Sheet Metal Bending: Bend Radius, K-Factor & Design Tips

Bending is the heart of sheet metal fabrication. A flat blank becomes a bracket, an enclosure, or a chassis through a series of precise folds — and the rules that govern those folds decide whether a part assembles cleanly or fights the technician on the shop floor. Two concepts sit at the center of every bend: the bend radius and the K-factor. Understand them and your sheet metal parts will be cheaper, more repeatable, and right the first time.

This guide explains how bends actually form, why the inside radius matters, what the K-factor is and how it drives your flat pattern, and the practical design rules — bend reliefs, hole spacing, minimum flanges — that keep parts manufacturable. It is written for design engineers and procurement teams specifying bent parts in aluminum, steel, and stainless.

What Happens When Metal Bends

When a press brake forms a bend, the metal on the outside of the bend stretches while the metal on the inside compresses. Somewhere between the two lies a layer that neither stretches nor compresses — the neutral axis. Because the outside lengthens more than the inside shortens, the flat blank must be slightly shorter than the sum of the finished leg lengths. Calculating that difference accurately is what lets a shop cut a flat pattern that folds up to the exact dimensions on your drawing.

Two values describe the geometry: the inside bend radius (the radius of the curve on the inner face) and the bend angle. Everything else — bend allowance, bend deduction, and the flat length — follows from these plus the material thickness and the K-factor.

Inside Bend Radius: Keep It Equal to Material Thickness

The single most useful sheet metal rule is this: make the inside bend radius at least equal to the material thickness. A radius smaller than the thickness forces the outer fibers to stretch so severely that the metal can crack, work-harden, or thin dangerously — especially in harder tempers and along the grain direction. A radius equal to or greater than thickness keeps the bend healthy and repeatable.

Equally important: use the same inside radius on every bend in a part wherever possible. Each unique radius may require a different press-brake tool and a separate setup, which adds cost. Standardizing on one radius lets the shop form the whole part with one punch.

The K-Factor Explained

The K-factor is the ratio that locates the neutral axis within the material thickness. It is defined as the distance from the inside surface of the bend to the neutral axis, divided by the total material thickness. Because the neutral axis always sits somewhere between the inside face and the middle of the material, the K-factor is a fraction between 0 and 0.5 — typically around 0.33 to 0.45 for common materials and tooling.

The K-factor matters because it determines the bend allowance — the actual arc length of material consumed in the bend — and therefore the length of the flat blank. Use too high a K-factor and the part comes out short; too low and it comes out long. A shop derives its K-factor empirically by bending test coupons and measuring, because it shifts with material, thickness, radius, and tooling. As a design engineer you usually do not need to compute it yourself, but you should know it exists so that flat-pattern dimensions and your folded dimensions agree.

Bend Allowance and Bend Deduction

Two related quantities turn the K-factor into flat-pattern numbers. The bend allowance is the length of the neutral axis through the bend, added to the flat legs to get total blank length. The bend deduction is the amount subtracted from the sum of the outside leg dimensions to reach the same blank length. CAD systems and brake software compute both automatically once the material, thickness, radius, angle, and K-factor are set — which is exactly why those values must be stated or agreed.

Springback

Metal is elastic as well as plastic, so after the punch releases, the part springs back slightly toward flat — the bend angle opens up a little. Springback grows with material strength, thickness, and bend radius. Shops compensate by overbending (forming to a slightly tighter angle so the part relaxes to the target) or by bottoming/coining the bend. Stronger materials such as stainless and high-temper aluminum spring back more and need more compensation, which is one reason consistent material and tooling matter for repeatable angles.

Design Rules That Keep Bends Manufacturable

Beyond radius and K-factor, a handful of geometric rules prevent the most common bending problems.

• Minimum flange length. A bent flange must be long enough to sit on the press-brake die — generally at least about 4 times the material thickness plus the bend radius. Flanges shorter than this slip into the die and form inaccurately.
• Bend reliefs. Where a bend ends at the edge of a feature, add a small relief notch so the metal does not tear at the corner. A relief at least as wide as the material thickness keeps the bend clean.
• Holes and slots away from bends. Keep holes at least about 2–3 times the material thickness (plus the radius) away from the inside of a bend. Holes too close distort into ovals as the metal stretches through the bend.
• Consistent bend orientation. Where possible, design so bends can be formed from the same side and in a sequence the brake can reach without collisions. Complex parts with bends boxing each other in may need more setups or become impossible to form.
• Mind the grain. Bending parallel to the rolling grain raises the risk of cracking, especially in hard aluminum tempers. Bending across the grain, or specifying a larger radius, is safer.

Tolerances You Can Expect

Bending is precise but not unlimited. A typical bend angle holds to about ±0.5 to ±1 degree, and bend-to-bend linear dimensions to roughly ±0.1 to ±0.25 mm, tightening or loosening with part size and the number of bends. Tolerance stacks across multiple bends, so dimensioning from a single datum edge rather than chaining bend to bend keeps accumulated error under control — the same discipline covered in our guides to GD&T and tolerance stack-up.

Putting It Together

Good bent-part design comes down to a few habits: keep the inside radius at or above material thickness, standardize on one radius across the part, leave room for flanges and bend reliefs, hold holes back from bends, and let the shop's measured K-factor drive the flat pattern. These choices cost nothing at the design stage and save real money and rework in production. They sit alongside the broader sheet-metal rules in our sheet metal fabrication guide and the cross-process limits in our wall thickness guide.

MechPart Pro fabricates bent sheet metal parts in aluminum, stainless and steel, with press-brake tooling and measured K-factors that make your flat patterns and folded dimensions agree. Upload your model and flat or 3D drawing and our engineers will review bend radii, flange lengths, hole positions and reliefs as part of our free design-for-manufacturability feedback — so your first bent parts come off the brake to print.