The punch cross section of the blanking die is generally smaller than the die hole.
There is an appropriate gap between the punch and the die, which is called the clearance. See figure below.
2. Influence of clearance on the quality of cutting section
From the analysis of blanking deformation process, when the blanking gap is reasonable, the upper and lower microcracks produced at the edge of punch and die can coincide with each other. In this way, the bright band of the blanking section is large, the collapse angle and burr are small, the section taper is moderate, and the part surface is relatively flat.
The quality of blanking parts can achieve satisfactory results. See figure below.
During blanking, if the clearance is too small, two bright bands will appear on the section of the blanking part, and the burr at the upper end is also large.
This is mainly due to the location of the upper microcrack at the edge of the punch due to the too small blanking clearance.
Stagger a certain distance from the position where the lower microcrack occurs at the edge of the die (see Fig. 2.3-2), so that the upper and lower cracks cannot be heavier than one line.
The material sandwiched between the two cracks will produce the second shear as the punch decreases, so a second bright band will be formed, and the burr will be further elongated, resulting in poor section quality.
If the clearance is too large during blanking, the position where the upper microcrack will occur at the edge of the punch.
The position of the lower microcrack at the edge of the die is staggered inward by a certain distance, so that the upper and lower cracks can not be heavier than one line.
The material sandwiched between the two cracks is greatly stretched with the decline of the punch, and finally torn and broken.
A large fracture zone appears on the blanking section, making the bright zone smaller and the burr and taper larger.
The collapse angle increases and the section quality is worse (see Fig. 2.3-4).
From the above analysis, it can be inferred that even if a reasonable clearance value is selected in the die design, the uniform distribution of the die clearance cannot be guaranteed due to the processing or assembly of the die.
In the same way, the ideal section quality and the side with small gap will not be obtained.
As mentioned above, the section feature of too small gap will appear, and the section feature of too large gap will appear on the side with large gap, which is particularly prominent for the die without guide post.
This must be paid attention to in production.
3. Impact of clearance on other aspects
(1) Influence of blanking clearance on blanking dimensional accuracy
As we mentioned earlier, elastic deformation and plastic deformation will occur in metal parts during blanking.
This means that there must be elastic deformation when the material is plastic deformation.
Because there is elastic deformation in the material during blanking, the elastic deformation of the material will recover after blanking.
The recovery of this elastic deformation will cause a certain deviation between the actual size of the blanking part and the edge size of the punch and die. See fig. 2.3-5 below.
The ordinate in the above figure is the elastic recovery of the blanking part.
The abscissa is the relative clearance of the blanking part.
During blanking, we can find from the size change curve of the blanking part that when the blanking clearance gradually increases, the tensile deformation of the deformed metal also increases due to the increase of the tensile stress at the deformed part.
After blanking, the compressed metal will recover elastically, so as to reduce the size of blanking parts.
This rebound increases with the increase of blanking clearance.
When the blanking clearance decreases gradually, the size of the blanking part also decreases. When the clearance is small to a certain extent (point B in Fig. 2.3-5), the deformation property of the blanking part will also change.
In addition to shear, there is extrusion deformation in the material, which changes the deformation zone from the original tensile state to the compressive state.
After blanking, the elasticity of the compressed metal will recover, so that the size of the blanking punch is larger than that of the die edge.
During punching, the deformation process and elastic recovery principle are the same as those above, but the measured objects are different.
Therefore, the conclusion is opposite to that of blanking parts, that is, the size of punching parts increases with the increase of blanking clearance.
When the clearance value is less than a certain value (point a in figure 2.3-5), the punching size will decrease, that is, the size of punching hole is less than that of punch.
It is pointed out that the dimensional accuracy of blanking parts mainly depends on the design and machining accuracy of blanking die.
The above analysis is carried out under a certain die manufacturing accuracy, and the influence of clearance on the accuracy is much smaller than that of the die itself.
(2) The impact of blanking clearance on blanking force
The smaller the gap, the greater the compressive stress component in the material deformation zone, the greater the deformation resistance of the material and the greater the blanking force required during blanking.
On the contrary, the larger the gap, the greater the tensile stress component in the material deformation area, which reduces the material deformation resistance and the blanking force required during blanking.
However, practice has proved that when the gap (single side) gradually increases in the range of 5% ~ 2% of the material thickness, the reduction of blanking force is not obvious.
(3) Influence of blanking clearance on unloading force and pushing force
The smaller the gap, the greater the elastic recovery of the material in the deformation zone, making the punching size smaller and the blanking size larger, so the unloading force and pushing force increase.
When the gap increases, due to the elastic recovery of the material, the punching size increases and the blanking size decreases, so it saves labor to unload the material from the punch or push out the parts from the die opening.
Generally, when the gap (single side) increases to 10% ~ 20% of the material thickness, the unloading is almost zero.
(4) Influence of blanking clearance on die life
Practice has proved that blanking clearance is the most important factor among many factors affecting die life.
In the blanking process, there is fierce friction between the punch and the punched hole, and between the die and the blanking part, and the smaller the gap is, the more serious the friction is, so too small gap is extremely unfavorable to the service life of the die.
The larger clearance will reduce the friction between the side of the edge of the punch and the die and the material, and can slow down the adverse impact of the uneven clearance caused by the manufacturing and installation error of the die, so as to improve the service life of the die.
4. Determination of clearance value
The so-called reasonable gap means that when this gap is used for blanking, it can obtain satisfactory workpiece section quality, high dimensional accuracy, minimize the blanking force (unloading force and pushing force), and make the die have a long service life.
However, it is impossible to meet many of the above requirements at the same time if the numerical requirements of one gap are adopted.
Therefore, in production, according to the specific requirements of parts, comprehensively consider the influence of various factors, and appropriately select an appropriate gap range as a reasonable gap.
The upper limit is the maximum reasonable gap and the lower limit is the minimum gap, that is, a reasonable gap refers to a range value.
In the specific design of the die, it can be selected according to the following principles according to the specific requirements of parts and production.
(1) When there are no special requirements for the cross-section quality of the counter cutting part, in order to improve the service life of the die and reduce the blanking force to obtain greater economic benefits, a larger clearance value can be selected.
(2) When there are high requirements for the cross-section quality of counter cutting parts, the smaller clearance value shall be selected.
(3) When designing the cutting edge size of blanking die, considering the actual situation that the die will be worn in the process of use, which will increase the cutting edge gap, the cutting edge size should be calculated according to the minimum gap value.
In the actual work, the die industry has accumulated a large number of empirical values for stamping parts with different thickness of various stamping materials, so the theoretical gap calculation method is only used as a reference.