When the hole is turned over, the deformation zone is basically limited to the fillet of the die. Under uniaxial or biaxial tensile stress, the tangential elongation deformation of the material in the deformation zone is greater than the radial compression deformation, which leads to the thinning of the material thickness. The mouth of the vertical edge of the flanging hole is thinned the most. When the thickness thinning is too large and the elongation of the material exceeds the ultimate elongation of the material, the so-called P fracture occurs (the crack caused by the excessive elongation and insufficient plasticity of the material is called anal fracture; Cracking caused by excessive forming force and insufficient material strength is called type A fracture). When punching and flanging, the smaller the flanging coefficient k is, the greater the deformation degree is, and the greater the thickness of the vertical flange is, the easier it is to break. Therefore, the thickness of the vertical cutting edge cannot be ignored when turning the hole.