Bottom Bending Sheet Metal

The lower tonnage requirements are attributable to the angular clearance between the complementary angle of the punch and the included angle of the v die see figure 2.

Bottom bending sheet metal.

Bottom bending requires more pressure generates less spring back and creates more accurate angles. In bottoming and coining bending methods that usually produce 90 degree bend angles the line curves to the right of the vertical axis showing how the metal springs forward to conform to the die angle. In this process the work piece is only in contact with the edge of the die and the tip of the punch. This is mainly for the purpose of reducing the ir of the workpiece so as to reduce the stamping into ir position of the workpiece by the punch tip.

Bottom bending is similar to coining when done properly except that tonnage is greatly reduced. Bottom bending is often confused with air bending and leads operators to ask themselves why there is a difference in radius when bending a 90 and 80 angle. The punch tip and the die shoulders fig. Bottom bending also uses a punch and bottom v shaped die but bends the metal by bringing the die and punch together.

Refer to the chart for values for folder as well as various press brake tooling combinations. Geometry of tooling imposes a minimum bend dimension. The final category of sheet metal bending is known as bottom bending or bottoming in this method the metal sheet or plate lays flat while the top punch and die are brought together above and below the work using minimal tonnage. The punch is them forced past the top of the die into the v opening without coming into contact with the bottom of the v.

Don t forget that we must have our sheet metal in direct contact with the die shoulders at all times during the bending process. In bottom bending the punch and die never make full contact with the metal sheet. If we fail to do this the smaller than required leg will fall into the v opening and our bend will be imprecise. So the larger the v opening the larger the minimum leg or flange we must have on a profile.

Figure 3 coining bottom bending and air forming affect stressstrain properties differently. The curved line in the air forming zone shows how springback changes at different bend angles. With air bending fig. Use the minimum bend dimension values in the charts below for your minimum closeness of cutout to a bend.

Because the material is pressed into the bottom of the die the desired bend angle determines the specific die to be used. The tooling only touches the material at three points. This is because when bending the 80 the punch tip is not pressed into the sheet metal in the way bottom bending is. And unlike coining bottom bending can reproduce an inside bend.

The diagram shows one such mechanism. The v opening width required by coining is smaller than bottom bending generally is 5x the thickness of sheet metal.