Holding fabric width on a narrow tolerance: what Cpk is for on a range

Buyers who write a width band of a few millimetres on a finished fabric are asking the range to hold still. Mills then hear the word Cpk from an auditor and either panic or print a number from a week when the range was on its best behaviour.

What the number is

Capability, in the ordinary mill sense, asks: if this stenter or compacting range keeps running as it ran while we measured, will width stay inside the band? It needs measurements taken in the mix you actually run — cotton one hour, a blend the next — not a single style lined up for the visit. It needs the spec the buyer wrote, not a kinder internal spec.

If we cannot see twenty honest width readings tied to machine ID, speed, and lot, we will not print a capability figure. A diary of speed versus width is then more use. In a Batumi mill the old production diary already showed width walking as soon as speed left a handwritten number. The capability study confirmed the diary; it did not replace it.

When not to bother

If the band is so tight that even the mill lab’s own ruler disagreement eats half of it, calculate the measurement error first. If lots arrive from the dyehouse with different residual moisture, the range is being asked to correct greige it did not make. Capability on the stenter will look poor, and the meeting will blame the wrong room.

How we use it on a visit

A line capability study on a range is two days of watching and measuring, then a briefing a supervisor can read. We will say whether the process is holding, hugging the edge, or regularly leaving the band. We will not promise that a printed Cpk will satisfy a named retailer. Their auditor has a preferred method; ask them for it before you pay anyone to imitate it.

If width is only one of the arguments — shade being the other — start with a Plant Quality Review so the meeting does not spend a week on the wrong machine.