a pocket opening sewing machine including a cutting device for producing two notching cuts at the ends of a pocket opening . the sewing machine has sewing needles and a belt driven clamping device for clamping to the sewn parts for advancing them in an advancing direction . the cutting device includes two knives spaced apart . the knife that is further from the sewing needles is movable toward the stationary knife that is closer to the needles . for adjusting the spacing between the knives , the movable knife is temporarily clamped to a moving belt which moves it toward the stationary knife . an adjustably positionable stop defines the furthest limit of spacing between the stationary and movable knives and a spring drives the movable knife against the stop . the clamping of the movable knife to the belt is initiated by a sensor which senses the passage of a sewn part and which operates a microcomputer with pocket length information stored in it to move the movable knife when required over the distance required . alternately , if the movable knife moves at a speed faster than the advancing speed of the sewn parts , adjustment of the movable knife stop can be dispensed with .