breadth of this movement is determined by the position of the slider 12 on the guide bar 11 more specifically the stroke of the differential feed dog 15 possesses maximum breadth when the slider 12 is disposed in correspondence with the free end of the guide bar 11 while the breadth of this stroke is reduced as said slider is moved downwardly toward the arm 10 the arm 10 supports the guide bar 11 which is displaced by the advance shaft 1 and thus to obtain a smaller minimum stroke of the differential feed dog than that which could be obtained if this guide bar were directly mounted on the feed advance shaft 1 accordingly by providing the arm 10 it is possible to obtain a shorter stroke of the differential feed dog 15 than that of the main feed dog 5 so as to obtain what is generally defined as 34 negative differential feed 34 the positioning of the slider 12 along the guide bar 11 is effected by means of an adjustment device which includes a forked slider 17 and control means 18 operatively associated with said forked slider the forked slider 17 is provided with two prongs 19 and 20 between which a slide element 21 which is pivotally connected to the slider 12 is adapted to extend the forked.