pivots the latter in an anticlockwise direction in accordance with fig3 counteracting the action of its helical spring 68 so far that the step 66a is pulled out from underneath the left end of the lever 40 since as described above the actuating lever 58 has already fallen back into its position of rest shown in fig3 the lever 40 can be pivoted in an anticlockwise direction in accordance with fig3 so that the friction wheel 48 pivots downwardly around the bearing pin 38 by the action of an elastic drive rope to be described later and moves away from the crank disc 22 the bar 30 carrying the material presser foot 28 comprises on one side toothing 144 of saw toothed configuration at the upper end of the bar 30 there is secured by means of screws 146 a bracket 148 which carries the handle 32 and in the lower end position of the material presser foot 28 comes into contact with the lower wall 10a of the front housing shell 10 mounted for rotation at the front housing shell 10 with the help of a screw 150 is a locking lever 152 coupled with the lever 40 via a connecting part 154 such that when the end 58a of the actuating lever 58 is pressed down the locking lever 152 is pivoted in a clockwise direction in accordance with fig3 if the material presser foot 28 is not at least almost in its lower end position the bar 30 prevents such a pivotal movement on the part of the locking lever 152 by way of a projection 152a.