Last spring a contractor in Doha sent us drawings for a retail complex and asked for "TDF flange throughout". Six pages later, one riser note said TDC. The two systems look similar from across a workshop — both are integral flanges rolled onto the duct end at roughly the same height — and people mix them up constantly. But they are not interchangeable, the machine setup is different, and getting it wrong means corner pieces that do not fit and joints that will not seal. I have spent enough evenings on video calls watching someone hold a corner up to a flange that it was never designed for. So this piece is about setting up a TDC flange forming machine properly: the die changeover, the numbers to check on the first piece, corner fit-up and the sealing practice that keeps a joint tight.
It helps to be precise about what differs, because the setup differences fall out of it.
On a TDF flange, the machine folds the duct sheet back on itself. The last 35 mm of the sheet is turned up, doubled over into a hem, and rolled flat, so the flange is made from the duct's own material. The edge you see is a folded edge, and the corner receives a stamped corner piece that slides over the flange on all four sides.
TDC — transverse duct clamp, sometimes called a raw flange — turns the sheet up the same 35 mm but does not hem it. The flange edge is the raw cut edge of the sheet, and the system depends on a gasketed corner plus close bolt spacing to seal and clamp. The corner piece on TDC carries the gasket and bolts; on TDF the corner is mostly a stiffener and a nut plate.
That single difference — hemmed versus raw edge — drives everything downstream: the die profile, the sealing method, the bolt spacing and the leak behaviour.
TDF is the default in North America and most of the markets that follow ASHRAE practice. It is fast, it uses no gasket on the flange face itself, and the folded edge is safe to handle. TDC is common in the Gulf, in much of Asia and on jobs specified to SMACNA where the gasketed corner gives a more predictable seal at higher pressure classes. On 20 mm of butyl tape and 100 mm bolt spacing, a TDC joint will pass a leakage test that a carelessly assembled TDF joint fails — but a well-made TDF joint with sealed corners is every bit as tight. The spec decides; the machine has to be able to do both, which is why most shops run a duplex setup.
Our duplex flange former carries both profiles on one stand, so the changeover is a roller set swap rather than a machine swap. It takes a good operator about 25 minutes. Here is the sequence we teach, and the places it goes wrong.
Run the machine to a comfortable working height and kill the main isolator. Set out the TDC roll set on a bench in order — on our machine the profiles are numbered 1 through 5, and out-of-order assembly is the single most common cause of a ruined first piece. Check the roll grooves for embedded zinc or aluminium debris from the last run; a sliver of aluminium pressed into a steel roll will print a line down every flange you form that day.
Change the rolls, then set the gap with feeler stock, not by eye. For 0.8 mm galvanized we run the forming gap 0.1 mm under material thickness; for 1.0 mm material, 0.1 mm under as well. If the gap is too open the flange comes out with a rounded root and the corner will not seat; too tight and you get flange lean — the flange tips out of square because the sheet is being dragged rather than formed. After setting, jog the machine and feed a 300 mm scrap piece, then measure.
Three measurements decide whether the setup is good. Flange height: 35 mm nominal, and I want it within 0.5 mm across the length. Flange squareness: stand the trial piece on a flat table and check the flange face against a square — more than 1 mm of lean over 300 mm and the corner will rock. Flange root flatness: run a fingertip along the root; a crisp root means the hem, on TDF, has closed fully. Then form a second piece and join them with a corner and two bolts. If the joint pulls square with hand pressure on the corner, the setup is done.
The flange former on our auto duct line has a 4 kW drive, and after a roll change I watch the ammeter more than the workpiece. A correctly gapped TDC pass on 0.8 mm sheet pulls around 6 to 8 amps at our line speed. If the current climbs past 11 amps on the same material, the gap has drifted closed or a roll is running dry — stop before the bearings pay for it. If it sits under 4 amps, the flange root is probably not closing, and the corner fit-up will tell you within the minute. Operators who learn to read current catch a bad setup in one piece instead of five.
The TDC corner is the whole sealing system in one part, so it deserves respect.
