## What Is Sheet Metal Setback?

Before understanding the sheet metal setback, it is important to be familiar with the definitions of the mold line and bend line.

The bend line refers to the straight line present on both sides of the bend plates and is located at the intersection of the bend area and the flange edge.

The mold line refers to the straight line formed at the intersection of the outer surfaces of two bent flanges and can be either an outside or inside mold line.

The setback is the distance from any bend line to the mold line and can also be described as the difference between the mold line length and the flange length.

In a 90-degree bend, the setback value is equal to the bend radius plus the sheet metal thickness.

By knowing the setback size, we can determine the bending tangent position of the workpiece.

The setback plays a crucial role in workpiece design. If the workpiece needs to be bent multiple times, the setback must be subtracted for each bend.

It is important to note that the bend allowance and bend deduction can change based on changes in the K factor value, but the setback remains constant regardless of changes in the K factor.

## Setback Calculation and Formula

The setback is divided into two types: inside setback and outside setback. The bend angle and radius are the factors affecting the setback.

The inside setback is the distance from the tangent point of the inside radius to the vertex of the inside mold line.

If the bend angle and radius change, the bend line and vertex will also move.

The outside setback is the distance from the tangent point of the radius to the outside bend vertex of the flange.

By knowing the values of outside setback and bend deduction, we can obtain the bend allowance.

**BA(Bend Allowance)=2OSSB-BD(Bend Deduction)**

The outside setback can be calculated by the following formula

**Outside Setback(OSSB)=Tan(A/2) × (T+R)**

The sum of bend deduction and bend allowance is equal to two times the outside setback.

This can be expressed as T (sheet thickness) + A (bend angle) + R (inside bend radius).

For a 90° bend angle, the setback value is equal to the bend radius plus the sheet thickness.

When the bend angle is less than 90°, the complementary angle is usually used, and when the bend angle is greater than 90°, the included angle or complementary angle is usually used.

## What Is the Sheet Metal Bend Radius?

The bend radius is the distance from the bend axis to the inner surface of the sheet, generally referring to the inner radius.

The value of the outside radius is equal to the inner radius plus the sheet metal thickness.

The smaller the radius, the higher the tension and compression on the material.

The size of the radius is determined by the metal material's properties such as tensile strength, ductility, thickness, and the size of the die opening.

As a general rule, the larger the die opening size, the larger the radius.

### Bend Allowance Chart

### Bend Deduction Chart

## Conclusion

This blog introduces the definition, calculation method, and related terms of the setback in sheet metal bending.

The setback is a crucial aspect of workpiece design and has close connections with the K factor, bend allowance, bend deduction, and other factors.

However, if the bend angle approaches 180°, the values of the inside and outside setbacks don't have to be considered because the setback value becomes close to infinity and the bend is nearly flat.

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