In the process of sheet metal processing, bending is one of the common processing techniques. In order to ensure the accuracy of the part size after bending, it is necessary to calculate the unfolded length of the sheet metal material before bending. Correct calculation of the unfolded length can not only improve material utilization but also reduce processing errors, enhance production efficiency and product quality.



One, Basic Concept of Unfolding Length



During the sheet metal bending process, the material undergoes plastic deformation under the action of external force, in which the outer material is stretched, the inner material is compressed, and there is a neutral layer in the middle that is neither stretched nor compressed. The unfolding length is the total length of this neutral layer before bending.



Two, Factors Affecting Unfolding Length



1. Plate thickness (T): the thickness of the material directly affects the deformation amount after bending.



2. Bending angle (θ): the larger the angle, the longer the deformation area.

　　3. Internal bending radius (R): usually determined by the mold, affecting the position of the neutral layer.



4. Bending coefficient (K factor): the position of the neutral layer relative to the plate thickness, generally between 0.25~0.5.



Three, Calculation Formula for Unfolding Length



The general calculation formula for the unfolding length of sheet metal bending is:



Unfolding length = straight edge length + bending compensation value (BA)



The calculation formula for the bending compensation value BA is:



BA = (π/180) × (R + K × T) × θ



Among which:



- R: Internal bending radius



- K: K factor (related to material and processing method)



- T: Material thickness

　　- θ: Bending angle (unit: degree)



Four, Determination of K Factor



The K factor is a key parameter that determines the unfolding length, which depends on the material type, bending method (air bending, bottom bending, pressing bending, etc.), and the size of the internal bending radius. For example:



- When R ≥ 5T, K ≈ 0.5



- When R < 5T, K < 0.5, the specific value needs to be determined according to experience or by looking up the table



In actual production, the K factor can be obtained through experiments or by consulting the standard sheet metal unfolding coefficient table.



Five, Case Analysis



Assuming there is a 1mm thick Q235 steel plate, it needs to be bent to 90°, with an internal bending radius R = 1mm, and a K factor of 0.45:



BA = (π/180) × (1 + 0.45 × 1) × 90 ≈ 2.28 mm



If the lengths of the two straight edges are 30mm and 50mm respectively, then the total unfolding length is:



L = 30 + 50 + 2.28 = 82.28 mm

　　Six, Conclusion



The calculation of the unfolding length of sheet metal bending is a key link to ensure the accuracy of the parts. Mastering the selection of the K factor, understanding the change law of the neutral layer, and adjusting them in combination with actual working conditions is an effective means to improve the quality of sheet metal processing. With the wide application of CAD/CAM software, many unfolding calculations can now be automatically completed by the software, but understanding its principles is still a basic skill that engineering and technical personnel must possess.