The tension settings on your sewing machine can vary widely depending on the fabric you use. Cotton is a medium-weight fabric that needs a moderate tension setting. For a perfect stitch, the upper and lower tension settings must be balanced.
Upper Tension
Your upper tension setting is controlled by a dial or LCD control located on the front faceplate of your machine. Cotton requires a moderate tension setting, usually between three and four. Always start adjusting your tension settings with your upper tension.
Lower Tension
The lower tension setting is controlled by a screw on the front of the bobbin casing. Only adjust this tension setting after you change and test the upper tension settings. Adjust the tension on the bobbin by turning the screw to the left or right 1/4 of a turn.
Testing the Tension
Each time you make an adjustment to your tension setting, test the setting by sewing a straight seam on a scrap piece of the cotton fabric you are using. If the tension is balanced, the seam will be completely flat on both the upper and lower sides of the seam. If your test seam has lumps or puckers, continue adjusting the tension until the seam is flat.