Estimated reading time: 25 minutes
Under normal blanking working conditions, the shear cracks produced by the edge of the punch and the shear cracks produced by the edge of the concave die merge with each other. At this time, the cross-section of the blanking part as shown in Figure 1-1 can be obtained. It has the following 4 characteristic areas.
- Slumped corners (rounded corners) area
This area is formed by bending and elongation deformation of the material near the edge of the punch when the edge of the punch is pressed into the material, and the material is formed into the gap between the punch and the concave mold. In the punching process, the collapse angle is located at the small end of the hole section; in the blanking process, the collapse angle is located at the large end of the workpiece surface. The better the plasticity of the sheet, the larger the gap between the convex and concave molds, and the greater the collapse angle formed.
- Bright band
This area occurs in the plastic deformation stage. When the cutting edge cuts into the sheet material, the sheet material and the side surfaces of the convex and concave die cutting edges are extruded to form a bright vertical section. It usually occupies 1/3~1/2 of the full section. In the punching process, the bright band is located at the small end of the hole section; in the blanking process, the bright band is located at the large end of the part section. The better the plasticity of the sheet, the smaller the gap between the convex and concave molds, and the wider the width of the bright band. The bright belt is usually the measuring belt surface, which affects the dimensional accuracy of the part.
- Fault zone
This area is formed during the fracture stage. The fracture zone is next to the bright zone, which is a tearing surface formed by the continuous expansion of microcracks near the cutting edge under tensile stress. The surface of the fracture zone is rough and has an oblique angle of 4°~6°. In the punching process, the fracture is located at the large end of the hole section; in the blanking process, the fracture is located at the small end of the part section. The larger the gap between the convex and concave molds, the wider the crack zone and the larger the oblique angle.
The formation of burrs is due to the fact that in the late stage of plastic deformation when the cutting edges of the punch and the female mold cut into the processed sheet to a certain depth, the material on the front of the cutting edge is compressed, and the cutting edge is in a state of high static pressure, making the starting point of the crack, not It will occur at the tip of the blade, but not far from the side of the mold. Under the action of tensile stress, the cracks will lengthen and the material will break to produce burrs. The distance between the point where the crack occurs and the tip of the blade becomes burrs. height. Burrs are inevitable in ordinary blanking.
There are many factors that affect the quality of the section of blanking parts, among which the most influential is the blanking gap between the convex and concave dies. Under the condition of blanking with reasonable clearance, the obtained blanking piece has a small cross-sectional collapse angle and a normal bright band. Although the fractured band is rough, it is relatively flat, with a small slope and not obvious burrs.
Dimensional accuracy of blanking parts
The dimensional accuracy of the blanking piece refers to the difference between the actual size of the blanking piece and the basic size on the drawing. The smaller the difference, the higher the accuracy. This difference includes two deviations: one is the manufacturing deviation of the mold itself, and the other is the deviation of the punching part relative to the size of the punch or die.
The dimensional accuracy of blanking parts is related to many factors, such as the manufacturing degree of the die, blanking gap, material properties, etc. The main factor is the blanking gap.
- The manufacturing precision of the die
The manufacturing accuracy of the die has a direct impact on the dimensional accuracy of the blanking parts. The higher the accuracy of the die, the higher the accuracy of the blanking part under other conditions. Under normal circumstances, the manufacturing accuracy of the die is 2 to 4 accuracy levels higher than the accuracy of the blanking parts. When the blanking die has reasonable clearance and sharp edges, the relationship between the die manufacturing accuracy and the accuracy of the blanking parts is shown in Table 1-2.
- Blanking gap
When the gap is too large, in addition to shearing during the blanking process, the sheet material will also produce greater stretching and bending deformation. After the blanking, the material elastically recovers and the size of the blanking piece shrinks in the actual direction. For blanking parts, the size will be smaller than the size of the die, and for punching parts, the size will be larger than the size of the punch.
