The K-factor is a key parameter used in sheet metal bending to calculate the unfolded length of material. It describes the position of the neutral layer (the layer that is neither stretched nor compressed) relative to the material thickness during bending.

The K-factor is defined as: the distance from the neutral layer to the inner surface of the bend, divided by the total material thickness. This ratio is typically between 0 and 1. For example, a K-factor of 0.4 indicates that the neutral layer is located at 40% of the material thickness from the inner surface.

This parameter is crucial because it directly determines the calculation of the bending coefficient. The bending coefficient refers to the actual arc length of the bent portion in its unfolded state. Only by accurately calculating this arc length using the K-factor can precise sheet metal cutting dimensions be obtained, avoiding dimensional deviations in the finished product.

In actual prototype making, the K-factor is not a fixed value; it is affected by factors such as material type, hardness, and bending radius. However, based on long-term processing experience, some typical values can be used as a reference:

For soft materials such as soft brass and pure copper, the K-factor is usually around 0.35. For medium-hardness materials such as semi-hard copper, mild steel (e.g., Q235), and aluminum (e.g., 5052/6061), the K-factor is approximately 0.41. For hard materials like bronze, hard copper, and cold-rolled steel, the K-factor is closer to 0.45.

Another important rule is that when the bending radius is much larger than the material thickness—for example, when the radius-to-thickness ratio is greater than or equal to 4—the tensile deformation of the material is very small, and the neutral layer tends to be located close to the center of the material thickness. In this case, the K-factor can be directly approximated as 0.5.