Why does the upper thread of a sewing machine get tangled under the bottom thread?

The problem of the upper thread of a sewing machine getting tangled under the bottom thread is usually caused by the following reasons, and the corresponding solutions are as follows:

1. Abnormal tension of the upper thread

‌The upper thread is too loose‌: The upper thread does not form enough tension and cannot be interwoven with the bottom thread normally.

‌Solution‌: Adjust the nut of the upper thread tensioner and tighten the upper thread appropriately.

‌The upper thread is not threaded correctly‌: The threading steps are wrong, resulting in the upper thread not being embedded in the tension system.

‌Solution‌: Rethread according to the instructions, making sure the thread passes through all guide rings and tensioners.

2. Bottom thread related faults

‌The bottom thread is too loose or too tight‌: The tension of the bottom thread is unbalanced, resulting in disordered stitches.

‌Solution‌: Adjust the shuttle screw (tighten clockwise, loosen counterclockwise), pull the bottom thread to feel the resistance, and match it with the tension of the upper thread.

‌Incorrect bobbin installation‌: The bobbin is not properly inserted into the bobbin case or the thread head is not pulled out.

‌Solution‌: Remove the bobbin and reinstall it, making sure that the thread head is reserved for 5-10cm and passes through the bobbin case guide groove.

3. Hook assembly problem

‌Thread accumulation or jamming in the hook: Residual thread or debris hinders the movement of the upper thread.

‌Solution‌: Disassemble the hook liner, clean the jammed thread and wipe it for lubrication, reassemble and test.

‌Hook damage‌: The hook tip is worn or deformed and cannot hook the upper thread.

‌Solution‌: Replace the new hook assembly.

4. Operation or component abnormality

‌The presser foot is not lowered‌: The presser foot is raised, causing the upper thread to be unable to form tension.

‌Solution‌: Make sure the presser foot is completely lowered before sewing.

‌Needle problem‌: The needle tip is blunt, bent, or installed in the wrong direction.

‌Solution‌: Replace the new needle and make sure that the needle tip is facing forward and fully inserted into the needle bar.

5. Other possible reasons

‌Poor wire quality‌: Using inferior thread is easy to knot or break, it is recommended to replace high-tenacity sewing thread.

‌Improper feed dog height‌: The feed dog is too low, resulting in poor fabric movement. Adjust it to a height suitable for the current fabric.

Operation suggestions

‌Test the adjustment effect‌: Use waste fabric to try sewing and observe whether the stitches are smooth and whether the tension of the upper and lower threads is balanced.

‌Regular maintenance‌: Clean the hook, feed dog and other parts to avoid dust or thread accumulation.

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