The top I've made up here was done in jersey knit fabric, but I've found that woven fabric works just as well, though you have to scale up your sizing to compensate for the lack of stretch.
If you are using knit fabric, remember to use a rounded tip needle to avoid tearing the fabric. The top I've made as an example for this tutorial was cut out of an old stretched out tee shirt and I've used the finished bottom edge as my bottom edge (always a bonus with knit).This top also uses lettuce edging. I've used the rolled hem setting on my serger (interlocker) to do lettuce edging on the knit by turning the differencial feed to low as it goes. To do lettuce edging without a serger, use a zig zag stitch to finish the edge of the material while stretching the fabric. Practice a bit before you start on your garment to find the width and length of zig zag stitch that appeals most to you.For this top, the front piece fits or is the same size as a regular tank top, minus the shoulder straps. I suggest using an existing tank top that fits well as a basis for sizing.The back piece is in the same shape as the front, but a few inches wider. It will be gathered with shirring to fit properly.
The straps should be approximately one and a half times larger then you would normally want. Cut these long, though, because you will be able to fit them at exactly the length you want and shirring does make exact dimensions hard to determine. For width, I've made my straps about 2" wide for a child's size 5/6. I wouldn't change this width too much, though, for any size - adults will most likely want a proportionally smaller strap than a child, lest ye end up looking like you have wings sprouting from your shoulders.
This is shape of the tank top:
I've made this one pattern piece for both the front and back pieces. To get the back piece, I cut around the pattern. To cut the front, I fold on the dotted line and cut around the remaining pattern.
After you've assembled your pattern pieces, put the right sides together of the front and back pieces and sew the sides together.Next, finish the underarm portion. You can use any type of finishing that makes you happy - lettuce edge or folded hem. May I suggest, though, my favorite thing to do with knit material when a bit of stabilization is needed, such as in a neckline or underarm. I like to sew in single-fold bias tape as interfacing to prevent misshaping the knit when sewing and through subsequent wear and washing.Cut an appropriate lenght of the bias tape and iron open the shorter side of the fold. Line up the open edge with the under arm edge, right sides together, and sew close to the edge with the bias tape on the bottom by the feed dogs. (If the knit portion is on the bottom, the feed dogs will stretch the knit in a most heinous way when you stitch.)
Turn the bias tape right side out and press. When pressing knit, do not move the iron back and forth but press one section, lift the iron, and then press another. Make use of a dress maker's ham if you have one - much friendlier for these curved areas. If you still find the knit becomes a bit out of shape, hover the iron about a 1/2" over your stitching and use steam to reshape the fabric.
Stitch again about 1/4" or 3/8" to finish the hem. Again, place the bias tape on the bottom while you stitch.
Next, hem the bottom of the top with either a regular folded hem or lettuce edging.
Finish the top edge of back piece and the straps with your lettuce stitch, treating it as one long edge.
Now that the straps are in place, replace your bobbin thread with elastic thread and shirr along the straps and the top of the back piece, again treating it as one long edge. If shirring is new to you, there is some instructions here.
With the front top edge finished, check the length of the straps for a proper fit and then stitch them in place along the top edge. And that completes the top.
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