When the gap is too small, in addition to shearing, the sheet material will be subjected to greater squeezing during the blanking process. After the blanking, the elastic recovery of the material causes the size of the blanking piece to expand in the opposite direction of the entity. For blanking parts, its size will be larger than the size of the die; for punching parts, its size will be smaller than the size of the punch.
When the gap is appropriate, during the punching process, the deformation zone of the sheet material is separated under the action of shearing, so that the size of the blanking piece is equal to the size of the die, and the size of the punching piece is equal to the size of the punch.
- The nature of the material
The nature of the material has a great influence on the amount of elastic deformation of the material during the punching process. The elastic deformation of mild steel is small, and the rebound value after punching is also small, so the precision of the parts is high. The situation with hard steel is just the opposite.
Shape error of blanking part
The shape error of blanking parts refers to defects such as warpage, distortion, and deformation. Excessive clearance can easily cause warpage (dome); uneven material, uneven clearance, and uneven friction between the back angle of the die and the material will cause distortion defects; the edge of the blank is punched or the hole distance is too small, etc., will be caused by bulging. Deformed.
The main factor that affects the shape error of the blanking part is the blade gap. Studies have shown that the general rule of the effect of the gap on the dome of blanking parts is that when the gap is small, the dome is larger; when the gap is the material thickness (5%~15%), the dome is smaller; as the gap increases, the dome will be Increase to reduce the flatness of the blanking piece.
When punching, it is not only required to punch out parts whose shape and size meet the requirements of the drawing, but also have certain quality requirements. The quality of blanking parts includes section quality, dimensional accuracy and shape error. The blanking section should be as vertical as possible, smooth and with small burrs. The dimensional accuracy should be guaranteed to be within the tolerance range specified in the drawings. The shape of the part should meet the requirements of the drawing, and the surface should be as vertical as possible, that is, the dome should be small.
The difference between the dimensions of the convex and concave edge of the punching die is called the punching gap, which is represented by Z, and also called the double-sided gap (the single-sided gap is represented by Z/2). The gap is a very important process parameter in the design of the blanking die. The blanking gap has a great influence on the quality, blanking force and die life of the blanking parts. In the long-term research, it is found that the law of influence is different. Therefore, there is no absolutely reasonable gap value, which can simultaneously meet the requirements of the best cross-sectional quality of the blanking parts, the highest dimensional accuracy, the longest life and the smallest blanking force. In actual production, the selection of the gap mainly considers the two main factors of the quality of the section of the blanking part and the life of the mold, which are closely related to the production cost and product quality.
- Blanking gap
The blanking gap has a great influence on the quality of the blanking part, die life, discharge force, etc., but the influence law is different, and there is no gap that meets the requirements of workpiece quality, die life and blanking force at the same time. In actual production, the choice of blanking gap mainly considers the quality of the blanking section and the life of the mold. At the same time, considering the deviation in mold manufacturing and the wear in use, select a suitable gap range, as long as Good blanking parts can be processed within this range. The minimum value of this range is called the minimum reasonable gap, which is represented by Zmin; the maximum value is called the maximum reasonable gap, which is represented by Zmax. Considering that the wear of the mold during use will increase the gap, the actual design and manufacture of the mold often use the minimum reasonable gap Zmin.
- Determination of reasonable blanking gap
At present, there are three methods to determine the reasonable blanking gap value: theoretical determination, empirical determination and look-up table method.
- Theoretical determination method.
The theoretical determination method is also called the formula method. The main basis of this method is to ensure that the upper and lower micro-cracks overlap and obtain a good blanking section.
Figure 1-3 shows the instantaneous state of cracks during punching. According to the geometric relationship in the figure, a reasonable gap can be obtained as
Z=2(t-h0)tanβ=2t(1-h0/t)tanβ (2 -1)
Here t—-material thickness;
h0—-The depth of the punch into the material when cracks occur;
h0/t—-the relative depth of the punch into the material when cracks occur;
β—the angle between the shear crack and the vertical
It can be seen from equation 2-1 that the reasonable gap Z is related to the material thickness t, the relative penetration depth of the punch into the material h0/t and the crack angle β, and h0/t is not only related to the plasticity of the material but also affected by the comprehensive thickness of the material. influences. The values of h0/t and β are shown in Table 1-4.
