WHAT HAPPENS WHEN A HOLE IS PUNCHED?
As a punch is forced through material, certain predictable events take place from the time the punch touches the material. The following is a step-by-step account of what occurs in the split second it takes to punch a hole. The above drawing shows a typical punched hole in mild steel with 15 to 20% die clearance.
The first state of punching occurs as the punch makes initial impact and penetration causing a plastic flow of material called ROLL-OVER. All metal is plastic (or ductile) to some degree. The harder the metal, the less roll-over. When the material reaches the limit of plastic flow. the punch starts to cut or SHEAR through the material creating a straight shiny surface known as the STRAIGHT WALL and is the same diameter or configuration as the punch. The straight wall area grips tightly around the punch and necessitates pulling force to withdraw the punch from the hole. (In some cases, on softer steel, the hole will shrink slightly after the punch is withdrawn. ) The final stage of punching occurs when the material FRACTURES and BREAKS OUT all around the hole on an angle from the punch size to the larger die size. After the fracture, the slug is free, but it is essential for the punch to continue travel to enter the die slightly (1/64" to 1/32" max.) so as to completely free the slug from the material.
The relation of ROLL-OVER, STRAIGHT WALL, FRACTURE AND BURR vary widely with the hardness of material, amount of clearance between punch and die, and condition of tools and press.
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