A fabricator in Klang called me about a TDF line that had been "fine for eight months". Then a hospital job came in with 1.0 mm galvanized, 1500 mm lengths, and the flanges started coming out 33 mm at the leading edge and 30 mm at the trailing edge. Corner inserts went in loose on one end and had to be hammered on the other. The installer sent photos. Nobody in the shop could explain why the same machine, same operator, same coil, produced a flange that changed height along the length of the duct.
The answer took twenty minutes with a feeler gauge and a straight edge. The upper forming head on station five had drifted 0.15 mm out of parallel after a bearing replacement six weeks earlier. On 0.7 mm material the error was invisible. On 1.0 mm it pushed the material past the point where it would recover, and the flange height walked. That is the nature of a TDF flange machine. It is a set of eight or nine roll stations that all have to agree with each other to within a tenth of a millimetre, and when they stop agreeing the machine keeps running and keeps producing scrap that looks almost right.
TDF stands for transverse duct flange. Instead of welding or bolting a separate flange onto the duct, the machine folds the duct's own material — the 30 to 40 mm that would otherwise be a raw edge — into a profile. That profile has three jobs: it must present a flat face for the gasket, it must hold a gasket in a groove so the gasket does not walk out under pressure, and it must accept a corner insert that locks two duct sections together at 90 degrees.
To get all three out of one strip of 0.6 to 1.2 mm steel, the roll set has to do this in a specific order. The first two stations notch and start the 90 degree bend. Stations three and four form the outer return leg. Station five sets the flange height. Stations six and seven press the gasket groove and the button pocket. The last station does the final calibration pass, which is where most of the arguments happen, because a calibration station can only correct a small error, not a large one.
Flange height, gasket groove depth, and button pocket position. If those three are inside tolerance, the duct assembles on site without a fitter standing over it. If any one is out, you get callbacks.
Flange height on a standard profile is 30 mm or 35 mm depending on the system you are running, with 20 mm used on small duct where the diagonal does not exceed about 600 mm. The tolerance that matters is not the absolute number but the variation along the length. I work to plus or minus 0.5 mm over a 1500 mm duct. Anything wider than that and the corner insert sits at an angle, which means the bolt hole pattern on the two mating flanges does not line up, which means someone on site is going to drill out a hole and then complain about the duct line.
Gasket groove depth should be about 60 percent of the uncompressed gasket thickness. For a 6 mm EPDM gasket that is roughly 3.6 mm of groove, plus 0.2 mm clearance. Too shallow and the gasket sits proud, so the joint bottoms out on metal before the gasket compresses, and the joint leaks at the corners. Too deep and the gasket has room to roll out of the groove when the duct is pressurised — you will find it lying inside the duct after the first air test.
Button pocket depth controls whether the corner insert clips in or falls out. Measured from the flange face, the pocket should be 1.8 to 2.2 mm deep on material up to 0.8 mm, going to 2.5 mm on 1.0 mm and above. I have seen shops open the pocket to 4 mm to make assembly easier and then wonder why the inserts pop out on a 500 Pa system.
The table below is the starting point I give to customers for a standard 30 mm TDF profile running galvanised steel. It is not a substitute for a test blank, but it will get you within a feeler-gauge pass of correct on the first try, which saves an hour of trial and error.
| Material | Thickness (mm) | Roll gap, forming stations (mm) | Roll gap, calibration (mm) | Flange height (mm) | Recommended line speed (m/min) |
|---|---|---|---|---|---|
| Galvanised steel | 0.5 | 0.45 | 0.48 | 30 +/- 0.4 | 14 - 16 |
| Galvanised steel | 0.7 | 0.62 | 0.68 | 30 +/- 0.4 | 12 - 15 |
| Galvanised steel | 1.0 | 0.88 | 0.96 | 30 +/- 0.5 | 9 - 11 |
| Galvanised steel | 1.2 | 1.05 | 1.15 | 35 +/- 0.5 | 7 - 9 |
| Stainless 304 | 0.8 | 0.68 | 0.74 | 30 +/- 0.4 | 6 - 8 |
| Aluminium | 0.8 | 0.70 | 0.76 | 30 +/- 0.4 | 10 - 12 |
Two notes on that table. First, the roll gap is not the material thickness. It is always slightly less, because the material thins at the bend and because you want a small amount of squeeze to control springback. The 0.88 mm figure for 1.0 mm material is a starting point — if the flange comes out short and the material shows a bright polished line at the bend, you are too tight and you should open up by 0.05 mm. Second, stainless and aluminium both need slower speeds than the numbers suggest if the roll surface is not polished. Stainless work-hardens at the bend and will crack the outer radius if you rush it; aluminium galls and picks up on the rolls, and once a roll has aluminium smeared on it, every subsequent flange carries a scratch.
