|My SM has a lot of problems making a decent, non-wavy double needle hem. I finally resolved this by using Wooly Nylon in the bobbin (threading it through the tension area, not bypassing it). I placed torn strips of newspaper under the fabric, next to the feed dogs; then sewed with matching poly thread in the twin needles. Tear away the newspaper after sewing. If the fabric is fragile (like onionskin), it's better to soak the hem and gently tear away the wet newspaper.