Good question! I serge over elastic a lot in both my projects and in my tutorials, but I don’t focus all that much on what that looks like. Here’s a tutorial to show how I sew elastic onto my projects with a serger.
3 thread overlock or 4:
3 thread overlock. It allows for more stretch.
What needle to use in the serger?
I use stretch needles.
To disengage the knife or not. I do it both ways, depending on the project. When I first started doing this technique, I always disengaged the knife. As I got familiar and more comfortable with the technique, I attempted the trimming as I serge.
It’s good to trim the fabric whenever serging. so when the project is more manageable, like yoga pants, I cut away a slight amount, but am careful to not trim the elastic. You will need to keep the edge of the elastic in a bit from the edge of the fabric.
I disengage when the project is fiddly, difficult to control while serging (like slippery fabric or a small area like a small person’s sleeve). Without the knife cutting away, you can keep the edge of the elastic right along the edge of the fabric.
Use wooly nylon thread in loopers or not.
I use wooly nylon thread in the loopers when I sew: Swimwear, underwear, camis, and compression shorts/shirts. In other words, I usually use wooly nylon when the garment is close fitting and the serger threads will be right against the skin. The wooly nylon makes it more comfortable–it’s super soft and gives the seams a bit more stretch.
I don’t use wooly nylon in the loopers when I sew: cotton lycra clothing, like shirts, shorts, yoga pants, etc. I don’t use it when I sew woven cotton, poplin, seersucker, jersey knit, cotton interlock, or sweatshirt fabric. I never use wooly nylon for loose fitting clothing or clothing that won’t be right against the skin.
UPDATE: Just a reminder, you use regular serger thread in the needle(s). You never use wooly nylon thread for the needles, only in the loopers.
Here are some photos I took for a quick how-to sewing elastic in the round with wooly nylon.
Machine is set up: 3-thread overlock, using a stretch needle. I am sewing in the round, and I have pinned the elastic to the fabric at the quarter points (fourths).
Begin an inch from the first pin. Place the fabric under the presser foot and angle the fabric toward the presser foot.
Begin serging and straighten the fabric out as the serger starts sewing the fabric and elastic.
If using the knife to trim the fabric as you sew, then make sure the elastic is a bit off the edge of the fabric:
Stop and remove the pin. Hold the fabric behind the presser foot and also in front of the presser foot at the next pin, stretching elastic to fit the area. Try to keep the edge of the elastic even with the edge of the fabric (or keep the fabric an even distance from the edge of the elastic if the knife is engaged and you’re trimming as you serge). Sometimes I need to hold in front of the next pin to be able to keep the edges even, especially on curved edges.
Serge until presser foot reaches the next pin, keeping the elastic stretched, of course.
Remove the pin and readjust your hold to stretch the elastic to fit the next section.
Serge until you reach the beginning point. At which, you will sew past for an inch or sew. Serge off the edge. (please overlook how bad this photo is)
Trim threads. The wooly nylon threads will be soft on the skin, which is ideal if serging elastic onto underwear or swimsuits.
I hope this tutorial helps. Let me know if you have any questions that it didn’t answer for you.