Minimum wall
No reading anywhere on the tube may fall below the stated figure.
The order must state the positions and number of readings, because the convention is about the lowest point found.
Steel Tubes · Technical Reference
Steel Tube Dimensions
Four measurements decide a tube order: outside diameter, inside diameter, wall thickness and length. Getting them onto the enquiry unambiguously — with the ordered convention for each — is what stops a dimensional question becoming a delivery dispute.
Conventions
State each dimension as what it actually is
Most dimensional arguments trace back to one of these four being stated in a form that meant two different things to two different readers. Each row below separates the term from what it is commonly mistaken for.
| Term | What it means | Not the same as | What to confirm |
|---|---|---|---|
| Outside diameter (OD) | The external diameter of the tube, measured across the outside surface. | Nominal bore or nominal size labels carried over from pipe practice. | Confirm whether the order is stated as an actual OD or as a nominal size designation. |
| Inside diameter (ID) | The internal diameter of the bore at the stated position. | OD minus twice the nominal wall — that is arithmetic, not a measurement. | If a component depends on bore fit, specify an actual ID requirement with its own limits. |
| Wall thickness | The thickness of the tube wall, subject to the ordered tolerance convention. | A single figure that means the same under every standard. | State whether minimum wall or average wall governs acceptance before applying any limit. |
| Length | The ordered length, whether random, fixed, or a cut-to-length requirement. | A surplus allowance that is assumed rather than agreed. | State the length and the allowed variation, and whether cutting is in scope. |
Nominal bore
Why OD minus twice the wall is not a bore guarantee
The calculation is useful. Treating its result as a measured bore is where it goes wrong.
- 1Nominal arithmetic assumes a perfect tubeOD minus twice the nominal wall gives the geometry a perfect tube would have. A real tube carries ovality, eccentricity and wall variation, so the derived figure is an estimate of nominal geometry — not a value any gauge will read.
- 2The wall convention changes the answerIf a standard sets a minimum wall, the actual wall may sit above it, and the real bore is correspondingly smaller than the arithmetic suggests. Under an average-wall convention the deviation runs the other way. The convention has to be stated before the figure means anything.
- 3Tolerances apply to each feature separatelyOD and wall each carry their own permitted deviation. Combining two tolerance bands arithmetically produces a range wider than either one, and that combined range was never agreed by anyone.
- 4Bore-dependent components need an ID requirementWhere a piston, insert, sleeve or fitting depends on internal fit, state the ID as its own controlled dimension with its own limits and measurement method. Do not leave it to be inferred downstream.
Wall convention
Minimum or average wall — the answer changes
A wall requirement is not meaningful until the convention is named. The same nominal figure accepted under one convention can fail under the other.
Average wall
The mean of the readings taken must meet the stated figure.
Individual readings may sit below it. The sampling plan therefore decides the outcome as much as the limit does.
What both need
The instrument, the number of readings, the positions and the evaluation rule.
Without these, two inspectors can apply the same convention and reach different results in good faith.
Enquiry
Carry the dimensional basis into the enquiry
Record the basis so the quotation, the mill and the receiving inspection all work from the same statement.
- The dimensional standard and edition the limits come from
- Whether the size is an actual OD or a nominal size designation
- The wall convention — minimum or average — and the sampling plan
- An explicit ID requirement wherever bore fit governs the component
- The length and its allowed variation, and whether cutting is in scope
- The measurement method, instrument and positions
Dimension basis summary
Related
Resolve the next decision
Dimensions are the first question. These carry the ones that follow.
Request a quote
Send the dimensional basis for review
Provide the standard and edition, the OD and whether it is actual or nominal, the wall with its convention, any explicit ID requirement, the length, quantity and destination.