Bend Allowance in Sheet Metal Design — A Plain English Guide

When sheet metal is bent, the material on the outside of the bend stretches and the material on the inside compresses. The neutral axis — the line through the material where there is neither tension nor compression — shifts toward the inside of the bend. Bend allowance is the length of material along this neutral axis.

Why Does It Matter?

If you design a flat pattern without accounting for bend allowance, your finished part will be the wrong size. The flat pattern will be too long — the material consumed in the bend is not free to contribute to the flat leg lengths. Get this wrong and your parts will not fit.

What is the K-Factor?

The K-factor is the ratio that describes where the neutral axis sits within the material thickness. A K-factor of 0.5 means the neutral axis is exactly in the middle of the material. In practice, K-factors range from 0.3 to 0.5 depending on the material, thickness, bend radius, and tooling used.

Common K-Factor Values

• ✓Soft copper / soft brass — K = 0.35
• ✓Aluminium — K = 0.38
• ✓Mild steel — K = 0.42
• ✓Stainless steel — K = 0.45
• ✓Hard materials, tight radius — K = 0.50

How TDM Handles This

At TDM, we always confirm the material, thickness, bend radius, and tooling before setting the K-factor in Autodesk Inventor. For critical parts, we ask for the fabricator's actual K-factor values from their tooling tables. This ensures the flat pattern we deliver is exactly right for your press brake.