Fine Blanking:
Fine blanking is a cutting-free processing technology and is a precision blanking method developed on the basis of ordinary stamping technology. The result of this processing method is that the punched surface of the stamped part is free of cracks and tears throughout its thickness. In addition, the tightest dimensional accuracy and flatness tolerances can be achieved.
Features: Compared with ordinary blanking, the mold structure of precision blanking has an extra ring gear plate and ejector, and the gap between the convex and concave molds is extremely small, and the cutting edge of the concave mold has rounded corners. During the punching process, before the punch contacts the material, the ring gear pressure plate presses the material against the die through force, thereby generating lateral pressure on the inner surface of the V-shaped teeth to prevent the material from tearing in the shear zone and the metal from breaking. Lateral flow, while the punching die presses into the material, the counter-pressure of the ejector is used to compress the material. In addition, the extremely small gap and the rounded die edge are used to eliminate stress concentration, thereby making the shear zone The metal inside is in a state of three-dimensional compressive stress, which eliminates the tensile stress in this area, improves the plasticity of the material, and fundamentally prevents the bending, stretching, and tearing phenomena that occur in ordinary blanking, allowing the material to move along the concave direction. The edge shape of the mold is punched into parts in the form of pure shear, thereby obtaining a high-quality smooth and flat shear surface. During fine blanking, the pressing force, blanking gap and die edge fillet complement each other and are indispensable. Their effects are interrelated. When the gap is uniform and the fillet radius is appropriate, a smooth cross-section can be obtained with a small pressing material. picture
Fineblanking characteristics
1. The collapse angle is extremely small;
2. The length of the bright strip is about 90% of the plate thickness;
3. The broken section height is extremely small;
4. Improves the processing accuracy of the cut surface, which is beneficial to the assembly function of the cut surface of the product.
The difference between general punching and fine blanking lies in the different mold structures, which leads to essential differences in process. The picture below shows a comparison of the mold structures of the two processes.
There are certain requirements for molds:
1) Fine blanking pressure is large, and the gap between the convex and concave molds is small. During mold manufacturing and assembly, the gaps must be evenly distributed and maintain neutrality. At the same time, the mold base must be precise and the guidance must be accurate and reliable. Each sliding part must have a matching gap. It is 0.002~0.005mm.
2) The main parts of the mold must have sufficient strength and rigidity, and must cooperate with each other with high accuracy. The working part has high wear resistance and no elastic deformation is allowed during operation.
3) Strictly control the depth of the punch entering the die (generally controlled at 0.025~0.05mm) to avoid damaging the cutting edge.
4) Properly consider the exhaust design of the mold and pay attention to the wear and fatigue wear of the working parts of the mold.
Concave and convex mold
What kind of parts are considered fineblanked parts?
The craftsmanship of fine blanking parts
What kind of parts are fine blanked parts?
The craftsmanship of fine blanking parts
The processability of fine blanking parts mainly refers to ensuring the technical and usage requirements of the parts, and under certain batch production conditions, they should be the simplest and most economical to manufacture. The main factors affecting it are:
(1) The craftsmanship of the part structure;
(2) Dimensional tolerances and geometric tolerances of parts;
(3) Material properties and thickness;
(4) Blanking surface quality;
(5) Mold design, manufacturing quality and lifespan;
(6) Selection of fine blanking machine, etc.
The processability of the fine-blanking part structure refers to the structural units that constitute the geometric shape of the part, including: the determination of the minimum fillet radius, hole diameter, wall thickness, ring width, groove width, punching tooth module, etc. is particularly important.
As shown in Figure 1, the limit values of the structural parameters of fine blanking parts can be selected. They are all smaller than regular punch parts. This is determined by the principle of fine blanking. However, reasonable part structural parameters are conducive to improving product quality and reducing production costs.
Figure 1 Geometric unit and difficulty level of fine blanking parts
A-aperture; B-slot width, overlap; C-tooth module; D-fillet radius.
Difficulty level of fineblanking parts
According to the part geometry and its structural units, it is divided into three levels: S1, S2 and S3 in each figure of Figure 1.
S1-simple, suitable for fine blanking material shear strength Ks=700N/mm2
S2-medium, suitable for fine blanking materials. Shear strength Ks=530N/mm2
S3-complex, suitable for fine blanking material shear strength Ks=430N/mm2
In the range below S3, fine blanking is not suitable, or special measures must be taken. When using the S3 range, the condition is that the blanking components are made of high-speed steel and the tensile strength of the fine-blanking material is
δb≤600 N/mm2 (shear strength Ks≤430N/mm2).
Example: The switch cam in Figure 1 is made of Cr15 (spheroidized), Ks=420N/mm2, determine its difficulty level.
·Aperture diameter d＝4.1mm S1
·Take edge b＝3.5mm S3
·Tooth module m＝2.25mm S2
·Corner radius Ra＝0.75mm S1/S2
The most difficult part for this part is edge b, so the total difficulty is S3 and fine blanking is possible.
