K-Factor Calculator

The ShincoFab K-Factor Calculator reverse-engineers the neutral-axis position used for sheet metal flat-pattern development. Enter material thickness, inside bend radius, bend angle, and either a measured bend allowance or test-coupon dimensions. The calculator returns a dimensionless K-factor plus neutral-axis offset, bend allowance, bend deduction, outside setback, and (for the coupon method) flat blank length. Use the result as a process-specific starting value, then validate it with the same material, tooling, and machine used for production.

K-Factor Calculator
01 / Parameters

Measure the bend

Calculation method
mm
T / thickness before bending
mm
R / measured inside radius
90°
Included bend angle in degrees
mm
BA / arc length consumed by the bend
02 / Output

Neutral axis position

LIVE CALC
K-factor
0.380
dimensionless ratio
Neutral axis offset0.760 mm
Bend allowance3.707 mm
Bend deduction4.293 mm
Outside setback4.000 mm
Flat blank length— mm
Bend section / neutral axis90°
R + K·TTθneutral axisK locates thestretch line
Use a test bend to validate the result for production.
K-factor from BA
K = (BA / θrad − R) / T

The neutral axis is measured from the inside surface as a fraction of total thickness.

Bend allowance
BA = θrad × (R + K × T)

The bend allowance is the arc length along the neutral axis.

Coupon method
BA = Lt − A − B + 2 × OSSB

OSSB = (R + T) × tan(θ / 2). All lengths must use one unit system.

This calculator reverse-engineers a starting K-factor from measured geometry. Material temper, grain direction, tooling, springback and measurement conventions can change the production value.

How to Use the Calculator

1. Choose units

Use MM for millimetres or IN for inches. Switching units converts every length already entered, including thickness, radius, bend allowance, flange legs, and flat blank length. The bend angle and K-factor do not have units.

2. Choose a calculation method

Select Measured bend allowance when you have the neutral-axis arc length from a bend table, CAD calculation, or measured test. Select Test coupon dimensions when you have a physical coupon and measured outside legs and flat blank length.

3. Enter material thickness

Enter T, the thickness of the flat stock before bending. Measure the actual sheet rather than relying only on a nominal gauge designation when the value is important.

4. Enter inside bend radius

Enter R, the radius on the inside surface of the finished bend. In air bending, the actual radius can differ from the punch nose radius, so use a radius gauge, section measurement, or validated supplier value where possible.

5. Set bend angle

Move the bend-angle slider from 1° to 179°. The calculator treats this as the included bend angle used in the bend-allowance equation. Confirm that your drawing and shop convention use the same angle definition.

6. Enter a measured bend allowance, or coupon dimensions

For Measured bend allowance, enter BA, the material length consumed along the neutral axis through the bend.

For Test coupon dimensions, enter:

  • Outside leg A and Outside leg B, measured from the outside bend tangencies to the ends of the two legs.
  • Measured flat blank length, the total length of the coupon before bending.

The coupon method first derives bend allowance from those three measurements and the calculated outside setback. Keep all three dimensions in the same unit system.

7. Review or copy the result

Results update as inputs change. Reset values restores a representative 2 mm, 2 mm radius, 90° example in the active unit system. Copy results copies the K-factor and supporting values; Copy K-factor copies only the primary ratio. Clipboard access depends on the browser and iframe permissions.

What the Results Mean

K-factor

The K-factor is a dimensionless ratio between 0 and 1:

K = neutral-axis offset / material thickness

A lower value places the neutral axis closer to the inside surface. The number is specific to the material, thickness, radius, bend method, tooling, and measurement convention used for calibration.

Neutral axis offset

This is the distance from the inside surface to the neutral axis:

neutral-axis offset = K × T

It is shown in the selected length unit and is useful when checking the geometry behind a CAD bend table.

Bend allowance

Bend allowance (BA) is the arc length along the neutral axis. In the measured-allowance method it is an input; in the coupon method it is derived from the test dimensions. It is the length consumed by the bend when developing a flat pattern.

Bend deduction

Bend deduction (BD) converts two outside leg dimensions into a flat-length estimate:

BD = 2 × outside setback − BA

The value is not a machine guarantee. It depends on the same radius, angle, and K-factor assumptions used for the calculation.

