47.4 BENDING

---

After basic shearing operation, we can bend a part to give it some shape.

Bending parts depends upon material properties at the location of the bend.

Some of the things that may/do occur in bends,

- material at the outer bend radius undergoes tensile plastic deformation

- material at the inner bend radius undergoes compressive plastic deformation

- the width (along the bend axis) will reduce in length based on poissons ratio

- if the bend radius is too small the plastic deformation at the outside of the bend will result in fracture.

The basic calculations for a bend radius are shown below,

The strain on the outermost fibers of the bend is,

After sheet metal is bent it will tend to spring back to a lesser angle. The following formula relates bend radius before and after release.

There are a variety of methods for springback compensating,

- trail and error to over bend

- calculated over bend

- special punch/die designs

The maximum bending forces may be calculated using the relationship below,

Press Brakes a small presses that will bend a sheet metal piece over several feet.

Some other operations done to sheet metal parts,

beading - the edges of the sheet metal are bent back (in a rounded shape) to stiffen the edges and eliminate the sharp edge.

flanges - a hole can be formed by punching through, and a lip (the flange) remains about the edge of the hole

Hemming - like a bead, but the edge is bent back as a flat.

Roll Forming - Bends can be made along long axes by using rollers.

When planning to bend a piece, car must be take not to punch holes too close to the bend.

Quality problems that occur in rolling are,

flatness

twist

straightness