There are many ways for CNC bending machines to bend plates, and there are many classification methods for different principles. This article specifically introduces several common bending methods from the different relative positions of the upper and lower molds and the different folded shapes during bending, and details the bending process and precautions.
1. The relative position of the mold: gap bending and bottom pressing
First of all, according to the difference in the relative positions of the upper and lower molds during the bending process, the bending process is divided into two forms: gap bending and bottom pressing. The characteristics and differences between the two are as follows:
The CNC bending machine is not pressed between the upper die and the lower die during the bending process. The required bending angle is obtained by adjusting the depth of the upper die entering the opening of the lower die. This bending method is called gap bending.
The deeper the upper die enters the lower die, the smaller the bending angle; otherwise, the larger it is. Due to the elasticity of the material, it is also necessary to consider excessive bending to control the amount of springback when bending.
The advantage of gap bending is that a smaller number of molds can be used to realize various angles of forming and processing, and the required processing pressure is small. Usually, in order to obtain the best bending effect, the ratio of the thickness T of the material to the width V of the V-shaped opening of the lower die can be selected as follows:
(1) When the material thickness is below 12.7mm, T:V is 1:8;
(2) When the material thickness is 12.7~22.2mm, T:V is 1:10;
(3) When the material thickness is above 22.2mm, T:V is 1:12.
The above three ratios are standard mold ratios, the material is low carbon steel, and the material strength is 43.4kg/mm2. When compiling a bending processing program, the above parameters can be set in the numerical control system, and the processing program will be generated by the system after automatic processing.
When the CNC bending machine adopts the bottom bending, the plate is pressed between the upper and lower molds, so as to obtain the required bending angle and elbow radius. CNC bending machine press bottom bending is suitable for processing metal sheets with a thickness of less than 2mm in batch and large-scale production in enterprises.
Its bending radius is small – high bending accuracy – good accuracy. It should be noted that the working pressure of bottom pressing is greater than that of clearance bending, generally more than three times.
The angle of the bottom pressing die should be adapted to the angle of the plate and the material. Usually, when bending low carbon steel at the bottom, the angle of the upper and lower molds should be consistent with the required angle of the plate. When the bottom pressing method is used for processing, the die ratio, that is, the ratio of the sheet thickness B to the opening distance V of the lower die, is B:V=1:6.
During the bending process of determining the working tonnage, the force between the upper and lower dies is applied to the material, causing the material to plastically deform. Working tonnage refers to the bending pressure during bending. The influencing factors for determining the working tonnage are: bending radius, bending method, mold ratio, elbow length, thickness and strength of bending material, etc.
2. Fold different shapes: L fold, N fold, Z fold, reverse fold and flatten, press hardware
According to the different shapes formed after bending, the bending process is divided into L-fold, N-fold, Z-fold, reverse folding and flattening, pressing hardware and other forms. The characteristics and differences are as follows:
According to the angle, it is divided into 90° fold and non-90° fold. According to the processing, it is divided into general processing (L>V/2) and special processing
1) The mold is selected according to the material, plate thickness and forming angle.
2) The principle of relying on position:
①The principle of two post-fixed positions is used, and the positioning is based on the shape of the workpiece.
②When a back gauge is in position, pay attention to the deflection, and it is required to be on the same center line as the bending size of the workpiece.
③When small bends are made, it is better to process them in the reverse position.
④ It is better to be at the back and lower in the middle. (The rear gauge is not easy to lift when it is in position)
⑤ It is better to be close to the rear set.
⑥It is better to lean on the long side.
⑦ Use a fixture to assist the positioning (irregular positioning of the hypotenuse).
3) Precautions: Pay attention to the positioning method during processing and the movement mode specified in the various positioning processing methods. The mold is bent when it is installed, and then it is required to be pulled back to prevent the workpiece from being deformed during bending. When a large workpiece is internally bent, the shape of the workpiece is large and the bending area is small, making it difficult to overlap the tool and the bending area, resulting in difficult positioning of the workpiece or damage to the bending workpiece. In order to avoid the above situation, a positioning point can be added in the longitudinal direction of the processing, so that the positioning and processing are carried out in two directions, which makes the processing and positioning convenient, and improves the processing safety, avoids the damage of the workpiece, and improves the production efficiency.
N folds should be processed in different ways according to different shapes. When bending, the inner size of the material should be greater than 4mm and the size of the X dimension is limited by the shape of the mold. If the inner size of the material is less than 4mm, special methods are used for processing.
1) Select the mold according to the material thickness, size, material and bending angle.
2) The principle of positioning: to ensure that the workpiece does not interfere with the tool
①Ensure that the reclining angle is slightly less than 90 degrees.
②It is better to use two back-sets to be in position, except in special cases.
①After bending and L-folding, the angle should be guaranteed to be 90 degrees or slightly less than 90 degrees to facilitate processing and positioning.
②When the second fold is processed, it is required that the locating position should be centered on the machining surface.
Z-fold is also called step difference, that is, one positive and one negative bending. According to the angle, it can be divided into hypotenuse and straight edge. The minimum size of the bending process is limited by the processing die, and the maximum processing size is determined by the shape of the processing machine. Under normal circumstances, when the inner size of the Z-folded material is less than 3.5T, the differential die processing is used. When it is larger than 3.5T, the normal processing method is adopted.
1) The principle of relying on position:
①It is convenient to rely on and has good stability.
②Generally, the position is the same as the L-fold.
③The secondary position requires the workpiece to be flat with the lower die.
①The processing angle of the L-fold must be in place, generally 89.5~90 degrees.
②When it is required to pull back, pay attention to the deformation of the workpiece.
③The order of processing must be correct.
④For special processing, the following methods can be used: centerline separation method (eccentric processing), small V processing (need to increase the bending coefficient), easy mold forming, grinding lower mold.
Reverse folding and flattening is also called dead edge pressing. The processing steps of the dead edge are: first bend and insert to a depth of about 35 degrees, and then use a flattening die to flatten it until it is flat and tight.
1) Die selection method: select the V-groove width of the lower die with a depth of 30 degrees according to 5–6 times the thickness of the material, and select the upper die according to the specific conditions of processing dead edges.
2) Matters needing attention: Pay attention to the parallelism of both sides of the dead edge. When the processing size of the dead edge is long, the flattened edge can be folded first and then flattened. For shorter dead edges, dunnage can be used.
The use of a folding machine to press and hold hardware parts generally requires the use of auxiliary molds such as concave molds and jigs. In general, there are: pressure nuts, pressure studs, pressure screws and some other hardware.
①When the shape of the workpiece needs to be avoided for processing, it should be avoided.
②After processing, check the torque, whether the thrust reaches the standard and whether the hardware and the workpiece are flat and tight.
③Press after bending. When pressing beside the machine tool, pay attention to the machining avoidance and the parallelism of the mold.
④If it is expanding, it should also be noted that there should be no cracks on the expanding edge, and the expanding edge should not be higher than the surface of the workpiece.