Why blank length uses the neutral axis
When plate is rolled, the outside surface stretches and the inside compresses — the same behaviour as a press brake bend, wrapped through 360°. The length that stays constant is the neutral axis, and this calculator approximates its position at mid-thickness. Treat the result as a starting estimate and validate the finished blank for the actual job.
Blank length = π × (ID + t)
- ID — finished inside diameter in mm
- t — material thickness in mm
Adding one thickness to the inside diameter places the calculation on the mid-thickness line. Using ID alone gives a blank that is too short; using outside diameter gives one that is too long.
Partial arcs
For anything less than a full cylinder, the blank is that fraction of the full circumference:
| Shape | Blank length |
|---|
| Full cylinder (360°) | π × (ID + t) |
| Half cylinder (180°) | 0.5 × π × (ID + t) |
| Quarter cylinder (90°) | 0.25 × π × (ID + t) |
Seam allowance
The calculator does not add a seam or trimming allowance. Determine any allowance from the joint design, edge preparation and approved fabrication procedure; there is no single allowance suitable for every shell. Check partial-arc trimming requirements separately.
What this calculator does not include
- Flat ends — the leading and trailing edges of a plate cannot be rolled without pre-bending. Plan for pre-bend or plan to trim.
- Springback — plate relaxes slightly after rolling, so machines typically over-roll to land on target diameter.
- Cone geometry — a tapered section develops as an annular segment, not a rectangle, and is a different calculation entirely.
Frequently Asked Questions
How do you calculate the flat blank for a rolled cylinder?
Blank length = π × (ID + t), where ID is the finished inside diameter and t is material thickness. Adding one thickness to the inside diameter puts the calculation on the neutral axis at mid-thickness, which is the length that stays constant through rolling.
Should I use inside or outside diameter?
Neither on its own. Use the neutral axis — inside diameter plus one material thickness. Calculating from inside diameter alone produces a blank that is too short, and from outside diameter one that is too long, with the error growing as material gets thicker.
How much weld seam allowance should I add?
The calculator excludes seam and trimming allowances. Determine them from your drawing, joint design and fabrication procedure; do not apply a universal allowance to every shell.
Does this work for cones?
No. A conical section develops as an annular (curved trapezoidal) blank rather than a rectangle, derived from the top diameter, bottom diameter and slant height. That is a separate layout calculation, normally done in CAD or nesting software.
Check machine capacity before choosing a plate roll
Blank layout does not establish whether a machine can form the part. Check rolling and pre-bending capacity at your material, width and finished diameter. Use the supplier review checklist when requesting a machine quote.
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