This guide has been provided by sewing tutor & author Lorna Knight. Take a look at her website to find out where you could learn new sewing techniques with her in person: http://www.lornaknight.com/
When you buy a T-shirt the hem will probably have been finished with a ‘cover stitch’ machine. This looks like two parallel rows of stitching on the right side with overlocking loops on the wrong side. A standard overlocker cannot carry out this technique and a specialist machine is required but since few home dressmakers or hobby sewers have one we need to find another technique.
If you have sewn with stretch knit fabric you will be aware of the difficulties in handling it. Firstly, if you sew a stretch fabric with a straight stitch as soon as the fabric is pulled the threads in the seam will break. Secondly, if the fabric is not supported when being sewn on a standard sewing machine a seam or hem is likely to extend creating a wavy, rippled edge.
The answer, when hemming a stretch knit T-shirt or leggings, is to use a twin (sometimes called a double) needle. The single shaft has two needles to create the parallel lines on the right side similar to the cover stitch effect and on the reverse the bobbin thread produces a zigzag as it travels between the two needles. This works because the zigzag stitch of the bobbin straightens out when the fabric is pulled so that the thread does not break!
Choosing a twin needle
The space between the needles of a twin needle ranges from 1.6mm to 6mm. Use twin needles at the smaller end for decorative tucks and choose 4mm and wider for hemming. It is also important to buy a stretch twin needle for sewing knit fabrics because a standard/universal twin needle will cause missed or skipped stitches on the surface.
Sewing a Twin Needle Hem
1. Mark the required hem line with a row of pins and cut away any excess allowance leaving a hem depth of approximately 2cm (¾”).
2. Press the hem to the wrong side and hold it in place with a few pins. (Pin on the right side).
3. Fit a 4mm twin needle to the sewing machine and thread up with two reels. Feed each thread through the same guides but leave the second thread out of the last guide just above the needle to help prevent the threads from tangling and knotting.
4. Place the hem under the presser foot with right side facing upwards. Sew round the hem, approximately 2cm (¾”) from the folded edge.
5. Check the inside of the hem. The zigzag stitches should cover the cut edge neatening it in the process. Press to finish.
If the hem stretches and ripples as you sew either fit a walking foot or use a tear away stabiliser below the hem as you stitch. The walking foot will control the amount of fabric being fed under the presser foot to prevent it from stretching while the tear away stabiliser supports the fabric and feeds it steadily under the foot. Always try a sample before you start to find the best method for the fabric you are sewing with.
Want more useful sewing techniques? Then why not take a look at Lorna's books: http://www.lornaknight.com/profile.htm
Looking for more dressmaking techniques? Read Lorna's guide to interlining here