Direct answer
A flat pattern combines straight flange dimensions with bend allowance or bend deduction for each bend, using conventions and parameters validated for the actual material, thickness, radius, angle, grain, tooling, and process.
What this calculation tells you
Bend calculations connect the finished part to a cut blank and help shops standardize brake setups, nesting, quotations, and first-article correction. A bend table preserves proven results better than repeatedly guessing a factor.
The calculation cannot prove tooling capacity, minimum flange, cracking risk, springback control, tolerance capability, or safe press-brake operation.
Where it is used
Sheet-metal fabrication
Develop flat patterns for brake-formed brackets, enclosures, and panels.
CAD/CAM
Configure material- and tooling-specific bend tables for unfold operations.
Estimating
Compare blank size, sheet yield, cut length, and forming steps.
Quality improvement
Record trial-bend measurements and correct process-specific bend data.
When this guide helps
- Changing punch and die alters the achieved inside radius.
- The drawing gives outside dimensions but the formula assumes tangent lengths.
- High-strength material springs back differently from the previous batch.
- Several bends interact with flange and tooling access.
Fix the dimensional convention
Identify finished angle, inside radius, thickness, and whether flange dimensions are inside, outside, or to tangent points. A correct formula with mixed references still gives a wrong blank.
Document the bend direction and sequence for asymmetric parts.
Use process-specific bend data
Air bending, bottoming, coining, roll forming, and other methods create different neutral-axis behavior and radii. Material family alone is not enough.
Prefer qualified bend tables or trial coupons made with the intended material lot, grain direction, tooling, and setup.
Check manufacturability
Minimum flange, hole-to-bend distance, relief, hemming, tool interference, backgauge access, tonnage, and sequence can prevent the nominal geometry from being formed.
Springback compensation belongs to the controlled process, not an invisible length allowance.
Close the loop with measurement
Form a sample, measure angle and critical dimensions with an agreed method, update the bend data, and preserve the result by machine, tool, material, and revision.
- Follow press-brake guarding and setup procedures.
- Verify tooling ratings.
- Do not reach into an unsafe point of operation.
Worked case: 90-degree bend
Inside radius 2 mm, thickness 1 mm, K = 0.4 and angle 90°.
BA = 1.5708x(2+0.4) = 3.770 mm.
Entered bend allowance is about 3.77 mm.
K-factor is a process/material assumption, not a universal constant.
Reproduce this worked caseOpen Sheet-Metal Bend Allowance Calculator
Worked case: larger radius
Inside radius 4 mm with other inputs fixed.
BA = 1.5708 × 4.4 = 6.912 mm.
Larger radius adds 3.142 mm to bend allowance.
Tangent lengths and bend deductions must use the same geometry convention.
Reproduce this worked caseOpen Sheet-Metal Bend Allowance Calculator
bend allowance: compare assumptions, not just answers
Calibrate with shop coupons, tooling and grain/material data. Springback and forming limits are separate.
| Field scenario | Changed input | Calculated consequence |
|---|---|---|
| R 2 | K = 0.4, T = 1 | BA 3.77 mm |
| R 4 | K = 0.4, T = 1 | BA 6.91 mm |
bend allowance: calculation checklist
- Angle converted to radians
- Inside radius used
- K source recorded
- Thickness consistent
- Shop validation
Practical questions
Frequently asked questions
What is K-factor?
It locates the neutral axis through sheet thickness for a defined bending model; its useful value depends on the actual process.
Are bend allowance and bend deduction interchangeable?
They describe related but different dimensioning methods. Use one consistent convention with matching inputs.
Can one bend table cover every machine?
Not reliably. Tooling, process, material, setup, and measurement method can produce different results.
Further reading
Authoritative sources
Use these primary and professional resources to check definitions, conventions, or requirements that may extend beyond this guide.