Precision Shaped Wire: What the Tolerance Number Actually Means | ERQI WIRE

±0.02 mm is the most quoted number in shaped wire enquiries and the least examined. Tolerance on what face? Measured with what, and where on the coil? Paid for how? A tolerance callout is only meaningful once those three questions have answers, and this article walks through them.

Tolerance is per-face, not per-part

A shaped section has several dimensions and they do not need the same tolerance. Take an asymmetric seal carrier: the ramp that compresses the seal and the width that keys into the slot are functional faces — those carry the tight number. The back face and the height carry commercial tolerance, often two to three times looser. The part works exactly the same, because fit is decided by the functional faces alone.

Specifying tight tolerance everywhere buys nothing your assembly can feel, but it costs die life, pass count and yield — all of which land in the price. The efficient drawing marks the functional faces and says “rest commercial”. If you cannot mark them, the mating part can: whatever slides, seats or seals decides which face matters.

What the number physically buys

Each tightening step costs in a predictable order:

• Commercial tolerance (roughly ±0.05 mm at these sizes): the default of a well-run die. Fits most guides, tracks and welded assemblies.
• ±0.02 mm on functional faces: needs gauge-controlled drawing and more frequent in-process checks. The standard choice for precision parts.
• Beyond ±0.01 mm: enters territory where the die alone no longer guarantees the dimension — selection of coils, more scrap, a real price step. Sometimes justified; often not.

The honest question to ask your supplier is not “can you hold ±0.01?” but “what does holding it do to the price, and which faces need it?”. A supplier who answers with the face question first is the one measuring their own wire.

Measured how, and where

A tolerance implies a measurement method. On a drawn section, dimensions are taken with section-specific gauges or an optical comparator, at points along the coil — not one spot. Wire varies slightly along its length; a report from one point hides that, a report from several points with min/max values shows it. When you vet a supplier, ask for a sample inspection report and look for the number of measurement points, not just the values. How the rest of a precision spec fits together is on the precision shaped wire page.

The tolerance that gets forgotten: the die correction history

Batch one holds ±0.02. Batch five, from a die that was re-cut by a different toolmaker, holds ±0.04 — and nobody notices until the assembly station starts complaining. Consistency is a tolerance property too, and it is owned by whoever keeps the die and the pass schedule together. This is the argument for mills that cut their own dies, made in detail in the vetting checklist.

A practical way to specify

• Mark the functional faces on the drawing and put the tight number there.
• Leave everything else commercial — and let the quote show what that saves.
• Say how you will inspect on receipt, so the supplier’s report matches your check.
• For repeat volumes, ask what keeps batch five inside the same band as batch one.

Ordering by section rather than gauge is the other half of getting quotes that mean something — the reasoning takes two minutes. The service built around all of this is on custom precision shaped wire. Send the drawing with the faces marked and the quote will show exactly what each tolerance choice costs.