The stool shown here was created for the Precision Machine Design class, 2.75. The prompt was to create something which used a kinematic coupling. Kinematic couplings use special geometries to constrain the degrees of freedom between two parts. This allows for a repeatable relative positioning of the two parts with very high precision.

I made a wooden stool in which the base and the seat were connected with a kinematic coupling. The base has three legs which end in a sphere. The seat has triangular grooves cut at 120 degrees to each other. The three spheres have six points of contact with the seat (2 in each groove) and can only fit in one position when a load is applied. I chose a trianglar seat to prevent someone from sitting on the edge and flipping the seat off of the base. One interesting detail regarding the construction of this stool is that the loading force required for a kinematic coupling is provided by the occupant. Also, the tree legs with the floor can be considered a kinematic coupling with 3 degrees of freedom.