Blanking is a stamping process in which one part of the closed contour of a plate is separated from another part through the use of a die.
The term “cutting gap” refers to the difference in dimensions between the top edge and bottom edge of the die during the cutting process.
This is a critical technical parameter in die design, manufacturing and production.
To ensure die longevity and molded part quality, as well as improve production efficiency, it is essential to properly manage and optimize the molding gap during actual production.
Blanking deformation process analysis
The blanking deformation process can be roughly categorized into three phases: the elastic deformation phase, the plastic deformation phase and the fracture phase.
The voltage state of the plate during this process is shown in Figure 1.
Fig. 1 Plate stress analysis during blanking deformation
In the figure,
- F C is the shear force acting on the upper matrix
- F C' is the shear force acting on the lower matrix;
- F I is the transverse force acting on the upper die, F L ' is the transverse force acting on the lower die;
- μ is the friction coefficient;
- F H is the horizontal component force received by the plate from the upper edge of the die, and F H' is the horizontal component force received by the plate from the lower die;
- F V is the vertical component force received by the plate from the upper edge of the die, F V ' is the vertical component force received by the plate from the lower die;
- M g is the bending moment of the plate;
- I is the lever arm;
- C is the suppression clearance.
Elastic deformation stage
During this phase, after the edge of the upper die comes into contact with the plate, the plate is initially flattened and then the edges of the upper and lower dies are pressed into the plate.
Due to the “C” gap, the combined force of the upper die and the combined force of the lower die are not aligned, causing the plate to experience a bending moment “Mg'” and bend slightly under elastic compression.
As the upper die continues to descend, the stress on the cutting edge of the material will reach its elastic limit.
Plastic deformation stage
As the upper die continues its downward movement, the stress in the plate increases, reaching the yield point and causing plastic deformation.
As the degree of plastic deformation increases, the tensile stress and bending moment within the plate continue to increase, causing the material to harden further. Material close to the edge will reach its strength limit first.
Fracture stage
As the upper die continues to descend, cracks initially appear on the sides of the upper and lower edges of the die.
At this point, the energy stored in the elastic and plastic deformation phases is released, spreading inward along the direction of maximum shear stress.
When the primary cracks at the top and bottom edges of the die align, the material is cut and separated.
If the blade edge clearance is unreasonable and the two primary cracks do not align, a third primary crack will appear.
Influence of blanking clearance on the section and its selection
Based on the analysis of the cutting deformation process described above, the cross section of the blunt parts mainly consists of the collapse angle “R”, the bright zone “B”, the fracture zone and the burr “h”, and has a fracture angle “α” as shown in Figure 2.
Fig. 2 composition of the blind parts section
Influence of cutting clearance on angle collapse
In the elastic deformation phase, the material near the cutting edge forms a free surface that is pulled into the cutting gap, creating the collapse angle. The height of the collapse angle increases with increasing suppression gap.
The presence of cutting clearance means that the force resulting from the upper and lower edges of the plate is not in a straight line, resulting in the generation of a bending moment (mg).
As the cutting gap increases, the bending moment of the plate also increases, leading to a greater bending effect on the plate. This, in turn, causes the height of the collapse angle to increase proportionally.
Influence of suppression gap on bright range
In the plastic deformation stage, the plate undergoes shear and bending deformation at the cutting edge, mainly shear, which creates a shiny stripe. The glossy strip has a smooth surface and excellent perpendicularity, making it an ideal shape for a board section.
However, as the suppression gap increases, the height of the bright band decreases. If the filling gap is too small, the upper and lower main fissures will not line up, and this results in the formation of a long, narrow second bright band. This strip is characterized by long burrs, irregular toothed edges and small cones, which lead to the creation of debris that is easily removed and transported to the subsequent process. This, in turn, causes poor indentation, which is one of the main causes of production downtime.
An increase in the shear clearance increases the tensile and flexural effect on the plate, reducing the relative strength of the shear effect. This makes the plate more likely to be pulled apart and form a fracture zone, and the height of the bright zone is also reduced.
Influence on burr
At first, the burr height increases gradually, but then increases steadily as the cutting clearance increases.
