Blanking Clearance: How To Select And Abnormal Handling Method

By analyzing the blanking process of the stamping die, the stress of the sheet, the deformation mechanism and the state of the formed section, this paper discusses the influence of different clearance on the blanking section and the influence of the blanking clearance on the quality of the blanking parts, so as to show the die blanking process and its abnormalities more specifically, and explore how to select the appropriate blanking clearance, then conduct targeted treatment on the exception of hedging.

Blanking Clearance

Blanking process and section analysis

The cross-section of stamping parts has regional characteristics

Fig. 1 shows the stress diagram of the stamping die when stamping the plate.

stress diagram of stamping die when stamping plate

Fig. 1 Stress diagram of stamping die when stamping plate

During blanking, the plate produces complex deformation (bending, stretching, shearing and extrusion) under the action of vertical pressure, transverse extrusion pressure, friction, bending moment and tension.

The main process includes the elastic deformation stage, plastic deformation stage and shear fracture separation.

The result is that there are obvious regional characteristics at the section of the stamping part, which can be divided into four parts: fillet zone, plastic shear zone (bright zone), shear crack zone (fracture zone) and burr, as shown in Fig. 2.

Fig. 2 the section of stamping parts has obvious regional characteristics

The fillet band, i.e. the corner collapse band, is the side where the sheet is in contact with the die.

At the beginning of blanking, it is squeezed into the hole of the die.

It is formed due to the action of tension and gradually increases with the increase of stress until cracks appear;

It is the free surface part pressed down when the die is cut in;

When the blanking clearance increases, the corresponding angle collapse zone will also increase accordingly.

Plastic shear band (bright band) is formed by plastic deformation caused by die extrusion and cutting materials.

It is the part with the best quality on the whole section.

Its proportion in the whole section area will vary with the difference of sheet material, die gap, sharpness of cutting edge and working conditions. 

Among them, the structural form of the die and the gap between the male and female die has the most obvious influence on the quality of the cut section.

When the blanking gap increases, the width of the plastic shear band will decrease accordingly.

The shear crack zone, also known as the rough zone, is formed by the continuous propagation and fracture of the microcrack at the edge under the action of tensile stress.

Its surface is rough and tapered.

When the gap value of the die is appropriate, the microcrack near the edge of the concave-convex die continues to develop, and finally the surface is rough and lusterless;

Since this part is broken due to cracks, a grain-like surface appears here.

The rough shear band increases with the increase of blanking clearance;

However, to a certain extent, there will be a decreasing trend.

Burr area is an irregular tearing burr formed around the fracture zone, and the proportion of each part changes with the difference of material performance, material thickness, sharpness of cutting edge, die structure and concave convex die gap.

For materials with poor plasticity, the fracture tendency is serious, the fracture zone is widened, the proportion of bright zone fillet zone is small, and the burr is also small;

Materials with good plasticity are on the contrary.

To improve the section quality of blanking parts, it can be realized by increasing the height of the bright belt and increasing the shaping process.

Increasing the height of the bright band is achieved by prolonging the plastic deformation stage and delaying the generation of cracks.

At the same time, because this area is a stress concentration area, it is also an important parameter index to measure the blanking quality of sheet metal.

The die clearance is adjusted through the presentation form of sheet metal blanking section and burr, so as to obtain better blanking sheet metal.

Influence of blanking gap on blanking section

Blanking clearance is the main factor affecting the section quality.

Only when the blanking clearance of the concave-convex die is appropriate, the upper and lower cracks can coincide.

At this time, the section slope of sheet metal is very small, relatively straight, the surface is smooth, the burr is small and there is no crack delamination.

Different blanking clearances have different effects on the sheet section.

Die clearance is too small

The crack generated by the material near the edge of the punch enters the compressive stress area under the punch and stops developing so that the crack generated by the material near the edge of the punch and die does not coincide.

At this time, the section material has a secondary tensile crack at the place where the distance between the two cracks is close, resulting in the second bright band – double bright band, and some materials are extruded to form burrs on the surface.

Or there is no second bright band, but there are intermittent small bright blocks on the fracture surface, that is, the bright band on the section of the workpiece is too wide and there are toothed burrs. The section and burrs are shown in Figure 3.

Excessive die clearance

The tensile stress in the material will make the tensile fracture occur prematurely, which makes the fracture surface wide and the bright band narrow.

When the gap is too large, the cracks do not coincide.

The slope of the fracture layer caused by the second tensile crack will increase the two slopes of the blanking section.

Sometimes there is an obvious concave shape in the middle of the section, and the burr is not only high but also triangular, that is, the section of the workpiece is rough, the fillet is large, the bright band is small, and there are elongated burrs.

The burr and section are shown in Fig. 4.

Uneven die clearance

The corresponding blanking plate will also have part of the gap is too large or too small, that is, the bright belt of the workpiece section is uneven or there are inclined burrs on one side.

Fig. 3 cross section and burr caused by too small gap

The burr condition and section are shown in Fig. 5.

 burr and section caused by excessive clearance

Fig. 4 burr and section caused by excessive clearance

Fig. 5 burr and section caused by uneven clearance

Sharpness of die edge

The sharpness of the die edge will also have an impact on its blanking process.

When the die edge is worn into a fillet and becomes blunt, the contact area between the edge and the material increases, the stress concentration effect is reduced, and the extrusion effect is increased, thus delaying the generation of cracks.

Therefore, the fillet of the workpiece is large, the bright bandwidth is wide, but the burr height also increases.

Selection of blanking clearance value

In the stamping die, the blanking size depends on the size of the female die (as the reference side), and the gap value is taken out from the male die;

The punching size depends on the punch size (as the reference side), and the clearance value is taken out of the die.

The so-called reasonable gap is that when using this gap for blanking, it can obtain satisfactory workpiece section quality, high dimensional accuracy and 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 and comprehensively considering the influence of various factors, the appropriate gap range is selected as the reasonable gap, the upper limit is the maximum reasonable gap and the lower limit is the minimum gap.

Reasonable clearance refers to a range value, which 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, and 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 the blanking die, considering that the die will be worn during use, which will increase the cutting edge gap, the cutting edge size should be calculated according to the minimum gap value.

Because the calculation method is inconvenient to use in production, the look-up table selection method is widely used at present.

Application scope of different blanking clearance:

(1) Small clearance.

It is required to have the largest straight wall section and the smallest collapse angle, no secondary processing and high requirements for molds.

(2) Small clearance.

It is applicable to the process of trimming after blanking forming.

(3) Medium clearance.

Due to the small residual stress, it can reduce deformation and fracture. It is suitable for occasions requiring continuous bending, stretching, forming and flanging.

(4) Large clearance.

It is required to have the highest die life, but it is not required to have a large straight side section.

(5) Large clearance.

Where the service life of the die is required to be long and the quality of the cut section is not required to be high.

Concluding

Through the stress form of sheet metal and the formation process of each area of the blanking section in the blanking process, it can be understood that the blanking gap is a very important parameter in the blanking die.

The value and uniformity of the gap directly affect the quality, dimensional accuracy and die life of the workpiece.

In order to ensure the service life of the die and improve the quality and accuracy of the product, a reasonable gap value must be adopted, and the uniformity of the gap must be adjusted and controlled.

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