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> Defects and countermeasures that are easy to appear in the process of punching & flanging of machined parts!

2021/07/06

The punching and flanging of stamping products is generally mainly for the next step of tapping or other processes. A series of problems such as cracks, burrs, deformation, etc. often occur in the process of general flanging and flanging. In fact, punching and flanging is very simple. Pay attention to these points to turn out beautiful edges:

**(1) The degree of deformation of the inner hole flanging of the ****stamping part****s hould not be too large**

Punching and flanging is a punching method in which a hole is punched in advance on a blank (sometimes it may not be punched in advance), and a vertical flange is formed along the edge of the hole. The leading and mandatory deformation of the hole flanging is the tensile deformation of the material along the tangential direction, and the closer it is to the mouth, the greater the deformation and the greater the thinning. Therefore, the defect of hole edge cracking is prone to occur. In order to prevent the edge of the hole from cracking, the degree of deformation of the inner hole flanging of the stamping part should not be too large. If the flanging height is large, it can be divided into several flangings.

**(2) The flanging coefficient of punching and flanging should not be too small**

In punching and flanging, the degree of deformation is expressed by the ratio of the aperture before flanging to the aperture after flanging, that is, the flanging coefficient K. Obviously, the larger the K value, the smaller the degree of deformation, the smaller the K value, the greater the degree of deformation, and the more likely the edge of the burr hole is to crack. The full value of the maximum degree of deformation that can be achieved without breaking the edge of the hole during flanging is called the allowable limit flanging coefficient.

In order to prevent edge cracking, the flanging coefficient of hole flanging should not be too small, and should be greater than the limit flanging coefficient. Production practice shows that the limit flanging coefficient is not only related to the type and performance of the material, but also to the processing properties and state of the prefabricated hole (drilled or punched, with or without burrs), the relative thickness of the blank, and the shape of the flanging punch, etc. The factor is related.

The maximum deformation degree (also known as the hole drilling coefficient K) is related to many factors, but in actual production calculations, the maximum elongation δ can be used to approximate the limit hole drilling coefficient, and the degree of deformation of the prefabricated holes can be used to preliminarily judge whether cracking defects will occur. The formula is as follows:

During the process of boring (see Figure 2), the material deformation zone mainly undergoes elongation deformation and thinning in the tangential direction, while the radial deformation is not large, so a simple bending method can be used to keep the length of the neutral layer unchanged. The principle is to determine the diameter of the prefabricated hole approximately, and the formula is as follows:

In the formula, d is the diameter of the prefabricated hole, D is the diameter of the neutral layer of the vertical side after drilling, H is the height after drilling, r is the radius of the fillet, and t is the material thickness in mm.

**(3) The flanging height of punching and flanging should not be too large**

The height of punching and flanging should generally not be greater than the limit value, otherwise, the edge of the flanging will be easy to crack. If the height of the stamped part is required to be greater than the limit value, it cannot be directly flanged at one time. At this time, if it is a small hole flanging of a single blank, a flanging with a thinner wall should be used, such as a self-tapping screw for flanging. If it is a large hole flanging, use the method of deep drawing, punching the bottom hole and then flanging.

The hole turning height is the main performance index of the part, and the formula of the limit hole turning height can be derived from the formula (1) and formula (2):

**(4) The pre-holes of punching and flanging should not have large burrs**

The processing quality of the punching and flanging pre-hole has a greater influence on the limit flanging coefficient. Pre-holes that are deburred after drilling have a small limit flanging coefficient, which is beneficial to flanging. For pre-holes punched with a punching die, if there are burrs, the limit flanging coefficient is large, which is unfavorable for flanging. At this time, if the required flanging coefficient is small, it is very easy to cause the flanging to crack. Take the side with the burrs up, and then carry out the flanging to reduce the phenomenon of flanging cracks.

**(5) The fillet radius of the punching and flanging punch should not be too small**

For flanging with prefabricated holes, the corner radius of the flanging punch should be as large as possible, preferably spherical or parabolic. In this way, the hole turning force is small, and the hole turning quality is also good.

**(6) The gap between the ****punch** **and the die of the hole flanging should not be too large**

In order to avoid or reduce shrinkage, the gap between hole flanging convex and concave die should not be too large. If the mold gap is too large, the material does not move close to the die during flanging, resulting in greater shrinkage, and residual bending deformation may occur, which will affect the flanging quality of the part.

**(7) When turning the hole, the thickness of the vertical side mouth can not be ignored**

When turning the hole, the deformation zone is basically limited within the radius of the die. Under the action of unidirectional or bidirectional tensile stress, the tangential elongation deformation of the material in the deformation zone is greater than the radial compression deformation, resulting in a thinner material. The thinning of the vertical edge of the hole is the largest. When the thickness is too thin and the material elongation exceeds the ultimate elongation of the material, the so-called p-cracking (cracking caused by excessive elongation and insufficient plasticity of the material is called Force anus rupture; the cracking caused by excessive forming force and insufficient material strength is called a rupture). When punching and flanging, the smaller the flanging coefficient K value, the greater the degree of deformation, and the greater the thickness of the vertical edge mouth is reduced, and the easier it is to break. Therefore, the thinning of the vertical edge mouth thickness cannot be ignored during hole drilling.