Cover Hem Stitch machines do an interlock stitch on hems, necklines, sleeves and seams. The stitches have double or triple needle topstitch with a serger type stitch underneath. You can see on the hems and sleeves of t-shirts and other knitwear. Fabric and stitch have a lot of stretch without breaking the seams.
Sportswear/knits can be seamed together on the inside with 3 or 4 thread sergers.
Bottom hems, sleeves and necklines need a bottom coverstitch with two or three needle top stitching showing on the outside
Here.
You can get seperate serger and coverstitch machines, or start with a
combination
5 thread serger and coverstitch machine.
You can construct Tshirts with combination serger (side seams and set in sleeves) and coverhem stitch (hems and necklines) machines Here.
Then you can use a heat transfer press to imprint designs and lettering Here. John Douthat, Owner and Tech, AllBrands