Flatlocking compared to cover stitching and overlocking

While everyone’s away at Market -some designers say the technical stuff I write makes their eyes glaze over but then that’s how I feel when they start talking about marketing or fashion in general so that makes us even- we’re going to talk more about our favorite subjects, sewing and patterns. Yeah! Today specifically, the topic is flat lock machines. Flatlock, without a space is also correct, you get different results with both in internet searches. If you don’t know, we all covet flatlock machines, yes we do. Everybody wants one -whether they need one or not, not mentioning any names. From the forum:

Can someone tell me what the difference between flat-lock/flatseam construction and a serge seam with coverstitch? I am going for the type of seam you would find on technical base layers (brand names redacted).

I’m not familiar with these brands but my performance running tights have flatlock seams. As to the difference:

A coverstitch is basically 2 or 3 needle bobbin-less top stitching that provides some elasticity. It doesn’t have to be used on an overlocked (serged) seam but usually is. The back side of the seam forms a ladder pattern or similar configuration joining the lines of stitching to and fro. This is similar to the effect you’d get on a home sewing machine with a double needle. The latter differs from a true double needle because these have two bobbins so their lines of stitching are not joined on the back side from side to side. Below is a drawing of a coverstitch seam on the left with photos of an overlock on the right (courtesy). There are various models, this one attaches ribbing and coverstitches in one pass. For this reason we also covet coverstitch machines, perhaps more than flatlocks.

A flatlock is different in several respects. Unlike an overlock, there are no layers to the underside, the seam is butted together (usually, exceptions). On a flatlock, there is no seam allowance with layers folding to the underside. Think of it this way, in the application you’re describing there is no seam allowance per se because the cut edges of fabric are butted against each other and joined flat in a single layer with thread. The flatlock stitching on the top and bottom joins the two butted pieces. Below is a photo of a flatlock seam (courtesy).

My contractor does not have a Flat-lock machine, but can’t you get the same effect by doing a serge seam and then go over the top with a 5 thread coverstitch? I have technical base layer wear and I have my own samples with serge/coverstitch and I am not seeing much of a difference.

The seam of the overlock/coverstitch as compared to flatlock is thicker because it has layers of fabric. First you serge a seam meaning there’s two layers of fabric the width of seam allowance on the underside. Then that is top stitched down with the cover stitch or if you’re attaching binding it’s done in one pass but there’s still extra layers there from the allowances. There’s a lot of thread concealing this on the underside which kinds of cushions and flattens it. However, the flatlock while also with lots of threads, has no seam allowance layers turned to the underside because the cut edges are butted against each other.

I want to get into some technical stuff, but I hear that a flat-lock machine goes for more than 30k.

Flat locks are definitely pricier than the average machine but I’ve heard price quotes closer to $3K-$4K, a tenth that price. Just this morning, JC showed me a site selling used Union Specials for $1,850. They also have a few Wilson & Gibbs flatlocks going for $500 but I think the price tells you all you need to know. If something sounds too good to be true, it usually is.

What would be the drawbacks of just going with serge/coverstitch or is that the same darn thing as flat-lock seams?

Flatlocking is used primarily for performance goods, it has fewer layers to chafe. It depends on what you want to do. If you’re running a marathon, you definitely want tights with a flatlock. An overlocked coverstitched seam, while it won’t pull apart, will chafe along your legs and cause bleeding. There’s other uses too; if you’re making apparel for children with sensory challenges, a flatlock seam is more comfortable. An overlock may or may not bother children (or even adults) depending on thread type but in active wear, you’d only use overlocking on seams of garments intended for light training.

It should be easier to find these machines than it is. These are also called “off the arm” or sometimes even “flat bed” or “flat arm” machines. Manufacturers are Union Special, Yamato, Kansai, Juki, Pegasus, Wilson & Gibbs and I’m sure others. Another way to shop is by knowing the seam class. I’ve updated the Seam class specifications entry to include a nifty six page chart that lists seams by the most common vernacular. The particular type you’re looking for is a FSa-1 seam.

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