Introduction: Galvanized sheet thickness on a rectangular duct line usually runs from 0.5 mm to 1.5 mm, and that ceiling comes from machine structure rather than shop habit.
Anyone comparing a 1250 mm duct line with a 1500 mm model runs into the same question sooner or later. The drawing calls for 1.2 mm galvanized sheet, the lighter machine is listed at 0.5–1.2 mm, and the heavier one goes to 1.5 mm. That gap is not a marketing tier. It reflects how much bending force, shear load, and frame stiffness each machine was built around. Once that link is clear, a thickness range stops looking like a number on a sheet and starts reading like a description of the machine itself, which is what makes it possible to see why running past the published figure creates forming problems and safety problems at the same time.
Sheet metal resists forming in proportion to how much material sits in the cross-section being worked. In bending, the force needed to fold a given width of galvanized sheet climbs much faster than the thickness number itself, roughly with the square of the thickness. Going from 1.0 mm to 1.5 mm is a 50 percent increase in thickness but closer to double the bending effort. Shearing and punching behave differently. Cut force rises more or less in a straight line with thickness, but it focuses into a narrow blade edge or die opening, so the pressure at the tool is far higher than the tonnage figure alone suggests. Operators notice these changes before any specification sheet does. Thicker galvanized sheet sounds heavier at the shear, produces a different burr, springs back further after the fold, and leaves the blank slightly different in length than the same part run in thinner material. Multiply that across a coil line feeding at 15 m/min and the machine has to hold alignment through every one of those cycles. A frame that flexes by a fraction of a millimetre under load will not show a problem on the first blank, but it shows up later as diagonals that drift and flanges that no longer sit square. Thickness capability is really a statement about the whole structure, not just the tooling.
The 0.5–1.5 mm range that appears on heavier galvanized configurations is not an arbitrary round number. It is the band where a machine of a given frame size, motor rating, and shear design can keep running without fighting itself. Four separate load paths decide where that band ends.
The published COORIG Auto Duct Line 3 variants follow exactly this logic. The 1250 mm and 1500 mm models share a 9 kW drive and a 15 m/min feed rate, yet the heavier galvanized configurations are listed at 0.5–1.5 mm, weigh 5,500 to 6,000 kg, and run 9,200 mm long, while the lighter ones sit at 0.5–1.2 mm, 3,500 kg, and 8,200 mm. The extra mass is the structure that carries the extra load.
The immediate result of running heavier strip than a line is rated for is usually a stop. Overload protection trips, the shear stalls mid-stroke, and a partly cut blank is left sitting in the machine. That is the good version of the event. The expensive version is mechanical: chipped shear blades, cracked punch dies, leaking hydraulic seals, motor overheating, and gradual deformation in the frame or the feed rolls. Once alignment is lost, a new blade does not bring the machine back to its previous accuracy, because the error now lives in the structure. Recovery means realignment and sometimes replacement parts. Forming quality degrades before any hard failure appears. Sheet that is too thick for the die opening does not bend to a clean radius. It stretches, cracks along the fold line, or springs back differently from the rest of the run. Blank lengths vary from part to part because the feeder is slipping rather than gripping. On a duct line, an out-of-square blank becomes an out-of-square duct, and a flange joint that should close tightly ends up with a gap that someone has to seal on site. The visible defect usually shows up downstream, far from the machine that caused it, which is why thickness problems are often misread as assembly problems. Safety deserves more attention here than it usually gets. Thick strip stores more energy, and that energy releases suddenly when a stalled shear breaks through or when a folded edge springs back. Operators also tend to step in when the feeder cannot pull a heavy strip, which puts hands close to rollers and blades designed for material. Work equipment regulations such as PUWER place duties on employers to keep equipment suitable for its intended use and properly maintained, and running outside a published rating sits inside that duty. A thickness limit is, in practice, a safe operating band as much as a forming limit. Treat figures like 0.5–1.5 mm as the published range for a specific galvanized configuration, not a blanket duct standard or an industry-wide machine ceiling. Different frame sizes and drive packages land on different limits, so the exact figure for a given line is worth confirming with the supplier's technical team before the coil is ordered.
Sheet thickness and machine structure are two halves of one decision. Bending force, shear load, frame rigidity, and coil handling all shift together as galvanized sheet gets heavier, which is why a line rated to 1.2 mm and one rated to 1.5 mm differ in weight and length as much as in a specification line. Reading a thickness range correctly means reading the machine behind it. Anyone comparing a 1250 mm duct line with a 1500 mm model can start from the published galvanized range for each configuration and then look at how the forming load was designed to be carried. The full specifications of the Auto Duct Line 3 are a useful reference point for that comparison.
A：That is roughly where the forming load stops being practical for a compact coil line frame. Bending force climbs with the square of thickness, and shear and punching loads concentrate into small tooling areas, so going past 1.5 mm on galvanized sheet calls for a heavier frame, a larger drive, and stronger tooling than most shop-floor lines are built around. Heavier machines exist, but they are a different class of equipment rather than a setting change.
A：Bending load rises much faster than thickness, roughly with the square of the thickness at the same width, so 1.5 mm galvanized sheet takes close to twice the bending effort of 1.0 mm. Shearing and punching force rise more steadily, closer to a straight line with thickness, but they act on a narrow blade edge or die opening, so tool pressure climbs even when the tonnage figure looks modest.
A：Not automatically. Sheet width and sheet thickness are separate design decisions. A narrower machine can be built around a heavier frame and carry the same 0.5–1.5 mm galvanized range as a wider one, and a wide machine with a lighter frame tops out near 1.2 mm. The published configuration matters more than the nominal width, so the galvanized range for the specific model is the figure to check.
Provision and Use of Work Equipment Regulations 1998 (PUWER) - HSE
