The traditional approach to opto-mechanical design is a top-down process where the optical designer communicates to the mechanical designer, “these are the tolerance requirements I need” to meet my system performance requirements. The optical designers typically communicate these tolerance requirements as tilts, decenters, and locations of optical elements (lenses, mirrors, prisms, lens cells, etc). The mechanical designer uses these optical tolerance requirements as inputs to his/her mechanical design process. The output of the mechanical design process is a set of piecepart dimensions and tolerances. These dimensions and tolerances communicate to the manufacturing shops “this is what I need” to meet the optical performance requirements. A significant drawback to this top-down process is that we have no way of knowing how well the manufacturing process can build parts that meet these tolerances. This paper proposes a different approach, where we the mechanical designers use the knowledge of the manufacturing processes as inputs to the mechanical design. With this new knowledge, we can predict the mechanical performance of the system. The mechanical performance, in turn, becomes an input to the optical design. This paper also shows how we have automated this new approach and applied it to the design of an optical system design.