The rules on seam allowances pt.2
I decided to write a follow up to my first entry because comments from two people who have every reason to expect they’d understand what I was talking about, didn’t. Perhaps my entry was too brief (1,500 words on seaming is brief) but I didn’t want to get too technical lest legions of readers expire from abject boredom, their hands affixed to the armrests of their chairs in the final stages of rigor mortis.
I have a silly question on overlock seam allowances. Are 3/8″ and 1/2″ the seam allowance as marked on the pattern or the finished seam allowance after sewing? I think most of my RTW knit garments have 1/4″ finished overlock seam allowances, and I’m pretty sure 1/8″ is trimmed during sewing. That would mean a starting seam allowance of 3/8″.
When we talk about allowances, we mean the total from nett (the sewing line) needed to complete the seam successfully. In the course of doing an overlock (serged) seam, it is presumed that successfully completing the seam means it is necessary to trim off a scant 1/8″ portion of the edge (or edges). In other words, the seam allowance (3/8″ for 3-4 thread, or 1/2″ for five thread safety stitch) will mean that after sewing and required trimming of the edge, the remainder will be 1/8″ less the finished viewable seam. So yes, a knit garment with a 3 or 4 thread overlock seam will finish at 1/4″ width from nett (the seam line) although 3/8″ was used to make it.
I understand where the confusion comes in. In the typical course of sewing a seam with a regular machine, the seam allowance is not trimmed away. The seam you see is the seam allowance you started with. Another way to explain it is if you were to dissasemble a knit garment (neatly undoing all the seam threads) to make a pattern from it, you would need to add another 1/8″ (that had been trimmed away) to all overlocked seams on the pattern to permit successful seam formation of the new garment.
Is 3/8″ cut off (and then the stitching takes up more on overlock) or is the actual seam at 3/8″ (so you cut off less).
I understand how this can be confusing. It is a matter of orientation, your point of reference, what you consider to be the zero point. With basic seaming, the point of orientation is (nearly always) the needle path, aka the sewing line. In the case of the single needle machine, it’s the position of needle (middle of the throat plate). This is the zero point. With usual handling, everything to the left of the needle is garment or product. Everything to the right of the needle is seam allowance. If it’s a two or more needle machine, the farthest left needle is the zero point of orientation (the sewing line). The space between the left and right needle and the fabric to the right of the right needle is all seam allowance. This is confusing because many people are oriented to lining things up on the right because there are (often) little lines on the throat plate and so, align the cut edge as their zero point or frame of reference on those markings. I just checked the machines I use most often. None of the industrials have these markings but all of the home machines do.
If you align the edges to cut off 3/8″, the needles to the left are seaming 1/4″ into the area that belongs to the body of the garment or product. To prevent this confusion, the guidelines on the throat plate of overlocks (only on my home machine) are measured and aligned in accordance with the needle path (the farthest left needle). If you do not have markings on the little guide that holds the knife of the overlock, you can get one of those pattern rulers and measure the distance between the left most needle and the knife. On my five thread overlock, this distance is 3/8″ -but that’s because it is a safety stitch. In order to to have a bit to trim away, I need to make the allowance 1/2″. [My problem is I forget to. My previous machine was a 3/4 thread that finished at 1/4" (total 3/8") so for the past year, I've consistently made my seam allowances too small for the new machine.]
Again, the above refers to most (but not all) basic seam formations aka “joining”. If you’re top stitching a patch pocket, the edges of the pocket are the point of orientation (the zero point). You’d stitch inside the area. This is similar to top stitching, say a collar edge. The edge of the collar becomes the zero point. This may not seam logical but think of it this way: you don’t need to add seam allowance to a pattern for top stitching. Top stitching is not a joining seam, it’s “ornamental”. Ornamental is the name of a separate and specific seam class. Not needing separate seam allowance is but one reason ornamental stitches are an entirely different seam class.
In the section on industrial sewing in my book (specifically 131-132), I explain the anatomy of seam class pictographs to analyze the manner used to illustrate them. You can also find out more about seam classes here or read related topics on sewing machine attachments. I was sure I’d written more about seam classes before but I’m not finding anything. Should I do something like that? I suppose it’s the only way you’ll see what I mean when I say that seam allowances vary according to the seam class and the equipment used to form them. If I ever write a production pattern making book, this will be a big part of it mostly because I don’t like having to figure it out over and over, asking around or measuring or whatever. I don’t know why somebody hasn’t cooked up a chart and posted it around long before now. Sure it’s minutia and a tedious task; where’s an autie when you need one?