As one of the more traditional methods for sheet metal fabrication processing metal materials, the press brake, also commonly known as a brake press, has been around for over a hundred years.
Fast forward to today, press brakes are an essential machine for bending metal. They’re used to create more complicated bends that other plate bending machines aren’t generally equipped to handle, and press brakes have evolved into highly-advanced machines powered with hydraulic systems and computer technology. These modern brakes efficiently bend sheet and plate metal with ease, supplying a variety of industries with simple and complex parts.
Press brake bending requires different methods of approach to achieve the desired results needed. From forming wind tower poles to intricate electrical cabinet components, press brakes are a vital tool for the fabricator, and knowing that not all bending is the same is a key to their successful operation of them. Understanding the process, the tooling and the material (as all metals being bent will respond differently to each bending process) is vital to gaining accurate parts quickly and repeatedly.
How Does a Press Brake Work?
Press brakes work by using a punch and die that are set at predetermined angles to bend or cut metal into a variety of shapes and forms.
The basic principle of press brake forming relies on force, otherwise known as tonnage. This determines the total amount of pressure that can be applied by the punch during a bend. The higher tonnage, the thicker the materials that can be bent and vice versa.
Along with the tonnage comes bending length, which is the maximum length of sheet metal that can be bent. For example, if you have a machine with a 14′ bending length, any sheet metal larger than 14′ will be too long for that machine to process.
Depending on the application and material size/thickness, different machines with different tonnages and bending lengths are required for proper fabrication. In fact, these factors are important because they help determine the press brake’s load limit – a limit calculated in tons per inch.
The Bending Process of Press Brake
To cover the basics a press brake has a Ram fitted with a punch that forces the material being worked into a die. The material is then formed into an angle corresponding to the angle of the die or the depth at the punch forces it into the die. Press Brakes can be powered by Mechanical methods (Large Flywheel Turning a Crankshaft), Hydraulic Methods, Electric Methods, or a combination of these processes. Press Brakes can also be operated by manual or CNC Controls.
Manual press brakes would be set up by hand with each individual piece of sheet metal being bent to the shape needed one angle at a time. With CNC press brakes, the machine can be pre-programmed to bend the sheet metal through many various bends completing one part at a time vs only one angle. Regardless of the type of bending machine, press brakes all use a V blade (called a punch) to push on the sheet metal with enough pressure against a die to cause it to bend. These machines can bend sheet metal work from just a few inches to some that are many feet long.
Whether the press moves up or down, both of these methods will produce the same sheet metal components and there are no restrictions on the design of your component to suit either machine.
3 Types of Bending in Press Brakes
If you’re a newbie to metal fabrication, here’s a refresher on the essentials of bending. Air bending, bottom bending, and coining are the three main types employed by precision metal fabricators, each requires different strategies in tooling and press brake operation. Discovering how these bending processes work will best help you the fabricator, understand which method is best for your application.
Coining is one of the most popular bending methods used in metal fabrication today. The term comes from the process of pressing coins. In order to get the images, numbers, and lettering on a coin, there needs to be a great deal of force used to indent the metal.
Operators use a press brake that features a punch and dies to form the metal into specific angles. Here, the entire piece is put under the full tonnage of the press brake. This method requires each bend to be exactly the same, regardless of how many pieces are being made.
In this process, the sheet or plate lays down flat on top of a die. The metal is then compressed between the punch and die with an extreme amount of tonnage to create a precise bend angle in the metal. The die can be formed to have a wide variety of shapes or angles falling in the categories of obtuse, acute, and right angles; the bend options aren’t heavily restricted. But no matter what type of angle is used with the die, the sheet metal will bend precisely to that angle. Coining allows for a precise, consistent bend every time.
The coining method requires the largest amount of tonnage in a press brake, usually 3X to 5X that of the other bending methods.
Air Bending is the most common form of press brake forming we see today for a combination of reasons.
Air bending works by forcing the material only far enough into the die to achieve the desired angle plus a little bit more to compensate for the material springback. Again, different bend angles can be done using the same set of tools which is a highly financially beneficial benefit. The thickness of the sheet metal, its ductility, hardness and springback will determine the angle of the punch and die. This method allows for more accuracy because of its flexibility but requires an accurately positioned machine and accurately ground tooling to achieve success easily.
Air bending requires far less tonnage to achieve, the tooling only touches the material at the punch tip and die shoulders, so the actual tooling angle is relatively unimportant. The bend angle is determined by how far the punch descends into the die. The further the punch descends, the more acute the bend angle. Because the depth of stroke determines the bend angle, a simple 85° Die can achieve multiple angles. However, the bend angle cannot be equal to, or smaller than, the angle of your punch and die.
Since tonnage doesn’t produce the bend in air bending, you don’t need as much as with coining. And as with bottom bending, air bending will entail some degree of springback, so you may need to bend to a slightly more acute angle to get the desired bend.
The final category of sheet metal bending is known as bottom bending or “bottoming.”
The bottom bending process doesn’t require as much tonnage as the coining method, , the material doesn’t entirely conform to the tooling’s bend angle. But does require more tonnage than the air-bending method. Here, the metal is pushed down into the V-Die to make a bend. Because it doesn’t use as much pressure, there is a slight spring back. Bottom Bending is common practice on mechanical press brakes as the accuracy is again obtained from the toolset and not necessarily the positioning accuracy of the press brake.
In bottom bending, the metal sheet or plate lays flat while the top punch and die are brought together above and below the work using minimal tonnage, so the material makes contact with the punch tip and the V-opening sidewalls. It differs from coining in that the punch and die don’t fully contact the metal, and tonnage isn’t enough used to actually imprint or thin the metal.
In fact, the metal experiences “springback”, naturally relaxing to a wider angle after bending. Different materials and their thicknesses at the time of the bend will respond differently to springback, so it’s important that the machine operator fully understand the unique properties of each metal used.
In bottom bending, the punch and die never make full contact with the metal sheet. This creates a bend that is liable to spring back inward toward its original shape. To compensate for this, tooling must be used to create a slightly more acute angle than what is normally requested in the job specs. Once the sheet metal is released, it’ll naturally move back out a bit into the desired bend angle. For example, you may need your punch and die to be at 88° to achieve a 90° finished form. Different materials and thicknesses result in different amounts of springback.
Air Vs Bottom Bending
Once you’ve settled on a main type of press brake, you’ll have to decide which of the two bending methods you’ll prefer: air or bottom bending.
Air bending has a weaker force impact and requires the bending angle of the sheet metal to take the spring back into consideration. This is done by determining the thickness of the punching blade as well as the sheet metal itself. An air bending press brake is a great choice for people working with thinner materials such as sheet metal as they do not need too much power to be shaped.
Bottom bending shapes the metal with much more force and it also seriously reduces the spring back that occurs with air bending. Depending on the press brake you have, it may get rid of it altogether. This method is preferred by people who use metal sheets thicker than 2mm or those who need a lot of angular precision.