Bending process
The metal bending process is one of the most common sheet metal fabrication operations.
The bending process is done through the application of force on a workpiece. The force must exceed the material’s yield strength to achieve a plastic deformation. Also known as press braking, flanging, die bending, folding, and edging, this method is used to deform a material to an angular shape.
Folding and bending are terms used to describe techniques that plastically deform a sheet/plate/bar along straight lines. ‘Folding’ is typically said with respect to (thinner) sheet, and ‘bending’ when dealing with (thicker) plate/bar.
Whether or not a material can be folded or bent depends on both its Yield Strength and Ductility, as well as how likely it is to Strain Harden. Brittle materials cannot be folded/bent, and the power/strength/stiffness (and thus cost) of the machine required increases for stronger materials.
Bending machines
Many types of sheet bending involve the use of a machine called a brake, sometimes called a bending machine or a sheet metal folder. Force can be applied manually or with, for example, hydraulics. A hydraulic press can exert high forces and can therefore bend greater sheet thicknesses.
Such machines include both press brakes and panel benders, but the most important piece of sheet metal bending equipment is the brake, which comes in several distinct forms:
Cornice brake
Cornice brake is a simple bending machine — and the most widely used sheet metal brake in manufacturing — that clamps a piece of sheet metal to a flat surface, then, via movement of a moveable bending leaf, uses force to make straight bends or simple creases.
Press brake
A press brake is a bending machine that uses a moving punch and a corresponding die. During the bending operation, sheet metal is placed on the die and the punch is moved with force into the metal, forcing it into the die opening. Depending on the shape of the die, a press brake can be used to make V bends, U bends, and other shapes.
Finger brake
A box-and-pan brake (also known as a finger brake) is another kind of sheet metal brake that uses a row of metal “fingers” to create multiple bend lines. As the name suggests, the box-and-pan brake is often used to make custom-size boxes.
Folder
A bar folder is a small and simple bending machine with a single handle that clamps the sheet metal and bends it in a single motion.
Know What Materials Are Good For Bending
Some materials are more malleable than others. That means that some will bend, whereas others will weaken and crack. For less malleable materials, it might be practical to heat up the workpiece to reduce the risk of cracking.
Here are some common materials that you might run into in sheet form and some information on how easy they are to bend without cracking.
Some common materials
| Mild Steel | This is very malleable, and you should be able to bend it cold without a problem |
| Spring Steel | Very bendable when fully annealed. You’ll need to heat treat it again to get it to perform like spring steel once it’s formed in the shape you want. If you try to bend it when it’s hardened, it’ll probably snap. |
| Annealed Alloy Steel | Wildly variable, unless you know the exact alloy. 4140 is usually pretty malleable, In general, you’ll always want it to be annealed if you’re planning on bending it or else it’ll crack. |
| 6061 Aluminum | Miserable to bend, cracking is very common, and cold bending will always weaken the metal. Proper bending can be done by annealing the aluminum first, although it’s not an ideal candidate for formed parts. |
| 5052 Aluminum | Highly formable, and one of the best kinds of aluminum to bend. Typically cracking or fatiguing is not an issue unless it needs to be unbent and reworked, but that’s pretty common of almost any malleable metal. |
| Copper | Super malleable, very easy to bend. |
| Brass | Formability will be affected by how much zinc there is in the alloy – the higher the zinc, the less malleable to brass. For simple bends in sheet metal it’s usually not a problem, but for something more complicated you may need to use heat to soften it up. |
| Bronze | Generally stiffer and more likely to crack. Use heat to improve formability. |
| Titanium | This is strong stuff, so you’ll need to be careful not to break your tools. To avoid cracking it, use a larger internal bend radius that you would for other metals. It also has a low modulus of elasticity, so you’ll need to overbend it considerably so it springs back in the shape you want. |
Press brake
Bending technology is often associated with various processes and machines employed in sheet metal fabrication.
Process
Bending using a press brake tooling is often accomplished in various steps.
The sheet metal is first held through clamps between a die and a matching punch. The ram of the machine, fixed with a punch, moves downwards thereby forming the desired bend.
