A stitching head engagement and release apparatus for releasable engagement of the driver rail with the staple driver and releasable engagement of the bender rail and the stable driver to allow a driver rail to communicate staple driving force to a staple driver and to terminate the staple driving force in response to changes in the thickness of the workpiece the apparatus comprising: a frame having slidably mounted thereon a driver rail and a bender rail and a driver, a driver-bender rail interlock (84, 86, 94, 96)for releasable engagement of the driver with the bender rail and the driver-bender rail interlock comprising a rotatable flange plate connected to the driver, the flange plate rotating to engage a shoulder on the bender rail to communicate a downward force from the driver to the bender rail, characterized by a cam extending from the frame to engage a cam follower on the flange plate the cam acting on the cam follower to rotate the flange plate for release of the flange plate from engagement with the shoulder to terminate the downward movement of the bender rail and to allow continued downward movement of the driver for insertion of the stable into the workpiece, and a driver-driver rail interlock (90, 91, 92, 93) for releasable connecting the driver to the driver rail the driver-driver rail interlock comprising a rotatable flange plate connected to the driver, a hip extending from the flange plate, and a flexible rod extending to contact the flange plate to rotate the flange plate to bias the hip of the flange plate against a shoulder of the driver rail to engage the driver with the driver rail for communication of downward staple driving force from the driver rail to the driver, the driver being responsive to resistance from driver contact against the workpiece to communicate an upward force to the hip to disengage the hip from the shoulder of the driver rail to release the driver from engagement with the driver rail to terminate the downward force of the driver against the staple. a frame having slidably mounted thereon a driver rail and a bender rail and a driver, a driver-bender rail interlock (84, 86, 94, 96)for releasable engagement of the driver with the bender rail and the driver-bender rail interlock comprising a rotatable flange plate connected to the driver, the flange plate rotating to engage a shoulder on the bender rail to communicate a downward force from the driver to the bender rail, characterized by a rotatable flange plate connected to the driver, the flange plate rotating to engage a shoulder on the bender rail to communicate a downward force from the driver to the bender rail, characterized by a cam extending from the frame to engage a cam follower on the flange plate the cam acting on the cam follower to rotate the flange plate for release of the flange plate from engagement with the shoulder to terminate the downward movement of the bender rail and to allow continued downward movement of the driver for insertion of the stable into the workpiece, and a driver-driver rail interlock (90, 91, 92, 93) for releasable connecting the driver to the driver rail the driver-driver rail interlock comprising a rotatable flange plate connected to the driver, a hip extending from the flange plate, and a flexible rod extending to contact the flange plate to rotate the flange plate to bias the hip of the flange plate against a shoulder of the driver rail to engage the driver with the driver rail for communication of downward staple driving force from the driver rail to the driver, the driver being responsive to resistance from driver contact against the workpiece to communicate an upward force to the hip to disengage the hip from the shoulder of the driver rail to release the driver from engagement with the driver rail to terminate the downward force of the driver against the staple. a rotatable flange plate connected to the driver, a hip extending from the flange plate, and a flexible rod extending to contact the flange plate to rotate the flange plate to bias the hip of the flange plate against a shoulder of the driver rail to engage the driver with the driver rail for communication of downward staple driving force from the driver rail to the driver, the driver being responsive to resistance from driver contact against the workpiece to communicate an upward force to the hip to disengage the hip from the shoulder of the driver rail to release the driver from engagement with the driver rail to terminate the downward force of the driver against the staple.