How to Determine and Optimize the Blanking Clearance?

Blanking is a stamping process in which a part of the closed contour of a plate is separated from another part by using a die.

Blanking clearance refers to the difference between the dimensions of the upper die edge and the lower die edge in the blanking process.

It is an important technical parameter in die design, manufacturing and production.

Therefore, in order to meet the requirements of die life and the quality of blanking parts and improve production efficiency, it is necessary to manage and optimize the blanking gap in actual production.

Analysis of blanking deformation process

The blanking deformation process can be roughly divided into three stages: elastic deformation stage, plastic deformation stage and fracture stage.

The stress state of the plate in this process is shown in Fig. 1.

Stress Analysis of plate during blanking deformation

Fig. 1 Stress Analysis of plate during blanking deformation

In the figure,

  • FC is the shear force acting on the upper die
  • FC ‘is the shear force acting on the lower die;
  • FL is the transverse force acting on the upper mold, FL ‘is the transverse force acting on the lower die;
  • μ is the friction coefficient;
  • FH is the horizontal component force received by the plate from the upper die edge, and FH’ is the horizontal component force received by the plate from the lower die;
  • FV is the vertical component force received by the plate from the upper die edge, FV ‘is the vertical component force received by the plate from the lower die;
  • Mg is the bending moment of the plate;
  • I is the lever arm;
  • C is the blanking clearance.

Elastic deformation stage

At this stage, after the edge of the upper die contacts with the plate, the plate is flattened first, and then the edge of the upper die and the lower die are pressed into the plate respectively.

Due to the blanking gap C, the resultant force of the upper die and the resultant force of the lower die are not collinear, so the plate will receive a bending moment Mg’, which makes the plate slightly bend under elastic compression.

As the upper die continues to descend, the stress on the cutting edge of the material will reach the elastic limit.

Plastic deformation stage

With the continuous downward movement of the upper die, the stress of the plate increases, and the stress of the material reaches the yield limit, resulting in plastic deformation.

With the increase of the degree of plastic deformation, the tensile stress and bending moment in the plate continue to increase, the hardening of the material intensifies, and the material near the edge first reaches the strength limit.

Fracture stage

The pressing depth of the upper die continues to increase, and the cracks first appear on the sides of the upper and lower die edges.

At this time, the strain energy stored in the elastic and plastic deformation stage is released, and it extends to the interior of the material along the direction of the maximum shear stress.

When the main cracks at the upper and lower die edges coincide, the material is cut off and separated.

If the knife edge gap is unreasonable and the two main cracks cannot coincide, a third main crack will appear.

Influence of blanking clearance on section and its selection

According to the above analysis of blanking deformation process, the section of blanking parts mainly includes collapse angle R, bright zone B, fracture zone and burr h, and has fracture angle α, as shown in Fig. 2.

section composition of blanking parts

Fig. 2 section composition of blanking parts

Influence of blanking clearance on angle collapse

In the elastic deformation stage, the free surface formed by the material near the cutting edge being pulled into the blanking gap is the collapse angle, and its height increases with the increase of the blanking gap.

Due to the existence of blanking clearance, the resultant force from the upper and lower die edges of the plate is not in a straight line, so a bending moment mg is generated.

The larger the blanking gap is, the greater the bending moment of the plate is, the greater the bending effect of the plate is, and the height of the collapse angle increases accordingly.

Influence of blanking clearance on bright strip

In the plastic deformation stage, due to the shear and bending tensile deformation of the plate at the cutting edge, which is mainly shear, a bright band is formed. Its surface is smooth and has good perpendicularity, which is an ideal plate section shape.

The height of bright strip decreases with the increase of blanking clearance.

However, when the blanking gap is too small, the upper and lower main cracks will not coincide, and a long and narrow second bright belt will be formed.

There are long burrs, uneven toothed edges and small cones on the second bright belt, which will form debris that is very easy to peel off and bring it into the subsequent process to produce poor indentation, which is one of the main sources of actual production time loss.

The increase of blanking clearance will increase the tensile and bending effect of the plate, so the shear effect is relatively weakened, the plate is easy to be pulled apart to form a fracture zone, and the height of the bright zone is also reduced.

Influence on burr

The burr height increases slowly at first and then continuously with the increase of blanking clearance.

In the fracture stage, because the cracks appear from the side of the cutting edge rather than in the middle of the blanking gap, the section is bound to produce burrs.

