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7. MECHANICAL DESCRIPTIONS
S-7200C
65
７-６-２.
Upper and lower thread trimming sequence
1. The needle rises 1.8 mm from its lowest position and the
rotary hook tip catches the loop made by the needle.
(For heavy-weight materials, it rises 2.2 mm.)
2. The thread trimming signal turns on and the thread
trimming cam moves the movable knife. The upper
thread is held by the rotary hook, and it passes
underneath the inner rotary hook.
3. The tip of the movable knife pierces the middle of the triangular loop formed underneath the needle plate by the tip of the
rotary hook as shown in the illustration, and the upper and lower threads are separated. At this time, the thread take-up is
slightly higher than its lowest position. (approximately 330° of angular rotation of the upper shaft)
* If the timing for this is too fast, it will affect the separation of the threads by the movable knife, and thread trimming
errors may occur.
4. The upper and lower threads being held by the movable knife in step 3 are spread out gradually by the spread knives
and the lower thread finger and are cut by the tip of the fixed knife. At this time the thread take-up reaches to about its
highest position.
When the knives are spreading the threads, the tension release is activated at the same time so that the upper thread is
fed out smoothly and excessive force is not applied to the upper thread. The amount of thread which is held by the
movable knife is the trailing length of upper thread from the needle after thread trimming and the length of lower thread
trailing out from the bobbin case, and these amounts affect the formation of the seam at the next sewing start.
1644M 1643M
Movable
knife
Lower
thread finger
Lower
thread
Fixed
knife
Upper thread
(The illustration shows the fixed knife removed.)
1647M
1645M 1646M
1648M
Movable
knife
Movable
knife
Upper thread
Movable
knife
Lower
thread
Fixed
knife
Lower
thread
finger