The tooling only touches the material at three points.
Bottom bending sheet metal.
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.
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.
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.
So the larger the v opening the larger the minimum leg or flange we must have on a profile.
Don t forget that we must have our sheet metal in direct contact with the die shoulders at all times during the bending process.
3a even less contact is made with the metal than with bottom bending.
Because the material is pressed into the bottom of the die the desired bend angle determines the specific die to be used.
In bottom bending the punch and die never make full contact with the metal sheet.
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.
If we fail to do this the smaller than required leg will fall into the v opening and our bend will be imprecise.
This is because when bending the 80 the punch tip is not pressed into the sheet metal in the way bottom bending is.
With air bending fig.
The v opening width required by coining is smaller than bottom bending generally is 5x the thickness of sheet metal.
In this process the work piece is only in contact with the edge of the die and the tip of the punch.
The diagram shows one such mechanism.
Geometry of tooling imposes a minimum bend dimension.
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.
Air bending is the most common type of bending process used in sheet metal shops today.
Bottom bending is similar to coining when done properly except that tonnage is greatly reduced.
For this reason the actual angle of the tooling is relatively unimportant.
Use the minimum bend dimension values in the charts below for your minimum closeness of cutout to a bend.
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.
Refer to the chart for values for folder as well as various press brake tooling combinations.
The punch tip and the die shoulders fig.
Bottom bending requires more pressure generates less spring back and creates more accurate angles.
Bottom bending also uses a punch and bottom v shaped die but bends the metal by bringing the die and punch together.
