The thread tension is adjusted depending on the sewing materials, layers of fabric and sewing method.
Needle thread tension is too tight
The bobbin thread (bottom thread) will appear on the right side (top side) of the fabric
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Loosen the needle thread tension by moving the dial to a lower number
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Needle thread tension is too loose
The needle thread (top thread) will appear on the wrong side (bottom side) of the fabric
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Tighten the needle thread tension by moving the dial to a higher number
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