A sewing machine with a work supporting surface convertible into a free arm, comprising a bed provided with a cantilevered arm containing a bobbin hook, a door pivoting on the cantilevered arm to rotate from a closed position to an opened position for access to the hook, and a movable work supporting surface which can be moved to an operatively raised position and an operatively lowered position at which lowered position the free arm is completely accessible. The support work surface comprises a U-shaped element, a plate slidable on suitable guides positioned on the work supporting surface in the front portion of the work supporting surface, a latch connected to the plate and a wall defining a relief and tooth connected to the door to couple with the plate latch to allow the opening and/or closing of the door every time that the plate is urged to take the extreme position of its stroke.