Exact constraint design
Exact constraint design, see also [Skakoon2008], is one of the most important lessons I had to learn in my career as a Mechanical Engineer. I had to learn it the hard way by making many mistakes. In short:
Never limit more degrees of freedom than absolutely needed, when mating parts, with an absolute maximum of 6 namely 3 translational and 3 rotational degrees of freedom.
For each free degree of freedom of movement (R=Rotation, T=Translation) needed between two parts one less degree of freedom needs to be constrained. The sum of constrained and free degrees of freedom of movement is always 6, namely 3 translational and 3 rotational degrees of freedom of movement.
Use force-closure to mate parts, never use form-closure, thus never use matching contra-forms to mate parts.
Mechanical products very often consist of many parts which need to be assembled or need to move/rotate relative to each other. Therefore, these parts need to be:
positioned relative to each other and if needed
fixated relative to each other or
they need to move in 1 or more directions relative to each other.
The generic cases of suppression of degrees of freedom of a cube are shown in Figure 1.