In short, the greater the material thickness, the lower the plasticity of hard and brittle materials, the larger the required gap Z value; the thinner the material thickness, the better the plasticity, the smaller the required gap value.
Because the theoretical calculation method is inconvenient to use in production, empirical data is widely used at present.
- Empirical determination method
The following empirical formula is commonly used in production to calculate the value of the reasonable blanking gap Z.
In the formula, t—-material thickness, (mm);
c—-Coefficient, related to material properties and thickness, when t<3mm, c=6%~12%; when t>3mm, c=15%~25%.
When the material is soft, take the small value; when the material is hard, take the large value.
- Look-up table method
Generally, there will be empirical data provided by a special table for the initial blanking gap of blanking and punching dies, which can be used for blanking under general conditions. The minimum value Zmin of the initial gap in the table is the minimum reasonable gap, and the maximum value Zmax of the initial gap is to take into account the manufacturing error of the punch and the die, add a value on the basis of Zmin. During use, the gap will increase due to the wear of the working part of the mold, so the maximum gap (maximum reasonable gap) may exceed the value listed in the table.
- The selection principle of reasonable punching gap
Production practice has proved that when the blanking gap is set to a small value, the cross-sectional quality of the blanking part is better, but if the gap is too small, the blanking force and the return force will be increased, and the service life of the mold will be reduced. Therefore, when selecting the blanking gap, various factors should be considered comprehensively.
- When the quality of the punching parts is not high, the gap should be as large as possible within a reasonable range, so as to help extend the life of the mold and reduce the punching force, pushing force, and unloading force.
- When the quality of the punching parts is high, the smaller value should be selected within the reasonable clearance range, so that although the life of the die is reduced, the quality of the blanking of the parts is guaranteed.
When designing the die, Zmin is generally taken as the initial gap, mainly considering that the die should be sharpened after a period of time. After grinding, the gap will increase and make the transition from Zmin to Zmax. Therefore, in order to enable the mold to punch out qualified parts in a relatively long period of time, increase the utilization rate of the mold, and reduce the production cost, Zmin is generally used as the initial gap when designing the mold.
Convex and concave die cutting edge size calculation
The die edge size and tolerance are the main factors that affect the dimensional accuracy of the blanking parts. The reasonable gap value of the die is also guaranteed by the convex and concave die edge sizes and their tolerances. Therefore, the correct determination of the dimensions and tolerances of the cutting edges of the convex and concave dies is a key task in the design of the blanking die.
The existence of the gap between the convex and concave dies makes the cross-section of the blanking part tapered, so the size measurement and use of the blanking part are based on the size of the bright belt. The bright band of the blanking part is produced by the cutting of the material by the cutting edge of the die, and the bright band of the punching part is produced by the cutting of the material by the cutting edge of the punch. Therefore, the design of convex and concave edge sizes should distinguish between punching and blanking, and follow the following principles.
- Determine the size of the cutting edge of the reference die.
The blanking die is designed to first determine the size of the cutting edge of the concave die. The gap is taken on the convex die based on the concave die, and the blanking gap is obtained by reducing the size of the convex die. When designing the punching die, first determine the size of the punch blade, take the punch as the benchmark, and take the gap on the die. The punching gap is obtained by increasing the size of the die.
Follow the law of wear of the die during use
During the blanking process, the convex and concave molds rub against the blanking parts or waste. The contour of the convex mold becomes smaller and smaller, the contour of the concave mold becomes larger, and the gap between the convex mold and the concave mold becomes larger. When designing the blanking die, the original size of the die should be close to or equal to the minimum size of the workpiece; when designing the punching die, the basic size of the punch should be close to or equal to the maximum limit size of the workpiece hole. Regardless of punching or blanking, the blanking gap is generally selected as the smallest reasonable gap value Zmin.
