Brother T-8421D Instruction Manual page 104

Problem
Upper and lower threads
are breaking.
Incorrect thread trimming
Broken needle
Possible cause
• Is the needle tip bent? Is the needle tip blunt?
If the needle tip is bent or blunt, replace the needle.
• Is the needle properly installed?
If it is incorrect, install the needle correctly.
• Is the machine properly threaded?
If it is incorrect, thread the thread correctly.
• Is the oil tank filled with lubricating oil?
If the level of lubricating oil is below the bottom
reference line, add more oil.
• Is the upper or lower thread tension too strong or too
weak?
Adjust the upper thread or lower thread tension.
• Is the rotary hook blocked with dust or thread
scraps?
Clean the rotary hook.
• Is the upper thread loose because the thread take-up
spring stroke is too small?
Adjust the position of the stopper.
• Is the rotary hook, feed dog or other part damaged?
If the thread path is damaged, smooth it with
sandpaper or similar. Alternatively, replace the
1
damaged part.
• Is the thread path damaged?
If the thread path is damaged, smooth it with
sandpaper. Alternatively, replace the damaged part.
• Is the thread take-up amount correct?
Adjust the thread take-up amount.
• Is the tip of the movable knife damaged or bent?
Replace the movable knife.
• Is the material being pushed or pulled with excessive
force during sewing?
• Is the needle properly installed?
If it is incorrect, install the needle correctly.
• Is the needle tip bent? Is the needle tip broken, or is
the needle hole blocked?
Replace the needle.
• Does the needle drop into the center of the needle
hole in the feed dog?
Check the needle drop position. If the needle does
not drop into the center of the needle hole in the feed
dog, reinstall the feed dog.
If the needle bar has rotated, rotate the needle bar to
1
the correct position.
1
1
1
1
Page
See Installing the needle
on page 17.
See Installing the needle
on page 17.
See Threading the upper
thread on page 22.
See Lubrication on
page 68.
See Adjusting the thread
tension on page 62.
See Cleaning on page 68.
See Adjusting the thread
take-up amounts of the
thread tension springs on
page 74.
See Adjusting the thread
take-up amount on
page 66.
See Installing the needle
on page 17.
See Installing the needle
on page 17.
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