JUKI MF-7800D Series, Engineer'S Manual

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1. Thread breakage 1-1) Threading 1-A) Thread caught in thread guide, Incorrect Refer to threading diagram.
threading
1-2) Thread path 2-A) Resistance produced by flaw, burr, rust etc. Remove the flaw, burr etc. and process the thread
around needle entry of throat plate, stitch guide finish. However, replace such important parts
tongue, looper, spreader, needle thread as looper or throat plate etc. with the new part bec-
take-up auxiliary thread tension adjustment, ause their shape is changed by being processed.
needle guide, thread tension disc etc.
1-3) Needle guard 3-A) Strong contact of needle against needle In case needle exchanger, or looper needle guard
guard produces a sharp edge in needle is worn out, replace it with the new part.
guard resulting in thread breakage.
1-4) Needle 4-A) Too thin needle for the thread used Replace it with an appropriate needle.
1-5) Needle heat 5-A) Needle is heated depending on fabric Use thinner needle. Reduce the sewing speed.
type, number of fabrics, sewing speed Use silicon oil lubricant device.
resulting in thread breakage.
1-6) Thread 6-A) Poor quality and weakness of thread Replace it with the thread of good quality.
1-7) Thread tension 7-A) Too strong thread tension Reduce thread tension. needle thread guide rod is
positioned too high making the thread tension too
strong.
1-8) Interference 8-A) Interference with feed dog, throat plate Mount it in the correct position.
due to the incorrect mounting height of
looper
1-9) Chain-off thread defect 9-A) Flaw produced in stitch tongue in throat Remove the flaw, burr etc.
plate, feed dog, tongue in presser foot,
underside in presser foot
2. Looper thread breakage 2-1) Thread guide 1-A) Resistance produced by flaw, burr, rust Remove the flaw, burr etc. and process the thread
etc. in stitch tongue in throat plate, looper, guide finish. However, replace such parts as looper
looper thread cam, thread guide, thread with the new part because its shape is changed by
tension disc being processed.
2-2) Looper thread cam adjustment 2-A) Excessive tension applied due to the Refer to the standard adjustment figures.
incorrect position of looper thread cam
timing, thread guide
2-3) Thread tension 3-A) Too strong tension of looper thread Reduce the thread tension while checking to see the
tension balance against the needle thread, top cov-
ering thread.
Trouble Cause (1) Cause (2) Corrective measures
7. TROUBLES AND CORRECTIVE MEASURES
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