In the design of a stamping die, it is necessary to conduct a reasonable process analysis to prevent design defects and scrap of the die.
There is no shortcut to practical design. How can we avoid process defects?
1. Bending radius should not be too large
When a workpiece is bent, in addition to plastic deformation, there is also elastic deformation and springback. As a result, it’s important to keep the radius of bending parts from being too large. Otherwise, the bending angle may become unstable.
2. Bending radius should not be too small
If the bending angle R is too small, the outer fiber can easily crack.
For low carbon steel, the minimum bending radius should be about 1.0T.
The minimum bending radius for brass and aluminum is about 0.6T.
For medium carbon steel, the minimum bending radius should be about 1.5T.
3. Bending straight edge should not be too small
To guarantee the bending quality of the workpiece, it is essential to ensure that the straight edge height H of the bending product is not too small. It should be greater than or equal to the minimum bending height Hmin.
H≥Hmin=r +2t
Where:
r — bending radius
t — plate thickness of bending parts
4. Bending edge distance and hole position distance should not be too small
When bending a blank with holes, it is important to ensure that the edge distance between the holes and the bending edge is sufficient. If the hole position is too close to the bending edge, the shape of the hole may be altered during the bending process.
To determine the appropriate distance, the formula L shall be followed, where L represents the distance between the hole edge and the bending edge.
- When T < 2, L ≥ R + t
- When t ≥ 2, L ≥ R + 2T
Where:
R — Bending radius;
T — thickness of bending part.
5. The symmetry of bending shape and size should not be too different
To prevent deformation, the height difference of bending parts should not be too significant.
Furthermore, it is advisable to keep the shape and size of the bending parts as symmetrical as possible. Otherwise, abnormal distortion may occur at the small end.
If improving this result in the design proves to be challenging, it is crucial to ensure that:
H>R+2T
Where:
H — height of small side;
R — Bending radius;
T — thickness of bending part.
6. Punching and unloading slot should not be ignored in local bending edge
When bending a certain section of an edge, in order to avoid tearing at the junction due to stress concentration, it is important to consider punching an unloading hole, cutting a groove, or shifting the bending curve to a certain distance.
7. Process incision should not be ignored in narrow edge bending
When the narrow side is bent, the cross-sectional shape of the deformation zone will be distorted. Specifically, the width of the inner surface increases, while the width of the outer surface decreases. This effect is particularly pronounced when the plate width b is less than 3T (where T is the plate thickness).
If it is necessary to achieve an accurate width b for the bending part and prevent bulging, it is essential to make an incision along the bending line before the bending process. Ignoring this step can result in unwanted distortions.
8. Springback cannot be ignored in bending process
Simultaneously existing during bending are plastic deformation and elastic deformation.
Once the external load is removed, the elastic deformation vanishes, and springback occurs.
The value of springback depends on various factors, such as material properties, the relative bending radius R/T, bending angle, etc.
As the yield point of material (σs) increases, the springback also increases, provided E and R/T decrease, and the bending angle increases.
It is essential not to overlook springback while bending to ensure the precision of the workpiece.
Several techniques can reduce springback, including repairing the die, using different springback directions for distinct components, incorporating local triangular ribs, and utilizing the concave die structure of the swing block.
9. The workpiece with large bending radius cannot be bent by ordinary method
The ordinary bending method cannot be used for parts with a large bending radius. This is because the large elastic deformation involved makes it impossible to obtain the required shape and size using this method. Instead, the method of stretch bending can be employed.
Prior to bending, an axial tension is applied, which causes the stress in the blank section to be slightly greater than the yield point of the material. This allows for bending to be carried out simultaneously, resulting in the desired shape and size.
10. The bending of complex shape parts cannot be completed at one time
For parts with complex shapes, it may not be possible to form them in a single bend and multiple bending operations may be required.
The principle for arranging these bending processes is to begin with bending the outer corners first. It’s essential that the subsequent bends do not affect the deformation of the previously bent parts.
The number of bends required for the desired shape may vary, ranging from two to three or even more.
