CNC punching machine and CNC press brake are the main equipment for sheet metal processing. They are known for their convenience and flexibility and are often used in processing various sheet metal parts.
NC punch press
The CNC punching machine can perform various complex hole shaping and shallow drawing processes. Using small punching dies in a stepped punching method, it can make large round holes, square holes, waist-shaped holes and various curved contours. It can also perform special processes such as louvering, shallow drawing, drilling, hole flanging, reinforcing and embossing.
Through simple mold combinations or multi-coordinate connections, the CNC punch can also process complex parts. Compared with traditional punch machining, CNC punching machine saves mold costs and has a larger machining scope and capacity, making it more suitable for changing market and product demands, with the ability to produce small and diversified batches with low cost and short cycle.
CNC press brake
The CNC press brake can be used to bend metal sheets. Most bent parts can be machined using a combination of multi-purpose upper punch and V-shaped lower die. Additionally, most press brakes have the capacity to process parts longer than 2 m, offering high flexibility.
However, when manufacturing sheet metal parts using these two types of equipment, the part may often suffer from scratches, crush marks, and abrasions, especially when processing aluminum and stainless steel plates. These scratches can significantly affect the appearance quality of the product, especially for pieces without a lacquered surface. To eliminate these scratches, a grinding process is often added, which is time-consuming and ineffective.
In this content, I will analyze the causes of scratches and provide detailed solutions to avoid them during the handling, cutting and bending process.
Transport process
Causes:
In the process of raw material inventory, logistics and parts process, parts will inevitably collide with surrounding objects and scratch.
Solutions:
There are several solutions for scratches during the handling process:
- To increase the operator's sense of responsibility, inspect the surface quality of incoming materials before processing and reject those that are not suitable.
- Train operators to handle parts carefully and not to stack them during manufacturing.
- Use cardboard or foam board to separate parts during stacking and shipping and avoid dragging artifacts on the board. The workpieces must be placed evenly and orderly, with consistent quantities in each layer, and the height must not exceed 1 meter.
- Avoid collisions between workpieces.
Erase process
Causes:
⑴ Risk of friction
a) During the manufacturing of copper, aluminum and stainless steel, the material can adhere to the tool and produce tiny, hard particles similar to the built-up edges of the mold. When the plate is dragged back and forth in the bottom mold, it can be damaged, resulting in deep scratches that are difficult to remove.
b) During machining, friction between the plate and the lower mold can cause scratches, especially when using special molds such as shutters or diving dies. The larger the bottom die, the more severe the scratches will be.
c) During processing, friction between the plate and the surface of the work table may cause scratches on the bottom surface of the plate.
⑵ Recoil
a) There are impurities on the surface of the input material, the blade and the blade disc turret. Although the impurities are not many, they can easily spread on the surface of the raw material during machining. When the discharge plate presses the area where the impurities are spread, the surface of the plate will have obvious indentations.
b) The mold material is magnetic.
c) During stamping, if the discharge plate is harder than the plate, it may cause indentations when pressing the plate, especially when manufacturing pure aluminum or copper plates.
Solutions:
To avoid risks during the blanking process, there are several solutions to consider:
⑴ Before processing, inspect the surface of the incoming material for debris and clean it with a blast gun and cloth if necessary.
⑵ Before installing the mold, check the upper and lower mold and tower for iron debris and clean it if found.
⑶ Grinding molds and molds used for a long time should be demagnetized before reuse.
⑷ When processing viscous metals such as copper, aluminum or stainless steel, reduce the punching speed to minimize edge build-up. Shops with adequate budgets can use coated molds to manufacture viscous metals and prevent buildup.
⑸ Regularly check and clean the equipment.
⑹ Use air blowing injection molds, which prevent foreign bodies from entering and reduce waste and material transportation.
⑺ Machines that are not equipped with air injection devices can be lubricated by regularly brushing the mold with oil.
⑻ Replace the ball table with a brush table and install anti-scratch brushes so that the bottom mold changes from hard friction to smooth friction between the plate and the work surface and the plate and the mold. When processing different materials and thicknesses, choose different brushes. To facilitate brush replacement, replacement should mainly be done at the bottom mounting bracket of the mold. Use a hard brush for heavy plates and a soft brush for thin, light plates. When using special forming molds such as shutter bending molds, lift the brush due to the high position of the lower mold. The brush scratch-proof mold is shown in the drawing.
⑼ Using polyurethane as a discharge device, replace the rigid pressure material with an elastic pressure material or a special plastic discharge plate.
The causes of scratches in the blank manufacturing process are complex. This is just a preliminary analysis, and the most effective solution can only be determined through further research and analysis.
Bending process
Causes:
In most of the bending process, V-shaped die bending is adopted. See the diagram below:
In the bending process, points A and B are the areas of tension in the plates, resulting in indentations. During manufacturing, the punch moves downward, causing the plate to slide along points A and B. In sliding range L, the surface of the plate will show visible signs of abrasion.
To avoid this problem, some factories use a protective film on the surface of the board, which helps to a certain extent. However, if the bending force is strong, the indentations will be deep and the protective film will not eliminate them effectively.
Solutions:
⑴ Use a reusable, pressure-resistant and wear-resistant film in a common mold.
Place a polyurethane membrane mat on the V-shaped die, and use a polyurethane membrane pad between the sheet and the mold during the process to avoid direct contact between the sheet and the die, thus avoiding indentations and scratches.
The cushion membrane protection process is shown in the figure.
⑵ Design anti-indentation mold. There are three commonly used forms of anti-scratch molds:
a) Hard rubber V-shaped mold.
This material has low hardness, so it does not cause marks or scratches on the board during processing. Due to its low hardness, the mold is usually made in a single V shape.
b) Invert the folded matrix V.
This type of mold uses rotary forming and turning as the main movement, and its main component is the rotating block and rotating pivot. During processing, when the press brake drills downward to contact the workpiece and continues moving downward, the right end of the bending block rotates along the axis of rotation, driving the bending of the workpiece.
Due to this rotation, there is almost no relative sliding of the part surface, avoiding indentation and abrasion. The mold structure is suitable for bending plates with a thickness of t ≥ 1mm, but when the thickness is greater than 1mm, the root segment cannot be fully bent. Therefore, I suggest using the first method for scratch-free sheet metal fabrication.
c）Replaceable rubber strip V dies
The principle of the mold involves opening the opening in the traditional V-shaped die and mounting a rubber strip into the opening. During the bending process, the force support point of the part comes into contact with the flexible rubber strip. Since the hardness of the rubber is lower than that of the workpiece, it will not produce indentations or scratches. This type of scratch-resistant mold is not widely used and is mainly used in the reconstruction of old molds.
To complete
When processing sheet metal, the causes of scratches are diverse. The analyzes and solutions above are the result of years of technical work and have shown significant results in manufacturing practice. Sheet metal craftsmen can adopt the suitable solution based on the actual situation.
