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Punching force and its reduction measures

1. Calculation of blanking force

The punching force is exerted by the punch on the sheet during the punching process, and it is one of the important factors for selecting the press and designing the mold. Throughout the blanking process, the size of the blanking force is constantly changing, as shown in Figure 1-1. The OA section in the figure is the elastic deformation stage, and the blanking force on the sheet increases linearly with the downward pressure of the punch. Section AB is the stage of plastic deformation. Point B is the maximum value of the punching force. When the punch is pressed down again, cracks form in the material and expand rapidly, and the punching force decreases, so BC is the fracture stage. When reaching point C, the upper and lower cracks overlap, and the sheet has been separated. The pressure used by the CD is only to overcome the frictional resistance and push out the separated material. Blanking force refers to the maximum resistance of the sheet material on the punch. When the sheet material acts on the punch to produce the maximum resistance and cracks (point B in Figure 1-1), the shear in the shear deformation zone of the sheet material is used as the material’s shear strength (MPa).

For blanking with ordinary flat blades, the blanking force F can be calculated by the following formula.

F=KLtτ_b

In the formula F-punching force;

L—-The length of the punching periphery;

t—-material thickness;

b—-shear strength of material;

K—-Coefficient. The coefficient K is a correction coefficient that takes into account the influence of factors such as the fluctuation and unevenness of the mold gap value, the wear of the cutting edge, the mechanical properties of the sheet, and the thickness fluctuation in the actual production. Generally, take K=1.3.

In general, the tensile strength of the material σ_b=1.3τb. For the convenience of calculation, the punching force can also be calculated by the following formula.

F=Ltσ_b

2. Measures to reduce blanking force

When punching high-strength materials or thick materials and workpieces with large dimensions, the required punching force is larger, which exceeds the nominal pressure of the selected equipment. The following methods are commonly used to reduce the punching force.

• Step punch punching

In a multi-punch mold, different heights can be made according to the size of the punch, so that the working end faces are arranged in a stepped shape. As shown in Figure 1-2.

The force reduction principle of step punch punching is that it prevents several punches from being punched at the same time, avoiding the simultaneous occurrence of the maximum punching force of multiple punches, thus reducing the total punching force.

The height difference H between the punches depends on the material thickness.

Thin material: when t<3 mm, H=t;

Thick material: when t>3 mm, H=0.5 t.

When using a stepped punch, the thin punch should be as short as possible, which is beneficial to its strength; in addition, the punch should be arranged as symmetrically as possible to prevent the deflection of the mold. Step punch punching can reduce the punching force, reduce vibration, without affecting the accuracy of the workpiece, and avoid tilting and breaking of the small punch that is close to the large punch. When all the punches are of the same height, the small punch that is close to the large punch is affected by the material flow caused by the large punch, and it is easy to tilt or break the small punch. The disadvantage of this method is that the long convex mold is inserted deeper into the concave mold, which is easy to wear, and it is troublesome to sharpen the cutting edge. It is mainly used for molds with multiple convex molds and relatively symmetrical positions.

The punching force of the step punch is generally only calculated according to the step that produces the largest punching force.

• Blanking with oblique blade

Flat-blade blanking is to simultaneously punch the material along the entire periphery of the cutting edge, so the punching force is relatively large. If the punch (or die) cutting edge plane is made into an inclined plane that is not perpendicular to the direction of movement, the cutting edge will not be in contact with the periphery of the blanking part at the same time during punching, but will gradually cut the material away, which can significantly reduce Punching force.

For punching with oblique blades, in order to obtain flat parts, the punch should be flat blade when blanking, and the concave mold should be oblique blade. When punching, the concave die should be flat blade and the punch should be oblique blade. The oblique blades should also be arranged symmetrically, so as to prevent the die from shifting due to unidirectional lateral pressure during punching and gnawing on the cutting edge. The cutting edge forms of various oblique blades are shown in Figure 1-3.

Figure 1-3 shows the value of the height H of the inclined blade. When the material thickness t<3mm, H=2t; when the material thickness t=3~10mm, H=t.

The calculation formula of the blanking force of the oblique blade is

F _oblique = K _oblique Ltτ

In the formula, F _oblique —- blanking force of oblique blade;

K _oblique —- the parameter of force reduction, its value is related to the height H of the oblique blade. When H=1, K _slope=0.4~0.6; when H=2 t, K _oblique=0.2~0.4.

The advantage of oblique blade blanking is that the press can work under soft conditions. When the blanking parts are large, the force is reduced significantly. The disadvantage is that the mold is complicated to manufacture, the cutting edge is easy to wear, and it is difficult to grind. The blanking parts are not flat enough, and they are not suitable for blanking parts with complex shapes. Therefore, in general, try not to use them, and only use them for large stamping parts or thick plates Blanking.

When using oblique blade punching or step punch punching, although the punching force is reduced, the punch enters the fourth mold deeper, and the punching stroke increases, so these molds save effort and no effort.

• Heat punching (red punching)

Heating blanking is also called red blanking. Metal has a certain shear strength at room temperature, but when the metal material is heated to a certain temperature, its shear strength is significantly reduced, so heating and punching can reduce the punching force (heat the metal material to 700~900 ℃, The punching force is only 1/3 of normal temperature or even less).

The advantage of heating blanking is that the force is reduced significantly, but the disadvantage is that heating is easy to produce hydrogenated skin and damages the surface quality of the workpiece; and because of heating, the working conditions are poor. Heating blanking is generally used for the blanking of thick materials and the blanking of workpieces with low tolerance levels.

