Key takeaways
- For 90° air bending, minimum outside flange length is V/2 + t, so a 2 mm part on a 16 mm V die needs at least 10 mm of flange.
- With V equal to 8t, the practical minimum flange is about 5t, which gives 5 mm at 1 mm, 10 mm at 2 mm and 50 mm at 10 mm.
- A 4,450 mm bend in 10 mm CRCA needs about 225 tonne-force, just over the 200 T brake rating, so full-bed bends at that gauge are limited to roughly 3,900 mm.
- Flange height also has to clear the punch: a 60 mm box needs a punch no taller than about 55 mm, or a gooseneck and sectional tooling.
Minimum flange length for press brake parts equals half the die opening plus the sheet thickness: a 2 mm part on a 16 mm V die needs at least 10 mm of outside flange, and a 10 mm plate on an 80 mm V opening needs at least 50 mm. Below that the flange cannot reach both die shoulders, the part drops into the die and the angle becomes a guess.
The rule is simple and it is still the most common drawing error we see on bent parts. A 6 mm return flange looks like a design choice on screen; on the brake it is a tooling constraint that forces a 6 mm V die, roughly three times the bending force, a tighter inside radius and a different flat length.
This page gives the numbers by thickness, the two other clearances that decide whether a flange can be formed at all, and what to change on the drawing when the flange is below the limit.
What is the minimum flange length for press brake parts at 90°?
V/2 + t, where V is the die opening in millimetres and t is the sheet thickness. That is the geometric minimum: the flange edge must reach the far shoulder of the die and the bend line must sit over the centre of the opening.
For 2 mm CRCA on a 16 mm V die: 8 + 2 = 10 mm outside. The inside dimension at that flange is 8 mm, which is the figure a designer usually draws. Work from the outside and convert, because the outside is what the operator measures.
The die opening itself follows thickness. The working rule on mild steel is V = 8t for material up to about 4 mm and 10t above it. Thinner dies lower the minimum flange and raise the tonnage, which is why a part with one very short flange often drives the entire tool choice.
Minimum flange by thickness
The table uses the standard die opening for each gauge and a 350 MPa cold-rolled steel. Force is per metre of bend, calculated as 1.42 × σb × t² ÷ (1,000 × V), with a bed 4,450 mm long and a 200 T brake.
| Thickness | Die opening V | Minimum outside flange | Bending force, kN/m | Maximum bend length at 200 T | Ganesh limit or note |
|---|---|---|---|---|---|
| 1.0 mm | 8 mm | 5 mm | 62 | 4,450 mm bed limit | 200 T, 5-axis CNC brake |
| 1.5 mm | 12 mm | 8 mm | 93 | 4,450 mm bed limit | standard tooling |
| 2.0 mm | 16 mm | 10 mm | 124 | 4,450 mm bed limit | most elevator and genset panels sit here |
| 3.0 mm | 24 mm | 15 mm | 186 | 4,450 mm bed limit | check hole-to-bend on the flat pattern |
| 4.0 mm | 32 mm | 20 mm | 249 | 4,450 mm bed limit | 5-axis control keeps the angle on long parts |
| 6.0 mm | 60 mm | 36 mm | 298 | 4,450 mm bed limit | force per metre drops as V widens |
| 8.0 mm | 80 mm | 48 mm | 398 | 4,450 mm bed limit | inside radius grows with V |
| 10.0 mm | 100 mm | 60 mm | 497 | about 3,900 mm | 10 mm is the practical brake maximum |
Two readings matter here. The minimum flange column is the answer to the design question, and the last column is the answer to the production question: at 10 mm the brake can still bend, but only to about 3,900 mm of length before the 200 T rating runs out.
Punch clearance: the second limit nobody draws
Flange length is limited twice. The die shoulder sets how short a flange can be; the punch sets how tall a flange can be and how deep a box can be formed.
