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Sheet Metal Design (DFM): Designing the Bend, Not the Drawing

What are bend allowance, K-factor and minimum bend radius in sheet metal design? Practical DFM rules for correct opening and crack-free twisting.

25 May 2026Updated: 25 July 2026Author: Ömer Faruk Top

Three concepts determine dimensional accuracy in sheet metal design: bend allowance, K-factor and minimum bend radius. When the sheet metal is bent, its outer surface lengthens, its inner surface shortens, and only a thin layer called the "neutral axis" maintains its length; Therefore, the straight expansion of the bent part is not the simple sum of the side lengths. A drawing that does not handle these three concepts correctly will produce defective parts, no matter how precise the laser cut is..

The most common design mistake in sheet metal is to draw the part in its bent form and assume that the expansion will come out correct on its own. Sheet metal design is as much about drawing the finished part as it is about designing how it will be bent to that state..

What is K-Factor?

When a sheet metal is bent, the outer part of the material is subjected to tension and the inner part is subjected to compression. There is a layer between the two that neither lengthens nor shortens: the neutral axis.. K-factor is the ratio of the position of the neutral axis to the material thickness and in practice it takes values between 0 and 0.5; It forms the basis of the twist allowance calculation..

K-factor varies with material, thickness and twisting method. Protolabs' guide to sheet metal design, shows that typical values differ depending on the twisting method (air twisting and bottom twisting do not give the same K value). The most common mistake here is to use the CAD software's default K-factor without verifying it with the real material and the real press brake mold; The result is an incorrect opening and an incorrectly sized part. Before mass production, it is most reliable to verify the K-factor by trial bending with your manufacturer's machine and mold..

How to Calculate Twist Allowance?

Twist allowance is the length covered by a twist in straight unfolding and is calculated on the neutral axis.:

Bend allowance = (π/180) × bend angle × (inside bend radius + K-factor × material thickness)

Its practical meaning is this: for correct expansion, the share of each twist is calculated separately and added to the straight lengths. If the opening is wrong, the bent part will come out with the wrong size, even if the cut is correct. Dimensional accuracy in sheet metal is determined not in the drawing, but in the bend allowance calculation..

Why Does Minimum Bend Radius Exist??

If the sheet is forced to bend with a radius that is too sharp, the outer surface will crack. In Protolabs' guide The common starting rule is to take the inner bend radius as at least the material thickness (1T). This is a lower bound; the actual value varies depending on the material and temper: aluminum alloys generally start around 1T, while harder and hardening materials such as stainless steel require larger radius. For exact values, refer to your manufacturer's material-thickness table.; SendCutSend's material-based bend guides is a good example of this type of tables.

Pushing this limit can lead to invisible microcracks and premature fatigue of the part. Choosing the appropriate radius for the material at the design stage prevents scrap and rework in production.

Other Practical DFM Rules

In addition to dimensional accuracy, there are also rules that determine manufacturability.:

Rule	From where?
Keep flange length adequate (common practice: at least ~4 times the thickness)	Short flange cannot hold properly on the press brake
Keep holes away from bend line	Holes close to the bend line are deformed during bending
Collect bends in the same direction if possible	The part is rotated less on the press brake, the margin of error decreases
Add corner relief cuts	Prevents tearing at the edge of the bend line

These values vary from manufacturer to manufacturer; Ask for the design guide of the workshop you are working in for exact boundaries. Rules, In our DFM article It carries the same logic as the principle: the geometry must be designed according to the realities of the production method. In sheet metal, this fact is the physics of the press brake and the material.

What Should You Clarify Before Receiving a Quote??

When sending a sheet metal part to a contract manufacturer, having the following information in the file speeds up both the quote and production: material and thickness, internal bend radii, which critical dimensions are (not all), surface treatment and quantity. It is often better to let the manufacturer deduce the expansion; because the expansion must be calculated with the manufacturer's own K-factor values. In our article comparing contract manufacturer offers We discuss this issue in detail.

Conclusion

Sheet metal design is not just about drawing the finished part; It is about designing how it will be bent, how the expansion will come out, and whether the material can handle the bending. When the K-factor, bend allowance and minimum bend radius are handled correctly, the part comes out to the right size and bends without cracking..

Is the expansion of your sheet metal parts incorrect, are you experiencing cracks or measurement deviations in the bending lines? Tactfully, we examine your sheet metal parts with a DFM eye; We verify K-factor, bend allowance and radius selection according to material and process. Our design and development service browse or contact us.