3. Calculation of discharge force, thrust force and ejector force

When punching, there is elastic deformation before the material is separated. At the end of punching, due to the elastic recovery of the material and the existence of friction, the blanking parts or punching waste are blocked in the die, and the remaining material is blanked. Hoop tightly on the punch. In order to continue the punching work, the material hooped on the punch must be unloaded, and the material stuck in the die must be pushed out. The force required to unload the hoop material from the punch is called the unloading force F _unloaded; the force that pushes the workpiece or wastes out of the punching direction from the die is called the force F_push. The force required for the workpiece or waste to be ejected against the punching direction is called the ejector force F _top.

It is difficult to accurately calculate these forces. The following empirical formulas are commonly used in production.

F _unload=K _unload F

F push = nK push F

F top=K top

In the formula F—-punching force;

F _unloading, F _pushing, F _top—-unloading force, pushing force, ejecting force;

K _unloading, K _pushing, K _top—-discharge force, pushing force, ejector force coefficient, see Table 1-4;

n—-The number of blanking pieces (or scraps) stuck in the die at the same time.

Note: Unloading force coefficient K is used when unloading holes, large edges, and complex contours, take the upper limit.

n=h/t

In the formula, h—-the height of the straight edge wall of the cavity of the cavity;

t—-The thickness of the sheet.

4. Determination of the nominal pressure of the press

The unloading force, pushing force, and ejecting force are transmitted by the press and the mold unloading device or ejecting device. Therefore, when selecting the nominal pressure of the equipment or designing the die, it should be considered separately.

When punching, the nominal pressure of the press must be greater than or equal to the sum of the various punching process forces F_total. The total calculation of F should be treated separately according to different mold structures.

When using the elastic pressure unloading device and the mold with the lower discharge method,

F _total==F+F _unloading+F _pushing

When using the elastic pressure unloading device and the mold with the upper discharge method,

F _total==F+F _unloading+F _pushing

When using a rigid discharge device and a mold with a lower discharge method,

F _total = F + F _push

Calculation of blanking pressure center

The pressure center of the mold is the point of action of the resultant force of the pressing force. The pressure center of the mold must coincide with the centerline of the pressure slider through the axis of the mold handle. Otherwise, the slider will be subjected to eccentric load during stamping, resulting in abnormal wear of the slider guide rail and mold guide part, and the reasonable gap will not be guaranteed, which will affect the quality of the parts and reduce the life of the mold, and even damage the mold.

1. Determining the pressure center of simple geometric figures

• The pressure center of the straight line is located at the center of the straight line.

• The pressure center of the symmetrical blanking part is located on the geometric center of the contour figure of the blanking part.

• When punching the arc line segment, the position of the pressure center, as shown in Figure 1-5, is calculated by the following formula.

x_o =180Rsina/πa =Rb/l

Here l—-arc length.

The meanings of other symbols are shown in Figure 1-5.

2. Determination of the pressure center of the multi-punch mold

To determine the pressure center of a multi-punch mold is to determine the pressure center of each punch and then calculate the pressure center of the mold. Figure 1-6 shows the position distribution of punches for punching multiple holes. The steps to calculate the center of pressure are as follows.

• Draw the position of the contour of each punch’s edge according to the scale.

• Draw the coordinate axis x, y at any position. When selecting the position of the coordinate axis, try to take the coordinate origin as the pressure center of a certain edge contour, or make the coordinate axis pass through the pressure center of the punch edge contour as much as possible. The coordinate origin should preferably be several punch edges. The center of symmetry of the pressure center of the mouth profile, which can simplify the problem.

• Calculate the pressure center and coordinate positions x₁, x₂…x_n and y₁, y₂…y_n of the punch edge contour respectively.
• Calculate the punching force F₁, F₂…F_n of the punch edge contour and the perimeter L₁, L₂…L_n of each punch edge contour respectively.

F₁=KL₁t_b

F₂=KL₂t_b

…

F_n=KL_nt_b

For the parallel force system, the resultant force of the blanking force is equal to the algebraic sum of the forces. That is, F=F₁+F₂+…+F_n.

According to the theorem of mechanics, the moment of the resultant force on a certain axis is equal to the algebraic sum of the component forces on the coaxial moment, then the pressure center coordinate calculation formula can be obtained.

Substitute F₁, F₂…F_n into the above formula respectively, then the pressure center coordinates become

3. Determining the pressure center of the mold of complex shape parts

When punching complex-shaped parts, the calculation principle of the die pressure center is the same as that of the multi-punch punching pressure center, as shown in Figure 1-7. Specific steps are as follows.

• Select the x-coordinate axis and the y-coordinate axis at any location in the edge contour.

• Divide the contour line of the cutting edge into a number of simple line segments according to the basic elements, and find the length of each line segment L₁, L₂…L_n.

• Determine the center of gravity position x₁, x₂…x_n and y₁, y₂…y_n of each line segment.

• Calculate the pressure center coordinates (x₀, y₀) of the edge profile according to the formula.

To determine the pressure center of the blanking die, in addition to the above-mentioned analytical method, it can also be used as a drawing method and a suspension method.

The drawing method is the same as the analytical method. It can find the pressure center of punching with multiple punches and the pressure center of the punching of complex shape parts. However, the precision of the drawing method is not high and the method is not simple, so Subject to certain restrictions in the application.

In production, the suspension method is often used to determine the center of pressure of complex blanking parts. Use uniform fine metal wires to bend along the punching contour to form a simulated part, then hang the simulated part with sewing thread and draw a plumb line from the hanging point; then take another point of the simulated part and make another part, in the same way, A vertical line, the intersection of the two vertical lines in the center of pressure. The theoretical basis of the suspension method is to replace the blanking force uniformly distributed on the contour of the blanking part with a homogeneous metal wire, and the center of gravity of the simulated part is the pressure center of the blanking.