There are three types of bends used to form metal that any sheet metal engineer or press brake operator must be familiar with; Air Bending, Bottom Bending and Coining.
Air Bending is the most common type of bending process used in sheet metal shops today. In this process the work piece is only in contact with the edge of the Die and the tip of the Punch. 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. The v opening is typically deeper than the angle which is sought in the work piece. This allows for over bending, compensating for the Spring Back of the work piece. Typically 30 degree tooling can be used to fully air bend and 90 or 88 degree tooling can be used to partially air bend. There has recently been the introduction of 75 degree tooling to allow for full Air Bending, without the tooling restrictions of Acute Angle Tooling. (Acute punches are almost always knife dies with no goose-neck.) Because the punch tip does not penetrate the work piece the inside radius of the bend is controlled almost entirely by the size of the v-opening of the bottom die. The larger the v-opening the larger the radius will be. This has both pros and cons, the pros being that the operator can control the radius of a bend even when working with the same material and thickness just by changing the bottom die. This can be used to compensate for errors in the layout or achieve a wider variety of design options. This same versatility can work against you as well as it can lead to bad parts if the wrong dies are used. Dies should be clearly marked for their intended gauges. The determination of tooling is done with the assistance of an Air Bend Force Chart. A further note on Air Bending is that it should almost never be used on older, mechanical, Brake Presses because of their inherent margin of error, even the difference of a few thousands of an inch can result in bad parts.
Coining is a very basic type of bending in which the work piece is stamped between the punch and die. The material is put under enough pressure that the punch tip penetrates the material and it begins to flow into the die. This method produces excellent accuracy and repeatability, and does not require sophisticated machines to execute. It does however very large tonnages compared to Air Bending. Often times in excess of 50 tons per inch of material, compared to 1 or 2 tons per inch for Air Bending. Because of these tonnage requirements wear and tear on the machines will be much greater than air or Bottom Bending. Tooling required for Coining must be robust and this can limit your tooling and geometry options. Because of the tooling restrictions and the large tonnages required to coin this process is rare in the press brake world.
Bottom Bending has similarities to both Air Bending and Coining. In this process the die angle should match the intended angle of the work piece, adjusting a few degrees for Spring Back, hence the existence of 88 degree tooling to achieve 90 degree angles. The work piece is first bottomed against the die, then the radius of the punch is forced into the work piece which achieves the angle of the punch, it is then released and the work piece springs back to meet the die again. Unlike Coining however the material is not under so much tonnage that the metal flows. Because of this there is still Spring Back which must be compensated for. In order to do compensate the angle of the punch can be smaller than the angle of the die by a few degrees allowing an over-bend when the punch tip is forced into the work piece, it should not be larger or else you will damage the tooling. This method can give an operator excellent repeatability if properly set up. Setting up a Bottom Bending operation accurately requires knowledge in the tooling, material, tonnages and even the timing of the different steps so it lends itself to a more skilled operator.