Rolled Cylinder Blank Calculator
The flat blank for a rolled cylinder, shell course or pipe from plate: from the inside, outside or mean diameter and the thickness, with an optional seam gap — and the blank as a DXF.
The method behind it: How long a blank does a rolled cylinder need? — step by step, with sources.
Dimensions
Results
Preview
The blank
How this tool is verified
- Reproduces the worked examples of the books it follows: Atkins (2nd ed.), 7 ft inside in ¾ in plate → 84¾ in × 3.1416 = 266¼ in; Neubecker (1920), 48 in inside in ½ in plate worked on 48½ in.
- Inside, outside and mean diameters describing the same part give the same blank, and the classic inside-diameter error is shown at exactly π × thickness.
- The DXF is one closed rectangle on CUT, read back in the tests.
The method, the reports and the responsibility boundary are on the Verification page.
Built by Ruthen Systems
Free, with no account, and the dimensions you type never leave your browser. If a result looks wrong, or the option your shop needs is missing, say so — reports of a wrong result are dealt with first.
Report a problem or request a toolAssumptions and limits
Read these before you cut anything. They describe exactly what this tool does and does not account for.
- The blank is the circumference at mid-thickness, where rolling neither stretches nor squeezes the plate.
- The seam gap, if any, is taken off the length. No allowance is made for flat ends left by the rolls or for trimming; add your shop’s.
- One course. Where courses lap, the outer one is longer by about 2π × thickness.
Output is a planning and fabrication aid. You are responsible for verifying dimensions, tolerances and material behaviour before production — full disclaimer.
Worked example
1000 mm inside, 10 mm plate, 1500 mm wide.
- Mean diameter1010 mm
- Blank3173.0 × 1500 mm
By hand: 1000 + 10 = 1010; π × 1010 = 3173.0. With the inside diameter straight, π × 1000 = 3141.6 — 31.4 mm short.
Method
Sources
- E. A. Atkins, Practical Sheet and Plate Metal Work, 2nd ed. (Pitman, undated), Ch. XXXII “Plater’s Work, Tanks, Shells, etc.”, section “Allowance for Metal Thickness”, pp. 350–352: the neutral line at the middle of the thickness, and the worked example; pp. 356–358: lapped courses about 2π × thickness longer.
- W. Neubecker, Sheet-Metal Work (1920), pp. 116–117: add the thickness to an inside diameter, subtract it from an outside one — which he calls theoretically correct, recommending instead seven times the thickness on the circumference because material is lost in forming. This tool uses the theoretical rule.
- J. Bourne, A Catechism of the Steam Engine (1850), p. 152: bent edgewise, add the breadth of the bar to the diameter.
- T. Moore, Engineers’ and General Smiths’ Work (1906), pp. 153–154 and 160–161: a 3 ft ring of 3 × 2 in bar bent edgewise (10 ft 2½ in before the upsetting allowance); the angle-ring rule, and that no rule should be relied on too implicitly.
- International Correspondence Schools, Machine Molding … Iron Forging (1906), §57, p. 37: inside diameter plus the thickness of the stock, times 3 1/7, plus a small amount for upsetting and scarfing (1/8–1/4 in for its 2 in round ring).
- J. Courtney, The Boilermaker’s Assistant, ed. D. K. Clark (1885), pp. 47–49: angle rings, flange outside and inside.
- J. Marquardt, The Smith’s Pocket Companion (1893), pp. 107–109: angle rings — the rules in the books “none of them agree”; bend one half and check it first.
Questions
How long a blank do I need for a rolled cylinder?
π × the mean diameter: the inside diameter plus one thickness, or the outside diameter less one thickness. For 1000 mm inside in 10 mm plate that is π × 1010 = 3173 mm.
Why not π × the inside diameter?
Because the plate does not keep its inside length when rolled: the inside is squeezed and the outside stretched, and only the middle of the thickness keeps its length. Using the inside diameter leaves the blank short by π × thickness — 31 mm on 10 mm plate. (Neubecker, who gives the same rule, recommends adding seven times the thickness to the circumference in practice, because some material is lost in forming; check against your own rolls.)
What about the seam?
Enter the gap you want left open for welding, and it comes off the length. For a lap joint, add the lap as material instead.
Does it work for a ring rolled from flat bar?
Yes — the old smithing books measure a ring on the middle of the bar, as this tool does. Rolled the easy way (flat), enter the bar’s thickness. Rolled the hard way (edgewise), enter the bar’s width as the thickness: it is the depth of the bar across the ring that counts (Bourne, 1850, p. 152). Moore (1906, pp. 153–154) works a 3 ft ring of 3 × 2 in bar bent edgewise on a 39 in mean diameter — 10 ft 2½ in round, before his allowance for upsetting; bent flat, the same rule gives 38 in. For a forge-welded ring ICS (1906, §57) adds a small amount for upsetting and scarfing (1/8 to 1/4 in in its example); this tool adds no such allowance, so add your own.
What about rings of angle?
Not covered. The old shop books give different rules for an angle rolled with the flange outside or inside: one adds twice the root thickness to the inside diameter with the flange outside, and takes it from the outside diameter with the flange inside (Courtney, 1885, pp. 47–49; Moore, 1906, pp. 160–161); another adds twice the width of the bar to the length with the flange outside and deducts the width once with the flange inside (Marquardt, 1893, pp. 107–109). Marquardt says the books “none of them agree” and advises bending one half of the ring and checking it before cutting the rest. Until one rule can be backed properly, this tool does not work out angle rings.
Missing an option?
A different datum, an offset, a shop allowance, another shape altogether — say what this tool does not cover yet. Requests decide what gets built next, and a report of a wrong result is dealt with before anything new.
Rather have the whole part drawn — a shop drawing or a cut file from your sketch? That is paid drafting, quoted per job: drafting for small shops.
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