The mold wear reserve is related to the manufacturing accuracy of the workpiece. Expressed by xΔ, the Δ is the tolerance value of the workpiece, and x is the wear coefficient, and its value is between 0.5 and 1. The following selection principles are based on the manufacturing accuracy of the workpiece.
The accuracy of the workpiece is above IT10: x=1;
The accuracy of the workpiece is IT11~IT13: x=0.75;
The accuracy of the workpiece is IT14: x=0.5.
Consider the relationship between workpiece accuracy and mold accuracy
When selecting the manufacturing tolerance of the die edge, it is necessary to consider the relationship between the accuracy of the workpiece and the accuracy of the die, not only to ensure the accuracy of the workpiece, but also to ensure that there is a reasonable gap value. Generally, the accuracy of the die is 2~4 higher than the accuracy of the workpiece. For simple circular and square cutting edges, the manufacturing deviation can be selected according to IT6~IT7; for complex cutting edges, the manufacturing deviation can be selected according to 1/4 of the tolerance value of the corresponding part of the workpiece; for cutting edges If the size of the mouth does not change after wear, the manufacturing deviation value can be 1/8 of the tolerance value of the corresponding part of the workpiece and prefixed with “±”.
- Tolerance labeling follows the principle of “into the body”
The workpiece size tolerance and the manufacturing deviation of the die edge size should in principle be marked as one-way tolerance according to the principle of “entering into the body”. The so-called “human body” principle means that the index should be marked in the direction of the material entity when the workpiece size tolerance is specified. However, for dimensions that do not change after wear, the bidirectional deviation is generally marked.
The calculation of the cutting edge size of convex and concave molds should consider the characteristics of mold manufacturing.
Calculation of the cutting edge size of punch and die
Due to the different processing methods of the die, the calculation method of the cutting edge size is also different, which can basically be divided into two categories.
- The method of processing separately according to the pattern of the punch and the concave mold.
This method is mainly suitable for round or simple and regular-shaped workpieces. Because the convex and concave molds for blanking such workpieces are relatively simple to manufacture and the accuracy is easy to ensure, separate processing is adopted. When designing, the dimensions and manufacturing tolerances of the punch and die cutting edges should be marked on the drawings.
Suppose the diameter of the punched part hole is d0+Δ. According to the calculation principle of the cutting edge size, the calculation formula is as follows.
Convex mold: dp=(1+xΔ)0-δp (2-3)
Die: dd=(d+xΔ+Zmin)0+δd (2-4)
Suppose the blanking size of the blanking part is D0-Δ. According to the calculation principle of the cutting edge size, the calculation formula is as follows.
Die: Dd=(D-xΔ)0+δd (2-5)
Punch: Dp=(D-xΔ-Zmin)0-δp (2-6)
The center distance is a dimension that remains basically unchanged after wear. In the same step, the hole distance is punched on the workpiece, and the center distance of the concave model hole can be determined by the following formula.
Ld=L+1/8 Δ (2-7)
In formula (2-3) ~ formula (2-7):
D, d-the basic size of blanking and punching workpieces, mm;
Dp, Dd—-blanking convex and concave die cutting edge size, mm;
dp, dd—-punching convex and concave die cutting edge size, mm;
Ld, L—-the nominal size of the center distance of the workpiece hole and the center distance of the die hole, mm;
Δ—-workpiece tolerance, mm;
δp, δd—-the manufacturing tolerance of convex and concave molds, the tolerance of the punch is removed, and the tolerance of the concave mold is taken up. Generally, it is selected according to 1/3~1/4 of the part tolerance. For blanking parts with simple shapes (such as round parts, square parts, etc.), due to simple manufacturing and easy accuracy, the manufacturing tolerances can be selected according to IT8~IT6 levels, or check Table 1-7.
