links 26 and 28 have effective lengths which are equal to each other as seen in fig1 the workpiece clamping mechanism 3 which receives the workpieces hind part of trousers w and pocket cut t comprises a support plate 39 which is displaceable on the fabric supporting plate 15 of sewing machine 1 a clamping plate 40 is pivotally mounted on the supporting plate 39 both supporting plate 39 and clamping plate 40 are provided with a slot 41 corresponding to the configuration of the seam to be produced in the example shown a substantially u shaped pocket seam n defines a passageway for needle 8 it is to be noted that the diameter of the cylindrical extension 16 of workpiece support 15 is slightly smaller than the width of slot 41 so that in the piercing area of needle 8 extension 16 can support the workpiece supporting plate 39 of the workpiece clamping mechanism 3 is connected to two links 42 43 of a parallelogram guide of guide mechanism 2 link 42 is pivoted to a fixed pin 44 and carries a guide roller 45 projecting into a guide groove 47 which is provided in the underside of a control disc 46 and which for clarity is shown as a curved endless dash dotted line in fig1.