The sheet metal bending allowance is the amount of material that will be stretched at the bend during press brake work. During the bending process, the material is stretched and compressed. When bending sheetmetal, it is important to consider the elasticity of the material and its margin.
Press brake bend allowance plays a vital role in the processing of sheet metal. The material’s elasticity can cause bending angles to deviate from design.
1. What is the bend allowance for press brakes?
The bend allowance measures the additional material required to accommodate the bending of the sheet metal component. This method is used to determine the length of a flat sheet metal component that must be unfolded to create a bend with a specified radius and angle.
Calculation of the bend takes into consideration the bending angle and radius, the material thickness and other material properties, such as the Y and K factor.
The bend allowance is the amount of material needed to accommodate a particular bend between two panels.
In many cases, the bend allowance can be calculated and predicted fairly easily. The bend allowance is sometimes referred to as the “bending development”, or just “development”.
The calculation of the unfolded length also accounts for the stretching that occurs in the bending region. When bending metal parts or forming them, the material on the outside of the neutral bend axis will be stretched and the material on the inside of the neutral bend axis will be compressed.
This material characteristic can automatically be taken into account by establishing a material description that is appropriate and formulas for calculating the unfolded size.
It is possible to use the exact unfolding length calculation in order to capture the design intent of the solid model. It can also create an accurate flattened version that can be used by the manufacturer to manufacture the product. Make it a habit to first determine how to calculate the extended length.
The unfolding length is calculated using only Y or K factors.
Table of bends: Calculate the length by using standard, predefined bending tables.
Customized bend tables: Use the custom bending table to calculate the unfolded size in Pro/Table.
Common K-Factor Values by Material Type
According to the Machinery’s Handbook (31st Edition), the following K-factors are typically used for different materials:
| Material | Thickness | K-Factor |
|---|---|---|
| Aluminum | All gauges | 0.33-0.40 |
| Brass | All gauges | 0.33-0.38 |
| Cold Rolled Steel | < 0.060″ | 0.40-0.45 |
| Cold Rolled Steel | ≥ 0.060″ | 0.41-0.47 |
| Stainless Steel 300 Series | All gauges | 0.42-0.48 |
| Stainless Steel 400 Series | All gauges | 0.40-0.45 |
| Copper | All gauges | 0.38 |
| Spring Steel | All gauges | 0.42 |
Source: Machinery’s Handbook, 31st Edition, Industrial Press, 2020
2. How do you determine the bend allowance for a curved surface?
Calculating the bend in sheet metal fabrication is done using the press brake bend allowance.
The internal compression and external tension will cause a change in the total length of a bent metal plate. The neutral axis is a line that passes through the thickness of bending.
The length of the part will not change when it is bent. The material within the wire is compressed. The material on the outside of the wire is stretched.
The neutral axis position changes depending on the bending angle, bending radius and the bending angle.
The K factor is the product of the distance between the neutral line and the inner edge of a material divided by its thickness.
The formula for the specific formula is:
Bend allowance = π/180 × A × (IR + K × MT)
Note:
- MT is the material thickness
- A is the bending angle in degrees
- IR is the inside radius
- K is the K-factor (neutral axis factor)
The K factor cannot be more than 0.5 because the tension inside the bend can never be greater than the compression within.
The K factor is a measure of the ratio between compression and extension when bending. In other words, the higher the K-factor, the greater the compressive force. The K factor is dependent on the type of material and bending.
The bend allowance is then added to the material length needed to manufacture the bent component.
Brake presses, also known as presses, and box and disc breaks are commonly used machines to bend metal sheets.
There are different methods for bending brakes. Aluminum and mild steel are common materials for brake bending. Certain types of ductile polymers can also be bent with these processing methods.
Don’t forget, when consulting your press brake, to give Shenchong’s technical team the parameters you need. We will then calculate the bend allowance of your CNC press brake and provide the best professional advice.
3. How to use the sheet metal bending allowance correctly?
When bending sheetmetal, it is important to make specific calculations based upon the different materials and specifications. The bending operation must be done based on this allowance.
When producing products of higher precision, it is important to pay more attention to the calculation and monitoring of bending allowance.
If there is no bending allowance clearly indicated on the drawing, the routine can be used to calculate the allowance to ensure bending accuracy.
Check the equipment’s condition regularly to ensure accuracy and stability.
Understanding the concept of sheet metal bending allowance and the calculation method is essential. You should also know how to use it correctly and avoid the negative impacts and risks caused by misusing the allowance. To improve the quality of products and the efficiency of production, it is important to constantly review experience in sheet metal production.
Alternative Bend Allowance Calculation Method
The American Society of Mechanical Engineers (ASME) provides an alternative formula that is sometimes used in industrial applications:
Bend Allowance = A × (IR + (0.64 × MT))
Where:
- A is the bend angle in radians (degrees × π/180)
- IR is the inside radius
- MT is material thickness
Source: ASME Y14.46-2017, “Product Definition for Additive Manufacturing”
4. Misuse of sheet metal bending allowance can cause serious risks and consequences.
Misuse of allowances can cause the bend angle to diverge from the original plan and affect the quality.
The bending angle will be affected if the allowance is either too small or large.
The bending allowance may also change if the material strength or elastic modulus changes, or if the process condition error is significant. This change is not predictable during the design phase and can affect the product’s quality.
Real-World Bending Tolerances
According to research published in the Journal of Manufacturing Science and Engineering, typical manufacturing tolerances for sheet metal bending are:
| Material Thickness | Typical Angular Tolerance | Length Tolerance |
|---|---|---|
| < 1.0 mm | ±1° | ±0.2 mm |
| 1.0 – 3.0 mm | ±0.5° | ±0.3 mm |
| > 3.0 mm | ±0.3° | ±0.5 mm |
Source: Journal of Manufacturing Science and Engineering, Vol. 142, Issue 6, 2020
Bend Radius Recommendations
Research from the Fabricators & Manufacturers Association International (FMA) recommends minimum bend radius values to prevent material cracking:
| Material | Minimum Inside Bend Radius (× Material Thickness) |
|---|---|
| Aluminum 1100-O | 0.0 MT |
| Aluminum 2024-T | 4.0 MT |
| Aluminum 6061-T6 | 1.0 MT |
| Mild Steel (Low Carbon) | 0.5 MT |
| Stainless Steel 304 | 1.0 MT |
| Copper (Soft) | 0.0 MT |
| Titanium | 2.5 MT |
Source: FMA Practical Sheet Metal Fabrication Guide, 2018
