JUKI AP-874/IP-420, Instruction Manual

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2. TROUBLE SHOOTING
Phenomenon Cause Corrective measure
1. Thread
breakage
① The thread path, tip of needle, blade point
of hook, or bobbin case positioning finger
has scratches.
② The needle thread tension is too high.
③ The needle comes in contact with the
blade point of hook.
④ Lubricating oil in the hook is too small.
⑤ The needle thread tension is too low.
⑥ The thread take-up spring pressdre is too
high, and stroke of the thread take-up le-
ver is too small.
⑦ The timing between needle and hook is
too early or too late.
⑧ Idling amount of the bobbin is too large.
○ Re-sharpen the blade point of hook with
a fine emery paper to remove scratches.
Buff the bobbin case positioning finger.
○ Adjust the needle thread tension.
○ Refer to "III-1-2. Needle and hook ad-
justment" p.105 .
○ Adjust the oil amount properly. Refer to
"III-1-7. Hook oil amount (track of oil)
adjustment" p.108
and "III-1-8. Adjust-
ing the oil amount in the face plate"
p.109
.
○ Adjust the needle thread tension proper-
ly.
○ Decrease the pressure of the thread
take-up spring, and increase the stroke
of thread take-up lever.
○ Refer to "III-1-2. Needle and hook ad-
justment" p.105 .
○ Increase the spring pressure.
2. Stitch skip-
ping
① The clearance between the needle and
the hook point is too large.
② The timing between the needie and the
hook is too early or too late.
③ The holder pressure is too low.
④
The clearance between the top edge of
the needle eyelet and the hook point is
not correct.
⑤ The needle thread guide is not properly
threaded.
→ Refer to "I-4-2. Threading the needle
thread" p.14 .
○ Refer to "III-1-2. Needle and hook ad-
justment" p.105 .
○ Refer to "III-1-2. Needle and hook ad-
justment" p.105 .
○ Tighten the holder spring regulator.
○
Refer to "III-1-2. Needle and hook ad-
justment" p.105 .
3. Loose
stitches
① The thread has not been passed through
the forked section of the bobbin case ten-
sion spring.
② The thread path has been poorly finished.
③ The bobbin does not rotate smoothly.
④ The bobbin thread tension is too low.
⑤ The bobbin thread tension is too high.
⑥ The presser sponge is located too close
to the needle entry. As a result, the thread
comes in contact with the sponge.
→ Cut the section of sponge which
comes in contact with the thread.
○ Properly thread the bobbin case.
○ Grind the thread path with a fine emery
paper, or buff it up.
○ Replace the bobbin or the hook.
○ Adjust the bobbin thread tension proper-
ly.
○ Decrease the bobbin thread tension.