association with a detector element 33 of the proximity sensor or opto electronic type this detector element 33 serves to activate the electromagnet 26 so as to vary the position of the vertical wall 12 upon sensing an area of increased thickness in the workpiece and which is depicted by numeral 34 in fig4 more particularly the feeler 28 is located in a position whereat it will be engaged by that portion of the workpiece of increased thickness when the two folds 20 and 35 which will form a part of the hem pass between the worksurface 2 and the tip of the hemmer plate 23 for controlling the distance between the tip 29 from the feeler 28 the lower portion of the pin 24 is extended and defines a threaded rod 36 having a knob 37 fixed on its lowermost end and by rotating said knob in one direction or the other the hemmer plate 23 and its tip 29 can be raised or lowered as desired to summarize the operation and just prior to the start of the sewing cycle the hem as shown in fig4 is so disposed that the first fold 20 or lower fold is pre formed and inserted between the tip 29 and the worksurface 2 and the second fold 35 or upper fold is superposed on the hemmer plate 23 so that said tip 29 serves as an inner aligning guide for said second fold during the actual sewing cycle the lower fold 20 is in contact with the vertical wall 12 and in alignment with the needle 4 while it is being advanced and the edge portion of the upper fold 35 is disposed so as to be trimmed by the knife 9 when those portions of the.