JUKI MO-6800S Series Engineer's Manual page 59

WARNING :
Turn OFF the power before starting the work so as to prevent accidents caused by abrupt start of the sewing machine.
Trouble
3. Needle breakage
4. The needle point is crushed.
(Double chainstitch needle)
Cause (1)
3-1) Needle entry 1-A)
3-2) Upper looper posi- 2-A)
tion
3-3) Contact with the 3-A)
looper
3-4) Needle guards 4-A)
3-5) Needle No. 5-A)
3-6) Thread tension 6-A)
3-7) Height of the feed 7-A)
dog or needle
4-1) Needle guard 1-A)
4-2) Contact with the 2-A)
looper
Cause (2)
The needle entry has not been correct-
ly adjusted, and the needle strikes the
throat plate or presser foot.
The projecting amount of the upper
looper is too much or the height of the
upper looper is too low.
The needle strikes the looper, resulting
in needle breakage.
A needle guard has been improperly
positioned, causing the needle point to
strike it.
The needle is too thin for the materials.
The thread tension is too high.
The feed dog is too high, or the needle
is too low, causing the needle to deflect
with resultant needle breakage.
The needle guard is too low, or its longi-
tudinal position is not correct.
The tilt of the looper is not correct. The
longitudinal motion of the looper is not
correct.
Checking order and adjusting procedure
Correct the needle entry.
Refer to the pertinent Standard Adjustment.
Re-position the looper so that it does not come
in contact with the needle. Adjust the longitudinal
motion of the double chain looper for the contact
of its back with the needle.
Refer to the pertinent Standard Adjustment.
Replace the needle with a thicker one.
Reduce the thread tension.
Refer to the pertinent Standard Adjustment.
Increase the height of the needle guard. Check
the clearance between the needle and needle
guard.
Check the tilt of the looper. Correct the longitudi-
nal motion of the looper, and increase the clear-
ance between the looper and needle when the
looper reaches its most retracted position.