the two top wheels 94 and 96 being located between the side wheels 102 the side wheel 104 carried by the end portion 82 of the main frame is located midway between the two end wheels of the opposite end portion 80 and is moveable toward and away from its associated side surface 74 of the table further while the carriage 10 is in working relationship with the table this wheel 104 is urged toward its side surface 74 so that the table is laterally clamped between the two side wheels 102 of the end portion 80 and the one side wheel 104 of the end portion 82 to eliminate any lateral play of the main frame 30 relative to the table to achieve this functioning of the side wheel 104 it is carried by a mounting bracket 106 slidable relative to an intermediate plate 108 by virtue of the bracket having elongated slots 110 receiving support posts 112 fixed to the plate 108 the plate 108 also carries a pneumatic cylinder 114 which when supplied with pressurized air by the associated supply line 116 urges the mounting bracket 106 to the left as seen in fig2 to urge the wheel 104 into engagement with the adjacent side table surface 74 when air pressure is removed from the supply line 116 an internal spring in the actuator 114 moves the bracket 106 rightwardly to the broken line position shown in fig2 to remove the wheel 104 from engagement with the adjacent surface 74 thereby freeing the main frame 30 of the carriage for vertical movement relative to the table top without hindrance by the side wheels 102 and 104.