An apparatus for printing, cutting or otherwise working sheets of sheet material has a friction sheet advance mechanism and a sheet loading mechanism in advance of the feed mechanism having rows of sprocket pins cooperating with complementary rows of sprocket holes in a sheet to assure accurate insertion of a sheet into the sheet advance mechanism and subsequent accurate movement of the sheet through the advance mechanism and past a work station. A photosensor carried by the work head of the apparatus senses one side edge of the sheet as a reference for laterally positioning the work process relative to the sheet, and a transversely extending index edge of an index hole in the proof sheet is detected and used as a reference for the longitudinal positioning of the work process relative to the sheet. When working on both faces of the sheet, the same reference edges are detected and used as references for the work process on the second face as are detected and used as references for the work process on the first face so as to avoid registration errors between the two work processes.