which corresponds substantially to its cutting position by turning swing lever 24 toward stop screw 54 trimming knife 17 is moved down while compressed air cylinders 43 47 remain in an unchanged switching position and a first cutting stroke is performed immediately thereafter compressed air is alternately admitted by using a timing clock control means not shown into the two inlets in cylinder 43 which causes it to act as a vibration cylinder by alternately moving piston 44 up and down during the ascending movement of piston 44 into the top position shown in fig3 switching handle 40 is turned clockwise over guide 38 and also guide rod 23 so that joint bolt 22 is moved into a center position which is between the top position shown in fig1 and the stationary bolt 25 by turning joint bolt bolt 22 down guide rod 21 is also turned down lever 13 is turned counterclockwise and trimming knife 17 is raised into the position shown in fig3 with shaft 8 of sewing machine 2 standing still the drive of trimming knife 17 is effected by compressed air cylinder 43 through the admission of compressed air with adjustable frequency in alternating directions this results in a reciprocating movement of switching handle 40 and guide rod 23 which serves as a transmission element thus swing lever 24 is moved to a position.