What Is the Hem in a Sheet Metal Bend?

Bending

• To bend sheet metal merely means to deform it around a single axis -- folding back one of the edges. Manufacturers usually place the metal on a die, pinion it in place and use press brakes to bend the edge of the metal in the desired direction. When they bend the sheet metal, the inside surface of the bend compresses, while the outside surface is stretched. Manufacturers must take these changes into account when determining how much sheet metal they need for a given application.

Hemming

• The first step in making a hem is to deform the sheet metal until the edge is bent back on itself and 30 degrees off the horizontal. In some cases, a 45-degree angle will work as well. At this point the metal is placed under a flattening bar, and additional pressure applied, until the edge folds back against the metal, just like the cloth in a hem. The metal must be held in place during this process or it could slide out from the die and ruin the hem.

Special Designs

• Some kinds of dies and presses are specially designed to facilitate bending and hemming. A two-stage hemming punch and die combination, for example, punches the sheet metal into a V-shaped channel to make the bend, then crushes the resulting bend beneath the edge of the same punch. Three-stage punches start by bending the metal in the same way, but the hem is formed by compressing the bend between an upper and lower pad on the die, rather than the die and the punch.

Considerations

• Hems are a good way to strengthen the product, conceal any imperfections and make the edges of the product safer to handle. They can also be aesthetically appealing. Some hems have more complex profiles; a piece of metal with a reinforced-edge hem, for example, is hemmed and then bent at a 90 degree angle along the edge. Double-hem edges are hemmed twice so you have a hem that is essentially 3 layers of metal thick.