Fine Blanking: The Ultimate Guide

The difference between ordinary blanking and fine blanking process

Comparison of cut-off faces of ordinary stamping parts and fine-blanking parts

During the punching process, before the punch contacts the material, the ring gear pressing plate is used to press the material on the female die by force, thereby generating lateral pressure on the inner surface of the V-shaped tooth to prevent the material from tearing in the shear zone and the lateral flow of metal.

6mm thick FORD handbrake fan part (Hand brake sector)

While the punching dies are pressed into the material, counter pressure from the ejector is used to compress the material.

Coupled with the use of a very small gap and the concave die with a rounded edge to eliminate the concentration of stress, so that the metal in the shear zone is in the three-way compressive stress state, eliminating the tensile stress in the zone, improving the plasticity of the material.

The bending, stretching, and tearing phenomena that occur in ordinary blanking is fundamentally prevented, which makes the material be blanked into parts in the form of pure shear along the edge shape of the die, resulting in a high-quality, smooth, even shear surface.

6mm thick TESLA seat parts

In fine blanking, pressing force, blanking gap and die edge radius are complementary to each other and are indispensable.

Their influences are interrelated, when the clearance is even and the radius of the radius is appropriate, a smooth section can be obtained with little pressing material.

6mm thick TOYOTA tubing composition flange

Extremely high flatness requirements

Fine Blanking Overview

01 What Is Fine Blanking

Fine blanking is the abbreviation of precision blanking.

Fine blanking is a kind of precision stamping process developed on the basis of general blanking.

Although it belongs to the same separation process as the general blanking, it is a processing method that contains special parameters.

The parts produced by it also have different quality characteristics.

Especially in the fine blanking and cold forming (such as bending, deep drawing, flanging, upsetting and extruding, press sinkhole, half-punch and extrusion, etc.) processing technology combined, fine blanking parts have the possibility in many fields (such as automobiles, motorcycles, electronics industry, etc.) to replace the previous parts processed by blanking, machining, forging, casting and powder metallurgy, thus exerting its great technical advantages and economic benefits.

02 Fine Blanking Classification

The various different methods of fine blanking are categorized as follows according to their technological methods:

03 Fine Blanking Working Principle

1.Difference between blanking and fine blanking

The fine blanking we often talk about is not fine blanking in the general sense (such as trimming, finish blanking and high-speed blanking, etc.), but fine blanking with a strong pressure plate (see below figure).

• P_R-gear ring force
• P_S–punching force
• P_G-backpressure

The basic principle of fine blanking is to use a special (three-way force) press to produce plastic and shear deformation of the material with the help of a specially structured die to obtain high quality fine-blanked parts.

2.Fine blanking process characteristics

The following table shows the characteristics of the two different process methods: general blanking and fine blanking.

3.Working principle of the die

The fine blanking machine is special equipment for realizing the fine blanking process.

As shown in below figure, there are three kinds of forces (P_S, P_R, P_G) acting on the die during fine blanking.

Before the start of punching through the ring force P_R, through the shear line outside the guide plate (6), so that the V-shaped gear ring (8) pressed into the material and pressed on the die, thus generating lateral pressure on the inner surface of the V-shaped gear ring to prevent the material in the shear zone tearing and the lateral flow of metal outside the shear zone.

At the same time, the counterpressure P_G is pressed by the ejector (4) in the shear line, which presses the material against the cams, and in the pressed state, under the action of the punching force P_S.

The metal in the shear zone is in a three-way compressive stress state, which increases the plasticity of the material.

At this point, the material follows the shape of the die edge and punches the part in pure shear form.

At the end of punching, P_R and P_G pressure is released, the die is opened and the parts and scrap are ejected by the ejector force P_RA and the ejector force P_GA respectively, and are blown out with compressed air.

