What Is Press Brake Crowning and 2 Types of It

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What Is Press Brake Crowning

To guarantee accurate, repeatable forming results, it’s essential to compensate for the deflection that inherently occurs in the beam (ram) and table of the press brake when load is applied. Without deflection compensation, it’s likely that a workpiece will have some form of deformation at its center when it’s bent along the full length of the press brake. This is especially so for press brakes 8 feet or longer, 80 tons or more, and when bending long or large parts, but it also can be the case when forming shorter workpieces. To keep the bend angle consistent over the full length of the part, Therefore, it is necessary to take corresponding measures to reduce or eliminate the deflection caused by the deformation.The so-called deflection compensation device we call it crowning has preset a deformation in the direction of the opposite force-deformation in the ram and the upper die or the worktable and the lower die working table, and the deformation has the same amount of deformation generated in the actual work.a press brake needs a crowning system, either in the beam, in the table, or in both.

In this age of short batches, complex parts, and quick turnaround, crowning ensures efficiency, part accuracy, and repeatability. In high-tonnage applications, crowning helps produce straight bends in challenging, high-tensile-strength materials.

Types of press brake crowning

Hydraulic Crowning

The hydraulic automatic deflection Crowning mechanism of the workbench is composed of a group of oil cylinders installed in the lower workbench. The position and size of each Crowning oil cylinder are designed according to the deflection ram of the ram and the workbench finite element analysis. The hydraulic crowning realizes the bulge Crowning of the neutral version through the relative displacement between the front, middle and back three vertical plates. The principle is that the bulge is realized through the elastic deformation of the steel plate itself, so the Crowning amount can be adjusted within the elastic range of the workbench.

When the ram is down, the auxiliary cylinder is filled with liquid oil and goes downward.

During the bending process, hydraulic oil inlet into the auxiliary cylinder, so that the ram generate downward deflection for compensation.

Install the auxiliary hydraulic cylinder in the lower part of the worktable.

During the bending process, it generates an upward force on the worktable, which forms the automatic crowning system.

The pressure compensation device is composed of several small oil cylinders, comprising an oil cylinder, a motherboard, an auxiliary plate and a pin shaft, and a compensating cylinder is placed on the worktable.

The pressure compensation system is formed with a proportional relief valve.

During working, the auxiliary plate supports the oil cylinder, the oil cylinder holds up the motherboard up, just overcomes the deformation of the ram and the worktable.

The convex device is controlled by a numerical control system, so that the preload can be determined according to the thickness of the plate, the opening of the die and the tensile strength of the material when bending different sheet materials.

The advantage of hydraulic crowning is that it can realize the deflection compensation for continuous variable deformation with large compensation flexibility, but there are also some disadvantages of complex structure and in relatively high cost.

Mechanical Crowning

Mechanical crowning is a kind of new deflection compensation method, which generally uses a triangular oblique wedge structure.

The principle is that the two-triangle wedge block with α angles, the upper wedge moving is fixed at X-direction and can only move in Y-direction.

When the wedge moves the △x distance along the X-direction, the upper wedge moves up the H distance under the lower wedge force.

Regarding the existing mechanical compensation structure, two bolster plates are placed in full length on the worktable, the upper and lower plates are connected through the disc spring and bolts.

The upper and lower plates are consist of a number of oblique wedges with different slopes, through the motor drive to make them relatively moving and forming an ideal curve for a set of convex positions.

Three most commonly used bending methods-Air Bending,Bottom Bending,Coin Bending

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When we talk about press brake bending,we are familiar with three bending methods.Air Bending,Bottom Bending,Coin Bending.in this article we will explain all three bending methods.

Bending Method V-width IR Angle Accuracy Features
Air Bending 12T—15T 2t~2.5t ＞±45’ Can achieve a wider range of bend angle.
Bottoming 6T—12T 1t~2t ±15’—30’ The higher bending precision is obtained with the smaller press force.
Coining 4T—6T 0t~0.5t ±10’ It can achieve high bending precision, but the bending force is very large.

Air bending

Air bending means only part of the material is in contact with the toolings for bending.

From the above image, we can see that the toolings only touch A, B and C points of the metal during the bending process (the punch tip and the die shoulders). The rest position is not.

Because of the above reason, the actual angle of the toolings becomes unimportant.The factor that determines the bending angle is how far the punch descends into the die.The further the punch descends, the acuter the bend angle.Therefore, the fabricator can get a wide range of bending angles with only one set of tooling since the depth of the stroke (not the tooling) determines the bend angle.Besides, there will have a certain amount of spring back in air bending, so you need to bend a slightly more acute angle so as to get the desired bend angle.

