Sheet Metal Process:
Workpiece Material Preparation
After receiving the drawings, different methods of material preparation are chosen based on the unfolding diagram and batch requirements. These methods include laser cutting, CNC punching, shearing, and mold cutting. The corresponding unfoldings are made according to the drawings.
Due to the influence of cutting tools, some irregular workpieces and irregular hole processing may result in significant burrs on the edges. Post-processing is required to remove the burrs, which can also affect the precision of the workpiece.
With no tool limitations, the cross-section is smooth, making it suitable for processing irregularly shaped workpieces. However, it takes longer to process small workpieces. Placing the worktable next to the CNC machine or laser machine facilitates the placement of the sheet material for processing, reducing the effort required to lift the sheet. Some usable edge materials are placed in designated locations to provide material for trial bending. After material preparation, the edges, burrs, and contact points of the workpiece need to be properly trimmed and deburred. The contact points are smoothed using a flat file, while larger burrs are removed using a grinding machine. Small internal hole contact points are trimmed with corresponding small files to ensure a pleasing appearance. The trimming of the outer shape also ensures proper positioning during bending, allowing the workpiece to be consistently positioned on the bending machine, ensuring consistent dimensions for batch production.
Subsequent Processes
After material preparation is complete, the workpiece moves on to the subsequent processes based on the specific requirements. These processes may include bending, riveting, flanging, tapping, spot welding, bulging, and step differences. In some cases, when there are bulging or step differences that require molds, it is necessary to consider processing them first to avoid interference with other processes that may occur if they are processed later and hinder the required operations. When there are hooks on the upper cover or lower shell, if welding is not allowed after bending, they need to be processed before bending.
Bending
When bending, the tooling and die slot used for bending are determined based on the dimensions and material thickness specified in the drawings. This ensures that the product does not collide with the tooling, causing deformation. The selection of the upper die is crucial (different models of upper dies may be used for the same product), while the selection of the lower die is determined by the thickness of the sheet metal. The bending sequence is then determined, following the general rule of bending from the inside to the outside, from small to large, and from special to ordinary. For workpieces that require edge flattening, the workpiece is first bent to 30°-40° and then flattened using a flattening die.
Riveting
When riveting, the selection of the same or different molds is based on the height of the threaded stud. The pressure of the riveting machine is adjusted to ensure that the stud is flush with the surface of the workpiece, avoiding insufficient or excessive pressure that may cause the stud to come loose or protrude beyond the workpiece surface, resulting in rejected parts.
Welding
There are various welding methods, including argon arc welding, spot welding, manual arc welding, etc. For spot welding, the positioning of the workpiece for welding is a primary consideration. In batch production, fixtures are used to ensure accurate spot welding positions.
To achieve a strong weld, protruding points are created on the workpiece to ensure even contact with the flat plate before electric welding. This ensures consistent heating of each point and helps determine the welding position. Similarly, for welding, the pre-press time, holding time, maintenance time, and rest time need to be adjusted to ensure a secure spot weld. After spot welding, welding scars may appear on the surface of the workpiece, which can be treated using a flat grinding machine. Submerged arc welding is mainly used when two large workpieces need to be connected or for edge treatment of a single workpiece to achieve a smooth and polished surface.
The heat generated during welding can cause workpiece deformation. After welding, grinding machines and flat grinding machines are used for post-welding treatment, especially for edge and corner areas.
Surface Treatment
After processes such as bending and riveting are completed, surface treatment is carried out. Different types of sheet metal require different surface treatment methods. Cold-rolled sheet metal usually undergoes surface electroplating. After electroplating, no spray coating is applied, but phosphating treatment is performed before spray coating. Electroplated sheet metal is cleaned and degreased before spray coating. Stainless steel sheets (including mirror panels, frosted panels, and brushed panels) can undergo brushing treatment before bending, eliminating the need for spray coating. If spray coating is required, the surface needs to be roughened. Aluminum sheets are generally treated with oxidation. Different colors of spray coating require different oxide base colors, with black and natural oxidation being commonly used. Chromate oxidation treatment is performed on aluminum sheets that require spray coating. Pre-treating the surface in this way ensures a clean surface, significantly improves the adhesion of the coating, and greatly enhances the corrosion resistance of the coating. The cleaning process involves washing the workpiece by hanging it on a production line. First, it goes through a cleaning solution (alloy degreaser), then enters the water rinse, followed by a spray zone, and finally passes through a drying zone before being taken off the production line.
Spray Coating Process
After surface pretreatment, the workpiece enters the spray coating process. When the workpiece needs to be assembled before spraying, the threads or partially conductive holes need to be protected.
Protection Treatment
For threaded holes, soft rubber rods or screws can be inserted, and high-temperature adhesive tape is used to protect areas that require conductivity. In the case of large-scale production, fixtures are used for positioning and protection during the spraying process to prevent overspray inside the workpiece. Screws are used to protect visible nut (flange) holes on the outer surface of the workpiece to avoid the need for rethreading after spraying.
Some large-scale workpieces also require the use of protective fixtures.
When the workpiece is not assembled for spraying, areas that do not require coating are covered with high-temperature adhesive tape and paper, and exposed nut holes are protected using screws or high-temperature rubber. If both sides of the workpiece need to be sprayed, the same protection methods are used for nut (bolt) holes. Small workpieces are bundled together with lead wires or paper clips before spraying. For workpieces with high surface requirements, dust scraping is performed before spraying. Special high-temperature adhesive stickers are used to protect grounding symbols on some workpieces.
During the spraying process, the workpiece is first hung on a production line, and the surface is blown clean of dust using an air hose. It then enters the spray zone for coating and proceeds along the production line to the drying zone. Finally, the finished sprayed workpiece is taken off the production line.
There are two types of spray coating: manual and automatic, which require different fixtures.
Assembly Process
After spraying, the workpiece goes through the assembly process. Before assembly, the protective stickers used during spraying are removed, and it is ensured that no paint or powder has entered the threaded holes. Throughout the process, gloves are worn to prevent dust on hands from adhering to the workpiece. Some workpieces may also require cleaning with an air gun.
Packaging Stage
After assembly, the workpiece enters the packaging stage. It is inspected and then placed in specialized packaging bags for protection. Workpieces without dedicated packaging are wrapped in bubble wrap, etc. Before packaging, the bubble wrap is cut to the appropriate size to avoid the need for cutting while packaging, which would affect processing speed. For large-scale batches, custom-made cardboard boxes, bubble bags, foam pads, trays, wooden boxes, etc., are used. After packaging, the workpieces are placed in cardboard boxes, and the corresponding finished or semi-finished labels are attached to the boxes.
Sheet Metal Quality Inspection
In addition to strict requirements during the production process, independent quality inspection is necessary to ensure the quality of sheet metal components. This involves strict dimensional checks according to the drawings and rigorous inspection of the appearance quality. Non-conforming dimensions are repaired or scrapped, and the surface should be free from scratches or marks. The color difference, corrosion resistance, and adhesion of the coating after spraying are also inspected. This helps identify errors in the unfolding diagram, poor production habits, and process errors, such as programming errors in CNC punching or mold errors.
Sheet metal quality inspection is essential to ensure the quality of sheet metal components. In addition to stringent requirements during the production process, independent quality inspection is conducted. This includes strict dimensional checks according to the drawings and rigorous inspection of the appearance quality. Any non-conforming dimensions are either repaired or scrapped. The appearance should be free from scratches or marks, and the color difference, corrosion resistance, and adhesion of the coating after spraying are inspected. This process helps identify errors in the unfolding diagram, poor production habits, and process errors, such as programming errors in CNC punching or mold errors.