A TDC corner slides over the flange from the duct end, and its internal pocket has to accept four flange thicknesses plus a small clearance. Formed flange stock running 0.05 mm heavy per sheet sounds trivial; four sheets later the corner is 0.2 mm tight and someone is persuading it with a mallet, which splits the gasket. We check corners against a go/no-go gauge made from four offcuts of the actual flange stock. Corners out of tolerance go back to the supplier — no exceptions.
Standard practice is M8 bolts with the corner bolted at every hole, then flange bolts at 100 mm centres for pressure class up to 750 Pa, tightening to 25 to 30 Nm. Above that class, some specs want 75 mm centres near the corners. Two habits keep joints square: bolt from the centre of each side outward, and torque in two passes — snug everything, then final torque. One-pass tightening with an impact gun walks the flanges out of square and preloads two corners while the other two sit loose. On a 1200 mm joint the diagonal difference can reach 2 mm, which is enough to open a hairline leak the tester will find.
TDC corners carry a factory gasket — on decent corners it is a continuous bead, on cheap ones a glued strip that shifts during bolting. We buy corners with the continuous bead and reject any shipment where the gasket is applied with visible gaps at the bolt holes.
On the flange face itself, SMACNA practice for most classes allows the gasketed corner to do the sealing at the corners, with butyl or sealant on the flange faces mid-span where the spec calls for it. What I tell crews: read the seal class on the drawing before you reach for the cartridge. Class A wants continuous sealing on both flange faces. Sealing a joint that does not need it wastes an hour a shift; not sealing one that does fails the leakage test in front of the consultant.
| Parameter | TDF | TDC |
|---|---|---|
| Flange height | 35 mm, hemmed from duct sheet | 35 mm, raw edge with gasketed corner |
| Corner piece | Stamped corner, slides on, nut plate | Cast or fabricated corner with gasket and bolts |
| Bolt spacing | 100–150 mm typical | 100 mm typical, 75 mm at corners for high class |
| Gasket | Corner seal only on most classes | Gasketed corner; flange face sealing per class |
| Sheet range on our former | 0.5–1.2 mm galvanized | 0.6–1.2 mm galvanized |
| Best fit | Standard commercial, fast assembly | Higher pressure classes, Gulf/Asia spec work |
Flange lean. Almost always a closed gap or a worn drive-side roll bearing. Fix the gap first; if lean persists on a fresh gap, the bearing is next.
Wavy flange face. The sheet is slipping in the drive during forming. Check the drive roll knurl and the hold-down; a polished knurl on 1.0 mm sheet will slip long before an operator notices.
Short corners. The flange was formed 2 mm short because the stop datum moved after a blade change on the shearing station upstream. Re-zero the stop against a formed reference piece, not the rule.
Gasket shear at corners. Bolting one side tight before the others. Two-pass torque, centre outward. It takes thirty seconds longer and halves the callbacks.
Ask whether the flange former carries both profiles and how long the changeover takes — 25 minutes on a duplex stand versus 90 minutes on two single machines changes how your shop schedules mixed work. Ask what tolerance the machine holds on flange height and ask for a first-off report from the FAT. Ask which corner supplier they ship with and whether corners arrive with continuous gaskets. And ask for the factory test video showing an actual corner fit-up, because a machine's brochure never shows the joint, and the joint is what you are buying.
Can one machine form both TDF and TDC?
Yes, if it is a duplex or convertible model — the roll set swaps on one stand. Single-profile machines need two stands or a roll change that can take most of a morning. If your market mixes both specs, buy convertible from the start.
What flange height should TDC be formed to?
35 mm nominal is the standard for both systems, and corners are built to that. Hold it within 0.5 mm. Undersized flanges seat shallow in the corner and the gasket never compresses; oversized ones bind and tear the gasket on assembly.
Why do my TDC corners leak even with new gaskets?
In order of likelihood: corners out of tolerance against your flange stack, flange lean letting one corner carry the load, single-pass impact tightening walking the joint out of square, or gasket bead gaps at the bolt holes from a bad corner batch. Fix the fit-up before blaming the gasket.
Do TDC joints need sealant if the corner is gasketed?
Follow the seal class on the drawings. The gasketed corner seals the corner; mid-span flange faces need butyl or sealant only when the pressure class or the spec calls for it. Extra sealant on a joint that will not be tested is wasted labour, but leaving it off a Class A joint fails the leakage test.