A straight punch 100 mm tall will form a 100 mm flange in free air, because the flange stands beside the punch as the ram descends. Form a box with 100 mm walls and the punch body has to enter the box, so the box depth must be at least the punch height plus 5–10 mm of clearance. A 120 mm deep box needs a gooseneck, a sectional punch or a different bend sequence.
That is the practical reason that deep electrical enclosures are usually bent in two operations with the box mouth formed last, or produced as a welded fabrication. See box bending for electrical enclosures for the sequence, and bending long panels without distortion for the cases where a 4,450 mm bed is not enough.
What to do when the flange has to be shorter
There are five legitimate answers, and one bad one.
- Use a smaller V die. A 6 mm V on 2 mm sheet brings the minimum outline down to 5 mm, but the bending force rises by roughly 2.7 times and the inside radius falls to about 1 mm. Check the punch first.
- Use an acute tool. A 30° punch and die set forms flanges down to about 3 mm on 1–2 mm sheet by letting the flange sit inside the V rather than on the shoulders.
- Add a flange relief notch at the bend line, which lets the short section form without dragging the adjacent wall.
- Move the bend line with a joggle or an offset, so the short flange becomes a step rather than a bend.
- Make the short flange a separate part and weld it after forming. This works, but welding pulls the flange and the assembly tolerance becomes a fixture problem rather than a press problem.
The bad answer is to specify the flange and let the supplier "try". Tryout on a 4 mm flange costs a die set, a rejected first batch and a drawing revision, and the part usually comes back with a correction that was available at quotation.
Short flanges on long parts pull the ram
A 15 mm flange on a 4,000 mm part behaves differently from the same flange on a 300 mm bracket. On a long part the brake has to bend across its full span, the ram deflects in the middle and the flange height varies along the length by 0.3–0.8 mm on a single-axis machine.
The 5-axis control on our brake compensates for this by adjusting the ram position and the back gauge during the stroke. Without that, the answer on a long part is to bend in sections with a step gauge, which adds an extra operation and a visible witness line on the outside face.
Flat length moves when the die changes
A short-flange correction that changes the die opening also changes the flat pattern, and the change is larger than most designers expect.
Take a 100 mm flange on a 200 mm base in 2 mm CRCA, 90° bend, 2 mm inside radius, K-factor 0.42. Bend allowance is 1.5708 × (2 + 0.42 × 2) = 4.46 mm, and the bend deduction is 2 × (2 + 2) − 4.46 = 3.54 mm. Flat length is 100 + 200 − 3.54 = 296.46 mm.
Move the same bend onto a 6 mm V die and the inside radius drops to about 1 mm. The bend allowance falls to 1.5708 × (1 + 0.42 × 2) = 2.90 mm, the deduction falls to 3.10 mm and the flat grows to 296.90 mm. Half a millimetre across one bend, and more than a millimetre across a box with four bends. If the flat pattern was released with the drawing, the tool change has just invalidated it.
That is why the drawing should treat the flat pattern as reference and let the supplier's bend table govern, and why a die change at quotation is a drawing change, not a shop-floor adjustment.
How flange length becomes assembly tolerance
A single flange length holds about ±0.2 mm on a CNC brake with a good back gauge. The angle holds about ±0.5–1°, which over a 100 mm flange is 0.9–1.7 mm of tip movement. Two flanges at right angles multiply that error, and a welded frame made from four bent parts can accumulate 2 mm before any weld distortion is counted.
That is why a welded elevator counterweight frame or a genset base is checked in a fixture rather than on a surface plate. The fixture holds the datums that the customer uses, and the first part is set to those datums rather than to the drawing dimensions in isolation. Welded assembly tolerances covers how the press tolerance and the weld tolerance add.
Robotic MIG helps here for a different reason: the same weld sequence on every part produces the same distortion on every part, so the fixture correction stays valid across the batch. Manual welding varies more, and the correction has to be built into the fixture with more margin.