In the fracture stage, cracks form on the side of the cutting edge rather than in the middle of the cutting gap, which inevitably leads to the formation of burrs.
If the plug gap is less than a reasonable value, the main cracks in the plate will not match, resulting in small, difficult-to-remove burrs. However, if the plugging gap is greater than a reasonable value, the plate is pulled into the plugging gap by tension and bending, causing the main crack to appear on the side relatively far from the cutting edge and eventually break.
This results in a large burr height, which is another important cause of burr formation and a significant source of production downtime.
Suppression clearance selection
As shown in Figure 3, the relationship between cutting clearance and die life and part section quality was analyzed based on the information discussed above and relevant literature.
When selecting the cutting gap, it is necessary to consider both the quality of the part section and the service life of the die.
α represents the relative clearance that results in better part section quality, β represents the relative clearance that results in good part section quality, γ represents the relative clearance that results in good die life, and δ represents the clearance which results in the best useful life of the matrix.
Fig. 3 Effect of relative blanking clearance on section quality and die life
The relative gap can be expressed by formula (1), which shows the relationship between the blind gap and the plate thickness.
C=xt (1)
In the formula, “C” represents the cutting gap (mm), “x” represents the proportion coefficient and “t” represents the plate thickness (mm).
Based on practical production experience, it is suggested to use x = 6% ~ 8% when the body cover is made of steel plate and x = 10% when the body cover is made of aluminum plate. This balances the quality of the part section and the life of the molds.
Further reading:
- How to determine punch and die clearance?
A method to quickly measure and evaluate cutting clearance in practical production
Blind clearance measurement
There are several methods for measuring cutting clearance, including using a feeler gauge. However, this method has low measurement efficiency for complex blade shapes and is difficult to measure internal blades, leading to low operational efficiency.
Therefore, in actual production, it is important to use a quick and simple method to measure the cutting gap.
One of these methods is to use 0.06mm gap test paper and red lead coating, as shown in Figure 4. This method is suitable for parts with body covers made of steel plates with a thickness of around 0.7 mm, which is the case of this post. The thickness of the applied red lead coating is generally between 0.01 mm and 0.02 mm.
Figure 4 measurement tools
To begin with, it is necessary to determine the reasonable range for blanking clearance. Based on the information discussed above, using a proportional coefficient of x = 6% ~ 8%, the reasonable range can be calculated between 0.04mm and 0.06mm.
Next, the pressing plate must be removed and the mold installed in the press. The measuring point on the bottom mold should be selected, and the gap test paper should be evenly applied to the measuring point, as shown in Figure 5.
Fig. 5 Gluing the blank edge gap test paper of a die
After that, a layer of red lead coating should be evenly applied to the upper mold.
In terms of recording the offset cutting clearance, in this article it is recommended to record it based on the number of inserts on the top edge of the die, as shown in Figure 6. This helps to avoid confusion in data recording and ensures data accuracy collection.
| No. | Left | Quite | Right |
| 208 | 0.35 | 0.35 | 0.35 |
| 207 | 0.35 | 0.35 | 0.35 |
| 206 | 0.35 | 0.35 | 0.35 |
| 203 | 0.35 | 0.35 | 0.35 |
| 204 | 0.4 | 0.4 | 0.4 |
| 205 | 0.35 | 0.35 | 0.35 |
Fig. 6 data recording method
Finally, the pressing machine must be operated in a gradual stroke at the actual production speed and the state of the adhesive tape must be observed visually to determine the cutting gap.
The steps involved in this process are summarized in Table 1.
Table 1 Gap measurement operation steps
| NO. | stage | Operation |
| 1 | Calculate reasonable slack | Steel plate: x=6% ~ 8%; Aluminum plate: x=10%. |
| 3 | Unloading the pressing plate | Unload the pressing plate and load the die into the press. |
| 4 | Gap Test Paper Measuring Point Selection | The blind edge of the lower die should be evenly pasted with gap test paper, and the segment registration should be done according to the insertion number of the blind edge of the upper die. |
| 5 | Apply red lead coating | Evenly brush a layer of red lead coating on the upper mold, with the thickness increased by 0.01 ~ 0.02 mm. |
| 6 | Press forward | Adjust the target height of the slider to bottom dead center, increase the actual production speed in one motion, and visually observe the state of the tape. |
Suppression Authorization Judgment
After data measurement is complete, you need to review and analyze the data. The analysis is based on the state of the edge gap test paper. The suppression gap can be roughly determined by observing the condition of the adhesive tape.
