14 is mounted on the lower surface of the supporting member 10 and a piston 12 of the air cylinder 11 is connected at its one end with a gauge plate support 13 located on the front side of the supporting member 10 the gauge plate 14 for determining an external shape of the pocket piece p is detachably mounted to the gauge plate support 13 and the pocket piece p is adapted to be put on the gauge plate 14 the gauge plate 14 is operated by the piston 12 to be advanced onto the workpiece supporting table 4 and be retracted to the underside of the supporting member 10 the supporting member 10 is formed at its front end with a pair of shaft supports 15 projecting frontwardly and a horizontal support shaft 16 is supported to the shaft supports 15 a presser plate support 17 is rotatably mounted on the support shaft 16 the presser plate support 17 is formed with a pair of mount portions 18 projecting frontwardly for detachably mounting the presser plate 19 the presser plate support 17 is further formed with a connecting portion 20 projecting upwardly at an upper central position thereof the connecting portion 20 is connected to a piston 22 of the air cylinder 21 rotatably mounted on the upper surface of the supporting member 10 when the piston 22 is operated to advance and retract the presser plate 19 is rotated between an operative position where it is superimposed on the gauge plate 14 and an inoperative position where it is separated from the gauge plate 14 a pair of supporting blocks 23 are rotatably supported at both ends of the support shaft 16 and a substantially u shaped folding member mount plate 24 is detachably mounted.