X—-Wear coefficient, its value should be between 0.5 and 1, which is related to the accuracy of the blanking parts. It can be directly selected according to the tolerance level of the blanking parts or determined by referring to Table 1-8.
Zmin—-Minimum blanking gap.
Table 1-7 The manufacturing limit deviation of regular shape punching convex and concave molds
|Material||Non-circular workpiece x value||Round workpiece x value|
|t(mm)||Workpiece tolerance Δ(mm)|
Table 1-8 Wear coefficient x
This calculation method is suitable for round and regular-shaped blanking parts. When designing, the dimensions of the cutting edge and manufacturing tolerances should be marked on the convex and concave die drawings respectively. In order to ensure that the blanking gap is within a reasonable range, the following formula should be established.
|δp|+|δd|≤ Zmax– Zmin (2-8)
If the above formula does not hold, the mold manufacturing accuracy should be improved to reduce δd and δp. So when the mold shape is complex, this method is not suitable.
- Example 2-1
Punching the connecting piece as shown in Figure 1-9. The material of the known part is Q235, and the material thickness is t=0.5mm. Calculate the dimensions and tolerances of the convex and concave edge parts of the punching die.
Solution: It can be seen from Figure 1-9 that this part is a general punching and blanking part without special requirements, and the convex and concave molds are manufactured separately according to the interchange processing method. The outer dimension φ360-0.62 is obtained by blanking, and the inner hole size 2-φ60+0.12 and the size 18±0.09 are obtained by punching at the same time.
Determine the initial gap, look up the table to get Zmin=0.04mm, Zmax=0.06mm
Determine the wear coefficient x, check the table punching 2-φ60+0.12 wear coefficient x=0.75; blanking φ360-0.62, wear coefficient x=0.5.
Calculation of punching convex and concave die edge size.
Look up the table, -δp=-0.004mm, -δd=-0.006mm.
Punch cutting edge size: dd=(d+x Δ)0-δp=(6+0.75X0.12)0-δp=6.090-0.004mm
Die cutting edge size: dd=(d+Zmin)0+δd=(6.09+0.04) 0+δd=6.130+0.006mm
Check, |δp|+|δd|=0.004+0.006=0.01mm. Zmax-Zmin=0.06-0.04=0.02mm. Meet the requirements of |δp|+|δd|≤ Zmax– Zmin.
Blanking convex and concave die cutting edge size calculation.
Look up the table -δp=0.004mm, -δd=0.006mm.
Die cutting edge size: Dd=(D-x Δ)0+δd=(36-0.5X0.62)0+δd=35.690+0.006mm
Punch cutting edge size: Dp=(Dd-Zmin)0-δp=(35.69-0.04)0-δp=35.650-0.004mm
Check, |δp|+|δd|=0.004+0.006, Zmax-Zmin=0.06-0.04=0.02mm. Meet the requirements of |δp|+|δd|≤ Zmax– Zmin.
Center distance calculation.
Coordinated processing method of punch and die.
When the convex and concave molds are processed separately, in order to ensure a certain gap value between the convex and concave molds, the manufacturing tolerance of the punch must be strictly limited. Therefore, the manufacturing of the punch is difficult. For punching thin materials (due to the small difference between Zmax and Zmin), punching dies for complex-shaped workpieces, and punching dies for single-piece production, the punch and die cooperating processing method is often used.
The method of cooperating the punch and the concave mold is to first manufacture a reference part (punch or female mold) according to the design size, and then prepare another part according to the actual size of the reference part according to the minimum reasonable gap. The characteristic of this processing method is that the gap of the mold is guaranteed by the preparation, the process is relatively simple, it is not necessary to check the conditions of |δp|+|δd|≤ Zmax– Zmin, and it can also enlarge the manufacturing tolerance of the reference parts, making the manufacturing easy. When designing, the cutting edge dimensions and manufacturing tolerances of the reference parts should be marked in detail, and only the nominal dimensions are marked on the matching parts, and no tolerances are noted. Only the drawing should be marked: “The cutting edge of the convex (concave) die is as concave (convex) The actual cutting edge size of the mold is prepared to ensure the minimum double-sided reasonable gap value Zmin“. At present, most factories generally adopt this processing method.