What Ganesh runs
Bending runs on a 200 T, 5-axis CNC press brake with a 4,450 mm bed and Delem control, working to 10 mm thickness. The 5-axis control matters on long flanges because it compensates the ram and the back gauge for deflection across a 4 m part, which is where a single-axis brake loses angle towards the ends.
Welded parts move to three CLOOS robotic MIG cells or to one of 30+ manual MIG, MAG, spot and SPM stations, with fixtures designed and built in-house. That matters for a short flange that has to become a welded tab: the fixture controls the position, and the weld sequence is set before the first part is made rather than corrected afterwards.
Finished assemblies carry a ±0.5 mm general tolerance, with tighter features agreed against the drawing. First article inspection is standard, and PPAP documentation is available where the programme needs it.
What the formed and welded drawing has to settle
- Outside or inside dimensions, stated once, with the bend line shown. Mixing the two is the most common source of a short flange at first-off.
- Inside radius, or a note that the radius follows the tool. An R0.5 call-out on 6 mm sheet cannot be formed without coining.
- Bend angle with its tolerance, per bend, where the angle controls assembly.
- Flange relief and corner relief shown on the flat pattern, not left to the tool room.
- Hole-to-bend distance, which should be at least 2.5t plus the inside radius if the hole is to stay round.
- Cosmetic face and grain direction, because the punch side carries the marks and the bend line shows on a brushed finish.
- The flat pattern if one exists, marked as reference only, since the supplier's bend table governs.
Run the flange and bend deduction through the bend allowance calculator before releasing the drawing. If the flat length changes when the die opening changes, the part is sensitive to tool choice and the note should say so.
When the press brake is the wrong route
Above 10 mm the brake stops being the right tool, and heavy plate is cut and welded instead, or formed hot. Flanges shorter than 3 mm on thin sheet are usually better as a rolled or folded edge. Hems, curls and closed seams need a dedicated hemming tool that Ganesh does not run, and they should not be drawn as a tight bend with a note.
For a welded box where every wall is a separate part, the design may be cheaper as a fabrication than as a deep box on a brake. Compare both before freezing the drawing.
The bending capability page lists the brake, the bed and the thickness range. Minimum bend radius by material covers the radius side of the same equation, hole-to-bend distance deals with holes near the bend line, and how to calculate bend allowance gives the flat-length arithmetic that a flange change always disturbs. The bending, welding and assemblies hub indexes the rest.
Frequently asked questions
What is the minimum flange length for a 2 mm sheet?
About 10 mm outside dimension on a 16 mm V die, which is the standard 8t opening for 2 mm mild steel. A 6 mm V die brings it down to about 5 mm, but the bending force per metre roughly triples and the inside radius gets smaller.
Why does a flange shorter than the die opening fail?
The flange has to sit on both die shoulders to control the angle. If it is shorter than V/2 plus the thickness, the part drops between the shoulders, the bend opens, the edge marks and the angle varies from part to part. No amount of gauge adjustment fixes it.
Can a 5 mm flange be formed on 3 mm sheet?
Not on a standard V die. A 3 mm part needs about 15 mm of flange on a 24 mm V opening. A 5 mm flange requires a 6 mm V die with a sharp punch, an acute tool, or a redesign that moves the bend line. On 3 mm stainless, tool load becomes the limit before geometry does.
Does the flange length affect the bend allowance?
Yes, through the die opening rather than the flange itself. A different V changes the inside radius, which changes the K-factor and therefore the flat length. Recalculate the flat pattern whenever the die opening changes, and never reuse a bend table from a different tool.
How tall can a flange be on a 4,450 mm press brake?
Flange height is limited by punch height and, on box parts, by the punch body. A straight punch of 100 mm lets a 100 mm flange form freely; a 200 mm flange on a box needs a gooseneck or sectional tool. The bed length limits the bend to 4,450 mm, and load limits the length at 10 mm.
Sources
Have a flange that looks tight on paper?
Send the drawing with the bend lines and the flat pattern if you have one. We check the flange against the die set and the punch clearance before quoting the part.
Send a drawing