In the world of modern sheet metal processing, laser cutting and press-brake bending go hand in hand. A truly accurate end result is only achieved if the cut part stays dimensionally accurate after bending. The K-factor plays a key role here: it shows where the neutral fibre lies within the sheet thickness during bending.
We created our K-factor table from measured data of a Trumpf press brake. It applies to the ST37 mild steel, stainless steel and aluminium sheets we process, so our customers can be confident of accurate flat-pattern dimensions and error-free production for every material.
K-factor table for mild steel, stainless steel and aluminium sheet metal parts – based on Trumpf machine data
| Thickness (s) | Inner radius (Ri) | Shortening | K-factor* |
|---|---|---|---|
| 0.5 mm | 1.0 mm | 0.66 mm | 0.139 |
| 1.0 mm | 1.7 mm | 0.94 mm | 0.541 |
| 1.5 mm | 2.5 mm | 1.41 mm | 0.532 |
| 2.0 mm | 3.3 mm | 2.10 mm | 0.387 |
| 2.5 mm | 4.1 mm | 2.63 mm | 0.382 |
| 3.0 mm | 4.7 mm | 3.13 mm | 0.373 |
| 4.0 mm | 4.7 mm | 3.76 mm | 0.397 |
| 5.0 mm | 7.7 mm | 4.64 mm | 0.512 |
| 6.0 mm | 7.7 mm | 5.61 mm | 0.433 |
| 8.0 mm | 8.7 mm | 7.59 mm | 0.362 |
| 10.0 mm | 10.6 mm | 9.63 mm | 0.337 |
| 12.0 mm | 13.7 mm | 11.69 mm | 0.345 |
| 15.0 mm | 18.2 mm | 14.85 mm | 0.344 |
| 20.0 mm | 18.5 mm | 20.22 mm | 0.239 |
*The values are for guidance only and were derived for optimal bottom tools. It is advisable to ask the manufacturer for technical support after setup as well, to refine the values.
Why is an accurate K-factor important?
The K-factor determines the accuracy of the flat patterns. If we do not calculate with the correct value:
- the size of the bent part may differ from the design,
- fitting problems may arise during assembly,
- unnecessary correction or remanufacturing may be required.
We always rely on measured, real data, so we can guarantee that parts are made exactly to the dimensions of the drawing – whether it is mild steel, stainless steel or aluminium.
What else should you pay attention to when bending?
Besides the K-factor, several other factors influence the result of bending. The TRUMPF bending tool catalogue shows in detail, for example, the required pressing forces for different material thicknesses and grades.
A few important points:
- Material grade: higher-strength material (e.g. stainless steel) requires more bending force than mild steel or aluminium.
- Tool selection: choosing the right bottom and top tool affects the inner radius and the flat-pattern accuracy.
- Flange length: with flanges that are too short, the sheet can distort more easily.
- Optimal V-opening: the size of the V-opening of the bending tool directly determines the required force and the bending quality.
Based on the catalogue, the manufacturers give recommended values for every thickness and material, which help in selecting the tools correctly and avoiding production errors.
Forums and practical experience
On international professional forums (e.g. r/sheetmetal) experts also stress that there is no universal K-factor. The value always depends on the material, the thickness, the bending radius and the tool setup.
The best practice is for every manufacturer to build a database measured on their own machines and materials – as we do with our Trumpf machines.
Summary
An accurate K-factor is essential for error-free flat patterns and cost-effective production. Our table, based on data from our Trumpf press brake, can be used equally for mild steel, stainless steel and aluminium sheets.
If you also want precise, dimensionally accurate parts, please feel free to get in touch: Contact us →
Frequently asked questions about press-brake bending
What happens if the wrong K-factor is used when calculating the flat pattern?
Answer: If the wrong K-factor is used, several problems can arise:
Dimensional errors: The final size of the bent part may differ from the design.
Fitting problems: The part will not fit precisely with other components during assembly.
Waste and delays: Unnecessary corrections, rework or even remanufacturing of the part may be required, which increases time and cost.
Solution: Always check that you are using a measured K-factor for the given material, thickness and inner radius. Ideally, as in our case, rely on data calibrated to the machine and the material.
The values in the K-factor table are for guidance only. How can I guarantee accuracy?
Answer: The K-factor table mentioned in the article was indeed derived for optimal bottom tools and is for guidance only.
Problem: If you rely solely on these general values in your own production environment, where machines, tools or material grades may differ, accuracy is not guaranteed.
Solution: The best practice is for every manufacturer to build a database of measured, real data for their own machines and materials, as we do with our Trumpf machines. Always ask the manufacturer for technical support after setup as well, to refine the values.
Why can a part distort during bending even if I used a good K-factor?
Answer: The K-factor is only one factor among many. Other setup errors can also cause part distortion:
Problem:
Flange length too short: Flanges that are too short do not provide enough material for a stable bend and can easily distort.
Wrong tool selection: An incorrect choice of bottom and top tools (for example the V-opening size) can affect the inner radius and the bending quality.
Non-optimal pressing force: A pressing force that does not match the material and thickness can cause over- or under-bending.
Solution: Pay attention to the material grade (higher-strength materials such as stainless steel require more force), select the right bottom and top tools, and make sure the flange length is adequate. The Trumpf bending tool catalogue provides detailed guidance on this.
Does the material grade (e.g. mild steel, stainless steel, aluminium) affect the K-factor and the bending?
Answer: Yes, absolutely! The material grade plays a key role:
Problem: If you use the same K-factor or bending settings for different materials (for example values used for mild steel applied to stainless steel), errors can occur. Different materials have different mechanical properties.
Solution: The article highlights that our company’s K-factor table, based on Trumpf data, applies specifically to ST37 mild steel, stainless steel and aluminium. This ensures material-specific accuracy. Keep in mind that higher-strength materials (e.g. stainless steel) require more bending force.
My K-factor matches the one in the table, yet the result is still inaccurate. What could be wrong?
Answer: Besides the K-factor, other elements of the manufacturing process can also affect accuracy:
Problem:
Machine wear or calibration: Machine wear, tool condition or inaccurate machine calibration can all affect bending accuracy.
Material thickness variations: If the thickness of the incoming sheet is not completely uniform, it can cause deviations in bending.
Bending speed: The bending speed can also influence how the material behaves.
Solution: Regular machine maintenance and calibration are essential. Make sure the quality and thickness of the raw material are within the specified tolerances. Ask the manufacturer for advice, as they know the characteristics of their machines and the correct settings precisely. As the article emphasises, there is no universal K-factor; data measured for your own machines and materials is the most reliable.
