I just got this free ebook. It has lots of information on hemming. Get your copy!
I’ve been sewing knit hems on T-shirts lately with a double needle (also called a twin needle). One suggestion in this ebook is to use Wonder Tape (page 94) to stabilize knit hems. I try to avoid buying expensive consumable sewing notions – so my cheap natural solution is to use home-made spray starch. To make heavy starch, shake up 1 tsp (5 mL) cornstarch (aka cornflour in the UK) in 1 cup (240 mL) of water in a spray bottle.
Shake the bottle each time before you spray. Spray the fabric until it is wet and then press up the hem. Your iron needs to be set on the wool setting (about 300°F / 148°C). Any hotter than that and you will burn the starch. Hold the iron down in one spot for a few seconds until the fabric is most of the way dry, gently twist the iron back and forth to loosen it, then lift it and move to the next spot. If you try to lift the iron too soon it will be stuck to the fabric. This isn’t commercial starch with additives to make it slippery! However, the stickiness can be useful – the heavy starch not only stiffens the fabric; it also temporarily glues the layers of fabric together if you use enough starch. Starch really only works well on natural fibers, which is pretty much all I sew with. It makes sewing a perfect knit hem really easy, then it washes right out.
Starch residue does build up on the iron. I use my old iron for ironing on starched fabric and fusibles. To clean it after each use, I turn the heat all the way up and iron over an old wet towel. This doesn’t get all of the build-up off, so very occasionally I will use commercial iron cleaner to completely clean the iron.
When I was using lightweight cotton jersey that really wanted to curl at the edges, I made boiled starch and starched all of the fabric before I cut it. It really helped to make the fabric behave better without making it stiff. For medium starch: Put ¼ cup (40 g) of cornstarch in a cooking pot. Measure out 2 quarts (2 L) of cold water. Pour a little of the water into the cornstarch, sir out the lumps, then add more water until about half of the water is in the pot. Heat on medium heat, stirring occasionally. When it starts to thicken, start stirring constantly. When it starts to boil, turn down the heat and boil for 1 minute while stirring. Turn off the heat and mix in the rest of the water.
Wet the fabric with plain water, wring it out, put it in the starch mixture and swish it around. If you have more pieces of fabric to starch, wring out the first piece, then put the next piece of fabric in. For small pieces of fabric, squeeze out the liquid, then roll in a towel. For larger pieces of fabric, put the fabric in the clothes washer and set the washer to spin only (I used a low speed spin cycle to retain more of the starch). You can save some of the liquid to put in a spray bottle if areas of your fabric need additional starching later, or you can mix up some raw starch when you need it. Store the starch mixture in the refrigerator – it’s a food product, and it will spoil. It’s the price you pay for using starch without preservatives such as formaldehyde, which is perfectly fine with me!
Either let the fabric dry flat, or put in in the dryer and take it out when it is still damp. Put a towel or protective cloth over your ironing board. Iron the fabric dry with an iron set to the wool setting. Set the iron down in one spot for a few seconds, then slightly rotate it back and forth to unstick it before lifting. If you try to lift the iron too soon, the fabric will stick to it. If you have so much fabric that ironing the whole thing would take hours, just iron any curled edges or wrinkled areas, then hang the fabric on a drying rack or lay it flat to finish drying.
I didn’t realize this when I first started sewing knit hems with a double needle: In order for the stitches to stretch, you need to increase the upper thread tension and/or reduce the bobbin tension until you get a zig-zag on the underside. The problem with increasing the upper thread tension is that you can get a raised channel (aka tunneling) between the rows of stitching. Stitching on heavily starched fabric will help prevent tunneling. Loosening the bobbin tension sounds like a better idea, but I haven’t tried it, since I try not to adjust the bobbin tension too often (I don’t want to wear out that little screw). Although, now that I’m thinking of it, I have an extra bobbin case – I should just leave that one set to a looser tension. With starched fabric and a 3 mm wide double needle, I haven’t had any problems, but I imagine tunneling would be more of an issue with double needles set further apart, and you would really need to loosen the bobbin tension.
Did you know that double needles can be used with any zig-zag machine, whether or not they are mentioned in your sewing machine manual? Here’s how I use them in my Singer Fashion Mate 252. I fill up a bobbin and set it underneath my upper thread spool, with the threads unwinding in opposite directions to keep them from tangling. I thread both threads through as if they were one, stopping before the last thread guide. I pull out a few inches of thread to get the twists out, then thread one thread through the final thread guide and into the left needle. Finally, I thread the other thread directly into the right needle, skipping the last thread guide.
When you buy double needles, make sure you are not buying needles set any further apart than the widest zig-zag stitch your sewing machine makes. You can get extra wide 6 and 8 mm double needles now that will only work on certain sewing machines. Get the right kind (universal, ballpoint, stretch) and size of needle for the fabric you are using, too.