1. Definitions
The cross-sectional area of the punch in a blind die is typically smaller than the size of the die hole. There is a specific space, known as clearance, between the punch and the die. This can be seen in the illustration below.
2. Influence of clearance on the quality of the cutting section
From the analysis of the blanking deformation process, it was determined that when the blanking gap is reasonable, the microcracks produced on the edge of the punch and the die will coincide with each other. This results in a large bright band in the cutting section, a small collapse angle and burrs, a moderate taper of the section, and a relatively flat workpiece surface.
As a result, the quality of blind parts can be achieved satisfactorily, as shown in the figure below.
When molding, if the gap is too small, two distinct bright bands will appear in the blank section and the burr at the top end will be substantial. This is mainly due to the presence of an upper microcrack located at the edge of the punch, which is caused by insufficient blanking clearance.
To avoid this, it is recommended to stagger the position of the lower microcrack on the edge of the die to a certain distance from the position where the upper microcrack occurs (see Fig. 2.3-2). By doing this, the top and bottom cracks will be no heavier than a line.
As the punch decreases, the material sandwiched between the two fissures will undergo a second shear, resulting in the formation of a second bright band and further elongation of the burr, leading to poor section quality.
On the other hand, if the gap during cutting is too large, the upper microcrack will occur at the punch edge, and the position of the lower microcrack at the die edge will be staggered inward for a certain distance, so that the upper and lower cracks they will not be heavier than a thread.
The material sandwiched between the two cracks will be greatly stretched as the punch shrinks, eventually tearing and breaking. This will result in a large fracture zone in the blind section, making the bright zone smaller and the burr and taper larger. The angle of collapse will increase, further deteriorating the quality of the section (see Fig. 2.3-4).
Based on the analysis, it can be concluded that even if an appropriate clearance value is chosen in the die design, it cannot guarantee a uniform distribution of the die clearance due to processing or assembly problems. As a result, ideal section quality and a side with a small gap are unlikely to be achieved.
As noted previously, if the gap is too small, the section will exhibit the characteristic of a small gap, and if the gap is too large, the section will exhibit the characteristic of a large gap, which is particularly pronounced in dies without guide posts. .
Therefore, it is important to be aware of this during production.
3. Impact of authorization on other aspects
(1) Influence of cutting clearance on dimensional cutting accuracy
As discussed previously, both elastic and plastic deformations occur in metal parts during blanking. This means that elastic deformation must occur when plastic deformation is occurring.
Due to the elastic deformation of the material during blanking, there will be a recovery of this elastic deformation after the blanking process is completed. This recovery will result in a certain deviation between the actual size of the molded part and the size of the edge of the punch and die (see Fig. 2.3-5).
The vertical axis in the figure represents the elastic recovery of the blind part, while the horizontal axis represents the relative clearance of the blind part.
During blanking, as the blanking gap increases, the size change curve of the blank part reveals that the tensile deformation of the deformed metal also increases due to the increase in tensile stress in the deformed area.
After cutting, the compressed metal will elastically recover, leading to a reduction in the size of the blunt part. This recovery increases with increasing suppression clearance.
As the blanking gap gradually decreases, the size of the blind part also decreases. When the clearance is reduced to a certain extent (point B in Fig. 2.3-5), the deformation properties of the blind part also change. In addition to shear, there is extrusion deformation in the material, causing the deformation zone to move from a state of tension to a state of compression.
After cutting, the compressed metal will regain its elasticity, making the punch size larger than the die edge size.
During puncture, the same deformation process and elastic recovery principle apply, but the measured objects are different. As a result, the conclusion is opposite to that of blunt parts, which means that the size of punched parts increases with increasing cutting clearance.
When the clearance value is less than a certain value (point A in Fig. 2.3-5), the size of the punched part will decrease, meaning that the size of the drilled hole is smaller than the size of the punch.
It is important to note that the dimensional accuracy of molded parts mainly depends on the design and machining accuracy of the molding die. The above analysis was carried out under a certain manufacturing accuracy of the die, and the impact of backlash on accuracy is much smaller compared to the die itself.
(2) The impact of suppression clearance on suppression force
The smaller the gap, the greater the compressive stress component in the deformation zone of the material, leading to greater resistance to deformation of the material and an increase in the cutting force required during cutting. On the other hand, the greater the gap, the greater the tensile stress component in the deformation area of the material, reducing the deformation resistance of the material and the cutting force required during cutting.
However, practical experience shows that when the gap (on one side) gradually increases within the range of 5% to 2% of the material thickness, there is no significant decrease in cutting force.
(3) Influence of blind clearance on unloading force and thrust force
The smaller the gap, the greater the elastic recovery of the material in the deformation zone, causing the size of the punched part to be smaller and the size of the molded part to be larger. As a result, the discharge force and thrust force increase.
As the gap increases, due to reduced elastic recovery of the material, the size of the punched part increases and the size of the molded part decreases, making it easier to discharge material from the punch or expel parts from the die opening.
Typically, when the gap (on one side) increases to 10% to 20% of the material thickness, the unloading force is close to zero.
(4) Influence of cutting clearance on die life
Practical experience has shown that among the many factors affecting die life, cutting clearance is the most important.
During the cutting process, intense friction occurs between the punch and the drilled hole and between the die and the blank. The smaller the gap, the more severe the friction, which is extremely detrimental to the service life of the die.
However, a larger clearance will reduce the friction between the edge of the punch and the die and material, and can mitigate the adverse effects of uneven clearance caused by manufacturing and installation errors of the die, thereby improving its service life.
4. Determination of clearance value
The term “reasonable clearance” refers to a clearance that allows satisfactory part section quality, high dimensional accuracy, minimizes cutting force (discharge force and thrust force), and results in long die life when used for cutting.
However, it is not possible to satisfy all these requirements simultaneously using a single gap value. Therefore, in production, it is necessary to comprehensively consider the influence of various factors and select an appropriate clearance range as a reasonable clearance based on the specific requirements of the parts.
The upper limit of this range represents the maximum reasonable range and the lower limit represents the minimum reasonable range. In other words, a reasonable gap refers to a range of values.
When designing the die, it is recommended to select the clearance based on the specific part and production requirements, following the following principles.
(1) When there are no special requirements for the cross-sectional quality of the countercut part, a larger gap value can be selected to improve the die life and reduce the cutting force for greater economic benefits.
(2) When there are high requirements for the cross-sectional quality of countercut parts, a smaller clearance value should be selected.
(3) When designing the size of the cutting edge of the cutting die, it must be taken into account that the die will suffer wear during use, which will increase the clearance of the cutting edge. The size of the cutting edge must be calculated based on the minimum gap value.
In practice, the die industry has accumulated a large number of empirical values for stamped parts with different thicknesses of various stamping materials, so the theoretical gap calculation method is only used as a reference.
