Sheet Metal Bend Neutral Factor (K-Factor)

July 21, 2026

Data source: E:\FDM\折弯中性因子\折弯中性因子.xlsx (author: 光軍) Applicable to: UG NX bend coefficient table

What is K-Factor

When sheet metal is bent, the inside is compressed and the outside is stretched. There is a neutral layer in between that is neither compressed nor stretched. The position of this neutral layer is expressed as the K-factor:

K = distance from neutral layer to inner surface / material thickness

K = 0 → Neutral layer at inner surface (material fully compressed)
K = 0.5 → Neutral layer in the middle (ideal case)
Higher K → Neutral layer closer to outer surface, longer flat pattern

Comprehensive Table: R/T vs Material K-Value + K1/K2 General Values

R/T	Steel	Aluminum	Copper/Brass	Stainless	K1 (General)	K2
0.10	—	—	—	—	0.230	0.300
0.15	—	—	—	—	0.260	0.320
0.20	0.300	0.300	0.300	0.300	0.290	0.330
0.25	0.300	0.300	0.300	0.300	0.310	0.350
0.30	0.300	0.300	0.300	0.300	0.320	0.360
0.40	0.355	0.355	0.365	0.370	0.350	0.370
0.50	0.370	0.370	0.375	0.380	0.370	0.380
0.60	0.380	0.385	0.390	0.390	0.380	0.390
0.70	0.390	0.395	0.400	0.400	0.390	0.400
0.80	0.400	0.405	0.408	0.408	0.400	0.408
0.90	0.405	0.405	0.410	0.414	0.405	0.414
1.00	0.410	0.410	0.420	0.420	0.410	0.420
1.10	0.420	0.422	0.424	0.425	0.420	0.425
1.20	0.424	0.426	0.428	0.430	0.424	0.430
1.30	0.429	0.430	0.432	0.433	0.429	0.433
1.40	0.433	0.433	0.436	0.436	0.433	0.436
1.50	0.436	0.438	0.440	0.440	0.436	0.440
1.60	0.439	0.442	0.443	0.443	0.439	0.443
1.70	0.440	0.442	0.445	0.446	0.440	0.446
1.80	0.445	0.445	0.450	0.450	0.445	0.450
1.90	0.447	0.450	0.452	0.452	0.447	0.452
2.00	0.449	0.452	0.455	0.455	0.449	0.455
2.50	0.458	0.458	0.460	0.460	0.458	0.460
3.00	0.464	0.468	0.468	0.470	0.464	0.470
3.50	0.468	0.468	0.470	0.473	0.468	0.473
3.75	0.470	0.470	0.472	0.475	0.470	0.475
4.00	0.472	0.472	0.474	0.476	0.472	0.476
4.50	0.474	0.474	0.478	0.478	0.474	0.478
5.00	0.477	0.480	0.480	0.480	0.477	0.480
6.00	0.480	0.485	0.485	0.485	0.479	0.482
10.00	0.490	0.500	0.500	0.500	0.488	0.490
15.00	0.500	0.500	0.500	0.500	0.493	0.495
30.00	—	—	—	—	0.496	0.498

Note: Material-specific values (Steel/Aluminum/Copper/Stainless) are K1 values. For R/T < 0.2 and R/T > 15, use K1 general values.

Pattern: Higher R/T → K closer to 0.5 (neutral layer approaches center). For R/T < 0.3, K ≈ 0.3.

Quick Reference (by r/t ratio)

#	Inner R / Thickness (r/t)	K-Value
1	0.1	0.23
2	0.25	0.34
3	0.5	0.37
4	1.0	0.41
5	1.5	0.44
6	2.0	0.45
7	2.5	0.46
8	3.0	0.46
9	4.0	0.47
10	5.0	0.48
11	6.0	0.50

This table is for quick lookup; precision is lower than the comprehensive table above.

Usage

Standard Steps

1. Calculate R/T ratio (inner bend radius / material thickness)
2. Find the K-value for your material (from the material column or K1 column)
3. K-value × material thickness = neutral layer offset distance
4. Measure the bend line length after offset in CAD = flat pattern length

Bend Flat Pattern Formula

L = L1 + L2 + (π × (R + K × T) × θ) / 180

Where:
  L  = total flat length
  L1 = straight section 1
  L2 = straight section 2
  R  = inner bend radius
  K  = K-factor
  T  = material thickness
  θ  = bend angle (degrees)

Example: Thickness 1mm, inner R 4mm, bend angle 120°, two straight edges 30mm each

Flat length = 30 + 30 + (3.14 × (4 + 0.33 × 1) × 120) / 180
            = 60 + (3.14 × 4.33 × 120) / 180
            = 60 + 9.07
            = 69.07 mm

Setting K-Factor in UG NX

In NX Sheet Metal module:

File → Utilities → Sheet Metal Preferences
  → Bend Allowance
    → Type: Bend Table or Formula
    → Enter K-factor value

You can also create a custom bend coefficient table (.bnd file), import the data above, and auto-match by material.

K-Factor Pattern Summary

R/T Range	K-Value Range	Description
< 0.3	0.23 ~ 0.30	Small radius bending, severe material compression
0.3 ~ 2.0	0.30 ~ 0.45	Standard bending range, K increases with R/T
2.0 ~ 5.0	0.45 ~ 0.48	Large radius bending, neutral layer near center
> 5.0	0.48 ~ 0.50	Very large radius, neutral layer approximately at center

Last updated on July 25, 2026

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