is centrally provided at a top surface thereof with a rack plate 41 which is located higher than the x y surface the slide rack 40 is provided with an abutting protrusion 43 at one side thereof adjacent the base plate 10 the abutting protrusion 43 is located corresponding to the protrusions 13 of the base plate 10 and extends in the direction y the slide rack 40 is further provided with a passage 42 corresponding to the rack plate 41 in this embodiment the slide rack 40 is driven by a linear movement device not shown to move linearly along the direction y of the x y surface so as to move close to or away from the base plate 10 the driving element 50 as shown in fig1 is mounted on the slide rack 40 and comprises a servo motor 51 disposed on the rack plate 41 a driving shaft 511 located below the servo motor 51 and inserted in the rack plate 41 and a driving unit 52 connected to one end of the driving shaft 511 the driving unit 52 is a disc structure in this embodiment around the outer peripheral surface of the driving unit 52 is formed a threaded surface and the end of the driving shaft 511 is connected to the center of the driving unit 52 the driving unit 52 is located on the x y surface and extends out of the passage 42 of the slide rack 40 to mate with the threaded surface 23 of the transmission element 20 a control.