When the blanking gap is less than a reasonable value, the upper and lower main cracks of the plate will not coincide, resulting in small and difficult to peel burrs;

When the blanking gap is greater than a reasonable value, the plate is brought into the blanking gap by tension and bending.

The main crack appears on the side relatively far away from the cutting edge, and then it is broken.

Therefore, the burr height is large, which is also one of the main reasons for burr generation and the main source of time loss in actual production.

Selection of blanking clearance

According to the above analysis of the part section quality and the relevant literature, the relationship between the blanking clearance and the die life and the part section quality is shown in Fig. 3.

Weighing the quality of the part section and the service life of the die has also become a factor to be considered when selecting the blanking clearance.

Among them, α is the relative blanking clearance with the best section quality of parts, βis the relative blanking clearance with good part section, γis the relative blanking clearance with good die life, δis the relative blanking clearance with the best die life.

Effect of relative blanking clearance on section quality and die life

Fig. 3 Effect of relative blanking clearance on section quality and die life

The relative blanking clearance can be expressed by formula (1), that is, the relationship between blanking clearance and plate thickness:

C=xt (1)

Where, C is the blanking clearance (mm); x is the proportion coefficient; t is the thickness of the plate (mm).

According to the experience value of actual production, under the condition of balancing the section quality of parts and the service life of molds, it is recommended to use x=6% ~ 8% when the body cover is steel plate, and x=10% when the body cover is aluminum plate.

Further reading:

A method of quickly measuring and judging blanking clearance in practical production

Measurement of blanking clearance

There are many kinds of methods to measure the blanking clearance.

For example, a feeler gauge can be used for measurement, but the measurement efficiency of this scheme is low when measuring the more complex blade shape, and it is difficult to measure the internal blade, so the operation efficiency of this method is low.

In actual production, it is necessary to use a relatively fast and simple method to measure the blanking clearance.

Here is a method that can be used in production.

In this post, the body covering parts mostly use plates with a thickness of about 0.7mm, so the test is carried out with steel plates with a thickness of 0.7mm.

The material to be used is 0.06mm gap test paper and red lead coating, as shown in Fig. 4. Generally, the thickness of brushing a layer of red lead coating is 0.01mm to 0.02mm.

measuring tools

Fig. 4 measuring tools

First, determine the reasonable blanking clearance range.

According to the conclusion above, taking the proportional coefficient x=6% ~ 8%, it can be calculated that the reasonable range should be 0.04mm to 0.06mm.

Then remove the pressing plate and install the mold on the press, select the measuring point on the lower mold, and paste the gap test paper evenly on the measuring point, as shown in Fig. 5.

pasting of clearance test paper of blanking edge of a die

Fig. 5 pasting of clearance test paper of blanking edge of a die

Next, apply a layer of red lead coating evenly on the upper mold.

As for the recording of offset cutting clearance, it is recommended in this paper to record according to the insert number of the upper die edge, as shown in Fig. 6, so as to avoid confusion in the recording of data and reduce the authenticity of data collection.

data recording method
(a) No. of upper die blade insert
No.LeftMiddleRight
2080.350.350.35
2070.350.350.35
2060.350.350.35
2030.350.350.35
2040.40.40.4
2050.350.350.35
(b) Blanking clearance data recorded according to blade number in actual production

Fig. 6 data recording method

Finally, operate the press machine to inch a stroke at the actual production speed, and visually observe the adhesive tape state to judge the blanking clearance.

The operation steps are summarized in table 1.

Table 1 operation steps of gap measurement

NO.stepoperation
1Calculate reasonable clearanceSteel plate: x=6% ~ 8%; Aluminum plate: x=10%.
3Unloading the pressing plateUnload the pressing plate and load the die on the press.
4Selection of measuring points of gap test paperThe blanking edge of the lower die shall be evenly pasted with gap test paper, and the segment record shall be made according to the number of the blanking edge insert of the upper die.
5Apply red lead coatingEvenly brush a layer of red lead coating on the upper formwork, with the thickness increased by 0.01~0.02mm.
6Press inchingAdjust the target height of the slider to the bottom dead center, inch the actual production speed by one stroke, and visually observe the adhesive tape state.

Judgment of blanking clearance

After completing the data measurement, it is necessary to read and judge the data.

The judgment is based on the state of the gap test paper on the edge.

The blanking gap can be roughly determined according to the visualization of the adhesive tape state.

