Bend Deduction Calculator, Formulas & Charts

1. Bend Deduction Calculator

Related calculators:

• K Factor Calculator
• Y Factor Calculator
• Bend Allowance Calculator

2. What Is Bend Deduction?

Bend deduction is a crucial concept in the field of sheet metal fabrication, particularly in the bending process. It is commonly referred to as the “back-off amount” and plays a significant role in determining the accurate dimensions of a bent part.

Definition

Bend deduction (BD) is the amount subtracted from the total length of the flat sheet metal to achieve the desired final dimensions after bending. It accounts for the material that is displaced during the bending process.

Importance in Sheet Metal Bending

The bend deduction is essential for accurately calculating the flat pattern of a sheet metal part before it is bent. It ensures that the final dimensions of the bent part match the design specifications. Without accounting for bend deduction, the part may end up with incorrect dimensions, leading to potential issues in assembly and functionality.

Factors Influencing Bend Deduction

Several factors can influence the bend deduction value, including:

• Material Type: Different materials have varying bend properties.
• Thickness: Thicker materials require different deductions compared to thinner ones.
• Bend Angle: The angle of the bend affects the amount of material displaced.
• Bend Radius: A larger bend radius requires a different deduction compared to a smaller radius.

Practical Application

In practice, the bend deduction is determined through empirical data, manufacturer guidelines, or specialized software that takes into account the specific properties of the material and the bending process. Accurate bend deduction calculations are critical for ensuring that the final product meets design requirements and functions correctly in its intended application.

3. Bend Deduction Formula

Bend deduction (BD) is a critical factor in sheet metal fabrication, used to calculate the flat pattern length of a sheet metal part. It accounts for the material that is displaced during the bending process. This value is essential for ensuring that the final dimensions of the bent part are accurate.

Key Terms and Notations

• Lt: Total unfolded length of the sheet metal part.
• A and B: Lengths of the two flanges adjacent to the bend.
• BD: Bend deduction value.

Calculation of Bend Deduction

The bend deduction formula is used to determine the flat length of a sheet metal part before it is bent. The formula is as follows:

Lt=A+B−BD

Where:

• BD is the bend deduction value, which is subtracted from the total length to account for the material displaced by the bend.
• A and B are the lengths from the edge of the part to the tangent points of the bend.

The bending deduction in Solidworks is only used for the calculation of 90-degree bends in sheet metal.

However, it can also be used for the calculation of non-90-degree sheet metal unfolding, but the bending deduction value for non-90-degree bending needs to be used according to the bending coefficient table.

Each manufacturer has a different table, and there may be errors. Some sheet metal factories may not use non-90-degree bending often.

Today, I will share the calculation method for the bending deduction of 90-degree bending that I am familiar with.

Bending deduction is a crucial factor in sheet metal fabrication, particularly when using machines like press brakes. It represents the amount of material consumed during the bending process. Accurate calculation of bending deductions is essential for precise part dimensions and efficient material usage. This article discusses three common methods for calculating bending deductions.

Method 1: Simple Multiplication Factor

The simplest method uses a multiplication factor based on material thickness:

• General rule: Bending deduction = 1.7 × material thickness
• For aluminum: Bending deduction ≈ 1.6 × material thickness
• For stainless steel: Bending deduction ≈ 1.8 × material thickness

This method is quick and easy but lacks precision. It’s suitable for applications with lower accuracy requirements or for initial estimations.

Method 2: Industry-Standard Formula

A more refined approach developed by the sheet metal industry:

Bending deduction = 2 × material thickness + 1/3 × material thickness

This formula accounts for material elongation during bending. It’s derived from: Unfolded length = Length A + Length B – Bending deduction

Where the bending deduction considers both material thickness and elongation.

Method 3: Advanced Formula Considering Die Width

A more sophisticated formula derived from empirical studies:

Bending deduction = 2 × t – (0.72t – 0.075V – 0.01)

Where:

• t = Actual material thickness (measured, not nominal)
• V = Lower die slot width (typically 6-8 times material thickness)

This formula accounts for the influence of the lower die width on bending deduction. It’s particularly accurate for carbon steel plates but has shown good results with other materials like aluminum when the die width is about 4 times the material thickness.

Important Considerations

1. Material thickness (t) should be the actual measured thickness, not the nominal value.
2. Die width (V) significantly affects the bending deduction and should be accurately determined based on the specific tooling used.
3. While there are more complex methods based on neutral axis theory, the three methods presented here offer a balance of practicality and accuracy for most sheet metal fabrication applications.

Conclusion

Choosing the appropriate bending deduction calculation method depends on the required accuracy, material type, and available tooling information. For high-precision work, the advanced formula (Method 3) is recommended, while the simpler methods can be useful for quick estimates or less critical applications.

