Blanking questions have you on the edge?

The condition of a blanked edge can affect the success or failure of downstream stamping operations.
Blanking is the part of the stamping process in which the stock material is cut into a designated shape before it goes through subsequent forming processes. Blanking—like piercing, parting, notching, and trimming—is fundamentally a shearing process. Therefore, like any other sheared sheet metal part, a blank undergoes a predictable process and yields a predictable cross-sectional profile.

As the blank undergoes straining, stretching, bending, and lateral expansion, any imperfections on its edge that have occurred as a result of the shearing will be exaggerated. The blanked edge should be examined carefully, as its condition plays a role in the success or failure of subsequent forming of the blank.

  1. What edge characteristics should I look for?
    The sheared edge exhibits some distinctive features, each of which may affect the subsequent forming processes. They include rollover, burnish, fracture, and burrs (see Figure 1). Rollover occurs as the punch engages the sheet metal and pulls the material downward, drawing the material slightly into the clearance. As the punch continues to penetrate and shears the upper portion of the material, the material becomes locked and burnished in the punch and die clearance.

Unstable shearing of the material can roughen the surface and create microcracks along the edge or cause the edge to fracture. When it fractures, the material is sheared at an angle against the blanking direction. This inclined fracture on the edge creates burrs, or protruding ragged edges (see Figure 2). Burrs often interfere with the success of subsequent forming processes. When the fractured edge is subjected to stretching or bending during forming processes, the microcracks can develop rapidly into cracks. The stress concentration can split these cracks instantly. Theoretically, the stress concentration in the cracked tip is infinite (see Figure 3).

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