Hemming (Flattening) bending is a common bending operation method, which usually bends the edge of the workpiece into parallel edges or rounded edges, as shown in Fig. 1.
Fig. 1 Schematic diagram of hemming and bending
The purpose of hemming and bending is to eliminate sharp edges, increase workpiece strength and improve workpiece aesthetics.
Flattening bending basically consists of the following two steps:
First, pre-bend the workpiece to 30 °; the second step is to flatten and bend the workpiece; as shown in Fig. 2.
Fig. 2 Steps of hemming and bending
In order to select an appropriate flattening bending solution, you first need to consider the material and thickness of the workpiece and the frequency of flattening bending.
Second, the hemming die can also be used for normal bending when it’s not needed for hemming.
According to different bending conditions, there are four different hemming bending solutions:
(1) Flat bottom hemming die;
(2) U-shaped hemming die;
(3) Spring type hemming die;
(4) Hydraulic hemming table;.
Fig. 3 Four hemming and bending solutions
(1) Flat bottom hemming die
The flat bottom hemming die realizes hemming and bending through two sets of dies, that is, one set of upper and lower standard dies and one set of upper and lower flat bottom dies are used together, as shown in Figure 4.
Fig. 4 Schematic diagram of flat bottom hemming die
The first step is to pre bend the workpiece to 30 °with a standard die;
The second step is to flatten and bend on the flat bottom die.
Because the flat bottom hemming die needs two-step bending, it is mainly applicable to the working condition where hemming and bending are not frequent and has high-cost performance.
However, there is a certain lateral force during hemming, and the maximum thickness of flattened carbon steel plate is 2 mm.
In order to solve the limited thickness of flattened bending plate, we innovatively launched a ball-type movable flat bottom lower die, and the maximum thickness of flattened carbon steel plate / stainless steel plate can reach 4 mm, as shown in Fig. 5.
Fig. 5 Schematic diagram of ball type movable lower die
(2) U-shaped hemming die
A set of U-shaped hemming dies can be pre bent and flattened, as shown in Fig. 6.
Fig. 6 Schematic diagram of U-shaped hemming die
The advantage of U-shaped hemming die is that when it is not used for hemming and bending, it can be used for conventional air bending, with high utilization and cost performance.
The maximum thickness of flattened carbon steel plate is 1.5 mm, the maximum thickness of flattened stainless steel plate is 1 mm, and the minimum flange size that can be flattened is about 14 mm.
(3) Spring type hemming die
The spring hemming die consists of a 28 °standard upper die and a 30 °hemming lower die.
The hemming insert of the hemming lower die is supported by a spring.
In the hemming process, the pre bent workpiece is placed between the hemming insert and the guide insert of the hemming lower die, and the hemming insert follows the descent of the upper die to press down until the pre bent workpiece is flattened.
The maximum thickness that the spring hemming die can flatten the carbon steel plate is 2 mm.
Similarly, the spring hemming die can also be used for conventional air bending when it is not used for hemming bending.
Fig. 7 Schematic diagram of spring hemming die
(4) Hydraulic hemming table
The working principle of the hydraulic hemming table (as shown in Fig. 8) is similar to that of the spring hemming die, but the hydraulic cylinder is used to replace the spring, and the lifting of the hemming insert is controlled by the hydraulic cylinder.
Fig. 8 Schematic diagram of hydraulic hemming workbench
The oil pressure is 100 bar, making it possible to bend thicker and heavier workpieces.
In order to further improve the service life of the hydraulic hemming table, an additional hemming insert with hardening can be equipped.
The standard lower die can be placed on the hydraulic hemming workbench, and the V opening of the standard lower die can be selected according to needs, up to v = 40 mm.
The lower die can be clamped manually or automatically to improve the die change efficiency.
The hydraulic hemming table can flatten the carbon steel plate with a maximum thickness of 3 mm.
The spring hemming die / hydraulic hemming table can be clamped on the fixture / mechanical compensation table.
The hydraulic hemming table can also be directly installed on the lower beam of the bending machine, which can obtain greater opening height and flexibility.
To sum up, the four different hemming and bending solutions have their unique advantages. For different hemming and bending application scenarios, you can choose the corresponding solutions.