11. The edge of bending parts should not be notched
If the edge of the notched bending part is punched out of the blank, a fork may appear during the bending process. This issue can become particularly severe, resulting in the inability to form the desired shape.
To avoid this problem, it is necessary to leave a connecting band at the notch and remove it only after bending.
12. The structure of bending die can’t ignore the blank offset
Offset is an important factor that affects the accuracy of the workpiece during the bending process. Therefore, before bending, it is crucial to securely fix a part of the blank onto a part of the die to prevent any offset.
It is recommended to utilize holes in the part for positioning as much as possible.
However, if there are no available holes on the part, the option of processing hole positioning can be considered.
13. The structure of bending die should not cause large local thinning and scratch of material during die closing
During the die closing process, the bending die should only bend at the position of the bending curve determined by the part.
The blank should not exhibit large local thinning or scratches.
When using the structure depicted in the left figure for bending, the position of the bending curve at the outer corner C changes during the bending process – first at point B, and finally to point C. This results in inaccuracies in the shape of the outer corner of the part and causes the straight arm portion to become thinner.
14. The die structure shall not hinder and prevent the blank from turning and moving during the die closing process
When designing the bending die, it is crucial to take into account the space required for the blank to rotate and move during bending. If the die structure impedes this movement, it can negatively impact the shape and size of the workpiece.
This consideration is particularly crucial when dealing with complex shapes that require multi-angle bending.
15. The material with small elastic modulus should not be used for bending workpiece
The amount of springback is directly proportional to the elastic modulus of the material.
Materials with smaller elastic modulus tend to experience larger elastic recovery after deformation, and hence, are not ideal for bending processes.
For materials with the same yield point, those with larger elastic modulus have a lesser degree of elastic recovery after deformation.
Annealed low carbon steel is more appropriate for bending as compared to soft manganese brass.
16. The material with high yield point is not suitable for bending workpiece
Springback is proportional to the yield strength of the material. Materials with a high yield point exhibit significant elastic recovery after deformation, making them unsuitable for the bending process. In other words, materials with the same modulus of elasticity and high yield point have a larger elastic recovery. Therefore, cold work-hardened steel is not appropriate for the bending process.
17. Ordinary bending method is not suitable for bending parts with large bending radius and arc angle
When it comes to bending parts with a large bending radius and arc angle, roll bending is a more suitable option compared to ordinary bending.
Roll bending involves placing the slab in 2-4 rollers, and with the rotation of the rollers, the slab is bent accordingly.
Moreover, since the position of the roller can be adjusted relative to the slab, it can also be molded into quadrilateral, elliptical, and other non-circular cross-section cylinder parts.
18. The common bending method is not suitable for the bending of long strip
To bend a long strip around the longitudinal axis, it is recommended to use roll forming instead of ordinary bending.
Roll forming involves placing the strip between forming rollers positioned in the front and back.
As the rollers rotate, the strip is fed forward and bent in the axial direction.
Roll forming is capable of producing parts with complex cross-sections.
The manufacturing process for the rollers is simple, resulting in lower costs and longer service life.
19. The general rolling forming method is not suitable for the parts with variable cross section
To reduce investment, rolling forming can be utilized for small and medium batch production of variable cross-section groove parts. In such cases, the forming roll moves not only longitudinally relative to the slab, but also transversely.
20. It is not suitable to use ordinary bending method for bending pipes and profiles
Although the deformation properties of tube and profile bending are similar to that of sheet metal, the methods and difficulties of the bending process are quite distinct.
In the bending of pipes and profiles, it is essential to prevent the shape distortion of the blank section in the bending deformation zone.
In production, the bending methods for pipes and profiles include stretch bending, roll bending, push bending, and winding bending.
It should be noted that the working surfaces of the punch for stretch bending, the roller for roll bending, and the fixed die for push bending and winding bending should be grooved to match the cross-sectional shape of the blank. This prevents rotation of the cross-section and distortion of the shape. If necessary, a corresponding mandrel should be added into the pipe.
Great over view of critical tooling design for straight forward parts!
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