The judgment method is shown in Table 2.

It is worth mentioning that the change of plate thickness will cause the error of the result within a certain range.

If the variation range of plate thickness is 0.7mm ± 0.05mm, the error can be ignored;

If the variation range of plate thickness exceeds this value, the results calculated in table 2 should be reconsidered.

Table 2 judgment standard of blanking clearance

NO.Red lead situationTape conditionGap range (mm)diagrammatic sketch
1Red lead completely scraped against the edge of the lower dieThe tape is completely crushed.0.03~0.04How to Determine and Optimize the Blanking Clearance? 1 
2Red lead scraping to the edge of the lower diePoor integrity of adhesive tape0.05~0.06How to Determine and Optimize the Blanking Clearance? 2 
3Red lead scrapes against the edge of the lower dieTape intact0.06~0.07How to Determine and Optimize the Blanking Clearance? 3 
4The red lead did not scratch the edge of the lower dieTape intact>0.07How to Determine and Optimize the Blanking Clearance? 4 

Section quality analysis and clearance optimization

Analysis of punching section

Record the data of the measured blanking clearance, as mentioned above.

At present, the reasonable clearance value of the steel plate used in the test should be 0.04mm to 0.06mm, but to further determine the optimal value of the blanking clearance, it is necessary to analyze the section of the plate.

The tool used in this article is model peak2008-50 × 50 times magnifying glass is shown in Fig. 7, and its parameters are shown in Table 3.

Table 3 magnifying glass parameters of peak2008-50 × 50

Parameters TypeMagnificationMinimum scale  Field of viewMeasuring range
value2008-50 ×50 ×0.02mm1.6mm1.6mm
magnifying glass parameters of peak2008-50 × 50

Fig. 7 magnifying glass parameters of peak2008-50 × 50

In this post, the influence of blanking clearance on the quality of plate section is analyzed.

The stainless steel blank with thickness of 0.7mm is used, and the sections with blanking clearance of 0.03mm, 0.04mm, 0.05mm, 0.06mm and 0.07mm are taken in turn, that is, five groups of data with relative blanking clearance of 4.3%, 5.7%, 7.1%, 8.5% and 10.0% are used for analysis. 

Use peak2008-‘s 50x magnifying glass to photograph its section and take the collapse angle height R, bright band height B and burr height h as the analysis indicators to obtain the relationship between the plate and the above analysis indicators under the conditions of different blanking gaps. The results are shown in Table 4.

Take the section with a 50x magnifying glass and take the collapse angle height R, the bright band height B and the burr height h as the analysis indicators to obtain the relationship between the plate and the above analysis indicators under different blanking clearance conditions.

The results are shown in Table 4.

Table 4 section analysis index of blanking parts

Blanking clearance (mm)Relative blanking clearance (%)Angle collapse height R (mm)Height of bright band B (mm)Burr height h (mm)Plate section photo
0.034.30.040.560.01How to Determine and Optimize the Blanking Clearance? 5 
0.045.70.050.460.02How to Determine and Optimize the Blanking Clearance? 6 
0.057.10.050.340.02How to Determine and Optimize the Blanking Clearance? 7 
0.068.50.060.280.02How to Determine and Optimize the Blanking Clearance? 8 
0.0710.00.090.160.04How to Determine and Optimize the Blanking Clearance? 9 

Draw the five groups of measured data into a scatter diagram and make regression.

It can be seen from Fig. 8 that with the increase of the blanking gap, the height of the collapse angle also increases.

The reason is as mentioned above.

As the blanking gap becomes larger, the bending moment of the plate becomes larger, and the bending and stretching effects become larger, making the height of the fillet belt become larger.

Influence of blanking clearance on collapse height R

Fig. 8 Influence of blanking clearance on collapse height R

As can be seen from Fig. 9, the height of bright band decreases with the increase of blanking clearance.

The bright strip is smooth, flat and perpendicular to the plate, which is an ideal section for blanking.

The reason for the decrease is that the shear action of the plate is weakened and it is easy to be torn to form a fracture zone, which increases the height of the fracture zone.

Effect of blanking clearance on height B of bright strip

Fig. 9 Effect of blanking clearance on height B of bright strip

As the blanking gap decreases, the height of the bright band increases, which is due to the weakening of the bending and tensile effects on the plate, the strengthening of the shear effect, and the extension of its plastic deformation stage.

In addition, under this gap condition, the upper and lower main cracks cannot coincide, resulting in secondary separation.

