There is a close relation between building design and fabrication. However still it is mostly a sequential process where the fabrication with its geometric chances and limitations is only to a small extend taken into account in early design phases. In this paper we describe a protoype form work project, realized out of styrofoam cut out with a CNC hot wire-cutting system and coated with the Claraporon coating system. Due to the linear nature of hot wire cutting the surfaces that can be created are ruled surfaces. They are discussed in a more general way as well as first exercises with ruled surfaces in teaching.