Three Rolls, One Pass: Making Round Duct the Way Sheet Metal Shops Actually Do It

Start With the Edge, Not the Middle

When someone says "plate roller," most people picture a machine that curls a sheet into a tube. That's true, but it's the easy half. The hard half is the two ends of the sheet — the edges that have to meet and become the longitudinal seam. A cylinder with flat ends isn't a cylinder yet: the seam has to land on metal that's already curved, and that's where the trouble starts. The machine that curls the middle beautifully can be the same machine that ruins the edges.

The geometry behind it is simple. In a symmetric three-roll arrangement, the two bottom rolls sit at equal distance either side of the top roll. The sheet is pinched in the middle and curved — but the parts of the sheet that never reached the pinch stay straight. So you get a nice arc in the middle and roughly 150–300 mm of flat metal at each end, depending on roll spacing. If your finished cylinder only needs to be a cylinder, you cut the flat ends off and throw away 5–8% of your material. If the seam has to land there, you fight it with hammers.

That's why an asymmetric three-roll machine is the standard answer in duct shops. The rolls are offset, so one edge is held while the other is bent in the same pass. The edge that matters — the one that becomes the seam — is pre-bent to radius before the sheet even enters the main pinch. One pass, both jobs done.

What "Pre-Bending" Buys You on the Floor

Strip the jargon away and pre-bending means one thing: when your duct section reaches the seam-closing station, the two edges are already at the right radius, so the lock or weld closes clean. Skip it, and the seam station has to pull two flat edges into a circle — which is why shops without pre-bending spend their afternoons re-hammering cylinders that never quite close.

On the Preda 2000 mm asymmetric machine, the sequence on a typical galvanized job goes like this:

1. Set the roll gap for your gauge — 0.8 mm galvanized needs a different gap than 1.2 mm.
2. Feed the sheet so the leading edge takes the pre-bend.
3. Run the full pass; the cylinder comes out with both edges already curved.
4. Close the seam at the seam closing machine and it fits without persuasion.

That last point is where the machine earns its keep. A seam that fits first time means your most experienced hand isn't tied up re-hammering, and the job isn't late because of it. Both of those are expensive in a duct shop, and they're what pre-bending protects.

The Spec Sheet, Read Like an Engineer Would

Spec sheets for plate rollers are written to impress, not to inform. Here's the Preda machine's table and, more usefully, what each number actually means for your work.

Watch out for two things when comparing quotes. First, "max thickness" is usually quoted for mild steel at a specific width — the same machine does less in stainless, because stainless work-hardens as it bends. Second, the price of a roller tracks its frame and roll hardness, not its motor. A 1.5 kW machine that's properly gear-reduced will out-work a 4 kW machine with a flimsy frame, every time. Ask for roll material hardness (45# forged steel with hard-chrome is the baseline we use) rather than asking for a bigger motor.

Sizing One for Your Shop: Three Numbers, In Order

Buyers tend to fixate on price. The three numbers that actually determine whether the machine survives contact with your work order are these, in priority order:

There's a rule of thumb we use with customers: the minimum practical forming diameter is roughly 1.2–1.5× the top roll diameter. A 2000 mm duct-class machine with a 130 mm top roll will comfortably make cylinders down to about 200 mm diameter — fine for the vast majority of round duct and fittings. If your shop lives on 100 mm fittings, you're shopping for a different class of machine, and we'll tell you so rather than sell you the wrong one.

First Week on the Machine

Most "the machine is bad" reports we get turn out to be setup problems. The first-week checklist is short and worth doing in order:

• Level it properly. A roller that rocks half a millimetre produces conical cylinders, and no adjustment of the rolls will fix a machine that isn't level.
• Set the gap before the bend. Gap controls thickness grip; bend controls radius. They're separate adjustments and people conflate them.
• Make a shop reference piece. Roll one cylinder at your most common diameter, mark it, and keep it. When an operator says "it's rolling big today," compare against the reference before touching settings.
• Lubricate the journals. The rolls themselves stay dry — oil on the roll face marks the sheet. Journals and drive bearings get grease on a schedule.

Five Mistakes We Keep Seeing on Install

After a few hundred installations, the same five come up again and again. None of them is the machine's fault.

Keeping the Rolls Honest

The maintenance list for a three-roller machine is short, which is exactly why it gets skipped. Weekly: grease the journals, check the roll faces for pick-up or scoring. Monthly: check drive chain tension and bottom-roll alignment with a straight edge. Annually: full alignment check, gearbox oil, and inspect the roll surfaces under good light — a single scored roll will transfer a line into every cylinder after it.

Roll surface condition causes more "the material is bad" calls than anything else on this machine. A tiny score picks up a burr, the burr marks every sheet, and the shop blames the coil. Re-grinding a roll is a fraction of the cost of replacing it, but only if you catch the score early. Deep scores that catch a fingernail mean replacement.

What It Actually Costs to Run

The honest annual cost picture for a duct-shop roller, single shift:

Run the payback on labour alone. If a hand-formed fitting took two workers 20 minutes and now takes one worker 4 minutes, you save roughly 35 labour-minutes per piece — about $15 at a loaded $25/hour rate. A $5,800 machine pays for itself around piece 390. Most shops hit that inside the first year on fittings and transitions, before counting the improvement in seam quality that ends up saving field rework.

Is a Four-Roll Machine Better? (Usually Not for Duct)

You'll hear that four-roll machines pre-bend both edges and hold the sheet better. Both true. Also true: they cost roughly double and, for 0.5–2 mm duct gauges, an asymmetric three-roll machine produces the same edge quality at half the price. The four-roll machine earns its premium at 3 mm and up, where sheet handling becomes a crane job. If your heaviest gauge is 2 mm, the fourth roll is capacity you're paying for and not using. We say this even though we sell both — the honest answer is what keeps customers coming back.