A three dimensional printer incorporates a kinematic coupling between the build platform and movable stage which holds the build platform, of three curved protrusions attached to one of the build platform or the movable stage and six locating features formed in receivers of the other. At least two flexures differentially change a Z position of each of two of the curved protrusions. 3D printing is paused at a preset level of completion, and the build platform may be removed for external operations. A print resume circuit resumes printing of additional printed layers at the previous position in response to a return detection circuit that responds to an input (e.g., a touch screen confirmation).