Upper And Lower Thread Trimming - Brother S-710A Service Manual

4-6-2. Upper and lower thread trimming

1. When the needle rises 1.8mm (2.2mm with heavy-
weight materials) above the down position the rotary
hook point catches the loop formed by the needle.
1645M
3. The tip of the movable knife passes through the middle of the triangular loop formed underneath the needle plate
by the rotary hook to separate the upper and lower threads. At this time, the thread take-up lever moves up slightly
from its lowest position. (The angular movement of the upper shaft is approximately 330°.)
* If the timing of this operation is advanced, it will affect the separation of the upper and lower threads by the
movable knife, and it may result in thread trimming errors.
1647M
4. The upper and lower threads caught by the movable knife in 3 above are gradually spread by the knife and lower
thread finger and cut by the fixed knife tip. The thread take-up lever has now approached the top of its stroke.
When the knife is spreading the thread, the tension release relieves the upper thread tension to prevent excessive
tension and to enable the upper thread to be smoothly extended. The length of thread spread by the movable knife
determines the length of thread remaining from the needle tip and bobbin when thread trimming is complete, and
effects stitch formation at the beginning of the next sewing procedure.
1643M
2.The thread trimmer signal is then relayed, and the
thread trimmer cam drives the movable knife. The
rotary hook catches the upper thread and passes it
through the inner rotary hook.
Movable knife
Lower thread
Fixed knife
Lower thread
Fixed knife
SL-710A
4. MECHANICAL DESCRIPTIONS
Upper thread
Upper thread
(As seen with fixed knife removed)
Movable knife
1644M
Movable knife
Lower thread
finger
1646M
Movable knife
Lower thread
finger
1648M
12