Similarly, in panel bending, the metal sheet is positioned and held below the blank holder’s tool which often descends on the metal sheet and clamps it in place. With the workpiece material protruding on one side of the machine the bending blades from above and below move to bend the material.
Metal Folding
Folding can be carried out on machines like a cornice brake or a bar folder. The sheet metal is clamped in place before a clamping beam rises to bend the metal around a profile. Folding achieves a V-shaped bend and allows positive or negative bend angles.
- The blank is placed on the CNC-controlled integrated sheet support back gauge system.
- Blank is clamped between an upper beam tool and a lower clamping beam.
- The folding beam then sweeps up or down in accordance with the programmed direction of the first flange.
- It takes only one set of universal tools to complete the job.
Decades ago folding systems created only positive bends, like edge or return flanges around a box or panel, but modern folding systems can create both positive and negative bends. The folding beam does this by swinging upward and downward. To swing downward, it must pivot outward by the width of the folding tool. Say the beam tool is 0.6 in. wide. For the first upward bend, the top edge of the tool contacts the sheet to make the bend. To bend downward, the tool’s bottom edge must fold the material surface. This requires the beam’s pivot point to move 0.6 in.
Comparative review of Metal Folding & Bending operations
Accuracy
The dimensional accuracy achieved by folding is controlled by the metal folding machine – the bending edge is utilized as a reference while the folding angle is controlled by the flange angle of the folding machine. This ensures high precision. Conversely, bending has a lower short edge dimensional accuracy owing to back gauge positioning. In this sense, errors may accumulate on the inner surface since the bending is controlled by the upper die pressing amount with respect to the thickness of the material.
Sensitive material surfaces
Bending induces surface scratches on the workpiece since the material workpiece often moves relative to the lower die of the machine. This results in the indentation on the material surface. Similarly, a large workpiece may require many turnovers during a complex folding process and this may inevitably induce scratches on the surface of the workpiece. Conversely, folding operations do not induce surface scratches as the material rests on the metal folding machine table and the process is performed at a shorter cycle time and without the need for turnovers. This results in high surface quality.
Drive
Metal folding machines often employ all-electric drive build designs which require lower maintenance and are not susceptible to environmental factors. This results in high folding accuracy. On the other hand, bending machines are often hydraulically driven, hence requiring high-level maintenance and also may be susceptible to environmental factors.
Material consideration in metal folding and bending
The most important material property considered during bending and folding operations is malleability. Malleability is a measure of a material’s ability to be deformed without cracking. Malleable materials are easy to bend and fold without cracking. However, less malleable materials often require heat treatment to achieve desirable bending and folding effects. Table 1 illustrates the materials considered for bending and folding.
Material considered for bending and folding
| Material type | Malleability |
|---|---|
| Aluminium 6061 | Has very low malleability hence difficult to bend as it results in cracking |
| Aluminium 5052 | Highly malleable and therefore easily folded and bent as desired |
| Annealed steel | Bendability and foldability depend on the alloy designation. 4140 steel exhibits good malleability |
| Brass | A high content of zinc reduces malleability. However small bending for a complex part may require heat treatment |
| Copper | Very malleable hence easy to fold and bend |
| Mild steel | Has high malleability |
| Stainless steel | Grade 430 and 304 stainless steel have good malleabilityGrade 316 stainless steel is austenitic with good formability and malleability characteristicsGrade 4003 stainless steel is ferritic stainless steel with high stiffness and excellent malleabilityHowever, 410 stainless steel is brittle and less malleable |
Material thickness
Generally, folding operations can be performed on relatively thicker material as compared to bending. However, the thickness of a material that can be achieved through folding and bending operations depends on the type of material. Table 2 illustrates the maximum thickness that can be achieved in folding and bending operations.
Maximum thickness for folding and bending processes
| Process | Material | Thickness | Max. Fold /Bend Length | Max. fold / bend Angle |
|---|---|---|---|---|
| Metal Folding | Mild steel | 15mm | 8m | 45 degrees |
| Aluminium | 15mm | 8m | 45 degrees | |
| Stainless steel | 10mm | 8m | 45 degrees | |
| Metal Bending | Mild steel | 6mm | 4m | 90 degrees |
| Aluminum | 8mm | 5m | 90 degrees | |
| Stainless steel | 4mm | 2m | 90 degrees |
Material thickness tolerances
In the metal folding sequence, the tool for the folding beam approaches the contact from the outside of the workpiece and the material often moves at an exact position as per the programmed angle. As such, the reference side of the material is only on the outer surface of the material hence ensuring high precision. The movement of the metal folding machine as per the programmed folding angle ensures extremely high accuracy. The tolerances of the material thickness do not affect the respective bending angle hence no angle measuring instrument is required.
