The adoption of industrial robots and collaborative robots (cobots) requires significant development in robot integration. Besides the abundant instructions for programming and improved usability, the mechanical design aspects for robot integration still depend on engineers’ individual expertise. This paper provides an easy-to-learn set of models to support the fixture design. It exposes the challenges in modeling unilateral constraints, i.e. detachable contacts between fixture and workpiece, and proposes the concept of Unilateral Constrained Degree of Freedom, which combines conventional degree of freedom and unconstrained movements in one expression. To facilitate the design process of robot fixtures, the paper clarifies the logical relations between movements used for load, constraint, freedom, entry and exit used in a two-step design process. The models presented have been developed and tested during the educational activities at the author’s institution.