Bend allowance calculator

A 160 mm blank does not make a 100 × 60 mm bracket

Two outside legs of 100 mm and 60 mm in 2.0 mm CRCA come from a 156.5 mm blank, not a 160 mm one. The 3.5 mm removed is the bend deduction. Get it wrong and the part is short on both legs, a hole has drifted toward the bend line, or a flange will not seat in the weld fixture. The flat pattern is a calculation, not a scaled view of the formed part.

What the neutral axis measures

When the punch drives the sheet into the V, the outside of the bend stretches, the inside compresses, and one surface in between keeps its length. Bend allowance is the arc length of that surface:

BA = π × (R + K × T) × θ / 180

R is the inside radius, T the thickness, θ the included bend angle in degrees, K the position of the neutral axis as a fraction of thickness. At 90° the expression collapses to BA = 1.5708 × (R + K × T).

Bend deduction is the number a shop needs to turn outside dimensions into a flat length:

BD = 2 × (R + T) × tan(θ/2) − BA

flat length = leg A (outside) + leg B (outside) − BD

For a channel with three identical bends, subtract one deduction per bend: flat = A + B + C + D − 3 × BD. When the bends differ in radius or angle, compute each deduction separately and track which dimension pair it applies to.

K-factor follows the radius, not the material data sheet

K is not a property you look up once per grade. It is where the neutral axis settles for a given R/T, material condition and tooling. The values below are the ones a shop converges on after trial bends on a standard V die.

The radius is set by the die, not the drawing

In air bending the inside radius follows the die opening, roughly R ≈ V / 6. The punch tip sets the radius only when it is larger than that value. A drawing calling R2.0 on 2 mm CRCA, formed in a 16 mm V die, produces about R2.7. The flat length moves by a quarter of a millimetre and the outside corner no longer matches the CAD model. Either accept the die radius and change the drawing, or bottom the bend and pay the tonnage for it. Ganesh air bends as standard, so a drawing radius is treated as a target to be checked against the die list, not as a guarantee.

Springback by material

Springback is the elastic recovery of the whole section after the punch lifts. It is measured on the machine with a digital angle gauge, not derived from a table, because it changes with the coil, the die condition and the amount of wrap around the punch.

On a 60 mm leg, 4° of residual angle moves the tip by 60 × tan 4° = 4.2 mm. That is an order of magnitude larger than any K-factor error on the same part, which is why the angle correction in the brake program matters more than the third decimal of the flat length.

Minimum flange and the tooling limit

L_min ≈ V / 2 + T

With a standard V = 8T die this becomes L_min ≈ 5T: 10 mm on 2 mm material, 30 mm on 6 mm. A shorter flange does not sit on both die shoulders, so the bend line wanders, the part twists and the angle drifts along the length. A smaller V die reduces the minimum flange but raises tonnage roughly as 1/V, and above the press rating it is not an option. A flange below 5T is a design change, not a tooling change. See minimum flange length for press brake parts for the tooling geometry.

The four errors that make the flat pattern wrong

1. Adding outside dimensions and calling the sum the flat length. On the 100 × 60 bracket that is 3.5 mm long, on every part, forever.
2. One K-factor for every material. CRCA to stainless at the same R/T moves K by about 0.04, worth 0.13 mm on the flat. Small next to springback, but it accumulates over four bends.
3. Ignoring bend reliefs and corner notches. They change the cut length, and if the DXF was generated from the formed model without them the blank is short at the root of the bend.
4. Reusing a program from a similar part. A stored program carries the die, the material batch and the angle correction of the part it was written for, not of yours.

What Ganesh runs

Bending runs on a 200 T, 5-axis CNC press brake with a 4,450 mm bed and Delem control, handling material to 10 mm. Each repeat part keeps its own stored program: die set, bend sequence, angle correction and backgauge positions. The flat length proven with that program is the one the laser cuts on the next release, so a change to a radius or a material grade is treated as a new program rather than an edit to an old one.

There is no universal K table here. The first article of a new part is checked on the brake with a digital angle gauge and the program is corrected before the batch runs. Formed features are typically held to ±0.5 mm, tighter on critical features by agreement. For the wider route, see the sheet metal bending capability page and the bending, welding and assembly guides.

What to put on the drawing

• Inside radius, or the note "radius to suit standard tooling".
• Bend angle with its tolerance and the face it is measured from.
• Flange lengths to a stated convention: outside, inside or bend line.
• A flat pattern DXF carrying the same revision string as the PDF.
• Material grade and temper, because both move K and springback.
• Whether coating thickness counts: an 80 µm powder coat adds 0.16 mm to an outside dimension measured after coating.

L-bracket, 2.0 mm CRCA (IS 513 D), outside legs 100 mm and 60 mm, inside radius 2.0 mm, one 90° bend, K = 0.42.

Step 1 — R/T = 2.0 / 2.0 = 1.0, so K = 0.42 from the table.

Step 2 — BA = π × (2.0 + 0.42 × 2.0) × 90 / 180 = 3.1416 × 2.84 × 0.5 = 4.462 mm.

Step 3 — BD = 2 × (2.0 + 2.0) × tan 45° − 4.462 = 8.000 − 4.462 = 3.538 mm.

Step 4 — Flat length = 100 + 60 − 3.538 = 156.46 mm. A 160 mm blank is 3.54 mm too long.

Step 5 — Bend line sits at 100 − (R + T) = 96.0 mm from the end, and the backgauge is set to 96.0 mm.

Now the die check. The shop picks a 16 mm V die for 2 mm stock, so the formed radius is 16 / 6 ≈ 2.67 mm, not 2.0 mm. Recompute with R = 2.67 and K = 0.43: BA = 1.5708 × (2.67 + 0.86) = 5.545 mm; BD = 2 × 4.67 − 5.545 = 3.795 mm; flat = 160 − 3.795 = 156.21 mm. The blank is cut at 156.2 mm.

The same bracket in 304 stainless raises K to 0.46: BA = 1.5708 × (2 + 0.92) = 4.587 mm, BD = 3.413 mm, flat = 156.59 mm — 0.4 mm off the CRCA blank. The angle is what moves. Without overbend the leg springs back to about 86°, putting the tip of the 60 mm leg 4.2 mm out of position. The program carries 4° of overbend and the flat stays at 156.6 mm.