How to Avoid Press Brake Bending Accuracy Problems?

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Publish Date: November 29, 2022

Various errors can occur during sheet metal bending using a press brake.

Inaccurate bending radius, insufficient bending force, and improper die clearance are some of the common causes of these errors.

Incorrect positioning of the back gauge and errors in bending calculations also contribute to inaccurate workpiece bending.

The actual bending angle of the workpiece may sometimes differ from the calculated angle.

This can be due to improper leveling of the press brake or a malfunctioning crowning mechanism.

Press Brake Parallelism

Various errors can occur when bending sheet metal using a press brake, including inaccurate bending radius, insufficient bending force, improper die clearance, positioning error of the back gauge, and errors in bending calculation.

The angle of the workpiece can also deviate from the calculated angle if the press brake is not level or if the crowning mechanism is not functioning properly.

Over time, the ram and workbench of the press brake will experience deformation, which can lead to uneven force on the sheet metal and decreased bending accuracy.

To counteract this, the crowning mechanism must compensate for the deformation of the ram and workbench.

There are two types of crowning mechanisms for press brakes: hydraulic crowning and mechanical crowning.

The hydraulic crowning mechanism, found on electro-hydraulic press brakes, compensates for the deformation through the use of hydraulic cylinders on the beam and under the workbench, which generate downward and upward forces respectively.

The compensation force can be adjusted based on the sheet metal thickness, tensile strength, and die opening size, and is controlled by the numerical control system.

Mechanical crowning uses a triangular wedge structure and requires placing two base plates, composed of several wedges, above and below the workbench.

The base plates are connected by disc springs and bolts, and a motor is used to move the wedge relative to the base plates, forming a curve that offsets the original bulge.

Appropriate Sheet Metal Bending Method

Sheet metal bending accuracy is also influenced by the bending method used. There are three main methods of bending: air bending, bottoming, and coining.

These methods are differentiated based on the relationship between the end die position and the thickness of the sheet metal.

The air-bending method does not require full contact between the die and workpiece.

This method requires relatively low bending force, and the punch presses the sheet metal into the U- or V-shaped die, using two points on the die shoulder.

The angle of air bending is determined by the shape and stroke of the punch and lower die, and a proper stroke depth results in more accurate bending.

However, the angle of air bending can change due to the springback after load release, which varies based on the compressive strength of the material.

To modify the angle, some pressure needs to be applied to make adjustments.

The bending angle error for air bending is typically around 0.5 degrees.

In the bottoming method, the workpiece is positioned at the opening of the punch and V-shaped die.

The size of the V-shaped die opening is 6 to 10 times the thickness of the sheet metal, and the opening size varies based on the bending angle and material thickness.

The springback of the sheet metal is less after the load is released, resulting in higher accuracy.

Finally, in the coining method, the punch presses the material into the lower die completely.

This method requires a high bending force, which can shape the material permanently.

The springback after coining is minimal, making this method highly accurate for bending.

Sheet Metal Bending Parameters

In addition to selecting a suitable bending method, it is crucial to determine the bending parameters of the workpiece.

During the process of metal bending, the inner surface of the metal undergoes compression while the outer surface is stretched.

To ensure the accuracy of the bend, it is necessary to know the tensile value of the material and calculate the minimum flange tolerance length.

The parameters involved include the bending radius, K factor, bending deduction, bending allowance, setback, etc.

If the material properties are inconsistent, the bending angle of the workpiece may be affected when using air bending.

Additionally, if the plate thickness remains constant and the die opening becomes narrower, the bending angle will be more pronounced.

Material Properties

If the material properties are inconsistent, the angle of the workpiece may vary when using air bending.

Additionally, if the plate thickness remains the same but the die opening narrows, the bending angle of the workpiece will change even more.

It's important to note that even though the material properties may be inconsistent, they may still fall within the thickness and strength tolerances of the mill.

This is because the tensile strength of many materials falls within a certain tolerance range.

Another factor to consider is the outer surface of the sheet, as different natural texture directions require different bending pressures.

It's crucial to keep in mind that these values may not be the most precise, so adjustments to the angle and length may be necessary during bending.

Press Brake Balanced Operation

For smooth and precise sheet bending, it is necessary to balance the press brake.

To do this, the intermediate frame of the bending machine should be supported on a sturdy bearing surface and clamped at one end while being supported at the other end.

During the operation, the lower two supporting claws of the press brake should be made to touch the workpiece's supporting surface evenly and then locked into place.

The upper cover should then be tightened and the position of the upper support claw adjusted until it is properly secured.

It is important to ensure that all the supporting claws of the intermediate frame are applied evenly throughout the process.

To avoid wear on the workpiece surface, a layer of pure copper sheet or fine emery cloth should be placed between each supporting claw and the supporting surface of the intermediate frame.

By following these steps, the press brake can be balanced and run smoothly.

Conclusion

This article presents various ways to enhance the accuracy of workpiece bending.

These methods include the selection of bending method, the leveling and compensation mechanism of the machine, the choice of bending materials, and the precision of bending parameters.

ADH is a sheet metal processing machine manufacturer and offers customers efficient and accurate bending with its press brake.

Our products are of high performance, competitively priced, and come with exceptional after-sales service.

To learn more about our press brake, you can explore our product page or get in touch with our product experts.

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