4. Bend Deduction Chart

(1) Sheet Metal Material Bending Deduction Table

Explanation:

• T: Material thickness;
  V: Lower die groove width;
  W: Lower die width;
  α: Internal angle after bending;
  H: Minimum distance from the bending center of the die to the edge of the material;
  Z: Minimum size of Z-bend.
• Unfold formula: L = A + B – K; (A, B: Outer dimensions of the part; K: Material bending coefficient);
• Expanded size of flattened steel plate: L = A + B – 0.45T;
• Expanded size of pressed steps: L = A + B – 0.3T;
• Minimum size of Z-shaped bend: Z = W/2 + 2T + 1;
• The deduction coefficients for cold-rolled steel plate, aluminum-zinc coated plate, stainless steel plate, electrolytic plate, and aluminum plate are the same;
• According to the current upper die conditions of the company, the bending coefficient of upper die R1 is suitable for cold-rolled steel plate, aluminum-zinc coated plate, and stainless steel plate; The bending coefficient of upper die R2 is suitable for aluminum plate, copper plate, electrolytic plate, etc.

(2) Mild Steel Bend Deduction Chart

(3) Stainless Steel Bend Deduction Chart

(4) Bend Deduction Chart for Cold Rolled Steel Plates at Various Angles

1. The following table shows the deduction factor for each 5-degree angle increment from 90 degrees outward: (180°-n°)÷90°×(deduction factor at 90°).
2. The inside bending radius is uniformly set to R0.2, and the gap distance is set to G0.2.

(5) 0°-180° sheet metal bending deduction table

1) The bending deduction table is applicable for sheet metal bending processes where no pressing plate is used and the sheet width is more than three times the thickness (t).
2) When bending on the bending machine, calculations can be made according to this table.
3) According to the dimensions indicated in the diagram, the calculation formula for the unfolded size of the bent workpiece is as follows:

L=a+b-y

Where:

• L – Unfolded size of the bent workpiece;
• a, b – Indicated lengths from the straight edge to the vertex of the bent workpiece in the diagram;
• y – Bending deduction value of the bent workpiece (correction value for the R part);

4) Due to the numerous factors influencing sheet metal bending, this sheet metal bending deduction value table is provided for reference only.

• Bend deduction values for bending angle 20°
• Bend deduction values for bending angle 25°
• Bend deduction values for bending angle 30°
• Bend deduction values for bending angle 35°
• Bend deduction values for bending angle 40°
• Bend deduction values for bending angle 45°
• Bend deduction values for bending angle 50°
• Bend deduction values for bending angle 55°
• Bend deduction values for bending angle 60°
• Bend deduction values for bending angle 65°
• Bend deduction values for bending angle 70°
• Bend deduction values for bending angle 75°
• Bend deduction values for bending angle 80°
• Bend deduction values for bending angle 85°
• Bend deduction values for bending angle 90°
• Bend deduction values for bending angle 95°
• Bend deduction values for bending angle 100°
• Bend deduction values for bending angle 105°
• Bend deduction values for bending angle 110°
• Bend deduction values for bending angle 115°
• Bend deduction values for bending angle 120°
• Bend deduction values for bending angle 125°
• Bend deduction values for bending angle 130°
• Bend deduction values for bending angle 135°
• Bend deduction values for bending angle 140°
• Bend deduction values for bending angle 145°
• Bend deduction values for bending angle 150°
• Bend deduction values for bending angle 155°
• Bend deduction values for bending angle 160°
• Bend deduction values for bending angle 165°
• Bend deduction values for bending angle 170°

5. Bending Dimensional Accuracy

The dimensional accuracy of bent workpieces is related to the positional precision of the press brake’s back gauge and the precision of the sheet metal cutting machine. Utilizing reliable machines for both cutting and bending processes can address these issues.

A critical factor affecting the dimensional accuracy of bent workpieces is the accuracy of the sheet metal’s flat pattern development. When a flat sheet is bent into a workpiece with a specific angle, measuring the dimensions of the bent workpiece reveals that they do not equal the dimensions of the flat sheet, as shown in the illustration.

This discrepancy is known as the bending deduction.

If the bending deduction is inaccurate, the flat pattern size will be imprecise, and regardless of the precision of subsequent operations, the final workpiece will not meet the required dimensional accuracy.

The bending deduction is complex, and a rudimentary method is to simply use twice the material thickness.

However, this approach is quite crude. A more refined method is to apply the neutral axis theory from the DIN 6935 standard, which involves calculating a factor ‘k’ and combining it with the thickness and angle of the sheet to be bent.

This formula yields a more precise bending deduction. Yet, even the bending deductions calculated according to the neutral axis theory from DIN 6935 may not be precise enough, as the actual deductions also depend on the characteristics of the material, the thickness, the bending angle, and the tooling used.

Different materials, thicknesses, and calculation methods yield varying bending deductions, as shown in Table below.

Table Deduction value for bending dimensions corresponding to different materials, thicknesses, and methods

For example, for a 4mm thick S235JRG2 plate using a V30 lower die, the bending deduction varies by method: twice the material thickness results in 8mm, the DIN 6935 formula yields 7.57mm, and the database empirical value gives 7.26mm.

There are discrepancies between the methods, which become even more significant when workpieces require multiple bends, leading to larger cumulative deviations. The empirical values from the database are derived from extensive practical testing and are stored in the database, providing the utmost precision.