Features of air bending:

  • Wide bending angle with one set of tooling. The angle can’t be smaller than the punch tip angle. If using a 30° punch, 180°-30° bending angle can be obtained.
  • The bending need less press force.
  • The bending angle is not in high accuracy.
  • The material has more spring back.

SEE ALSO: Air Bending Force Chart: The Most Authoritative Data From Amada

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Bottoming

Bottoming means the punch will descend to the bottom of the die so that the material makes contact with the punch tip and the sidewalls of the V-opening.

Bottoming is a method to obtain good bending precision with less pressure and is also a commonly used bending method.

V-opening width of the die can refer to below table:

T 0.5-2.6 3-8 9-10 ≥12
V 6T 8T 10T 12T

IR of workpiece

The interior radius of the workpiece is usually represented by IR.

During the bottom bending process, the IR is about 1/6 of the die’s V-opening (IR=v/6).

However, for different materials, the IR is also different, like SUS and Al has different IR.

Tooling accuracy of bottom bending

The angle after bottom bending will be affected by the spring back, so the bending spring back will be considered when choosing bottom bending.

The usual solution to obtain the target angle is by overbending.

  • Material, shape and thickness with small spring back – 90° tooling
  • Material, shape and thickness with big spring back – 88° tooling
  • Material, shape and thickness with bigger spring back – 84° tooling

When adopting bottom bending, the principle of using the same angle for both punches and dies should be abided by.

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Coining

The term “coining” is derived from the stamping method of the coin, which also means get very high accuracy.

For the coining process, enough tonnage of the press brake will be used to conform the sheet metal to the exact angle of the punch and die.

In coining, the sheet metal is not just bent, it’s actually tinned by the compress between the punch and die.The coining not only featured high accuracy, but also very small IR of the workpiece.The tonnage required by coining is 5-8 times higher than bottom bending.

V-opening width

The V-opening width required by coining is smaller than bottom bending, generally is 5X the thickness of sheet metal.

This is mainly for the purpose of reducing the IR of the workpiece so as to reduce the stamping into IR position of the workpiece by the punch tip.

Reducing the area of V-opening can obtain higher surface pressure.

Pressure limit

Because the pressure of bending is very large, the thickness of the SPCC should not exceed 2mm, and the thickness of SUS should not exceed 1.5mm.

The reason is that 2mm SPCC material need 1100KN pressure for bending which exceeds the allowable pressure of tooling 1000KN.

Note: different toolings have different allowable pressure, so not all toolings can be used to bend 2mm SPCC material.

Coining problem

The tonnage of the press brake needs to be increased due to the big bending force, and the abrasion of tooling will also become serious.

Therefore, only toolings with high allowable pressure can be used.

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Top 10 Considerations When Using Press Brake Toolings

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  1. Press brake toolings are made of high-quality steel products made by special heat treatment, with high hardness, not easy to wear and tear, able to bear great pressure, but each mold has its limits pressure: ton/meter, so choose the length of the mold when using the press brake toolings correctlyThat is, how much pressure per meter to bear, must not exceed the marked limit pressure.
  2. In order not to damage the die, we stipulate that align with the original point, it is necessary to use the upper and lower modes with the length of 300mm to do the alignment.The upper mold lower mold of the same height only can be used after the alignment.It is strictly forbidden to use the split small mold for the alignment, and the alignment must be based on the regulated pressure inside the press brake machine.
  3. In the use of the mould, due to the various height of the molds, the mold can only be used in a press brake machine with the same height, which can not be used in different heights.
  4. When using the press brake toolings, choose the right upper die and lower die according to the sheet metal hardness, thickness, and length.Generally, the slot width of the lower die should be 5-6 times the thickness of the metal plate, while the length should be longer than the sheet.The harder and the thickness of the sheet metal material, the wider of the lower die slot.
  5. When bending an acute angle or a dead angle, 30° punch should be chosen. Bending the acute angle first, then flattening.When bending R angle, R punch and R die should be chosen.
  6. In bending the long piece of work, it is better not to use a segmented mold in order to reduce the indentation at the mold connection position, and it is better to choose a single slot die, because the exterior angle R of lower die with single slot V is big, not easy to generate bending indentation.
  7. When selecting the top punch, we should know the type of die and the parameters well, and then decide which punch should be used based on the shape of the product that needs to be formed.
  8. When bending hard or too thick product, do not use press brake die to bend steel bar or other cylinder products.
  9. When using the press brake mold, the operators should have a clear mind.The upper die and lower die of press brake machine should be locked after alignment, in case the punch fall down to hurt the workers or damage the die.Attention should be paid when adding pressure during the operation process, do not add too much pressure and pay attention to the data changes on the display screen.
  10. After finish using the press brake toolings, put the die back in the press brake die cabinet (press brake store) according to the marking, clean the dust on the mold regularly, and apply the anti-rust oil to prevent rust which will reduce the precision of the toolings