Brother International DB2-B791 Service Manual - Page 18

3. Needle and bobbin threads trimming 0 (1) When the needle rises 2 mm (2.2 mm with thick materials) above from its lowest position, the rotary hook point catches the loop formed by the needle. (2) When the thread trimming signal is turned on, the thread trimming clutch drives the movable knife. The needle thread is caught by the rotary hook and fed around the shuttle body. Q-9 Bobbin thread O Fixed knife Needle thread Movable knife Lower thread finger II (3) The movable knife tip enters the triangular loop formed by the rotary hook point under the needle plate, and the loop spreading occurs. At this point the needle thread take-up is raised slightly above from its lowest position (the upper shaft has turned approximately 330°), and the needle thread is spread by the movable knife as shown in the figure above. If the timing of the movement is too early, loop spreading is not performed properly and results in thread trimming errors. Needle thread Bobbin thread Movable knife Lower thread finger As seen with fixed knife removed (4) The needle and bobbin threads caught by the movable knife in step (3) are gradually spread by the movable knife and lower thread finger to be cut by the fixed knife tip. At this time, the needle thread take-up approaches the top of its stroke. When the movable knife is spreading the needle thread, the tension release relieves the needle thread tension to prevent excessive tension and to enable the needle thread to be extended smoothly. -17-