How short can a flange be? Minimum flange length for air bending

Why a flange can be too short

On a press brake in air bending, the punch pushes the sheet into a V-die without bottoming it, and the sheet is carried on the die’s two shoulders while it bends. A flange has to reach past the shoulder on its side; if it does not, it slips into the V. Tooling makers’ charts (WILA, Mate p. 4) and Benson (2018) give a rule of thumb of about 0.7 × the die opening for a standard V-die; WILA’s chart gives it as an outside dimension — from the end of the flange to the outside of the bend. WILA also says the flange must be longer than half the opening.

1. Choose the die from the thickness

A common starting point is a die opening of a few times the thickness. Mate’s chart gives 6 × the thickness below 2.5 mm, 8 × from 2.5 to under 8 mm, 10 × from 8 to under 10 mm and 12 × from 10 mm (the table below uses those boundaries); WILA gives 6–8 × for steel up to about 3 mm. Dies vary a lot in practice — Mate’s own page also lists 12–15 × for air bending in another table — and a wider die needs a longer flange in proportion.

2. Check it against the radius

Air bending does not copy the punch: the inside radius it leaves is set by the die, about 0.16 × the opening in mild steel and 0.20–0.22 × in stainless (Benson 2015; WILA’s chart gives about 0.15 ×). So a radius on the drawing asks for a die of its own: about the radius ÷ 0.16 in mild steel, ÷ 0.21 in stainless. Use the wider of the two dies. Benson makes the same point: 8 × the thickness suits only a radius close to the thickness.

3. Allow about 0.7 × V

If your drawing’s radius is larger than the last column, the die has to be wider than the table’s — radius ÷ 0.16 — and the shortest flange grows with it. A radius tighter than the last column needs a narrower die than the table’s, which shortens the shortest flange; check that with your tooling.

A worked example

A U-channel in 2 mm mild steel with an inside radius of 2 mm, flanges of 8 mm to the outside and a 60 mm web. The table gives a 12 mm die, but a radius of 2 asks for 2 ÷ 0.16 = 12.5, so the die is 12.5 and each flange needs about 0.7 × 12.5 = 8.8. The 8 mm flanges are too short: lengthen them to about 8.8. A tighter radius alone does not save them — at 1.5 the table’s 12.0 die governs and the flange still needs 8.4, and that die leaves about 1.9 rather than 1.5. In stainless a radius of 2 asks for a narrower die, 2 ÷ 0.21 = 9.5, so the table’s 12.0 governs and the flanges need about 8.4 — and that die leaves a radius nearer 2.5 than 2.

Bends that are not 90°

The figures are for 90° bends. With θ the angle bent through, a flange crosses the die’s shoulder about (V/2) ÷ cos(θ/2) from the outside corner: 0.71 × V at 90°, 0.65 × V at 80°, 0.58 × V at 60°. A smaller bend needs less than a 90° bend does, so a flange that suits the 90° figure suits a smaller bend too. Past 90° you need an acute die, and Benson gives about 1.1 × the opening or more; how that length is measured is not stated, so treat it as a warning, not a number to cut to.

Common mistakes

• Measuring the flange to the inside of the bend: the limit is an outside dimension.
• Picking the die from the thickness alone when the drawing calls for a large radius.
• Expecting the mild-steel radius on stainless: the same die leaves a larger radius, 0.20–0.22 × V.
• Treating these as guarantees. They are tooling makers’ rules of thumb; your own tooling decides.

How this matches the tool

The profile builder runs the same check on both end flanges of any profile — die from the table or from the radius, about 0.7 × V to the outside of the bend — and says when a flange is short. The table and the worked example on this page were produced by that code when the page was built.

Sources

1. Mate Precision Tooling, TWS catalog LIT01120 Rev D (2017), p. 4 “Bending Basics”: V-die by thickness (6, 8, 10, 12 × t), minimum flange length and inside radius by die
2. WILA, “V-opening”: about 6–8 × the thickness for steel up to about 3 mm; minimum flange greater than half the V-opening
3. WILA, air bending tonnage chart (USA catalog p. 140): flange B as an outside dimension, inside radius ri, mild steel, 90° air bending
4. S. Benson, The Fabricator (2018), “Press brake bending tips for forming tapered bends and determining minimum flange lengths”: about 70 % of the die width for a standard V-die, about 110 % or more for an acute die
5. S. Benson, The Fabricator (2015), “Predicting the inside radius when bending with the press brake”: about 16 % of the die opening for 60-ksi cold-rolled steel, 20–22 % for stainless
6. S. Benson, The Fabricator (2013), “Finding the perfect die opening”: 8 × the thickness is right only when the inside radius equals the thickness