For blanking parts with complex shapes, the size properties of each part are different, and the wear conditions of the punch and the die are also different. Therefore, the cutting edge size of the reference part needs to be calculated by different methods.
Figure 1-10 (a) shows the blanking part. The die should be used as the basic part of the calculation. However, the wear of the die is divided into three categories: The first type is the increased size of the die after wear (in the figure) Type-A size); The second type is the reduced size after the die is worn (B size in the figure); The third type is the size that remains unchanged after the die is worn (C size in the figure). Figure 1-10(b) shows the punching part. The punch should be used as the reference part. According to the wear of the punch, the dimensions can be divided into three categories: A, B, and C according to the method shown in the figure. When the punch wears out, the increase or decrease of its size is also in line with the law that the size of the A-type increases, the size of the B type decreases, and the size of the C type remains unchanged. In this way, for blanking parts and punching parts with complex shapes, the cutting edge size of the reference part can be calculated by the following formula.
Type A size: A=(Amax-x Δ)0+δ
Type B size: B=(Bmin+x Δ)0-δ
C type size: C=C±δ/2
In the formula, A, B, C-basic size of reference parts, mm;
Amax —- The maximum limit value of A-type dimensions of blanking parts, mm;
Bmin —- The minimum limit value of B type size of blanking parts, mm;
δ —- Mold manufacturing tolerance, mm.
- Example 2-2
The blanking part shown in Figure 1-11, the material is No. 10 steel, the material thickness is 1mm, and the dimensions a=800-0.42mm, b=400-.034mm, c=350-.034mm, d=22±0.14mm, e=150-.012mm. Try to determine the size and tolerance of the punch and die edge of the punching die.
Solution: The blanking part is a blanking part, and the female mold is selected as the reference part, and it is manufactured according to the method of cooperating with the male mold and the female mold. The calculation only needs to determine the cutting edge size and manufacturing tolerance of the blanking die, and the punch edge size is manufactured according to the actual size of the die to ensure the smallest clearance fit.
Determine the initial gap: Zmin=0.10mm, Zmax=0.13mm by looking up the table.
Determine the wear coefficient x: look up the table a=800-0.42, wear coefficient x=0.5; size e=150-0.12mm, wear coefficient x=10; other wear coefficients press x=0.75.
Type A size: ad=(a-xΔ)0+δ=(80-0.5X0.042)0+0.42/4=79.790+0.105(mm)
Type B size: dd=(dmin+xΔ)0-δ=(22-0.14+0.75X0.28)0-0.28/4=22.070-0.070(mm)
C size: When the C size with constant wear is marked as a one-way deviation, there are two cases, C0-Δ and C0+Δ. At this time, the limit average size of C is taken into the equation, and then
The basic size of the blanking punch is the same as the basic size of the concave mold, respectively 79.79 mm, 39.75 mm, 34.75 mm, 26.07 mm, 14.94 mm. It is not necessary to mark the size deviation, but it should be noted in the mold: the actual cutting edge size of the punch It is formulated with a blanking die to ensure that the gap between the two sides is 0.10~0.13 mm. The dimensions of the blanking die and the punch are shown in Figure 1-12.
- The selection principle of manufacturing method.
1. When the blanking part has a complex shape (large number of dimensions), the die edge is made by the matching processing method.
2. When the blanking part is of simple shape (small number of dimensions), select the cutting edge manufacturing method according to the following discriminant.
When δp + δd> Zmax– Zmin, the die edge is made by the matching processing method.
When δp + δd ≤ Zmax– Zmin, the cutting edge of the die is manufactured by the separate processing method.