A 900 mm Cylinder That Will Not Close: Development Length and Roll Setting
You must make one cylinder, 900 mm internal diameter × 1200 mm long, from 1.2 mm galvanised sheet, closed with a 10 mm grooved (double) lock along the longitudinal seam. The hand rolls are rated for 1.25 mm × 1250 mm. The works calculation was π × 900 = 2827 mm, and the blank was sheared to that length with no seam allowance because the drawing does not call one up. After rolling, the two longitudinal edges fall short of each other by about 22 mm, leaving nothing to form the lock into. The cylinder is also oval by 9 mm when checked with internal callipers across two diameters at 90°, and both ends flare out over the last 80 mm. The rolls were set by eye, and the sheet was passed through in one direction only, with the rolls tightened on each pass. The customer collects tomorrow morning and this duct is the only item holding up the job.
1. Calculate what the blank length should have been and explain, step by step, where the 22 mm shortfall comes from.
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The blank is short, by two compounding errors.
- Wrong diameter used for development. A rolled cylinder develops on its neutral axis, not on the internal face. With t = 1.2 mm on a 900 mm diameter the radius-to-thickness ratio is enormous (r/t ≈ 375), so the neutral axis sits essentially at mid-thickness and the k-factor is about 0.5. Mean diameter = 900 + 1.2 = 901.2 mm; developed circumference = π × 901.2 = 2831 mm, not 2827 mm. That alone accounts for about 4 mm.
- No seam allowance. A 10 mm grooved lock needs roughly 10 mm of material on each edge to fold and interlock, and that material is additional to the finished circumference - it is not part of the 900 mm the customer measures. Add about 20 mm.
So the cut length should have been approximately 2831 + 20 = 2851 mm.
Check against the symptom: 2851 − 2827 = 24 mm, and the observed gap is about 22 mm. The two errors explain the shortfall within normal measurement and edge-straightness tolerance - which confirms the diagnosis rather than leaving it as a guess.
Method going forward: cut marginally long (2850–2860 mm), roll, form the seam, then trim to the finished diameter. The seam draws material and stiffens the joint, so the finished internal diameter is the dimension that must be checked, not the flat blank. Also confirm the rolls are within capacity: 1.2 mm × 1200 mm is comfortably inside the 1.25 mm × 1250 mm rating.
2. The cylinder is oval and the ends flare. Set out how you would correct the rolling operation, how you would verify the finished cylinder, and whether you would salvage this blank.
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Diagnose before touching the rolls. The 9 mm ovality and the flared ends point to set-up and technique, not material:
- Rolls set by eye and not parallel. If one end of the top roll is lower, that end over-rolls and the cylinder comes out barrel-shaped or oval. Check parallelism with a straight edge and feeler gauges across the roll, and equalise the adjusters before the next pass.
- Single-direction, progressively tightened passes. Each pass through tight rolls work-hardens and localises the bend. The correct technique is light, repeated passes alternating direction, with the rolls advanced in small increments, finishing with a spring pass at the same setting.
- No pre-bend. Feeding a flat leading edge into tight rolls produces the classic 80 mm flare. Pre-bend both ends with short back-and-forth passes at a slightly tighter setting before committing to the full length.
Verification: internal callipers or a trammel at three stations (each end, mid-length), two readings at 90° per station; check the seam gap is parallel along its full length; check the ends are square to the axis by standing the cylinder on a surface plate; check for twist by measuring diagonals.
Salvage decision: no. A blank 22 mm short cannot be closed by force - pulling the edges together ovalises the cylinder further, distorts the seam and leaves the diameter under size. Cut a new blank at about 2851 mm and keep this one as a check piece for setting the rolls.