The purpose of this study is to develop a series of response surfaces that define critical outcomes for welding in Al 7075 based on the tool geometry of a convex scrolled shoulder step spiral (CS4) friction stir processing tool. These response surfaces will be used to find critical minimums in forces which will decrease the required power input for the process. A comprehensive parameterization of the tool geometry is defined in this paper. A pilot study was performed to determine the feasibility of varying certain geometric features. Then a screening experiment eliminated those geometric features that were not as significant in determining the response surfaces. A central composite design with the five most important geometric features was used in order to develop response surfaces for nine different response variables. The nine response variables are the longitudinal, lateral and axial forces; the tool temperature, the spindle torque, the amount of flash, the presence of defects, the surface roughness and the ledge size. By using standard regression techniques, response surface equations were developed that will allow the user to optimize tool geometries based on the desired response variables. The five geometric features, the process parameters and several of their interactions were found to be highly significant in the response surfaces.