As with all knitwear and stretch fabrics, you can obtain the best result using a cover stitch machine and the clean or raw finish binder to make a binding. But if you do not have a cover stitch machine, don't despair. You can make perfect binding and straps with a serger, sewing machine and ball point twin needle.
How it looks on the right and the wrong sides you can see on the pictures below.
You can use for binding 100% cotton rib knit or rib knit with spandex (5%), which is preferable. If we sewed the rib knit binding like a bias tape binding on a woven garment, it would only add bulk. So, I recommend folding the binding only three times. It would work better.
Cut your binding 13/8” (35mm) wide. Make sure the ribs are perpendicular to the length of the strip. These strips have to be long enough to bind the neckline and armholes with the shoulder straps.
If you are using knit fabric for the straps - they tend to stretch while sewing. So first, you need to determine how long your straps will stretch while sewing.
1. Serge the one raw edge of the strip.
2. Fold the long raw edge of the binding on 1/3 width and press.
Fold over the serged edge of the binding, with serged edge just slightly lower than the folded edge. Press.
3. Mark the required length of the strap on a scrap of binding.
Using a ball point twin needle stitch the binding between marks. And measure the distance between the marks.
4.Using the formula below calculate the percentage of stretch.
X - starting measurement
Y – measurement after sewing
In my case:
(13*100)/12 -100 = 8%
Using the formula I got the 8% of stretch.
And now we need to calculate how long the shoulder strap needs to be before sewing.
Our formula is
required length - a percentage of stretch
In my case:
Required length = 12cm
percentage of stretch=8%
12cm – 8%=11,04cm
So, I already know the shoulder strap needs to be 11,04cm before sewing and can confidently make the strap in the required size.
Now we proceed to the binding.
First, you need to attach your neck and arm binding.
The front and back neckline are bound first, and then the armholes are bound, with the shoulder straps sewn as extensions of the armhole binding.
5. Prepare the binding as it shown in steps 1-3. Open up the serged edge of the binding.
6. Place the bodice piece right side down with the raw edges (and fold line) aligned as shown below.
Your binding (not the neckline) must be stretched a little bit to avoid the wavy edge after sewing.
8. Then fold the serged edge over the top and pin to prepare for basting.
9. Baste in place.
10. Edgestitch the binding with a twin needle. It will allow your binding to stretch and imitates the look of a cover stitch.
11. Cut off the extra binding.
12. On the wrong side, the binding looks like this.
13. Place the two pieces of bodice opposite each other right side down.
14. Pin the binding to the one bodice piece armhole. Then measure the length calculated earlier, and pin the binding to another bodice piece armhole.
16. Edgestitch the binding with a twin needle. Check the length of the strap.
17. Repeat steps 14-16 for another armhole.
18. Then sew the ends of the bias tape into the side seams when you sew those seams to join the front and back of the bodice together.
19. Feed the rest of thread chain back into the stitching.
20. Make tacks to hold the seam allowance of the binding down.
I hope this tutorial will be useful for you!