Marking Lines Mechanism and Problem Solving in the Sheet Metal Bending Process

Abstract

This article proposes ways to solve the marking lines problem on sheet metal workpieces graded SPCC (JIS) that have been bent to produce electrical cabinet parts, thereby minimizing the polishing process required to remove the marking lines on the workpiece surface before painting. The mechanism of the marking lines occurring on such parts was studied using simulation by finite element method for bending the workpiece material. The studies have shown that when sheet metal is exposed to force from punch, it takes the workpiece into the die. At the die shoulder area, the sheet metal will slide to raise, causing the surface of the sheet metal to be scratched with the die shoulder thus creating marking lines along the bending line on the surface of the workpiece in contact with the die shoulder. The proposed solution consists of: (1) using lubricant, (2) coating the die with a hard-thin film and (3) the use of a polyethylene sheet between the die and the workpiece surface while bending. All these methods are aimed at improving the tribological properties between the contact surfaces of the die and the workpiece. According to the comparison of the workpiece surfaces that have been bent, it has revealed that method (3) is the most effective in preventing making lines that occur on the workpiece after bending. It can also reduce the process of adjusting the workpiece surface through bending. The production cost to prevent marking lines by the above method is 0.21 baht/piece, however the process and operator in the electrical cabinet production line can also be reduced to 1 work station, resulting in a reduction of the total production time of such parts to 22 sec/piece. It can also reduce the chance of waste from the process of polishing to remove the marking lines on the workpiece surface after bending.