• Punch
• Die
• Internal punch
• Ejector
• Eject rod
• Guide plate
• Press plate
• Ring gear
• Fine blanking materials
• Fine blanking parts
• Internal waste
• Ps–Blanking force
• PR-Ring gear force
• PG–Back pressure
• PRA-Discharge force
• PGA-Ejection force
• SP–Blanking gap

4.Fine blanking work process 

(a) The die is opened and the material is fed;

(b) The die is closed and the material inside and outside the cutting edge (blanking line) is compressed by ring force and counterpressure;

(c) The material is blanked with the blanking force P_S, and the pressing force P_R and P_G are effectively pressed in the whole process;

(d) At the end of the ram stroke, the punch is in the die and the bore waste is flushed into the dropout die;

(e) The ring force P_R and counter pressure P_G are removed and the die is opened;

(f) In the position where the toothed ring force is applied, the effect is to eject the bore waste and to remove the discharge force P_RA from the punching lap;

(g) In the position where the counterpressure is applied, at this point the effect is: the topping force P_GA from the die.

Material starts to be fed;

h) Blow unload or remove waste materials of fine-blanking parts and inner holes.

Material feeding is complete.

• P_R-ring gear force
• P_G-back pressure
• Ps-blanking force
• P_RA–discharge force
• P_GA-ejection force
• 1- Press plate
• 2-Concave mold
• 3-Blanking (blanking) punch
• 4–Ejector
• 5–Fine blanking material
• 6–Fine blanking parts
• 7-Punching inner hole scrap

Fine Blanking Parts

01 Fine blanking parts technology

The technology of fine blanking parts mainly refers to ensure the technical and usage requirements of the parts, and it should be the simplest and most economical in manufacturing under certain batch production conditions.

The main factors affecting it are:

(1) Technology in the construction of parts.

(2) Dimensional and positional tolerances of parts.

(3) Material properties and thickness.

(4) Quality of the pressed surface.

(5) Die design, quality of manufacture and longevity.

(6) Selection of fine blanking machine, etc.

The technology of the structure of the fine blanking part refers to the structural units that make up the geometry of the part, which includes: the determination of minimum fillet radius, aperture, wall thickness, ring width, groove width and punching modulus etc.

The below figure shows the limit values for the structural parameters of the optional fine blanking parts.

They are smaller than the general blanking parts, which is determined by the fine blanking principle.

However, reasonable structural parameters of the parts contribute to the improvement of product quality and the reduction of production costs.

02 Difficulty level of fine blanking parts

According to the geometry of the part and its structural units, it is divided into S₁, S₂ and S₃ in each of the diagrams.

• S₁-simple, which is suitable for fine blanking materials with shear strength Ks=700N/mm²
• S₂-medium, which is suitable for fine blanking materials with shear strength Ks=530N/mm²
• S₃-complex, which is suitable for  fine blanking materials with shear strength Ks=430N/mm²

In the range below S₃, fine blanking is not suitable, or special measures are required.

When using the range of S₃, the condition is that the punching element must be 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, the material is Cr15 (spheroidization), Ks=420N/mm², which determines its difficulty level.

• hole diameter d = 4.1 mm S₁
• scrap b = 3.5mm S₃
• gear modulus m = 2.25 mm S₂
• fillet radius R_a= 0.75 mm S₁/S₂

The maximum difficulty of this part is lap b, so the total difficulty is S₃ and can be fine blanked.

03 Technical requirements for fine blanking parts

1.Dimensional tolerances

The dimensional tolerances of precision blanked parts depend on: part shape, quality of tooling manufacture, material thickness and properties, lubricants and press adjustments, which can be selected from Table 1.

2.Flatness tolerance

The flatness of a precision punching part is the deflection of the part plane, which has the value:

f = h – s

As the fine blanking material is in the pressed state, the fine-blanking parts have a good flatness.

And this flatness varies with the size, shape, material thickness and mechanical properties of the parts.

Generally,

1) Thicker parts are straighter than thin parts;

2) Low-strength materials are straighter than high-strength materials;

3) Materials with a higher pressing force than that with a lower pressing force.

The surface of the material on the convex die side is always concave and the concave die side is always convex.

However, if the part needs to be stamped, creased, notched, bent, etc., or punched with a continuous die, the flatness may fluctuate widely due to local deformation or different directions of punching on the part.

