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JA In the field of sheet metal fabrication, one of the biggest challenges for CAD/CAM design engineers is accurately calculating the dimensions of the flat pattern before feeding it into the bending machine. When bent, the material expands on the outside and compresses on the inside, resulting in a change in finished product dimensions compared to the original workpiece. To solve this problem, a thorough understanding and correct application of the K Factor in sheet metal bending, combined with tolerance standards, is essential.
In the article below, the Metaltech 1 technical team will share detailed instructions on how to calculate bending blanks and provide the most accurate K-Factor lookup table.
1. What is the K-Factor? The nature of material elongation.
When a metal sheet is bent, the cross-section of the sheet at the bending point is divided into three different load-bearing zones:
- Compression zone: The inner surface of the bend is compressed.
- Tension area: The outer surface of the bend is stretched.
- Neutral Axis: Located between the compression and tension zones, the material is neither compressed nor stretched, meaning the length of the neutral axis remains constant before and after bending.
K-Factor (K-Factor) This is the ratio between the distance from the inner surface of the bending angle to the neutral axis (denoted as ). t) compared to the total thickness of the metal plate (denoted as T).
Formula: K = t / T
In practice, the K-Factor typically ranges from 0.25 to 0.5. This coefficient is influenced by many factors:
- Type of material (Black steel, Stainless steel, Aluminum, Copper...).
- The thickness of the metal sheet.
- Inside Bend Radius (R).
- Shocking method (Air Bending or Bottoming).
2. Reference K-Factor table for common materials
For convenience in setting parameters in software such as SolidWorks, AutoCAD, or Inventor, engineers can refer to the following table of empirical K-Factor values (commonly applied to the air bending method on CNC machines):
| Material Type | Bending radius (R) and thickness (T) conditions | K-Factor |
| Mild Steel (SPCC / SS400) | R < 2T | 0.38 – 0.42 |
| R = 2T to 4T | 0.43 – 0.46 | |
| R > 4T | 0.50 | |
| Stainless steel (304/316) | R < 2T | 0.35 – 0.40 |
| R = 2T to 4T | 0.40 – 0.45 | |
| R > 4T | 0.50 | |
| Aluminum & Aluminum Alloys | R < 2T | 0.40 – 0.44 |
| R = 2T to 4T | 0.45 – 0.48 | |
| R > 4T | 0.50 |
(Note: The table above is for reference only. In actual machining, to achieve absolute accuracy, workshop engineers need to perform test bends to determine the precise K-Factor and Bend Deduction values for each specific batch of material and die system.)
3. Standard bending tolerances in mechanical machining.
Even if you have applied it correctly calculate the impact workpieceIn practice, achieving 100% accuracy is virtually impossible due to measurement equipment errors, material springback, and the thickness tolerance of the steel workpiece itself.
Therefore, technical drawings must always include requirements regarding... bending toleranceAt machining plants using modern CNC hydraulic bending machines like Metaltech 1, tolerance standards are typically controlled as follows:
- Angle tolerance: ± 0.5° to ± 1° (Depending on the length of the indentation and the material thickness).
- Dimensional tolerance after bending (Bending flange length): * For thin workpieces (T < 3mm): ± 0.2mm to ± 0.3mm.
- For thick workpieces (T > 3mm): ± 0.5mm.
4. Why should design engineers collaborate with a closed-loop manufacturing process?
The calculation of the flat pattern becomes meaningless if the initial workpiece cutting process is inaccurate in terms of dimensions. If you use plasma cutting with beveled edges, or stamping with protruding burrs, the CNC bending machine will not be able to accurately press the workpiece onto the die, resulting in incorrect bending angles.
That's why engineers always prioritize finding integrated machining workshops. Metaltech 1We provide a closed-loop embryo creation process. The embryos are produced by us. CNC Laser Cutting With extremely low tolerances (± 0.05mm), the cut surface is smooth, creating the perfect foundation for the CNC bending machine system to operate smoothly, closely adhering to all the K-Factor parameters that the engineers have meticulously calculated.
If you have a complex drawing and need a capable partner to realize it with the strictest tolerances, please contact Metaltech 1's engineering department. We are ready to assist you in drawing breakdown and advise on the most cost-effective machining methods.