The analysis method is shown in Table 2.
It is important to highlight that variation in sheet thickness can cause errors within a certain range. If the variation in sheet thickness is 0.7mm ± 0.05mm, the error can be disregarded. However, if the variation in plate thickness exceeds this range, the results in Table 2 must be reevaluated.
Table 2 Suppression clearance judgment pattern
| NO. | Red lead situation | Tape condition | Clearance range (mm) | schematic sketch |
| 1 | Red lead completely scraped off the edge of the lower die | The tape is completely crumpled. | 0.03~0.04 |
|
| two | Red lead scraping off the edge of the lower die | Poor adhesive tape integrity | 0.05~0.06 |
|
| 3 | Red lead scrapes off the edge of the lower die | Ribbon intact | 0.06~0.07 |
|
| 4 | The red tip did not scratch the edge of the bottom die | Ribbon intact | >0.07 |
|
Section quality analysis and clearance optimization
Punching section analysis
Measured clearance data must be recorded as described above.
Currently, the reasonable clearance value for the steel plate used in the test is 0.04 mm to 0.06 mm, but to determine the ideal cutting clearance value, it is necessary to analyze the section of the plate.
The tool used in this article is a magnifying glass model peak2008-50 × 50 times, which is shown in Figure 7. Its parameters are listed in Table 3.
Table 3 Pico2008-50 × 50 Magnifier Parameters
| Parameters | Type | Enlargement | Minimum scale | Field of vision | Measuring range |
| value | 2008-50 × | 50× | 0.02mm | 1.6mm | 1.6mm |
Fig. 7 Pico2008-50 × 50 magnifying glass parameters
In this paper, the effect of cutting clearance on plate section quality is studied.
A stainless steel blank with a thickness of 0.7 mm is used in the analysis, and sections are taken with gaps of 0.03 mm, 0.04 mm, 0.05 mm, 0.06 mm and 0.07 mm , respectively. This results in five groups of data, each with a relative slack of 4.3%, 5.7%, 7.1%, 8.5%, and 10.0%.
The white section is photographed with a Peak2008 50x magnifier. The collapse angle height (R), bright band height (B) and burr height (h) are used as analysis indicators to determine the relationship between the plate and these indicators under different cutting clearance conditions. The results are displayed in Table 4.
The blank section is examined with a 50x magnifying glass. The collapse angle height (R), bright band height (B) and burr height (h) are selected as analysis indicators to determine the relationship between the plate and these indicators under various cutting clearance conditions.
The results are shown in Table 4.
Table 4 blind parts section analysis index
| Blind clearance (mm) | Relative blanking clearance (%) | Angular collapse height R (mm) | Glossy strip height B (mm) | Burr height h (mm) | Photo of the board section |
| 0.03 | 4.3 | 0.04 | 0.56 | 0.01 |
|
| 0.04 | 5.7 | 0.05 | 0.46 | 0.02 |
|
| 0.05 | 7.1 | 0.05 | 0.34 | 0.02 |
|
| 0.06 | 8.5 | 0.06 | 0.28 | 0.02 |
|
| 0.07 | 10.0 | 0.09 | 0.16 | 0.04 |
|
The five groups of measured data are plotted in a scatterplot and a regression analysis is performed.
As can be seen in Figure 8, the height of the collapse angle increases with increasing plugging gap. The reason for this is due to the greater bending moment of the plate and the increased bending and stretching effects as the cutting gap becomes larger, causing the height of the fillet belt to increase.
Fig. 8 Influence of blind clearance on collapse height R
As shown in Figure 9, the height of the bright band decreases as the suppression gap increases. The glossy strip is characterized by its smooth, flat and perpendicular orientation to the board, making it an ideal section for erasing. The decrease in height is due to the weakening of the cutting action of the plate, which leads to the formation of a fracture zone and an increase in the height of the fracture zone.
