Brother S-710A - 4-6-2. Upper and Lower Thread Trimming

4. MECHANICAL DESCRIPTIONS
SL-710A
12
4-6-2. Upper and lower thread trimming
1. When the needle rises 1.8mm (2.2mm with heavy-
wei
g
ht materials
)
above the down
p
osition the rotar
y
hook point catches the loop formed by the needle.
2.The thread trimmer si
g
nal is then rela
y
ed, and the
thread trimmer cam drives the movable knife. The
rotar
y
hook catches the u
pp
er thread and
p
asses it
through the inner rotary hook.
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.
4. The u
pp
er and lower threads cau
g
ht b
y
the movable knife in 3 above are
g
raduall
y
s
p
read b
y
the knife and lower
thread fin
g
er and cut b
y
the fixed knife ti
p
. The thread take-u
p
lever has now a
pp
roached the to
p
of its stroke.
When the knife is s
p
readin
g
the thread, the tension release relieves the u
pp
er thread tension to
p
revent excessive
tension and to enable the u
pp
er thread to be smoothl
y
extended. The len
g
th of thread s
p
read b
y
the movable knife
determines the len
g
th of thread remainin
g
from the needle ti
p
and bobbin when thread trimmin
g
is com
p
lete, and
effects stitch formation at the beginning of the next sewing procedure.
Movable knife
Movable knife
Lower thread
finger
Lower thread
Fixed knife
Movable knife
U
pp
er thread
Lower thread
Movable knife
Lower thread
finger
Fixed knife
1643M
1646M
1648M
1647M
1645M
1644M
(
As seen with fixed knife removed
)
Upper thread