Flatlock, coverstitch, and the comment that changed the size

The situation

4 February, the pre-production sample review. Rhiannon Fenwick writes one comment against TG-620:

Side seams and inseam to flatlock for comfort. No other change.

It is a good comment. It is correctly worded, it names the reason, and everyone agrees with it. The sample room makes the pre-production sample in flatlock. It measures 88.2 cm on the hip against a specification of 88.0 cm plus or minus 1.0, and it is approved on 12 February.

On 5 March, day two of bulk, the audit on the line measures the hip at 90.8 cm. Every piece is out of tolerance by 1.8 cm, and 3,400 of them are already sewn.

Nobody made a mistake. This lesson is how that happens, and what the comment should have said.

The three seams, and what each one is really for

Course 4.2 covers the stitch and seam numbering itself. What this category adds is that the choice is made on what the seam does to skin — and that the comfort answer and the cost answer disagree.

Seam allowance is the strip of fabric taken up inside the seam. Butted means the two cut edges meet edge to edge, with nothing taken up at all.

TG-620 carries 2.94 metres of vertical seaming. Put money on it:

• Overlock: 0.911 minutes and 48.51 metres of thread — USD 0.146 a garment.
• Flatlock: 1.294 minutes and 79.38 metres of thread — USD 0.227 a garment.

The difference is USD 0.081 a garment, USD 1,944 over the order. That is the smallest of the three costs, and it is the only one that reaches a cost sheet.

The second cost is the machine bank. Six flatlock machines at 85% efficiency and 1.294 minutes a garment produce 236.5 garments an hour. One sewing line of 40 operators at a 21.4-minute SMV produces 95.3. SMV is standard minute value: the work content of one garment in minutes. So the flatlock bank feeds 2.48 lines — two, not three. Serimen can run this style on two lines and not on three, whatever the order book says, until it buys two more machines at USD 4,300 each. A machine class is a factory-level constraint, not a line-level one, and course 7.4 on capacity is where that argument gets made.

The third cost is the pattern, and it is the one that cost USD 18,428.

A seam type is a measurement

A butted flatlock seam has no seam allowance. The two cut edges meet and the stitch covers the join. An overlock takes 0.7 cm from each ply.

So on a garment with two side seams, the finished hip circumference under overlock is 2.8 cm smaller than the pattern. Under flatlock it is the pattern. Change the stitch without changing the pattern and the garment grows by exactly that: 88.0 becomes 90.8.

Why did the pre-production sample measure correctly? Because a sample machinist butting a seam trims the allowance away as she goes. She has always done it, she does not think of it as a change, and nobody has ever asked her to write it down. The sample room's skill hid the defect that the bulk line, cutting to a marker, could not. This is why a size set that passed does not prove a bulk that will pass, and it is a specific version of the general problem course 4.4 works through.

Why was the pattern not reissued? Because the comment said no other change. And because a construction comment goes to the sewing room's method sheet, while a seam allowance lives in the pattern room. The comment travelled to one department and its consequence lived in another.

What the 3,400 pieces cost

They cannot be corrected. Taking 2.8 cm out means re-sewing over a butted seam, which leaves the original needle line. On an elastane knit a needle line is not cosmetic. Every one of those holes is a place where a filament has been cut, and it will open into a run in the wash.

• Made cost at that point: fabric USD 4.12, trims USD 1.35, cutting and sewing USD 2.05 — USD 7.52 a piece.
• Cleared as seconds at USD 2.10.
• Net loss: 3,400 pieces at USD 5.42 — USD 18,428, plus two and a half days of line time.

The version of the comment that costs nothing is eleven words longer:

Side seams and inseam to flatlock, type 607, for comfort. Pattern room to reissue the affected panels at zero seam allowance and confirm the graded hip measurement before cutting.

Two more things this fabric does to a seam

The seam has to stretch further than the cloth. TG-620's fabric extends 118% at the load a body puts on it. A lockstitch, type 301, gives roughly a quarter to a third of that. So a lockstitch on a load-bearing seam here is not a sewing defect. It is a defect on the drawing, and it will burst the first time the garment is pulled on. An overlock with correct differential feed gives 110 to 140%, a flatlock 120 to 160%. Differential feed means the machine moves the fabric under the needle at two different speeds, front and back. Course 4.2 has the stitch-type figures. The rule this category adds is that the seam's extension is a specification line, checked before the machine is chosen.

The needle is a consumable, not a fitting. A round-point needle cuts elastane filaments instead of pushing between them, and needle cutting is invisible at final audit. The holes appear after the first wash, in the customer's hands. Serimen changes needles every 8 hours on elastane knits, against every 40 on cotton wovens. Across 46 sewing positions at USD 0.19 a needle and 12.6 line-days, that is USD 220 more over the whole order. Nine tenths of a cent a garment — against 3,400 pieces of seconds sitting in a cage on the other side of the same floor.