Leaf brake, box brake, finger brake, panel bender, etc.

This type of forming machine was originally a hand-operated machine and still is. But it has entered the 21st century with the addition of hydraulics and CNC controllers.

Figure 1
Figure 1

Usually the hand-operated leaf brake is limited to about 16 gauge material with a possible bend length limited only by the length of the forming beam. But in recent years the capacity of powered leaf brakes have increased dramatically.

The most common sizes for hand operated leaf brakes are: 24 and thirty 36 inch.

There are only a few types of bends that a leaf brake will not be able to perform and a few that it may do better than a regular press brake.

The leaf brake consists of a main housing, forming bar, forming bar and the clamping mechanism. The main housing is commonly cast iron or steel for durability and strength. A large flat area, known as the “platen” is machined into casting. Also, machined into the leaf brake are the various mounting features that are required for assembly.

Figure 2
Figure 2

The clamping bar is attached to the main housing body, lever-activated cam-driven that when engaged holds the material in place during the forming process.

This tooling, or “fingers” as it is commonly referred to, comes in a variety of radii, angles and lengths. These are either full, running the length of the forming bar, or sectionalized from .125 to 12.000 inches. These sections are then combined to build a specific tool length.

Once the clamping bar is engaged, the forming bar will then form the part, figure 2.

While this type of bending machine may look old and simple at a glance, perhaps even considered an unsatisfactory piece of equipment, but when it is operated by a skilled operator, this type of machine is very capable high quality and accuracy.

Figure 3
Figure 3

The leaf brake has three basic adjustments: one for the material thickness, one for the backgauge location, and the tooling setback. The thickness adjustments are located on both sides of the forming bar, usually a bolt with a locking nut. Without placing the material into the leaf brake, set the distance between the bottom of the platen and the bottom of the tooling to 10% less than the material thickness, with the forming bar locked down into position. This will give the forming bar its clamping action during forming.

Figure 4
Figure 4

Caution, the clamping bar can be “Sprung” or bent if the clamping force is too great; do not force the clamping, it is not necessary.

Setting up the leaf brake

There are many factors and adjustments that need to be reviewed to ensure quality in your project. First is the relationship between the nose of the tool and the forming bar. Some machines are fixed, but most are adjustable. This means they can get out of parallel to the forming bar. 

To correct any error an adjustment can be made on both sides of the frame assembly. Use you square or calipers to ensure the measured distance from the edge of the platen to the nose of the tool is the same on both ends.

The next consideration is the actual radius produced in the part.  The radius is set by the distance, from forming bar to tool nose. The greater the distance the greater the resulting inside radius.

There are two different styles of manual Leaf Brakes: the first is a fixed gauge brake commonly found in Heating, Ventilation and Air Conditioning (HVAC)

On these types of machines, the forming bar adjusts in and out and the tool is fixed.

Figure 5
Figure 5

The second style is the precision leaf, Pan or Box Brake; the leaf is fixed in position and the tool moves to adjust for various gauges of material and inside radii.

Under normal conditions a material thickness is the proper setting between the forming bar and the tool nose for a 90° bend.

The backgauge is set by taking the inside flange dimension from the part and setting that distance from tool nose to backgauge. This method ensures that the backgauge is square to the forming bar.

You can also take your part with the mold lines defined, line up the mark with the nose of the tool, clamp and set your backgauges. Either way, adjustments and test pieces will be necessary.

Figure 7
Figure 7

The bend angle and springback allowance are set next. This is done through the use of a collar that spins, or pin that is moved from hole to hole in 15° increments.

By placing the pin in the desired hole and fine tuning, an accurate and precise bend can be accomplished every time. The pin’s motion is stopped by an adjustable stop.

One final note: it is best practice to bend a test piece once the brake is set up. The test bend will reveal the true measurable radius, thus making correct bend deduction possible.

Figure 8
Figure 8

Panel benders

Modern Panel Benders, (circa 2013) as shown in figure 1, are common in many sheet metal shops; their big advantage is the weight and major surface area the sheet rests on and is clamped to the table so only the flange is bent.

This means the issues related to bending  panels on a press brake are eliminated: the need for multiple operators, the back breaking is eliminated and your flanges remain consistent over time.

These machines are CNC controlled and programmable, but physically, they operate in the same way that hand-operated machines always have. The stops are many and “pop” up or down as necessary to gauge the parts.

Generally used for 16 and 18 gauge steels, they are capable of up to .125 inch soft aluminum. There are plate forming machines that are capable of .625-inch thick steel.

Most just use the forming bar to complete the forming, but some, like the panel bender in the video below, may also use specially designed die sets to complete the forming.

Modern CNC machines are also capable of reading and adjusting the bend angle to maintain angular consistency. While the radius in the material is still established by the relationship between the forming bar and the nose of the tool, this too is CNC programmable.

Video courtesy of Fasti, Inc.

The Folding Machine and Bi-Directional forming

Up-Down forming is a variant of the CNC panel bender. The difference is this: rather a forming bar completing the bend, an upper and a lower punch move up or down in a forming process while the workpiece is held static by the clamping dies, figure 9.

Figure 9
Figure 9

This allows for forming in both directions without the need to turn the sheet or workpiece over during forming. CNC controlled backgauges are used for manual operation or a CNC controlled robot can locate the bend line and move the workpiece to a new forming location.
Figure 9

This machine finds the correct tool length for each bend, sets them in position and moves them out of the way when necessary to remove the finished part from the folding machine. inside bend radius (like the others) is determined by the relationship between the clamping tool and the forming tool; the greater the distance, the larger the resulting inside radius.

    Courtesy of Prima Power

Additional recommended reading: Why buy new tooling for your sheet metal hand brake?

After reviewing this material you should now be able to:

  1. Define the differences between box, pan and leaf brakes.
  2. Describe what a panel bender is.
  3. Describe the parts and features of this type of bending machine.
  4. Show how the tooling is set up.
  5. Explain how tool alignment is accomplished correctly and why.
  6. Explain the relationships between the tool radius and the forming bar.
  7. Describe Multidirectional Forming.
  8. Explain single gauge pan brakes vs precision box brakes.
  9. To set the bend angle.
  10. Explain the advantages ot the panel bender over the press brake.

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