Abstract: A printing machine includes a machine base, a lower box mounted on the machine base, an upper box mounted on the lower box in a vertically movable and adjustable position, a conveyor passing horizontally between the upper and lower boxes, and an upper printing device mounted in the upper box for printing a workpiece carried by the conveyor. The lower box has an adjustable support for adjustably supporting the upper box. The adjustable support has two upright support tubes mounted to the lower box, and driven shafts received rotatably in the support tubes. Each of the driven shafts has an upper end, a lower end which extends outwardly of a respective one of the support tubes, a threaded hole which extends longitudinally in the upper end, and an upright connecting rod with an externally threaded lower end that engages the threaded hole and an upper end connected to the upper box. The lower box further has a driving mechanism to drive the driven shafts.