Bend Radius, K-Factor and Springback: The Sheet Metal Design Basics That Decide Fit
Process Anjue Metal | Process | 11 min read | Updated Oct 8, 2026
A blank is cut perfectly, folded perfectly, and the finished part is 0.8 mm too wide to fit the hole it was designed for. Nine times out of ten the cause is one of three numbers that designers guess and factories calculate: bend radius, K-factor and springback. Here is what they mean in practice.
Minimum Bend Radius: The Number That Causes Cracks
When a sheet is bent, the outer surface stretches. Bend it too tightly and the outer surface simply tears - a crack that may be a visible split or a hairline that only opens after forming, coating or a drop test.
The safe rule is expressed as a multiple of material thickness (t):
| Material | Condition | Typical minimum inside radius |
|---|---|---|
| Mild steel (cold-rolled) | Standard | 0.5 t - 1.0 t |
| Stainless steel 304 / 316L | Annealed | 1.0 t - 1.5 t |
| Aluminium 5052 | H32 | 1.0 t - 1.5 t |
| Aluminium 6061 | T6 | 2.5 t - 4 t (often not bendable at tight radii) |
| Galvanized steel | Standard | 1.0 t - 1.5 t (watch coating cracking) |
| Brass / copper | Soft | 0.5 t - 1.0 t |
Note the orientation effect: bending across the rolling grain is safer than bending along the grain, because cracks propagate along grain boundaries. If a part has a single critical bend, designers should either specify the grain direction or allow a larger radius.
Practical shortcut: if you do not want to calculate, keep the inside bend radius at at least one material thickness for steel and 1.5 times thickness for aluminium and stainless. Most press brake tooling is built around a standard radius in that range anyway.
K-Factor and Bend Allowance: Why the Flat Pattern Matters
Bend it and the material does not simply fold along a line. The outer surface stretches, the inner surface compresses, and somewhere between them is a neutral axis that keeps its length. That axis is not at the middle of the thickness - it shifts toward the inside of the bend.
The K-factor is the position of the neutral axis expressed as a fraction of thickness:
- K = 0.5 would mean the neutral axis is exactly in the middle
- In reality K typically falls between 0.33 and 0.45, depending on material, radius and tooling
The K-factor feeds the bend allowance - how much material length the bend consumes. Change the K-factor and the flat pattern length changes. Get it wrong and the formed part is the wrong size even though every cut dimension is correct.
What this means for a buyer:
- Always supply the 3D model if you have one (STEP/STP). The model carries the design intent and the factory can derive the flat pattern on its own tooling.
- If you only supply a flat DXF, state the bending convention you used, or you and the supplier may be working from different K-factors.
- Do not mix DXF and drawing dimensions casually. If the DXF is the flat pattern and the drawing shows formed dimensions, say which one governs.
The common industry values - roughly 0.33 for tight bends in soft material up to about 0.45 for generous radii - are starting points. Serious workshops verify with a test bend of your actual material before running a batch.
Springback: Why the Angle Comes Out Wrong
Steel and aluminium are elastic before they are plastic. The press brake pushes the material past the target angle, then the material springs back part of the way when the load is released. The finished angle is smaller than the angle the tool reached.
Typical springback behaviour:
- Mild steel: small, generally 0.5-2 degrees
- Stainless steel: larger, often 2-5 degrees, and it work-hardens as it bends
- Aluminium: significant, especially in harder tempers like 6061-T6
- Thicker material: more springback than thin material at the same angle
How factories deal with it:
- Over-bending - programming the machine to bend past the target angle so the release lands on it
- Bottoming - pressing the material into full contact with the die, which reduces springback dramatically
- Coining - very high pressure that forces the material to conform; the most accurate and the most demanding on tooling
- Compensation in the CNC control - modern press brakes calculate springback per material and thickness
The practical consequence for you: an angle tolerance of ±0.5 degree is normal and achievable. Demanding ±0.1 degree on a bent part means coining, slower cycle times and higher cost - and often a part that still needs adjustment at assembly.
How to Specify Bending on a Drawing
A bend drawing that avoids arguments contains five things:
- Material grade and thickness, including the standard for thickness tolerance
- Inside bend radius for each bend, or a note such as "inside radius 1.0 t unless otherwise stated"
- Angle and direction for each bend, with a clear view marking bend lines
- Which dimensions are critical - the two or three that must be held. On real parts, not everything can be critical
- Flat pattern data or a 3D model, stating which governs in case of conflict
Note · A habit that prevents most bend problems: on the drawing, mark two or three dimensions as critical and leave the rest with a general tolerance note. Factories schedule inspection time where the risk is, and a drawing where every dimension is critical gets inspected less effectively than one where the important ones are flagged.
Realistic Bending Tolerances
| Feature | Typical achievable | Comment |
|---|---|---|
| Bend angle | ±0.5 degree | ±0.1 degree requires coining, add cost |
| Bend position | ±0.15 mm to ±0.3 mm | Depends on part size and bend count |
| Overall formed dimension | ±0.3 mm to ±0.5 mm | Accumulated error across multiple bends |
| Flat pattern dimensions | ±0.1 mm (laser cut) | Achieved before forming |
| Door/flange parallelism | ±0.3 mm | Depends on weld distortion if welded |
| Hole position after bending | ±0.3 mm | Better if holes are cut after forming, but that costs more |
Tolerance accumulates with each bend: a four-bend box has four opportunities for error. Setting a tight tolerance on the overall dimension after four bends usually means either a very capable press brake with a skilled operator, or rework.
Design Rules Worth Following
- Keep the inside radius between 1 t and 2 t for most work
- Keep the bend line at least 2.5 t plus radius away from any hole, or the hole will deform
- Keep flange length at least 4 t plus radius; shorter flanges cannot be formed reliably
- Avoid bends across a weld or across a large cut-out if you can - both distort
- Add bend relief slots where two bends meet in a corner, or the corner will tear
- Think in three dimensions. A part that is simple on a flat pattern can be impossible to form if two bends interfere with each other's tooling access
Frequently Asked Questions
For mild steel, roughly 0.5-1.0 times the material thickness inside radius. For stainless steel and aluminium, allow 1.0-1.5 times thickness. 6061-T6 aluminium needs 2.5-4 times thickness and often should not be bent at all at tight radii.
It is the position of the neutral axis in a bend, expressed as a fraction of material thickness - typically 0.33 to 0.45. It determines the bend allowance, and therefore the correct flat pattern length.
Springback - the material elastically returns part of the way after the load is released. Modern press brakes compensate by over-bending, bottoming or coining. The result is also affected by material grade, thickness and grain direction.
Send both if you have them, and state which governs. A STEP model is best because it carries design intent, and the factory derives its own flat pattern to match its tooling and K-factor.
±0.5 degree is standard and reliable. Tighter angles are possible with coining, but they slow the process and increase cost, and are rarely necessary for enclosures.
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