As we pointed out in the previous post metal sheet bending is the permanent deformation of the sheet under the effect of an external force, which has to be greater than the yield strength of the material. The force shapes the sheet beyond its elastic phase, until it reaches the plastic deformation zone.
The most common bending process is the V bending, in particular we’d like to underline the following techniques:
- Air bending: the punch forces the sheet into a V shaped die. The angle is defined by three contact points and it depends on the depth reached by the punch. It’s a simple and low-cost technique and it can be used with very thick sheets. Air bending requires the perfect positioning of the tools to avoid the possibility of sheet springback.
- Coining: differently from air bending, punch is pressed completely into the die. They have to fit together exactly so that there won’t be any springback. Punches and dies have to be specific for every angle and shape. It can be used with thin metal sheets.
- Hemming: it’s a two steps process. A 26°-35° pre-bend, by air bending process, and then a total or partial hemming depending on the force. This method ensures a greater stiffness of the sheet and an edge protection (no sharp edges). The applied force is greater than the air bending and it requires special tools. Hemming cannot be used on thicker sheets (up to 0.11in)