In the __design of stamping die__, reasonable process analysis is needed to avoid design defects and die scrap.

No practical design is shortboard, how to avoid process defects?

Table of Contents

**1****.**** **__Bending radius__** should not be too large**

When the workpiece is bent, besides plastic deformation, there is elastic deformation and springback.

Therefore, the radius of bending parts should not be too large. Otherwise, the bending angle is not stable.

**2****.**** Bending radius should not be too small**

If the bending angle R is too small, the outer fiber will crack easily.

For low carbon steel, the minimum bending radius is about 1.0T;

The minimum bending radius of brass and aluminum is about 0.6T;

For medium carbon steel, the minimum bending radius is about 1.5T.

**3****.**** **__Bending straight edge__** should not be too small**

In order to ensure the bending quality of the workpiece, the straight edge height H of the bending product should not be too small, and must be greater than or equal to the minimum bending height Hmin.

**H****≥****H****min=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 the blank with holes, the edge distance of holes should not be too small.

If the hole position is too close to the bending edge, the shape of the hole will change when bending.

The distance L from the hole edge to the bending edge, L shall conform to the following formula:

**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**

In order to prevent deformation, the height difference of bending parts should not be too large.

The shape and size of the bending parts should be symmetrical as far as possible, otherwise there will be abnormal distortion at the small end.

If this result is difficult to improve in design, it must be ensured that:

**H****>****R****+2****T**

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 edge, in order to avoid tearing at the junction due to stress concentration, it should not be ignored to punch the unloading hole, cut the groove, or shift 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-section shape of the deformation zone will be distorted, that is, the width of the inner surface becomes wider and the width of the outer surface becomes narrower. When the plate width b < 3T (t is the plate thickness), it is especially obvious.

If the width b of the bending part is required to be accurate and the bulging phenomenon is not allowed, it should not be ignored to make the process incision in advance on the bending line.

**8****.**** **__Springback__** cannot be ignored in bending process**

When bending, plastic deformation and elastic deformation exist simultaneously.

When the external load is removed, the elastic deformation disappears and spring back occurs.

The springback value is related to material properties, relative bending radius R / T, bending angle and other factors.

The higher the yield point of materialσs is, the smaller E is, the smaller R/T is and the larger bending angle is, the greater the springback is.

In order to ensure the accuracy of the workpiece, springback can not be ignored in bending.

The springback can be reduced by repairing the die, using different springback directions of different parts, adding triangular ribs locally, and using the concave die structure of swing block.

**9****.**** The workpiece with large bending radius cannot be bent by ordinary method**

For parts with a large bending radius, the ordinary bending method cannot be used.

Otherwise, due to the large elastic deformation, the required shape and size can not be obtained, so the method of stretch bending can be used.

Before bending, an axial tension is applied, which makes the stress in the blank section slightly greater than the yield point of the material, and then bending is carried out at the same time.

**10****.**** The bending of complex shape parts cannot be ****completed**** at one time**

For the bending parts with complex shape, it can not be formed at one time, but needs to be formed many times.

The principle of process arrangement is to bend the outer corner first.

The later bending can not affect the deformation of the previous bending part.

The number of bends can be two, three, or even more.

**11****.**** The edge of bending parts should not be notched**

If the edge part of the notched bending part is punched out of the blank, the fork will appear when bending, and it will not be formed when it is serious.

It is necessary to leave a connecting band at the notch, and cut off the connecting band after bending.

**12****.**** The structure of **__bending die__** can’t ignore the blank offset**

In the bending process, offset is an important factor affecting the accuracy of the workpiece, so before bending, a part of the blank should be reliably fixed on a part of the die to prevent offset during bending.

Holes in the part should be used as much as possible for positioning.

If there is no available hole on the part, process hole positioning can be considered.

**13****.**** The structure of bending die should not cause large local thinning and scratch of material during die closing**

The bending die should pay attention to bending only at the position of the bending curve determined by the part in the die closing process.

The blank should not have large local thinning and scratch.

When the structure in the left figure is used 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, so that the shape of the outer corner of the part is not accurate, and the straight arm part becomes thinner.

**14. The die structure shall not hinder and prevent the blank from turning and moving during the die closing process**

The structure of the bending die should consider the turning space of the blank when bending, and should not hinder and prevent the blank from moving and rotating, otherwise it will affect the shape and size of the workpiece.

This is especially important for the complex shape of multi angle bending.

**15****.**** The material with small elastic modulus should not be used for bending workpiece**

The springback is proportional to the elastic modulus of the material.

The material with small elastic modulus has large elastic recovery after deformation, so it is not suitable for the bending process.

For the material with the same yield point, the material with larger elastic modulus has smaller elastic recovery after deformation.

Annealed low carbon steel is more suitable for bending than soft manganese brass.

**16****.**** The material with high yield point is not suitable for bending workpiece**

The springback is proportional to the yield limit of the material.

The material with a high yield point has large elastic recovery after deformation, so it is not suitable for the bending process.

That is to say, the material with the same modulus of elasticity and high yield point has larger elastic recovery.

Therefore, cold work hardening steel is not suitable for the bending process.

**17****.**** Ordinary bending method is not suitable for bending parts with large bending radius and arc angle**

For the bending parts with a large bending radius and arc angle, roll bending is better than ordinary bending.

__Roll bending__ is to place the slab in 2-4 rollers, and with the rotation of the rollers, the slab is bent.

In addition, because the position of the roller can be changed relative to the slab, it can also be made 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**

The bending of long strip around the longitudinal axis should be formed by rolling instead of ordinary bending.

__Roll forming__ is to put the strip in the front and back row of forming rollers.

With the rotation of the roller, the strip is fed forward and then bent in the axial direction.

Rolling forming can produce parts with complex cross-section.

The manufacturing of the roller is simple, the cost is low and the service life is long.

**19****.**** The general rolling forming method is not suitable for the parts with variable cross section**

In order to reduce the investment, rolling forming can be used in small and medium batch production of variable cross-section groove parts.

In this case, the forming roll not only moves relative to the slab in the longitudinal direction, but also moves in the transverse direction.

**20****.**** It is not suitable to use ordinary bending method for bending pipes and profiles**

Although the __bending of tube__ and profile is the same as that of the sheet from the perspective of deformation properties, the process methods and difficulties are quite different.

The shape distortion of the blank section in the bending deformation zone must be prevented in the bending of pipes and profiles.

In production, the bending methods of pipes and profiles include stretch bending, roll bending, push bending and winding bending.

It should be noted that the working surface of punch for stretch bending, roller for roll bending and fixed die for push bending and winding bending should be made into grooves that are consistent with the cross-section shape of the blank, so as to prevent the rotation of the cross-section and the distortion of the shape. If necessary, the corresponding mandrel should be added into the pipe.