Die Science: The process of fineblanking

Fineblanking is a specialty type of metal stamping that can achieve part characteristics such as flatness and a full sheared edge to a degree that is nearly impossible using a conventional metal cutting or punching process.

In fineblanking, the manufactured product needs to have a full contact surface on the edge of the part. As an example, a gear, which often requires critical flatness and must have every tooth fully engaged with the gear that it mates to throughout the entire thickness of the part, is an ideal candidate for fineblanking.

Fineblanking also can pierce very small holes with respect to the metal thickness, as well as holes very close to the edge of the part and close to other holes.

Conventional Punching
Fineblanking is easier to understand if you also understand what takes place during conventional metal punching.

The first thing to remember is that all metals have a particular elastic behavior. During conventional punching, the metal deforms upon initial punch contact. Figure 1shows the first step in piercing or cutting a hole in a piece of sheet metal.

When the punch makes contact with the sheet, the metal begins to deform and bulge around the point of the punch. As the yield strength of the part material is exceeded by the downward force of the press, the point of the punch begins to penetrate the metal’s surface. Both the punch and matrix, or button, begin to cut from their respective sides.

When the ultimate tensile strength has been reached, the metal breaks or fractures from the edge of the punch to the edge of the matrix. This results in a cut edge that appears to be partially cut and partially broken or fractured. This cut edge condition often is referred to as the “cut band.”

In most cases, the cut edge has about 10 percent to 30 percent of shear, and the remainder is fractured. The fracture has two primary causes:

The distance between the punch and the matrix creates a leverage action and tends to pull the metal apart, causing it to rupture.
The deformation that is allowed during the cutting process also allows the metal to fracture prematurely.
Allowing the metal to deform severely during the cutting process results in straining of the metal, which in turn causes stress. Trapped stresses in a product cause it to lose its flatness, which is why it is very difficult to maintain a critical flatness characteristic using conventional methods.

Figure 2shows a cut band created with a conventional metal punching process. The cut surface is partially angled and has a rough appearance in the fracture zone of the cut.

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