Rolling a 1500 mm Shell: Flat Spots, an Open Seam and the Wrong Blank Length
A workshop must roll eight cylindrical shells from 6 mm S275JR plate, 1500 mm outside diameter and 1200 mm wide, with a single longitudinal butt seam. The plate is cut to 4700 mm length on the guillotine. Rolling is done on an initial-pinch three-roll pyramid machine with the bottom rolls 300 mm apart, and the operator rolls each plate through in a single pass, reversing direction but without any end pre-bending. The first shells come out with a visibly straight flat of about 150 mm at each end, a 4 mm gap at the longitudinal seam, and a diameter of about 1508 mm measured across the seam. The seam edges were also sheared square with no bevel. The customer measures diameter and roundness at four points and rejects out-of-round shells. Four shells have already been rolled and the workshop has two days to deliver.
1. Calculate the correct developed length for the shell and compare it with the 4700 mm cut. Show your reasoning about which diameter to use.
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Which diameter. The plate does not stretch or compress at mid-thickness, so the developed length must be based on the neutral axis at the mean diameter, not the outside diameter. Using the outside diameter would overstate the length by π × t.
Calculation.
- Outside diameter D = 1500 mm, thickness t = 6 mm.
- Mean diameter = 1500 − 6 = 1494 mm.
- Developed length = π × 1494 = 4693.5 mm.
Comparison with the 4700 mm cut.
- Using the outside diameter would give π × 1500 = 4712.4 mm - 12.4 mm too long, a mistake that would show immediately as a wide gap.
- The cut length is 4700 mm, so it is 6.5 mm longer than the neutral-axis length. If the shell were rolled to the correct diameter, that surplus would appear almost entirely as a longitudinal root gap of roughly 6.5 mm - which is consistent with the 4 mm gap actually observed once rolling slip and the flat ends are taken into account.
So the blank is not grossly wrong, but it is not right either. A practical allowance would be the neutral-axis length plus a weld shrinkage allowance of about 1–2 mm for a single 6 mm butt seam, giving roughly 4695 mm, with the bevel preparation taken into account. Cutting to 4700 mm leaves the fitter fighting an oversize gap, which in turn causes the welder to bridge a wide root and adds distortion.
Also note: the edges should have been beveled to about 30° with a 1.5–2 mm root face before rolling, while the plate was still flat - doing it afterwards on a curved shell is far harder to keep uniform.
2. Explain why 150 mm flat spots appeared at each end, and how the machine and technique should be used to eliminate them on the remaining shells.
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Why the flats occur. On an initial-pinch pyramid roll, the plate is gripped between the top roll and the two bottom rolls. Until the leading edge has travelled to a point where it is nipped between the top roll and a bottom roll, it receives no bending moment. That unbent length at each end is roughly half the bottom-roll centre distance - with rolls at 300 mm, about 150 mm of straight plate at each end. That matches the observed flat exactly. The error was rolling the plate straight through in one pass without pre-bending the ends.
Correct technique.
- Pre-bend each end. Insert the plate so the leading edge sits just past the top roll centreline, apply ram pressure, and roll a short distance back and forth to curve the end to the required radius. Check against a radius template or the finished shell. Repeat on the trailing end.
- Roll in stages, not one pass. Increase curvature progressively over several passes, alternating direction and shifting the plate slightly each pass so the whole surface is worked uniformly.
- Check continuously. Use a template at 0°, 90°, 180° and 270° and a trammel or large calipers across diameters; do not rely on the finished diameter only.
- Use an end allowance where flat ends are inevitable. If the machine cannot pre-bend, add 150 mm to each end of the plate, roll, then trim the flats off before welding. This costs material but saves the shell.
- Check machine capacity. Confirm the roll diameter and capacity are suited to 1500 mm diameter in 6 mm - a small top roll can limit the achievable curvature.
For the four shells already rolled, the flats cannot be rolled out once the seam is prepared; options are to cut out the flat sections, re-roll a replacement strip and add a circumferential seam (needs customer approval), or scrap and re-roll. Get the decision in writing before spending the remaining two days.
3. The diameter measures about 1508 mm across the seam and the customer rejects out-of-round shells. Explain what is happening and describe the fit-up, welding and checking regime you would use to bring the shells into tolerance.
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What is happening. Two effects combine. First, the oversize blank (6.5 mm too long) has been closed into the seam by force and the surplus has to go somewhere, so the shell bulges locally around the seam - measuring 1508 mm across the seam while the rest of the shell may well be nearer 1500 mm. Second, the longitudinal weld shrinks transversely as it solidifies, pulling the seam closed and flattening the shell across the weld, which produces an oval rather than a circle. Rolling the shell in a single pass without pre-bending also leaves residual curvature variation that the weld then locks in.
Fit-up and welding regime.
- Trim and re-bevel as required so the root gap is a consistent 2–3 mm over the full 1200 mm length; never bridge an oversize gap with weld metal.
- Tack on the outside at 150–200 mm spacing with a short, small tack, checking diameter before and after each tack.
- Fit a temporary strongback or internal spider across the diameter, or use a pair of stiffening rings, to hold the shell round during welding. Remove only after the seam has cooled.
- Weld with a balanced sequence: root, then fill alternately from each side if access permits, backstepping in 200–300 mm steps so shrinkage is distributed rather than concentrated.
- Control heat input and interpass temperature, and do not let a welder run a single continuous pass down the full length.
Checking. After cooling and removal of the strongback, measure diameters at four points 45° apart, check roundness against the drawing tolerance, and check the shell against a template. Record the measurements on the inspection sheet - an unrecorded check is treated as a check that did not happen when the customer's inspector arrives.