Mechanism for controlling the workpiece pressure grippers of buttonhole bar tacker sewing machines

In a buttonhole bar tacker sewing machine having a stitch forming mechanism including a needle, a cutting cam controlled by a device for selectively cutting the buttonhole either before or after bar tacking, a support plate for the workpiece, a device for exerting a motion of the support plate relative to the longitudinal axis of the needle in a shape corresponding to the required shape of bar tacking the buttonhole, pressing grippers which when lowered retain the workpiece in their gripping position on the support plate, and a device for stopping the machine upon completion of the buttonhole, all said devices operating automatically upon starting the machine, the improvement which comprises a first disengaging member coupled with mechanism for lifting the pressing grippers, a second disengaging member coupled with a device for selectively starting and stopping the machine, said two disengaging members being situated within the path of rotary motion of at least one section of at least one part mounted rotatably and coupled in its motion with the device for exerting a relative motion of the support plate of the workpiece relative to the longitudinal axis of the needle as well as with the cutting cam.