Fig. 9 Effect of blanking gap on bright stripe height B
As the suppression gap decreases, the height of the bright band increases due to the reduction of bending and tensile effects on the plate, the strengthening of the shear effect, and the prolongation of its plastic deformation stage. Furthermore, under these gap conditions, the upper and lower main cracks do not coincide, resulting in secondary separation.
The blank part forms a second bright band through friction on the side wall of the lower die. The surface of this second glossy strip is prone to peeling, as shown in Figure 10. This type of surface will be peeled off and partially attached to the surface of the pressing plate during subsequent processing, and the debris will leave a mark on the plate. during the next stroke of the mold.
The formation of these bad indentations leads to a significant increase in the number of failures and reduces production efficiency.
Fig. 10 plate section with 0.03 mm clearance
As seen in Figure 11, the burr height increases with increasing cutting clearance. Burr is a problematic aspect of the stamping process and can affect the normal use of stamped parts.
As previously discussed, when the filling gap is small, the upper and lower cracks of the plate align in the direction of maximum shear stress, resulting in a small flash height that is easily removable. However, when the cutting gap is large, the bending and stretching of the sheet metal increases, and cracks are more likely to form slightly away from the cutting edge of the upper and lower dies. This makes the sheet metal more prone to tearing, resulting in a higher height of burrs that are difficult to remove.
Burr results in a significant loss of production time and reduces efficiency, making it an important aspect of production management.
Fig. 11 effect of cutting clearance on burr height h
Blanking clearance optimization
The focus of this article is on the bright strip height and burr height and therefore the cutting clearance is optimized for these two parameters.
As shown in Table 4, when the suppression gap of the test plate is 0.06 mm (representing a relative blanking gap of 8.5%), the height of the bright strip represents 1/3 of the thickness of the plate. At this time, the fillet height and burr height are in an ideal state, with no indentation debris or high burrs.
In practical production, it is not feasible to strictly manage the cutting clearance according to this value, as indentation and burrs cannot be completely eliminated, but good product conditions can be achieved within a certain range of clearance values and the quality meets production requirements.
This article determines whether the gap is within the range of good products using the ratio of the bright band height to the plate thickness (the relative height of the bright band) and the suppression gap. Optimization can be performed within this range in actual production, as shown in Table 5.
Table 5 Section optimization scheme of blind parts based on the relative height of the bright strip
| Suppression clearance range (mm) | Relative height of the light zone | Burr status | graphic | Modification suggestions |
| 0.03~0.04 | >2/3 | Easily peeled burrs |
|
Need to increase suppression clearance |
| 0.04~0.05 | 1/3~1/2 | Removable burrs |
|
Need to maintain good product condition |
| 0.05~0.06 | 1/3 | Good condition of the product |
|
Need to maintain good product condition |
| 0.06~0.07 | 1/5~1/3 | Small burr |
|
Need to maintain good product condition |
| >0.07 | <1/5 | Burr gets bigger with tear marks |
|
It is necessary to reduce the blind clearance. |
Two sets of molds were optimized and managed using the test panel based on the clearance range indicated in the table, and their production performance was monitored.
Figure 12 shows flash failure statistics after optimized suppression gap management as of December 8. After a period of production, the failure rate decreased and stabilized.
Figure 13 shows the indentation gap statistics after optimizing suppression gap management as of December 8th. After a period of production, the failure rate decreased and stabilized.
Fig. 12 Burr failure statistics before and after optimization
Fig. 13 Indentation failure statistics before and after optimization
Mission completion
This article briefly examines the blanking deformation process and the structure and factors influencing the section of the blank part. It also introduces a method to quickly and easily determine the blanking gap in practical production. This method involves using 0.06mm gap test paper combined with red lead paint to visually assess the gap at the cutting edge of the die.
Section analysis of a 0.7 mm thick stainless steel plate of the GX220BDL+ZF brand is carried out under different blanking gaps and the optimal blanking gap scheme is established based on the relative height of the bright band. This improves the problems of poor indentation caused by too small a cutting gap and poor burr caused by too large a cutting gap.
Through subsequent production monitoring, it was confirmed that the failure rate had decreased and stabilized.