But in any case, the flatness of the precision stamped part is always much better than that of a normal stamped part.

The below figure shows the general straightness measured at a distance of 100 mm.

3.Perpendicularity Tolerance

The blanking surface of a fine blanking part and the base surface become a certain angle tolerance (inverted cone), which is called non-perpendicularity.

It is related to the thickness of the material and its performance, the state of the punching cutting edge, the rigidity of the die, the adjustment of the press, etc..

Generally, when the thickness of the material is 1mm, the non-perpendicularity is 0.0026mm, if the thickness of the material is 10mm, the burr side is 0.052mm larger than the sunk defect.

Below is the relationship between material thickness and non-perpendicularity.

4.Blanking surface quality

The blanking surface is the main indication of the quality of fine blanking parts.

It is related to factors such as material type, properties, metallurgical organization, die quality and cutting edge condition, lubricant and press adjustment etc.

The structural composition of the blanking surface includes: smooth surface, splitting surface, sunk defect surface and burr surface.

The representation and significance of the condition of the blanking surface are shown in below figure, with three quality characteristics.

In the figure:

• S – material thickness.
• h – minimum finish fraction as a percentage of the material thickness S at break (%).
• l – minimum finish fraction as a percentage of the material thickness S with fish scale fracture (%).
• b – the maximum permissible fish scale fracture width, where the sum of b is not greater than 10% of the relevant profile.
• t – allowed fracture depth is 1.5% S.
• e – burr height (mm).
• c – 30% of the collapsed corner width S (maximum).
• d – 20% S (maximum) for the depth of collapse (30% S for tooth shafts).
• E-Maximum width of the splitting tape.

(1) Blanking surface roughness

The finish of the blanking surface differs in the blanking direction and in different positions along the periphery.

That is, the collapsed side is better than the burr side.

The roughness of the blanked surface is represented by the arithmetic mean value aR.

Its value is generally Ra = 0.2 to 3.6, which is divided into six grades (see in Table 2), and the measuring direction is perpendicular to the punching direction;

The measurement location is in the middle of the blanking surface (see in Fig. 6a).

The relationship between the roughness of the blanking surface and the tensile strength of the material is shown in Fig. 6b.

Table 2 Blanking surface roughness

(2) Blanking surface integrity rate

There are five levels of intactness on the blanking surface of fine-blanking parts.

(3) Splitting grade of blanking surface

There are four levels of splitting on the blanking surface of fine-blanked parts.

(4) Method and significance of the quality of the blanking surface

The representation and meaning of the quality characteristics of the punching surface are shown in below figure.

For example,

• the roughness of the blanked surface is Ra = 2.4 μm;
• the finish is h = 90%S;
• l = 75%S;
• the tear grade is 2.

04 Collapse of precision blanked parts

Collapse angle refers to the sagging plastic deformation of the irregular convex curve at the junction of the inner and outer contour plane and the smooth surface of the fine punching parts (see Figure 8).

Collapse size is related to material thickness, material, part shape, back pressure and tooth ring height, etc..

The calculation method of the collapsed angle can be selected with reference to the below figure.

Generally, tE≈(5～10)S, bE≈(5～10)tE.

Calculate the value of collapse angle tE and bE

05 Burrs on precision blanked parts

Burrs are irregular protrusions on the end of the blanking surface of fine blanked parts.

Its size depends on the type of material, clearance, the condition of the die cutting edge, the depth of the die into the die and the number of times of blanking.

The burr produced during fine blanking is not a cutting burr but an extrusion burr.

The size of the burr is determined not only by the height of the burr, but also by the thickness of the burr root.

According to VDI3345 standard, when the die edge is sharp, only a thin burr is produced, e = 0.01 ~ 0.08mm; when the die edge becomes dull, a thick burr is produced, e = 0.1 ~ 0.3mm (see below figure).

 06 Dimensional tolerances

Because fine blanking is a flow-shear process, during fine blanking, the blanking die first causes a strong deformation of the crystals of the metal tissue, which then separates.

A certain fine blanking material affects the surface quality, dimensional accuracy and tool life of fine blanked parts.

