three-dimensional part is fabricated by progressively deforming a metal sheet. This is typically done via a robot with a single point tool following a defined trajectory. During this process a reaction force between the forming tool and sheet is generated. This force, denoted the forming force and defined as the force acting perpendicular to the sheet, has been modeled in several studies. Given a part with homogenous material, a fixed part geometry, and constant process parameters, these models predict the forming force will be constant. However, many studies have shown that this force fluctuates during the process. This paper augments the model by accounting for changes in process parameters due to robot geometric errors to describe these fluctuations. The model is experimentally validated, and the fluctuations of the forming force are reduced by 51% by modifying the tool path based on the identified robot geometric error.