How is the precision punching process force calculated
1、 Punching force P1
The magnitude of the punching force P1 depends on the total length of the inner and outer edges of the punching, the thickness of the material, and the tensile strength. It can be calculated according to the empirical formula:
P1=f1LtT·σb
In the formula, f1 represents the coefficient, which depends on the yield to strength ratio of the material;
Lt - total length of inner and outer perimeter (mm), Lt=Le+Li, where Le is the length of the outer perimeter and Li is the length of the inner perimeter;
T - Material thickness (mm);
σ b - the tensile strength of the material (N/mm2).
Considering that the clearance between the molds is small and the cutting edge has rounded corners during precision punching, the material is in a stress state of triaxial compression, which improves the deformation resistance compared to general punching. Therefore, the coefficient f1=0.9 is taken, and the punching force for precision punching is P1=0.9 Lt t σ b
2、 Edge pressure P2
The V-ring edge pressure has three functions:
Prevent materials outside the cutting zone from flowing with the punch during the cutting process;
Clamp the material and ensure that it remains perpendicular to the cutting direction without warping during the precision punching process;
Establish a stress state of triaxial compression in the deformation zone.
Therefore, the correct calculation and selection of edge pressure are closely related to ensuring the quality of the workpiece shear surface, reducing power consumption, and improving the service life of the mold
The edge pressure P2 is calculated according to the following empirical formula:
P2=f2Le·2hσb
In the formula, f2 represents the coefficient, which depends on σ b and can be obtained from the following table;
Le - length of the outer periphery of the workpiece (mm);
H - V-shaped tooth height (mm);
σ b - the tensile strength of the material (N/mm2).
3、 Counter pressure P3
The back pressure of the reverse pressure plate is also an important factor affecting the quality of precision punched parts. It mainly affects the dimensional accuracy, flatness, collapse angle, and shear surface quality of the workpiece. Increasing the back pressure can improve the above quality indicators, but excessive back pressure will increase the load on the punch and reduce its service life. Therefore, like the edge pressure, it needs to be adjusted to the formula as much as possible during the actual process while ensuring the quality of the workpiece:
P3=pF
In the formula, F represents the planar area of the workpiece (mm2)
P - unit back pressure (N/mm2), generally ranging from 20 to 70 N/mm2.
According to the above formula, the fluctuation range of the backpressure is relatively large, and it can also be calculated using another empirical formula:
P3=20% P1
4、 Total pressure Pt
The total pressure Pt required for the workpiece to complete precision punching is the main basis for selecting a precision punching press
Pt= P1 +P′2 +P3
In the formula, P1- punching force (N);
P'2- Holding pressure edge force (N);
P3- Counterpressure (N).
Why is the total pressure required to achieve precision punching not the sum of P1, P2, and P3? The reason is that in the practical process, it was found that the pressure P2 required for the V-ring edge pressing ring to press into the material is much greater than the pressure P'2 required for the V-ring edge pressing ring to maintain the quality of the workpiece shear surface during the precision punching process. Generally, P'2=30%~50% P2.