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9-7. Troubles and corrective measures (sewing conditions)
Trouble
1. The needle thread
slips off at the start of
bar-tacking.
2. Thread often breaks or
synthetic fiber thread
splits finely.
3. The needle often
breaks.
4. Threads are not
trimmed.
(Bobbin thread only)
Cause
① Stitches are slipped at the
start.
② The needle thread remain-
ing on the needle after
thread trimming is too short.
③ The bobbin thread is too
short.
④ Needle thread tension at
1st stitch is too high.
⑤ Pitch at 1st stitch is too
small.
① The shuttle or the driver
has scratches.
② The needle hole guide has
scratches.
③ The needle strikes the work
clamp foot.
④ Fibrous dust is in the
groove of the shuttle race.
⑤ The needle thread tension
is too high.
⑥ The tension of the thread
take-up spring is too high.
⑦ The synthetic fiber thread
melts due to heat generat-
ed on the needle.
① The needle is bent.
② The needle hits the work
clamp foot.
③ The needle is too thin for
the material.
④ The driver excessively
bends the needle.
⑤ Needle thread is stepped
on by the work clamp foot
at the start of sewing.
(Needle bend)
① The counter knife is dull.
② The difference in level
between the needle hole
guide and the counter knife
is not enough.
③ The moving knife has been
improperly positioned.
④ The last stitch is skipped.
⑤ Bobbin thread tension is
too low.
－
Corrective measures
◦ Adjust the clearance between the nee-
dle and the shuttle to 0.05 to 0.1 mm.
◦ Set soft-start sewing at the start of
bartacking.
◦ Correct the thread tension release tim-
ing of the thread tension controller No. 2.
◦ Increase the tension of the thread
take-up spring, or decrease the ten-
sion of the thread tension controller
No. 1.
◦ Retard the tension disk closing timing.
◦ Decrease the tension of the bobbin
thread.
◦ Increase the clearance between the
needle hole guide and the counter
knife.
◦ Decrease the tension at 1st stitch.
◦ Decrease the number of rotation at
1st stitch at the sewing start. (Extent
of 600 to 1,000 sti/min)
◦ Make the pitch at 1st stitch longer.
◦ Take it out and remove the scratches
using a fine whetstone or buff.
◦ Buff or replace it.
◦ Correct the position of the work clamp
foot.
◦ Take out the shuttle and remove the
fibrous dust from the shuttle race.
◦ Reduce the needle thread tension.
◦ Reduce the tension.
◦ Use silicone oil.
◦ Replace the bent needle.
◦ Correct the position of the work clamp
foot.
◦ Replace it with a thicker needle ac-
cording to the material.
◦ Correctly position the needle and the
shuttle.
◦ Widen the distance between the nee-
dle and the wiper. (23 to 25 mm)
◦ Replace the counter knife.
◦ Increase the bend of the counter knife.
◦ Correct the position of the moving
knife.
◦ Correct the timing between the needle
and the shuttle.
◦ In crease the bobbin thread tension.
－
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