Low roughness of free-form surfaces represents a challenge in production of forming tools. In the industry, the free-form surface is grinded and polished manually. The technology of hammer peening has the capability to replace the time consuming handcraft by automated machining with high productivity and other positive effects, like increased hardness and compressive residual stress in the surface layer. This leads to a high wear resistance of the machined surface. The hammer peening can be conducted with a piezo-actuated tool. The high frequency of such a tool allows achieving higher productivity than other actuator principles used in hammer peening tools. In this paper, a piezo-actuated hammer peening tool working in its mechanical resonance is presented. Furthermore, a cross-domain model including the process, the mechanical and the electrical part of the tool is explained. This model is further used for optimization of the process and the tool. The functionality of the tool is demonstrated by machining results.