The basic requirements for it are.

1. It must have good malleability and large denaturing capacity

This primarily allows the flow of material in the shear zone to continue until the end of the shear without tearing.

The best results of fine blanking are obtained with steels with a tensile strength δb ≤ 650 N/mm² and a carbon content of 0.35%.

[1] Fine blanking performance of the material

• -Tensile strength
• -Yield limits
• -Extension rate
• -Hardness

-Degree of deformation of carburizing bodies and carbides (spheronization)

[2] Deformability of materials

• -Low yield limit
• -Low tensile strength
• -High fracture elongation
• -High face shrinkage

Fine blanked materials with higher values of elongation at break and end-shrinkage have higher deformation properties.

A low yield limit means that the material starts to flow at low pressure.

The appropriate range for the strength of fine blanking materials is shown in the below figure.

The carbon content in the figure is calculated as equivalent carbon content.

2. It must have a good organizational structure

Fine blanking materials have higher requirements for the metallurgical organization.

The same material with different heat treatment, its metallurgical organization and elongation are different, which has a significant impact on the quality of fine blanking.

For carbon steel and alloy steel with a carbon content greater than 0.35%, The shape and distribution of cementite (Fe3C) have a decisive influence on the surface finish of shearing.

Among them, the carbides after spheroidization (after spheroidization annealing) are uniformly distributed in fine-grained form, and the chip pearlite structure is very good, which is difficult to punch out a smooth cut surface.

The below figure shows the carbon steel with 0.45% carbon, due to the different metallographic organization, different shear surface quality is got.

On the left is the untreated pre-ferrite pearlescent structure and on the right is the spherulitic carburized body after spheroidization.

3.Cold hardening during fine blanking

As the fine blanking is the extrusion – shear process of the composite, the material in the shear zone of the crystal tissue produces a strong cold deformation.

The prominent performance is that the hardness of the material in the cold work hardening zone is significantly increased than the hardness of the matrix.

For this reason, it is necessary to grasp the deformation law of cold hardening in fine blanking and determine the size, shape and depth of cold hardening as well as the actual effect on fine blanked parts.

Fig. 12 shows the cold hardening of materials during general blanking and fine blanking.

07 Selection of Fine Blanking Materials

1. Selection principle

It is technically necessary to meet the functional requirements of the fine-blanked parts, while considering economics to reduce costs.

It includes: type and state of supply, dimensional tolerances, surface quality, and precision blanking difficulty.

2. Material variety

Ferrous metals include: soft steel (C≤0.13%); unalloyed steel (0.12-1.0%C); alloy steel (0.15-0.20%C); stainless steel (C≤0.15%); fine grain steel (0.10-0.22%C).

Non-ferrous metals include: copper and copper alloys; aluminum and aluminum alloy.

3. State of supply

For steel requirements:

1. Types of supply: hot-rolled strips, cold-rolled strips, flat bars, but in different states, it has annealed, softened annealed, spheroidal annealed, etc.
2. Size: it is determined by the design of the die.
3. Thickness tolerance: It should be consistent with the parts.
4. Surface quality: different rolling methods are to get different surface quality, which has pickling, sandblasting, pickling, cold-rolled, etc..
5. Metallographic organization: according to the requirements of the product parts, it is divided into three levels:

FSG I: Maximum tensile strength, without the requirement for the metallurgical organization.

FSG II: after annealing treatment, material C>0.15%, containing about 80-90% spherical carburizing bodies.

FSGIII: Softened and annealed, material C>0.15%, containing about 100% spherical carburizing body.

For non-ferrous metals copper, aluminum and their alloys have a chemical composition and rolling state requirements.

4.Fine blanking evaluation

The evaluation of fine blanking materials and their selection is shown in Table 5.

Notes:

1. Very good-ideal fine blanking material, with a high fineness of blanking surface and long die life.
2. Good-suitable fine blanking material, with the smoothness of the blanking surface and normal die life.
3. Fairly well -barely fine blanking material, when used for parts with complicated shapes, the blanking surface is torn, and the die life is short.

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