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Fundamentals of Blanking Process


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Blanking Die Design

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Shearing Deformation and Sheared Edges in Blanking Process

Blanking is usually considered the easiest type of metal stamping as it only involves a cutting or shearing process of the stamping stock using shear deformation. (Click pictures to enlarge).



Unlike machining, the surface finishing of the cut face is not uniform. There are:

  • Penetration - part of the material which has been pushed in by the punch
  • Sheared face - face which has been sheared by cutting edge pushing into the material. This face is burnished, smooth and shiny.
  • Fractured face - face which has split due to crack development. This face is uneven and rough.
  • Burr - Sharp protrusion produced by the crack generated. This is located at the side of the workpiece where the cut is completed.

The Shearing process within Blanking:



  • After the leveling period, penetration occurs right up to point A.
  • From point A to C, the sudden increase in shearing load forms a sheared face.
  • At point C, when the maximum shearing load is reached, the cutting edge produces the crack.
  • At point D, the crack propagates completely through the workpiece.

Influence of Clearance between Punch and Die cutting edges on the Sheared Face during Blanking




  • If the clearance is too small, the initial crack stops forming and shearing continues, resulting in a second sheared face.
  • If the clearance is small, the cracks from both upper punch and lower die cutting edges spread considerably but the cracks do not converge at the same points, resulting in tongue-shaped protrusion on the face which will drop off.
  • When the clearance is adequate, the cracks extending from the cutting edges will converge at a point.
  • In the case of excessive clearance, the cracks do not meet and will rupture the face cross-section. The face has very poor squareness and considerable penetration and burring.
There are times when the stamping press cannot achieve the required blanking accuracy and proper sheared face. While reducing the clearance would give better blanking accuracy and finishes, it is also important to eliminate the generation of cracks. Eliminating the crack propagation would also eliminate fractured faces and correspondingly extends the sheared face. The process, called fine blanking, is shown below:



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Blanking Force Calculation

The maximum blanking force must be calculated beforehand in order to determine the capacity of the press machine needed to perform the blanking. This maximum shearing resistance of the material varies slightly with clearance, surface finish of the cutting edge, sheared profile, lubrication oil and press machine, but by and large, it is about 0.8~1 of the tensile strength of the material.

P = t * L * K1 * σB

P = Blanking Force (kgf)
t = Metal thickness (mm)
L = Total Length of all cuts (mm)
K1 = Correction Factor (0.8~1)
σB = Tensile Strength of metal (kgf/mm2)

Tutorial Example:
What is the blanking force required to stamp a circular part of diameter 30mm out of S30C with thickness = 3mm?

According to the Properties of Raw Material section, the shear strength of S30C is 36kgf/mm2. L = circumference of circle = 30mm* π , t=3mm. Thus, P = 3*30*π *36 = 10178kgf = 10.2T

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Stripping Force Calculation

This is usually less than 10% of the shear load, but is considerably higher in fine blanking where the minute clearance produces a large sheared face.

Ps = K2 * P

Ps = Stripping Force (kgf)
P = Blanking force (kgf)
K2 = Correction Factor; 0.05~0.2 for general application; 0.3~0.4 when the stamped part has flatness requirement and high speed stripping is required. See relationship between stripping force per unit area of cut surface/tensile strength ratio and clearance (modified from R. Tilsley and F. Howard). 
Stripping Force versus Clearance
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