In the operation of an interlock sewing machine, adjusting the bobbin thread tension is a crucial step to ensure uniform and secure stitching. By rotating the tension adjustment screw on the bobbin case, the tightness of the bobbin thread can be controlled — typically, turning clockwise increases the tension, while counterclockwise decreases it. For precise calibration, a tension gauge can be used, with a recommended range of 50–70 grams to accommodate different fabric characteristics. For example, when working with elastic fabrics, the tension should be slightly reduced to prevent thread breakage caused by excessive tightness; for thicker materials, the tension should be increased to strengthen the stitches. After adjustment, test sewing should be performed to check stitch consistency. If looseness appears, it may indicate insufficient tension, providing a basis for resolving loose stitch issues.