The blanking part forms a second bright band through friction on the side wall of the lower die, and the surface of the second bright band is easy to be stripped, as shown in Fig. 10.

This kind of surface will be peeled off and partially attached to the surface of the pressing plate when the subsequent process is stressed, and the debris will leave an indentation on the plate when the mold is in the next stroke.

The generation of poor indentation will produce a lot of failure time, which will greatly reduce the production efficiency.

plate section with blanking clearance of 0.03mm

Fig. 10 plate section with blanking clearance of 0.03mm

It can be seen from Fig. 11 that with the increase of blanking clearance, the burr height also increases.

Burr is one of the poor quality of blanking parts, which will affect the normal use of blanking parts.

According to the previous analysis, when the blanking gap is small, the upper and lower cracks of the plate will coincide with each other in the direction of the maximum shear stress, and the burr height is small, which is easy to remove.

When the blanking gap is large, the bending and stretching of the sheet metal increase, and the cracks are easy to be generated on the side slightly away from the cutting edge of the upper and lower dies.

The sheet metal is easy to be torn, so the burr height is large and difficult to remove.

Burr will waste a lot of production time and reduce production efficiency, which is also one of the important management projects.

effect of blanking clearance on burr height h

Fig. 11 effect of blanking clearance on burr height h

Blanking clearance optimization

The main parameters concerned in this post are the height of bright strip and the height of burr, so the blanking clearance is optimized for these two parameters.

It can be seen from table 4 that when the blanking gap of the test plate is 0.06mm, that is, the relative blanking gap is 8.5%, the height of the bright band accounts for 1/3 of the plate thickness. At this time, the fillet height and burr height are in an ideal state, and there will be neither indentation debris nor high burr.

In actual production, it is impossible to manage the blanking gap strictly according to this value, because the indentation and burr can not be completely removed, but have good product conditions within a certain gap range, and the quality can meet the production conditions.

In this post, based on the ratio of the height of the bright band to the thickness of the plate, that is, the relative height of the bright band, and combined with the blanking gap, we can get whether it is within the gap of good products.

In actual production, optimization can be carried out in this gap range, as shown in Table 5.

Table 5 section optimization scheme of blanking parts based on the relative height of bright strip

Blanking clearance range (mm)Relative height of bright zoneBurr statusgraphicModification suggestions
0.03~0.04>2/3Easily peeled burrsHow to Determine and Optimize the Blanking Clearance? 10 Need to increase the blanking clearance
0.04~0.051/3~1/2Peelable burrsHow to Determine and Optimize the Blanking Clearance? 11 Need to maintain good product conditions
0.05~0.061/3Good product conditionHow to Determine and Optimize the Blanking Clearance? 12 Need to maintain good product conditions
0.06~0.071/5~1/3Small burrHow to Determine and Optimize the Blanking Clearance? 13 Need to maintain good product conditions
>0.07<1/5The burr becomes larger with tear marksHow to Determine and Optimize the Blanking Clearance? 14 It is necessary to reduce the blanking clearance.

According to the gap range in the above table, optimize and manage two sets of molds using the test panel, and track their production performance.

Fig. 12 shows the burr fault statistics after the optimized management of the blanking gap from December 8.

After a period of production, the fault rate has decreased and stabilized.

Fig. 13 shows the indentation failure statistics after optimizing the management of the blanking gap from December 8.

After a period of production, the failure rate has decreased and stabilized.

burr fault statistics before and after optimization

Fig. 12 burr fault statistics before and after optimization

indentation failure statistics before and after optimization

Fig. 13 indentation failure statistics before and after optimization

Conclusion

This post briefly analyzes the blanking deformation process and the structure and influencing factors of the section of the blanking part, and introduces a method that can quickly and easily obtain the blanking gap in actual production.

This method uses a 0.06mm gap test paper combined with red lead paint to quickly judge the blanking gap through the visualization of the die cutting edge.

On this basis, the section analysis of the stainless steel plate with the brand of GX220BDL+ZF and the sheet thickness of 0.7mm under different blanking gaps is carried out, and the optimization scheme of blanking gap is determined based on the relative height of the bright band, which improves the poor indentation problem caused by the blanking gap being less than the reasonable value and the poor burr problem caused by the blanking gap being greater than the reasonable value in production.

Through the follow-up production tracking, it is confirmed that the failure rate decreases and tends to be stable.

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