bead and the corresponding cross stitches are terminated the motion to be transmitted by belt clasp 106 to cog belt 103 is reversed by appropriate means not shown so that nut 100 is driven oppositely to its previous direction of rotation since sliding member 74 is connected to supporting member 99 which is secured to threaded spindle 96 by a pressure locking contact between shaped part 89 which is pivotally mounted on member 74 and match 94 which is biased by spring 98 the reversal of the direction of motion of work clamp 21 is effected without any play at the end of the sewing operation the sewing machine is stopped with needle 10 in the lowermost position if there was no response of the thread monitoring device 68 during the formation of the buttonhole seam this seam has been terminated regularly in this case at the second point of interruption bridged by the thread monitoring device 68 the circuit of electromagnet 66 is closed the cam of the program control device closes the circuit of the magnet at its first point of interruption whereby voltage is applied to magnet 66 and lever 61 is moved downwardly thereby knife bar 63 is actuated and the buttonhole is cut open due to the downward movement of lever 61 angle lever 57 is swung clockwise as viewed in fig1 and its arm moves slide 49 to the lefthand side consequently the end portion of slide 49 engages recess 47 of pin 44 thereupon electromagnet 4 is energized whereby arm 2 is moved from its sewing position into its rest position as arm 2 moves upwardly first spring 46 producing the clamping pressure between.