Frames are an important part of the hull framework, and their bending quality directly affects the fairness of the hull shape. Traditional cold bending of frames relied on templates and manual experience, with operators repeatedly comparing and correcting, resulting in low efficiency and unstable accuracy. Jiangsu Haizhongzhou Shipping Industry Co., Ltd. has introduced CNC frame bending machines, achieving digitalisation and automation of frame bending.
The core of the CNC frame bending machine is the three‑point bending principle, where a concentrated load is applied to the profile through two fixed supports and one movable ram, causing plastic bending. Unlike traditional template bending, the CNC bending machine can automatically calculate the required ram stroke based on the input profile section parameters, material grade and bending radius. Haizhongzhou’s technicians first extract the theoretical shape of frames from the 3D design software, generating a bending data file for each frame, including chord length, camber, radius of curvature and twist angle, and then transfer the file to the bending machine’s control system via USB drive or network.
During the bending process, the machine gradually bends the profile in set steps, and automatically compensates for spring‑back after each step. Spring‑back compensation is a key technology in CNC bending – because the profile undergoes elastic recovery after unloading, the actual bending angle is always smaller than the angle applied by the ram. Using a spring‑back prediction model developed in cooperation with a university, Haizhongzhou calculates the theoretical spring‑back amount based on the profile grade, sectional moment of inertia and bending radius, and adds an over‑bend amount to the program. After extensive validation, the prediction accuracy of the model exceeds ninety percent, and the first‑piece acceptance rate has increased from less than fifty percent with traditional methods to eighty‑five percent.
For frames with twist (such as those in the bow and stern areas), the CNC bending machine can also apply torque synchronously. Combined bend‑twist forming is a feature of Haizhongzhou’s equipment. Through coordinated movement of the ram and clamps, the profile is simultaneously bent and rotated about its own axis, completing the bend‑twist forming in a single operation, avoiding the local distortion and stress concentration caused by manual twisting after bending in the traditional process.
After each frame is bent, Haizhongzhou uses a laser profiler to scan and inspect its shape. The scanned data are compared with the theoretical shape, and frames with deviations exceeding three millimetres are sent to a correction station for fine adjustment by local pressure or heating. Qualified frames are sprayed with identification marks and stored by category. The application of CNC bending technology has more than doubled frame processing efficiency while also providing accuracy assurance for allowance‑free hull assembly.