Technical requirements for fine blanking parts
3.1 Dimensional tolerances
The dimensional tolerance of fine blanking parts depends on: part shape, mold manufacturing quality, material thickness and performance, lubricant and press adjustment and other factors. Can be selected from Table 1.
3.2 Flatness tolerance
The flatness of fine blanking parts refers to the deflection of the part plane (see Figure 2), and its value is: f=h-s
Figure 2 Parts plan view
Since the fine blanking material is carried out in a compressed state, the fine blanking parts have good flatness. This flatness varies with part size, shape, material thickness and mechanical properties.
Generally speaking, thick material parts are straighter than thin material parts; low-strength materials are straighter than high-strength materials; and a large blank holder force is straighter than a small blank holder force. The material surface on the male side is always concave and on the female side is always convex. However, if the part also requires processes such as imprinting, creasing, notching, bending, or punching with a continuous die, the flatness will have a large fluctuation range due to local deformation or different punching directions on the part. But no matter what, the flatness of fine blanking parts is always much better than that of ordinary stamping parts. Figure 3 is the general straightness measured at a distance of 100mm.
3.3 Verticality tolerance
The blanking surface of fine-blanking parts has a certain angle tolerance (inverted taper) with the base surface, which is called non-perpendicularity. It is related to the material thickness and its performance, the status of the punching edge, the stiffness of the mold, the adjustment of the press, etc.
close. Generally, when the material thickness is 1mm, the non-verticality is 0.0026mm. If the material thickness is 10mm, the burr side is 0.052mm larger than the collapse angle. Figure 4 shows the relationship between material thickness and non-verticality.
3.4 Blanking surface quality
The blanking surface is the main indicator of the quality of fine blanking parts. It is related to factors such as material type, performance, metallographic structure, mold quality and cutting edge condition, lubricant and press adjustment. The structural components of the blanking surface include: smooth surface, tear surface, sag surface and burr surface. The expression method and significance of the blanking surface condition are shown in Figure 5, and its quality characteristics are expressed in three aspects.
Figure 5 Representation of punched surface
In the figure: S-material thickness; h-when fracture occurs, the minimum smooth surface part accounts for the percentage of material thickness S (%); l-when fish scale fracture occurs, the minimum smooth surface portion accounts for the percentage of material thickness S (%); b- The maximum allowable fish scale fracture width, the sum of b is not greater than 10% of the relevant contour; t - the allowable fracture depth is 1.5%S; e - burr height (mm); c - the collapse angle width is 30%S (maximum) ;d-The depth of the sag is 20%S (maximum) (30%S for toothed parts); E-The maximum width of the tear zone.
(1) Roughness of the blanking surface. The smoothness of the blanking surface is different in the blanking direction and at different positions along the periphery. That is, the sag side is better than the burr side. The roughness of the blanking surface is expressed by the arithmetic mean aR. Its value is generally Ra=0.2~3.6, which is divided into six levels (see Table 2). The measurement direction is perpendicular to the blanking direction; the measurement position is in the middle of the blanking surface (see Figure 6a). The relationship between the roughness of the blanking surface and the tensile strength of the material is shown in Figure 6b.
Table 2 Blanking surface roughness
roughness grade
1
2
3
4
5
6
Ra(µm)
0.2
0.4
0.6(0.8)
2.4
3.4
3.8(3.6)
code name
N4
N5
N6
N7
N8
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Figure 6 Relationship between blanking surface roughness and tensile strength
(2) Blanking surface integrity rate The blanking surface integrity rate of fine blanking parts is divided into five levels (see Table 3).
Blanking surface integrity rate
100%S
100%S
90%S
75%S
50%S
100%S
90%S
75%S
--
--
(3) Tear level of the blanking surface The tearing level of the blanking surface of fine blanking parts is divided into four levels (see Table 4).
Table 4 Tear grade of punched surface
E(mm)
level
0.3
1
0.6
2
1
3
2
4
(4) Expression method and significance of blanking surface quality Figure 7 shows the expression method and significance of blanking surface quality characteristics.
picture
Figure 7 Example of blanking surface length representation
In the example, the roughness of the blanking surface Ra=2.4μm; the integrity rate h=90%S; l=75%S; the tear level is 2.
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Figure 8 Find the collapse angle values tE and bE
The difference between fine blanking and general blanking
Fine blanking is a cutting-free processing technology. It is a precision blanking method developed on the general punching technology. It can obtain higher dimensional accuracy than the general punching parts in one stamping stroke. The blanking surface is smooth, the warpage is small, and the interchangeability is high. High-quality fine-blanking parts with good performance and improved product quality at a lower cost.
The difference between general punching and fine blanking lies in the different mold structures, which leads to essential differences in process. The picture below shows a comparison of the mold structures of the two processes:
