Millimetres, and a measurement that is a force

Two people, one bra, two right answers

11 May. Thalna has 9,000 bras finished and packed in cartons. Aubrette's inspector arrives. He measures the underband on the first twenty pieces and fails the whole lot on measurement.

Thalna's own in-line check runs four times a day through the whole run. It has the same bras passing on the same specification. Both measurements were taken correctly, by trained people, against the same document.

The document contained two numbers. Nobody had noticed that they described different things.

The tolerances this category runs on

A point of measure is one place on the garment that somebody puts a tape on. A tolerance of plus or minus 0.2 cm is 2 mm each way on a 214 mm seam, which is under 1%. Course 4.4 shows how a tolerance decides pass or fail, not the garment. What changes here is only the size of the window, and what that size says about who can sew inside it.

The measurement that is a force

The underband is elastic. That one fact breaks the idea of a measurement.

A woven shirt's chest is 52.0 cm whoever measures it, because the cloth does not move. An elastic band has no single length. Lay it flat and press it down and it reads one figure. Lay it flat and leave it alone and it reads another, 4 mm longer. Pull it out until it takes a stated force and it reads a third. There is no measurement here without a stated condition. That is why the standard method for measuring how far a fabric stretches works the way it does: you pull a sample until it reaches a stated force, and you record the length at that point.

The force is given in newtons, written N. It is the ordinary engineering unit of pull.

BR-6140's specification carried both figures, on the same page:

• Underband, relaxed, half measure: 32.0 cm, plus or minus 0.3 cm
• Underband, extended to 30 N, half measure: 39.5 cm, plus or minus 0.5 cm

Thalna measured the first. Aubrette's inspector measured the second, because Aubrette's inspection manual says every elastic point of measure is taken extended.

The bulk read 32.1 cm relaxed, which is inside the window. It read 38.2 cm extended, which is outside it by 1.3 cm. On a 9,000-piece lot that is a failure.

Both readings are of the same band. The stretch ratio the specification implies is 39.5 over 32.0, which is 1.234. The bulk delivers 38.2 over 32.1, which is 1.190.

Where the wrong number came from

The extended figure was never measured on a bra. It was calculated at pattern stage from the elastic supplier's data sheet. Halverston's 12 mm plush-back tape reaches 30 N at 23.4% extension, so the pattern engineer applied that ratio to the relaxed band and wrote 39.5 cm.

That was correct arithmetic on the wrong object. The tape on its own is one thing. The band is the tape plus a wire casing, a hook-and-eye tape, the frame fabric caught into the same seam, and two bartacks. A bartack is a short block of dense stitching that locks a stress point. Every one of those things stiffens the band. So the assembled band reaches 30 N sooner, and that means it reaches it at a shorter length.

Thalna measured twenty finished bands from bulk on a tensile frame, which is a machine that pulls a sample and reports the force. The mean was 38.3 cm at 30 N, with a standard deviation of 0.4 cm. The garment is right. The specification was wrong from the day it was written, and it was written by a competent person from a correct data sheet.

What it cost, and how it was settled

The lot was held for four days. Settling it took twenty bras destroyed on the tensile frame, one revised specification, and a full re-inspection of 9,000 pieces at EUR 0.11 a piece. That is EUR 990, plus the four days.

The settlement was not a concession. Aubrette re-issued the specification with the extended figure at 38.3 cm, and added one line: the relaxed measure is the production control; the extended measure at 30 N is the acceptance measure; where they conflict, the extended measure governs. That sentence is the whole lesson, and it costs nothing to write in February.

Millimetres imply a machine and an operator

A window of 2 mm is not held by inspecting harder. It is held by the machine, the attachment on it, and the person running it.

The standard minute value, or SMV, is the work content of one garment in minutes. Nine machine classes go into this one bra: plain lockstitch for the frame seams, a zigzag lockstitch for the strap junctions, a three-step zigzag for attaching the band and cup elastics, a bartack machine for the strap and channel ends, a coverstitch, a wire-inserting station, an automatic hook-and-eye station, a moulding press, and a hot-cut for the laminate blanks. Course 4.2 explains the stitch and seam classes themselves. The point here is the count, and what it does to a line.

The efficiency figure is the one to sit with. The same twenty-four operators reach 72% on the tee and 62% on the bra. The gap is not effort. The bra has more machine changes. It has more elastic-tension work, where the operator feeds two materials at different rates against a stretch guide. And it has more operations whose output has to be right to 2 mm.

Elastic attach is the operation that decides the line. Thalna's own record is that an operator moved from overlock to three-step zigzag elastic attach reaches standard performance in about 11 weeks. The same operator moved onto a tee overlock reaches it in about two. Course 5.6 does the arithmetic of a learning curve. The number that matters here is simpler: you cannot open a bra line for one order. The eleven weeks are spent before the order arrives, or they are spent inside it.

The routine that prevents it

1. Print every elastic point of measure with its condition attached. If the condition is missing, it is not a point of measure yet.
2. Take extended figures from sewn assemblies, twenty of them, once for each material combination. Record the standard deviation, not only the mean.
3. Name the governing measure in the specification, in a sentence, before the size set.
4. Measure the same points the buyer will measure, with the same instrument and the same load, in the in-line check.
5. Treat a 2 mm window as a capability question: which operator, which attachment, which machine, and what the spread in that section actually looks like.

Act as a lingerie technologist writing a measurement specification that two parties can be held to. Style facts: [STYLE], [DESCRIPTION], size range [LIST], base size [SIZE]. Paste my current points of measure with their values and tolerances: [PASTE]. The elastics in the garment, by position, width, construction and supplier: [PASTE]. My buyer's inspection manual says elastic points are taken [RELAXED OR EXTENDED, AND TO WHAT]. Do the following. First, sort every point of measure into three groups: rigid points, elastic points, and points on a moulded or bonded component — and say why each one is in its group. Second, for every elastic point, rewrite it as a value plus a CONDITION plus a statement of which condition governs acceptance, and tell me what has to be measured on SEWN assemblies before the extended figure can be written at all. Say plainly that a figure taken from an elastic tape's own data sheet is not a figure about the assembled garment. Third, tell me how many sewn samples to measure, what to record besides the mean, and how to word the sentence that settles a conflict between the two figures. Fourth, build the after-wash set: name each point that must be measured after washing, give it a DIRECTION as well as a limit, and say which points are expected to grow, which to shrink, and which must not move. Do not give me one percentage for the whole garment. Fifth, tell me which wash and dry cycle the specification has to name and what changes if the care label allows tumble drying. Sixth, list the points that cannot be judged by eye at inspection speed and belong on an in-line gauge check instead. Ask me for anything missing rather than assuming a value.