Outside setback

Outside setback (OSSB) is the distance from the outside tangent intersection to the bend tangent:

OSSB = (R + T) × tan(θ / 2)

The calculator uses the bend angle in radians inside the trigonometric function after converting the displayed degree value.

Flat blank length

The coupon method reports the measured flat blank length you entered. It derives bend allowance using:

BA = flat blank length − outside leg A − outside leg B + 2 × OSSB

The measured-allowance method shows — for this output because no flange dimensions were supplied.

Formula and Engineering Notes

The reverse calculation is based on the standard bend-allowance relationship:

BA = θrad × (R + K × T)
K = (BA / θrad − R) / T

Here, T is material thickness, R is inside bend radius, BA is bend allowance, and θrad is the included bend angle converted to radians. For the coupon method, the calculator derives BA from outside legs and flat blank length before applying the same equation.

The tool does not include a proprietary material lookup table. Published K-factor tables are process-specific starting points, while a measured test bend captures the actual material lot, press brake, die opening, punch, springback, and operator setup. A calculated K-factor outside 0 to 1 is treated as an input or datum error rather than silently clamped.

For related flat-pattern checks, compare the result with ShincoFab’s sheet metal bend allowance and bend deduction chart, bend allowance calculator, and bend deduction calculator.

Practical Fabrication Checklist

  • Measure the actual stock thickness at several points and record the material grade and temper.
  • Measure the finished inside radius instead of assuming it equals the punch nose radius.
  • Use the same bend angle convention in the drawing, CAD system, and shop measurements.
  • For a coupon, mark the outside tangencies clearly and use the same datum for both legs.
  • Use a coupon from the same material lot and the same grain direction planned for production.
  • Repeat the test after changing die opening, punch, tooling wear, bend method, or material supplier.
  • Check the calculated flat pattern against the actual first-off part before releasing a production batch.

Why Results May Differ

CAD systems may use a bend table, Y-factor, or a different outside-dimension convention instead of the exact inputs used here. Confirm whether a CAD value represents bend allowance, bend deduction, or neutral-axis arc length before comparing numbers.

Supplier data can use nominal thickness, nominal radius, or a standard material table that does not match your press-brake setup. Air bending, bottoming, and coining also move the neutral axis differently, and springback changes the final angle and radius after unloading.

For tolerance-sensitive work, pair this calculator with the sheet metal flat pattern calculator and the sheet metal tolerance chart, then request a drawing-specific review through ShincoFab’s sheet metal bending services.

Frequently Asked Questions

These answers address common situations where a preliminary K-factor differs from a CAD model, supplier table, test coupon, or manufactured part. Check the input datums and process assumptions before changing the value.

What is a typical sheet metal K-factor?

Many shop tables use values below 0.5, but there is no universal production value. Air-bend, bottoming, and coining setups, material strength, radius-to-thickness ratio, and grain direction can all move the result. Calibrate with a test bend when the fit matters.

Yes, if the CAD value is the arc length along the neutral axis and uses the same angle, inside radius, and thickness definitions. If the CAD system reports bend deduction or flat length instead, use the coupon method or convert the value first.

That usually indicates mixed units, an incorrect bend angle convention, an outside radius entered as an inside radius, or a measurement taken from inconsistent tangency datums. Recheck the test coupon and use the actual inside radius.

The equation treats K as a calibrated process parameter, but real material flow and springback can make the apparent value angle-dependent. If you use a wide range of angles, validate more than one representative bend.

Enter the finished inside bend radius. In air bending, the formed radius depends on die opening, material, thickness, and setup, so it may not equal the punch nose radius.

It reverse-engineers bend allowance from a measured flat blank length and the two outside legs after bending. This is useful for calibrating a specific material and press-brake setup, provided the tangency datums and radius are measured consistently.

No. It is a transparent engineering estimate and calibration aid. Run a first-off test and inspect the actual angle, radius, flange lengths, and fit before approving production.

Need a Production-Ready K-Factor?

A drawing-specific DFM review can reconcile your CAD bend table with actual material, tooling, bend sequence, and inspection datums. Share the part drawing or flat pattern, material and thickness, target bend angles, inside radii, quantity, and any critical tolerances so the fabrication team can review the assumptions.

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  • Whatsapp: +86 13392819510
  • [email protected]

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