Bending Accuracy of CNC Press Brake Explained

There are many factors affecting bending accuracy, among which are uneven plate thickness, plate hardness, workbench and ram deformation during bending, die opening selection, depth of upper die into the lower die, die wear, and workbench convexity. These factors can cause errors in the bending angle and straightness of the workpiece after bending.

The die opening and the depth of the upper die into the lower die are controlled by manual programming.

Influence of material quality on accuracy

The nominal bend is a free bend of low carbon steel plate with tensile strength σb = 450 N/mm², which bends the sheet into a 90° angle on the V-shaped die with an opening distance V = 8 × S.

When nominal bending is performed with plates of uneven thickness, angle errors may occur.

When the plate’s hardness is not uniform, the workpiece’s springback during bending is not equal.

Therefore, the quality of materials has a significant impact on processing quality.

Worktable convex compensation

Under the action of load, the workbench and ram will undergo elastic deformation.

The depth of the upper die into the lower die is uneven along the entire length, which can affect the bending angle and straightness of the workpiece.

To solve this problem, the machine adopts the NC convex of the worktable to compensate for its deformation, maintain the depth of the upper die into the lower die essentially uniform along the entire length, and improve the accuracy of the bending angle and straightness of the workpiece.

Eccentric load

Eccentric load refers to load operation on the left or right side of the ram. The machine has a strong ability to resist eccentric load.

Under the action of eccentric load force, tilt will be generated between the workbench and the ram.

The grating detection mechanism at both ends of the ram will detect the deviation and provide feedback to the computer. The computer controls the proportional servo valve to adjust the amount of oil entering the oil cylinder, to keep the position of the two pistons synchronized and maintain a small parallelism error between the workbench and the ram.

When processing special parts, one should consider the above eccentric load. In general, it is necessary to avoid operating under eccentric load.

Correction of bending angle error

After the bending part has been formed, and the angle is about α°, the measured angle error value △α° can be corrected by adjusting the position Y of the bottom dead center. The correction value △Y can be approximately calculated according to the following formula: △Y = K × V × △α


  • △ Y – correction value of bottom dead center (mm)
  • V – selected die opening (mm)
  • △ α – Angle error value (degrees)
  • K – Correction factor (degree /mm)

When α ≈ 90°, K ≈ 0.0055; for folded 8-12-sided steel rod, when α ≈ 135°-157.5°, K ≈ 0.004.

If the inspected part angle is greater than the drawing requirements, adjust the bottom dead center position downward according to the correction value △Y; otherwise, adjust upward.

Example 1:

The angles at both ends are equal to the middle angle.

For folding a dodecagon with α= 150°, select the opening V = 200mm, and measure the angle after bending, α= 151.5°. Using the above method, △α is calculated to be 1.5°, and K is 0.004.

△Y = K × V × △α = 0.004 × 200 × 1.5 = 1.2mm.

Input the bottom dead center position Y + △Y into the computer.

Example 2:

The angles at both ends are not equal to the middle angle.

For a workpiece bent at α=90° with opening V = 60mm, the actual measured angles at both ends are α=90°, and the middle angle is α=91° (the convex amount is not enough).

Using the above method: △α = 1°; K=0.0055

△Y = K × V × △α = 0.0055 × 60 × 1 = 0.33mm.

Add △Y to the original convex value and input it into the computer.

If the middle angle is smaller than the angles at both ends (the convex amount is too large), reduce the convex value by △Y from the original basis and input it into the computer.

Example 3:

The angles at the lower ends of the column are not equal. With opening width V = 60mm, a workpiece is bent at α= 90°.

The actual measured angle at the left end is α=90°, and the angle at the right end is α=91° (lower left and higher right).

Using the above method: △α=1, K=0.0055

△Y=K × V × △α=0.0055 × 60 × 1=0.33mm

Input the Y + △Y value at the right end’s zero point upward to change its reference position.

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