Why does my sewing machine keep jamming underneath?
However certain you are that the problem with the machine is most likely due to a huge tangled mess of thread in the bobbin underneath the fabric, the most common reason for the jamming is usually the lack of sufficient tension in the upper thread.
The tension could be too tight or too loose. Set the tension to the basic thread tension setting or adjust the tension manually. The combination of the needle size, thread size and fabric is incorrect. Be sure to use the correct size needle and thread for the type of fabric that you are sewing.
What setting should my sewing machine be on?
Most sewing is done in the 2.0 to 2.5 range. If you are foundation paper piecing, you may want to decrease your stitch length so that the paper tears away easier. Top stitching and quilting are usually done in the 3.0 to 3.5 range.
If your thread is pulled tight on the underside and not forming an even stitch then (counter-intuitively) it’s usually the top thread tension that’s wrong. Sometimes very lightweight fabrics such as sheers can get dragged down into the machine so it’s a good idea to use a straight stitch plate.