Bending Radius for Parts on a CNC Press Brake

When utilizing free bending, the bending radius is 0.156 times the die opening width. During the free bending process, the die opening width should be eight times the thickness of the metal material. For example, when forming low-carbon steel using a 1/2-inch die opening, the bending radius of the part will be approximately 0.078 inches. If the required bending radius is nearly as small as the material thickness, bottom bending (using a bottoming die) must be employed. However, the pressure required for bottom bending is approximately four times greater than that required for free bending.

If the required bending radius is smaller than the material thickness, a punch with a nose radius smaller than the material thickness must be used, and the coining method of bending must be applied. This approach necessitates a pressure level that is ten times greater than that required for free bending.

In the case of free bending, the punch and die are typically machined to angles of 85° or less. When using this die set, attention must be paid to the clearance between the punch and the die at the bottom of the stroke, as well as ensuring sufficient over-bending to compensate for springback and maintain the material at an angle of approximately 90°.

Typically, free bending dies on a new press brake produce a springback angle of ≤2°, and the bending radius is equal to 0.156 times the die opening width.

For bottom bending, the die angle is generally between 86° and 90°. At the bottom of the stroke, the clearance between the punch and the die should be slightly greater than the material thickness. The accuracy of the formed angle is improved because bottom bending requires a significantly higher tonnage (approximately four times that of free bending), which reduces the stresses within the bend radius that typically cause springback.

Coining is similar to bottom bending, with the distinction that the nose of the punch is machined to match the required bending radius, and the clearance between the punch and die at the bottom of the stroke is less than the material thickness. Because sufficient pressure is applied—approximately ten times that of free bending—to force the punch nose into full contact with the material, springback is essentially eliminated.

To select the lowest possible tonnage specification for the press brake, it is advisable to design for a bending radius that is greater than the material thickness and to utilize the free bending method whenever possible. When the bending radius is relatively large, it often does not affect the quality of the formed part or its subsequent use.