JUKI DDL-9000C-S Series Engineer's Manual page 175

Trouble Cause (1)
From the previous page
1-2) The R surface treatment
of the edge of a thread
trimming knife (other than
a cutting portion) lacks.
1-3) The low-intensity thread
(such as a high-count fila-
ment type thread or cotton
thread) is being used.
1-4) The bobbin thread is too
short at sewing start.
1-5) Knot tying between
the needle and bobbin
threads is not easy at
sewing start.
Cause (2)
From the previous page
1-G) The knife thread guide, moving knife,
or hook has a flaw.
1-H) The lifting amount of the auto-lifter
device (AK-154) is too large.
2-A) The thread makes contact with each
other and becomes so short at the
time of trimming thread that it becomes
difficult for the thread to make a knot
at the start of sewing.
3-A) When the thread trimming speed is
fast, the thread comes to be trimmed
short at the time of trimming the
thread.
4-A) Bobbin idling pulls the edge of the
bobbin thread into the bobbin case.
4-B) The pressure of the bobbin presser is
too high.
4-C) A flaw on the hook causes the bobbin
thread to be cut too short.
5-A) The sewing speed at sewing start is
too fast to knot the needle and bobbin
threads together.
Check and corrective measures
Check whether the hole portion of the hook (especially
the blade top) or knife thread guide has a flaw. If there
is a flaw there, buff the flaw portion sufficiently. If the
flaw portion is large, replace the part.
Excessive rise of the auto-lifter device causes the
thread guide with presser bar guide to rise rapidly and
the needle thread to be pulled by the thread take-up
spring resulting in shortened needle thread. Readjust
the rise amount of the AK device to approx. 8.5 mm.
Alternatively, set the remaining amount of the needle
thread relatively longer for use of the AK device.
The edge of a thread trimming knife (other than a cut-
ting portion) is buff-finished to the extent of R0.5 as an
R surface treatment.
The factory-configured thread trimming speed (300sti/
min) is lowered.
(Recommendation: 220sti/min)
Increase the pressure of the bobbin presser.
Increase the bobbin thread tension.
Reduce the pressure of the bobbin presser. However,
be careful not to idle the bobbin.
Repair the flaw on the hook, or replace the hook.
Conduct 1 or 2 stitches of soft-start (800 sti/min) at the
sewing start.