This invention is directed to a method and associated apparatus for allowing the carriage shafts of a pen plotter to be mounted and aligned quickly and accurately without the introduction of any bending stresses or intra-shaft misalignment. The shafts have conical indentations in the ends thereof. Laterally slidable end caps are bolted over the shaft ends and bolted in place in proper alignment with the shafts. An end cap on one end of the shafts has conical projections in the bottoms of bores therein aligned with and fitting into the shaft ends. An end cap on the other end of the shafts has bolts with conical ends threaded through the bottoms of bores therein aligned with and fitting into the shaft ends. The bolts through the end cap are tightened to hold the shafts in place. An alignment fixture holds the shafts in proper alignment while the end caps are aligned and tightened.