The present invention discloses and describes 3D printers that print 3D objects with acceptable precision and accuracy. Such 3D printers may comprise one or more of: an extrusion core with particularized temperature controls; an extrusion core with particularized filament pathway geometry; an extrusion core with a nozzle that is integral with a heating block; an extrusion core with nozzles of variable orifice-openings; a filament feeding system using smooth-faced-rollers; a build plate with an adhesion-layer for wetting between the extrudate and the adhesion-layer; an alignment-plane for anchoring axis positioning systems to a common plane to minimize tolerance stacking problems; and a single z-axis guide, a mono-rail, for guiding movement of a build plate in the z-axis direction; which may permit use of minimal mechanical fit tolerances between the mono-rail and a complimentary receiving sleeve.