What is the neutral axis in sheet metal bending?
The neutral axis is the surface inside the material that neither stretches nor compresses during bending. Its length is preserved, which is why the flat pattern is calculated from it rather than from the inside or outside face. Its position is described by the K factor.
What is the setback in bending?
Setback is the distance from the outside corner of a bend back to the tangent point where the bend arc begins. It is (R + t) × tan(θ/2). Two setbacks, one on each leg, make up the outside overlap that the bend deduction removes.
| Quantity | Formula | What it describes |
|---|---|---|
| Setback (OSSB) | (R + t) · tan(θ/2) | Corner to tangent point, outside |
| Bend allowance (BA) | θrad · (R + K·t) | Neutral axis arc length through the bend |
| Bend deduction (BD) | 2 · OSSB − BA | Overlap removed from outside dimensions |
| Flat length | A + B − BD | Blank size from outside leg dimensions |
What is bend allowance used for?
Bend allowance gives the length of material taken up by the bend itself, measured along the neutral axis. Add it to the straight portions of the legs and you have the flat blank length.
Bend allowance is the right correction when you know the leg lengths measured to the tangent points at the start of the bend arc, rather than to the outside corner.
What is bend deduction used for?
Bend deduction is the amount by which the sum of the outside dimensions exceeds the flat length. Subtract it from the total of the outside leg dimensions to get the blank.
Which method should I use?
Use whichever matches how the drawing is dimensioned, and use the same one for the whole part. The two are algebraically equivalent; mixing them is the single most common cause of an incorrect flat pattern.
| Drawing dimensions are given to | Use | Blank length |
|---|---|---|
| Outside faces of the legs | Bend deduction | Σ outside dimensions − Σ BD |
| Tangent points of each bend | Bend allowance | Σ straight legs + Σ BA |
| Mixed | Convert first | Redimension to one convention |
How do I get an accurate flat pattern?
Calibrate the K factor against a trial bend in the actual material with the actual die, then apply it consistently. Everything else in the calculation follows from the K factor and the inside radius.
- Cut a trial blank of known length in the production material.
- Bend it with the die that will be used in production.
- Measure the resulting legs and solve for the K factor.
- Confirm by predicting a second part and checking it.
Does the inside radius come from the die or the drawing?
In air bending the die determines the radius, roughly 0.16 times the die opening, so the radius usually has to be derived from the die rather than read from the drawing. If the drawing demands a specific radius, the die opening has to be chosen to produce it.
Related technical pages
Bend allowance calculator
Work out bend allowance, bend deduction and the flat length.
K factor calculator
Solve for K from a measured trial bend.
V-die opening selection chart
The opening that fixes the inside radius.
Minimum bend radius
How tight the radius can be before the material cracks.
Next step
Send us the drawing, material grade and the die you plan to use and we will check the flat pattern and recommend the tooling.
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