After a roll change, a bearing replacement, or a move to a new site, I do not trust the previous settings. The sequence takes about ninety minutes and it saves days.
Put a machinist level on the lower roll shaft, not on the machine frame. A frame can look flat while the shaft sits 0.3 mm out over its length. Then check that the material guide is square to the first station with a dial indicator on a magnetic base. A guide that is 1 mm out over 1 m will produce a flange that is consistently thicker on one side, and no amount of roll adjustment will fix it.
The dials on most Chinese and Turkish TDF machines are indicative, not calibrated. I set station one by running a 300 mm wide blank, stopping it halfway, and measuring the first bend with a vernier. The bend should be 90 degrees with no thinning at the outside. Then I run the blank all the way through, measure the flange, and adjust only station five if the height is out. Adjusting three stations at once is how people lose a day.
Push a 150 mm length of the actual gasket into the groove and look at the joint from the end. The gasket should sit with its top edge 1.5 to 2 mm below the flange face. If it sits flush, the groove is shallow and the joint will bottom out. If it disappears entirely, the groove is deep and the gasket will roll out under pressure.
Form two short flanges, fit corner inserts, and bolt the two sections together with the gasket in place. Then pull them apart by hand and look at the gasket witness mark. A correct joint shows a continuous compressed band all the way to the corner. A joint that has never been right shows a compressed band in the middle and nothing at the ends, which is where leaks start.
These are the ones I get called about, in roughly the order of frequency.
A TDF flange machine is rarely a standalone purchase. In most shops it sits at the end of a rectangular duct line, taking the notched and sheared blanks straight from the line and forming the two longitudinal flanges in the same pass as the lock. If you are specifying a new line, the flange former's maximum duct length and its speed relative to the line's shear cycle are the two numbers that decide whether it becomes a bottleneck. A former rated at 12 m/min paired with a line that shears a 1.5 m blank every 20 seconds will sit idle; the same former on a line shearing every 35 seconds keeps up comfortably.
If you run both TDF and TDC, put them side by side with a common infeed table rather than buying a duplex machine on day one. The TDF flange forming machine page lists the standard roll configurations, and the duplex version is worth the money only above roughly 800 square metres of duct a month, where the changeover time between profiles starts to cost more than the second machine.
For shops doing a lot of small duct — under 600 mm diagonal — the corner cost dominates, and a dedicated TDF corner making machine pays for itself faster than a faster flange former. I have watched a two-man shop cut corner assembly time by 40 percent with a corner machine while the flange former sat unchanged.
Grease the roll bearings every 500 operating hours with a lithium complex grease, not every month, because monthly greasing on a machine that runs two shifts means the bearings are under-greased and on a machine that runs one shift a week means they are over-greased and blowing seals. Check the drive chain tension at the same interval. Replace the feed rolls when the knurl depth measures under 0.3 mm; worn feed rolls slip on 1.2 mm material and the slip shows up as a length error at the shear, which everyone blames on the line controller.
Keep a log of every roll adjustment with the material thickness it was set for. When a new operator runs 0.6 mm after a 1.0 mm job and the flanges come out 1.5 mm tall, the log tells him which station to open and by how much, instead of him adjusting all nine.
Only if it was built with interchangeable roll sets or an adjustable height head. Most fixed-profile machines are 30 mm or 35 mm and stay there. Retrofitting a different profile means changing the height-setting station and the pocket station, which is a factory job, not a shop job. If you need two profiles, buy the second roll set with the machine.
Measure the flange face flatness, not the height. A flange can be exactly 30 mm and still be twisted, and a twisted face will not compress the gasket evenly. Put the flange on a surface plate and check with a 0.05 mm feeler; if the feeler goes in at one corner, the twist is in the machine, usually a worn bearing on one side of station five.
More than most operators expect. Going from 0.7 mm to 1.0 mm needs the forming gaps opened by roughly 0.26 mm and the calibration gap by 0.28 mm, and the flange height will still grow by 0.3 to 0.5 mm because of reduced springback. A shop that runs mixed thicknesses without a setup sheet will produce flanges that vary by a millimetre across a single order, and the installer will notice before you do.
For a machine that runs continuously behind a duct line, motorised with a variable frequency drive. Hydraulic drives are better where the machine is used intermittently and you want high forming force at low speed, for example on 1.2 mm stainless. The hydraulic unit also adds heat and a maintenance item, so on a line running eight hours a day I would still choose motorised and put the money into better rolls.
If you are still working out which combination of line, former and corner machine suits your order book, start from the auto duct line range and size the flange equipment to the duct sizes you actually sell. The machine list follows the order book. It never works the other way round.