Wednesday, February 23, 2011

Tutorial: Using your serger to 3-thread flatlock

Easy 3-thread Flatlocking

Some sergers have the ability to do a 2-thread, or "natural", flatlock. If your serger is not able to do that, you can do a 3-thread flatlock instead, and it is really easy! In flatlocking the idea is to make the needle thread tension so loose that the stitching pulls apart flat. This makes a really nice stitch with a flat seam allowance, which is great for things like boys swim shirts or for fleece socks.

1. Set your machine up for 3 thread serging. For me, this meant removing the right needle. With normal, balanced tensions, sew on scrap piece of fabric just to verify that your machine is set up and threaded correctly for a 3-thread stitch.

2. Now, turn your needle thread tension all the way down to its lowest setting.

3. Turn your lower looper thread tension all the way up to its highest setting.

4. Now, with wrong sides together serge while cutting off a small amount with your knife. It will look like the top line of stitching shown in this picture:

Can you see how it forms V's? That is because the needle thread tension is so loose that the needle thread is being pulled all the way to the edge of the fabric. The bottom line of stitching is just a regular, balanced 3-thread stitch, so you can see the difference.

5. Next, you will pull apart the stitching. So hold it with the seam allowance up like this:

And just tug on the two pieces of fabric.

6. It will flatten out to look like this on the right side:

And here is how it will look on the wrong side:

Some hints:
- Because of the extreme difference in tension between the needle thread and the lower looper thread, you should go slow or your threads might break.
- Depending on the thickness of your fabric and your exact serger, it might be possible to be less extreme in the tension settings and still get a good flatlock. I can keep my needle tension at 2 or 3 (instead of 1) and my lower looper at 7 (instead of 9) and still get a flatlock.
- It is not necessary to use the knife and cut off fabric if you don't want to. You could put your knife in the non-cutting position and flatlock on a fold of fabric for decorative topstitching.
- If you want the "ladder" side of the topstitching on the right side of the garment, you should flatlock with the fabric right sides together.
- If the "ladder" is on the right side of the fabric, it makes a nice sort of channel through which you can weave decorative, narrow ribbon.


  1. Thanks for the clear photos and the descriptive detail on how to flatlock.

    I appreciate that you pointed out that using the knife is optional. I was confused because some sites said to use the knife, some said don't. So it was good to get confirmation from you that you can cut the material if you want, but that you don't HAVE to.

    Thanks again.

  2. I am glad this was helpful! This is by far my most viewed post, but it has the least comments. :)

  3. Thanks for sharing :-)
    Gonna try some "top-stitching" with this decorative stitch.

    Very informative and clear pics as well!

  4. How sturdy is this? I have at the moment a sensory kid and I'm wonder about constructing a crotch seam this way....

  5. Sue, I have been wearing the socks a ton over the past year, and none of the seams have popped. However, a crotch seam gets a lot more stress. I have never heard of someone using flatlock for a crotch seam. Why don't you try something like a pair of pajama pants? That way, if the seam fails, at least he is home!

  6. I hope you don't mind, I am linking to this from Stitcher's Guild ( to illustrate, very well, the serger flatlock.

  7. Thanks for that: I had been searching for answers as to why my thread was breaking and yours is the only site that was at all helpful. Well done.

  8. Thank you so much for this site! I am taking a class on line and was really having a hard time understanding this stitch. Now I believe I will finally be able to do it correctly. I will return often. Thanks again.

  9. Hoping you are still able to receive this message. I am needing to sew together a teddy bear foot to its leg, would you recommend a flat lock stitch or using a sewing machine? I appreciate you giving your time and expertise to assist us all.


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