Hemming is a common metalworking process mainly carried out to reinforce an edge, hide burrs or just improve the overall appearance of sheet metal parts. https://www.shenchong.com/pressbrake/

1. What is sheet metal hemming in CNC press brake bending?
Hemming in sheet metal operations refers to the bending of a sheet metal edge onto itself. It is very similar to edge stitching in clothes. Similar to how an edge stitch strengthens the edge and makes it more durable, a hem imparts strength to the metal edge and improves its appearance. The edge of one part may also be folded onto another part to create a joint.
Hemming is usually done in two stages. The first stage creates an acute bend using acute tooling (V die) followed by a flattening of the return flange using a flattening die.

2. Flat or closed hem
In a flat or closed hem, the part of the edge that bends sits completely flush over the rest of the metal sheet. The angle between the returning flange and the sheet metal is 180 degrees. The inside radius is zero and thus, there is no gap between the returning flange and the metal sheet.
Closed hems require a lot more power and tonnage from the presses than open or teardrop hems and it is also not advised for metal sheets that are over 23mm in thickness since the sheets are likely to fracture.

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