Troubles And Corrective Measures - JUKI LH-3500A Series Instruction Manual

9. TROUBLES AND CORRECTIVE MEASURES

TROUBLES
1. Thread breakage
(Thread is untwisted
or scraped.)
(Needle thread 2 to
3 cm is left on the
wrong side of the
fabric.)
(Bobbin thread
comes out of the
bobbin.)
2. Stitch skipping
CAUSES
① There is a sharp edge or burr on
the thread path, needle point, hook
blade point or bobbin case resting
groove on the throat plate.
② Needle thread tension is too high.
③ Bobbin case opening lever
provides an excessive clearance
at the bobbin case.
④ Hook blade point hits the needle.
⑤ Hook is not lubricated properly.
⑥ Needle thread tension is too low.
⑦ Thread take-up spring is too tight
and its stroke is too small.
⑧ Needle-to-hook timing is wrong.
⑨ Thread untwines.
⑩ Uniform thread loops cannot be
formed when making chain-off
thread.
⑪ Bobbin is wound with excessive
amount of thread. (In particular,
filament thread)
① Clearance between the needle and
the hook blade point is too great.
② Needle-to-hook relation is wrong.
③ Pressing force of the presser foot
is not enough.
④ Needle bar height is wrong.
⑤ Needles are a little too thin.
⑥ Synthetic thread or thin thread is
used.
⑦ Stitch skipping occurs at the
beginning of sewing.
⑧ Stitch skips when sewing
multilayered parts of the material.
⑨ Stitch skipping occurs when the
material thickness changes, i.e.,
from a two-ply part to a multi-
layered part and vice versa.
CORRECTIVE MEASURES
◦ Remove sharp edge or burr using a fine
sandpaper. Polish the surface of the
bobbin case resting groove on the throat
plate using a buffing wheel.
◦ Adjust the needle thread tension.
◦ Reduce the clearance.
(Refer to
guide"
◦ Refer to
p.27.
◦ Increase the amount of oil supplied to the
hook according to
amount of oil in the hook"
◦ Adjust the needle thread tension.
◦ Reduce the tension of the spring and
increase the stroke.
◦ Refer to
p.27.
◦ Wind the thread on the
needle.
◦ Use the thread guide equipped with felt
pad.
◦ Use the optional needle clamp wire.
◦ Wind the bobbin with thread by 80 % of its
capacity.
◦ Refer to
p.27.
◦ Refer to
p.27.
◦ Tighten the presser spring regulator.
◦ Refer to
p.27.
◦ Replace the needle by thicker ones.
◦ Wind the thread on the needle.
◦ Use the optional needle clamp wire.
◦ Run the sewing machine under the soft
start mode by 2 to 3 stitches from the
sewing start.
◦ Use the needle thread guide and precisely
adjust the hook timing.
◦ Move the presser foot toward the operator.
At this time, be careful not to allow the
presser foot to come in contact with the
needle.
– 54 –
"6-5. Adjusting the inner hook
p.37.)
"4-17. Needle-to-hook relation"
"4-4. Adjusting the
p.9.
"4-17. Needle-to-hook relation"
"4-17. Needle-to-hook relation"
"4-17. Needle-to-hook relation"
"4-17. Needle-to-hook relation"