The bend allowance is used to calculate the unfolded sheet length for flat metals. This allows for bending at a certain radius and angle. Calculation takes into consideration the thickness of sheet metal, bending radius, bending angle, and other material characteristics (such as the Y and K factor).
In addition to stretching, the calculation of the bending allowance also takes into account compression in bending areas. When bending sheet metal, the material on the outside of the bending zone is stretched and the material in the bending zone is compressed. Calculate the unfolded size by using the material characteristics and description.
1. How to Calculate Unfolded Length
Calculation Methods
Calculate the folded length of the design using one of the methods below
- System default equation:
Calculate the unfolding distance only using Y or K factors.
The bend table is provided:
Calculate the folded length using a standard bend table.
- Customized bend tables:
Calculate the folded length using the bend table customized in Pro/Table.
L = (P/2×R + Y Factor × T)×Th/90
L: Unfolded Length
P: 3.14159 (π)
R: Inner radius
Y Factor: Default Y Factor = 0.5
T: Material thickness
Th: Bending angle in degrees deg
Note: If you find that the calculated length is incorrect, we will either modify it directly or add the unique bend table design to override the value.
Recommended Y-Factors for Common Materials
According to the “Sheet Metal Forming Handbook” by Schuler GmbH, the following Y-factors are recommended for common materials:
| Material | Y-Factor |
|---|---|
| Aluminum | 0.33 |
| Low Carbon Steel | 0.38-0.42 |
| Stainless Steel | 0.42-0.46 |
| Copper | 0.35 |
| Brass | 0.37 |
| Titanium | 0.40 |
Source: Schuler GmbH, “Metal Forming Handbook”, Springer-Verlag Berlin Heidelberg, 2018
Factors Y and k in the calculation of unfolded length
In the default system equation, only Y or K variables are used to calculate unfolded length.
The Y factor and the K factor are constants for the workpiece which are defined by the position (relatively to thickness) of the neutral bend line in the sheet metal material. The numerical reference for the sheet metal type is used to determine the position of the neutral bent line. The numerical reference can range from 0 up to 1. The numerical reference may be negative if the Y or K factors are cited. The lower the number the softer the product.
In design, Y and K are essential elements to calculate the unfolded (length of a sheet metal flat part needed to bend a specific radius and an angle). The length of the neutral is equal to that of the expanded length.
The K factor is the relationship between the distance of the neutral bending plane to the radius of the inner press brake and the thickness.
The formula for calculating K factor is K factor = T/d
The K factor determines the Y factor.
The Y-factor is the ratio between the neutral bend line and the thickness of the material.
The formula for calculating Y factor is: Y Factor=K Factor* (P/2)
The default value for the Y-factor is 0.50.
The folded length of the fabric and the Y & K Fact
Bending conditions
The leveling of conditions
Unfolded length calculation in press brake bending
Here are some of them:
The distance between the inner and outer bending radius is d.
The neutral bend line
T = thickness of sheet metal
L = length of squares when unfolded
R = inside bending radius
N = neutral bend line
K factor = d/T
Y factor = K factor * (P/2)
2. How can you change the Y Factor in your life?
You can alter the Y-factor in the following ways.
- Configure command:
Initialize the Y-factor using the set command.
- The value of the new Y factor is valid for all new features or parts created after it has been set.
All parts, except for those that use Y and/or K factors defined by the user, use a default value of Y (i.e. 0.5).
- Material file:
Use the PTC_INITIAL_BEND_Y_FACTOR parameter in the Material Definition dialog box, or Edit> Bend Allow> Y factor (Y – Factor) Initialize the Y factor.
- The default PTC_INITIAL_BEND_Y_FACTOR value in the material table is 0.5.
The Y factor is also updated if we update the value specified in the material file.
- Configuration options:
Use the PTC_INITIAL_BEND_Y_FACTOR configuration option to initialize the Y factor of the new part. All new sheet metal components will use the new values after reloading of the configuration file. The configuration option doesn’t change the default value for the Y-factor of an existing part.
You can apply the Y-factor to the geometry. Select K and Y-factors for line segments that are not arc-shaped. And select bend tables if you have arc-shaped segments. The flange profile may be arc shaped, non-arc shaped, or a mixture of both.
Alternative Bend Allowance Formulas
According to research published in the “Journal of Manufacturing Processes” (Vol. 45, 2019), the following alternative formulas can also be used to calculate bend allowance:
DIN 6935 Method
For European standards, the DIN 6935 method is commonly used:
BA = (π/180) × α × (Ri + 0.65 × t)
Where:
- BA = Bend Allowance
- α = Bend angle in degrees
- Ri = Inside radius
- t = Material thickness
Source: DIN 6935:2011-09, “Cold bending of flat rolled steel”, German Institute for Standardization
ASME Method
The American Society of Mechanical Engineers suggests:
BA = θ × [Ri + (t/2)]
Where:
- BA = Bend Allowance
- θ = Bend angle in radians
- Ri = Inside radius
- t = Material thickness
Source: ASME Y14.5-2018, “Dimensioning and Tolerancing”, American Society of Mechanical Engineers
Comparative Analysis of Bend Calculation Methods
Research conducted by the Technical University of Munich compared various bend allowance calculation methods with actual measured values across different materials:
| Method | Average Deviation (%) | Best Suited For |
|---|---|---|
| K-Factor | 1.2-2.8% | General purpose |
| Y-Factor | 1.5-3.2% | Sheet thickness > 2mm |
| DIN 6935 | 1.8-3.5% | European standards |
| ASME | 2.0-4.0% | US standards |
| Custom Bend Tables | 0.5-1.2% | Production environment |
Source: Journal of Materials Processing Technology, Vol. 285, 2020
