3 Types of Bending

There are three kinds of bends utilized to create metal that every sheet-metal engineer, or operator of a press brake should be aware of: air bending, bottom bend and coining.

Air Bending

Air bent is the most popular method of bending employed by sheet-metal shops of today. The workpiece is in contact with the edges of the die’s exterior and the edge of the punch. The punch is then pushed through to the edge of the die and into the v-opening, but not getting into contact with it’s bottom. The v-opening generally is deeper than the angle required in the workpiece. This allows over bending to take into account the spring-back effect on the piece. In general, 30 degree tools is able to fully air bend while 90 or 88 degree tools is a good choice to air bend a portion. Recently, there has been the introduction of tooling with a 75 degree angle to permit full air bending without the restrictions on the use that are associated with acute punches. (Acute punches are usually knives that have no gooseneck.) Since the tip of the die doesn’t penetrate the workpiece the bent radius is determined nearly entirely by the width of the v-opening in the die’s bottom pneumatic press machine.

The greater the v-opening size, the greater the radius. There are advantages and disadvantages, the advantages being that the user can regulate the bend’s radius even when using identical material, thickness and materials by altering the die that is on bottom. This is a great way to make up for mistakes in layout or provide more designs. The same flexibility can cause problems for you, too since it can result in poor quality parts if the wrong dies are utilized. I recommend that the dies that you are using are clearly identified to indicate the gauge they are intended for and that the use of larger dies for greater radii should only be done after obtaining engineering approval. Be aware that you shouldn’t utilize smaller dies to bend larger gauges because of the danger of causing damage to the die. Follow the general principle that the v-opening needs to be at the very least (Rp. + 2* Mt.) To find a list of radius compared with v-openings, look up the chart of force for air bends. Another note about air bends is that they is not recommended to use it on mechanical brake presses due to their inherent error margin even just a few thousandths of inches can result in damaged parts.

Coining

Coining is a fundamental form of bending, whereby the workpiece is pressed between the die and the punch. The material is then put under sufficient pressure so that the edge of the punch is able to penetrate the material before it starts inflowing into the punch. This technique is extremely precise and repeatability. It doesn’t require expensive machines to perform. However, it produces extremely large quantities of tonnages when compared to air bent. Most often, it is more than fifty tons for every inch as opposed with 1 to 2 tons per inch in air bent. Due to these requirements for tonnage, damage to machines is much more than bottom or air bending. The tools required for coining have to be sturdy and durable, which could limit your tooling and the geometry choices. Because of the restrictions on tooling and the huge tonnages needed for coining, this process is uncommon in the world of press brakes.

Bottom Bending

Bottom bending shares a lot of similarities with both coining and air bending. When doing this, the die’s angle must match the angle that is intended for the workpiece, and it is adjusted by a few degrees to springback, which is why you see the existence of tools with a 88 degree angle to create 90 degrees angles. The workpiece first is lowered against the die, and then the radius of the die is forced into the piece, which creates an angle that is the same as the die, it is released, and the piece of work is retracted to meet the die. In contrast to coining, the material is not subject to too much pressure, so the metal flows. Due to this, there is still a spring back that must be compensated. To compensate, your angle, you should consider that the die can be lower that the angle the die is by just a couple of degrees. This allows for an over-bend to occur when the punch tip is pressed into the workpiece. However, it is not necessary to be bigger or otherwise, you’ll harm the tooling. This technique can provide an operator a high degree of repeatability when correctly configured. The process of setting up a bottom bending operation correctly requires understanding of the tooling, materials and tonnages as well as the timing of each of the steps , so it can be a benefit to a skilled operator.