On the other hand, in a metal bending sequence, the machine’s punch often contacts the material from both sides of the die. This implies that contact points are on both material sides which may compromise the dimensional integrity. The bending angle is measured from the linear upper movement of the tool. The tolerances for the sheet thickness may result in variation in the bending angle hence, during bending, an inflexible measuring instrument is required for compensating the variances.
7 differences between press brake and folding machine
Work-principle
Bending machine: control the bending angle by controlling the amount of pressure on the upper knife. When bending the short side, the operator needs to hold up most of the external material. When bending large workpieces, two or more employees collaborate.
Folding machine: After the plate is laid flat on the worktable, the edge holder is pressed down to fix the plate, and the edge holder is turned up and down to achieve hemming. In all hemming processes of one edge, manual participation is no longer required. Positioning and auxiliary turning and positioning work.
Bending accuracy
Bending machine: the dimensional accuracy of the control is the dimensional accuracy of the short side of the back gauge positioning. After the bending is completed, the final error accumulates to the inner hollow size. At the same time, the bending angle is controlled by the lower pressure of the upper die. This angle is related to the thickness of the material.
Folding machine: The dimensional accuracy of the control is that after the first hemming is completed, the first edge is used as the positioning reference. The final controlled size is exactly the inner hollow size required by the customer, and the bending angle is directly controlled by the flanging angle, Has nothing to do with the thickness of the material.
Bending quality
Bending machine: when working, the material will move relative to the lower die, leaving indentations without surface protection. When bending large workpieces, it needs to be turned and moved many times, and scratches will inevitably occur in the process.
Folding machine: When working, the blank holder and hemming beam cutter do not move relative to the material, thus completely avoiding surface damage. When large workpieces are bent, because the plates are processed flat, all processing on one side of the workpiece can be completed at one-time positioning, completely avoiding surface damage.
Operator
Bending machine: the technical level of the bending operators is relatively higher.
Folding machine: programming can be achieved intuitively by drawing with fingers, or the engineer can use software to achieve offline programming and use a USB or network connection to import the processing program. After the programming is completed, the main work of the operator becomes simple loading and unloading Work without skilled bending operators.
Tool configuration
Bending machine: products with different thicknesses and shapes require different upper and lower molds. When realizing some special bending (such as arc) requirements, it is necessary to replace the tool or transfer it to another machine tool to achieve this, increasing the transfer and transfer of semi-finished products. Temporary work.
Folding machine: By configuring a rotatable blank holder beam and installing two sets of blank holder tools at the same time, a single station can complete all the bending procedures. For arc bending or other special bending requirements, there is basically no need to replace the tools, just It can be fully realized by making changes in the program.
Tool life
Bending machine-due to the relative movement of the workpiece in the die, the tool will wear out and need to be maintained or replaced.
Folding machine: basically avoids the relative movement between the material and the tool, the tool is almost free from wear for a long time, and the tool life is greatly extended.
Drive system
Bending machine: hydraulically driven, which brings more maintenance work and is easily affected by environmental factors.
Folding machine: adopts all-electric drive design, which reduces maintenance workload and has nothing to do with the use environment, thereby achieving higher bending accuracy.
Conclusion
A review of the operational techniques shows that Folding Machines are well suited to handling large panels, multiple-setup components, and surface-sensitive materials. When the overall weight of the part is taken into consideration, it becomes easier to maneuver and position large panels when a sheet support system and back gauge system are used. Thus, the cost of production can be minimized.
It has also been determined that folding offers outstanding advantages over bending. Due to its machine-controlled folding angle, folding provides high precision and accuracy; thereby, performing well with thicker materials than bending. Compared to the bending operation, folding produces high-quality products that are free of surface scratches.