How to Avoid Deviations in Bending Angles and Dimensions on CNC Sheet Bending Machines

For the bending process of a cnc sheet bending machine, the quality of bending forming mainly depends on two important parameters: the bending angle and size of the bending machine. The parameter setting of a cnc sheet bending machine directly determines accuracy: a 10% pressure deviation will cause an angle deviation of more than 1°, and every 0.01mm reduction in positioning accuracy will expand the dimensional tolerance by 0.02mm. However, most enterprises are stuck in a “trial-and-error-rework” cycle due to improper parameter matching, resulting in significant cost losses. This article systematically analyzes the causes of deviations and provides deviation avoidance solutions from equipment selection, daily maintenance to upgrading, helping enterprises control accuracy from the source and reduce later debugging costs.

Three Core Causes of Angle and Dimensional Deviations in CNC Sheet Bending Machines

1. Equipment Factors

• Servo motor positioning error: When the positioning error of the servo motor is ≥0.02mm per pulse, it directly affects the accuracy of the slider movement. Over time, this error accumulates, leading to obvious dimensional deviations in long workpieces.
• Guide rail clearance: If the guide rail clearance exceeds 0.05mm, it will intensify the shaking of the slider during movement. This instability causes inconsistent pressure application during bending, resulting in angle deviations of up to 0.5° in severe cases.
• Die wear: For every 0.1mm increase in the fillet radius of the die, the angle deviation increases by 0.3°. Worn dies also lead to uneven force distribution on the sheet, further worsening the deviation.

2. Material Characteristics

• Fluctuation in yield strength: Take Q235 steel as an example. When its yield strength fluctuates within ±10MPa, the difference in bending springback can reach 1.5°. This is because the higher the yield strength, the greater the internal stress of the material, resulting in more obvious springback after bending.
• Thickness tolerance: A thickness tolerance of ±0.1mm in the sheet causes a 15% deviation in bending force. For example, a 3mm sheet with an actual thickness of 3.1mm requires 15% more force than the standard to achieve the same bending effect; otherwise, the angle will be too small.

3. Operation and Environmental Factors

• Improper pressure calibration: Operators who fail to calibrate pressure parameters often apply insufficient pressure, leading to springback exceeding 2°. Excessive pressure, on the other hand, may cause plastic deformation of the sheet, resulting in dimensional shrinkage.
• Temperature changes: Every 10℃ temperature difference in the workshop changes the elastic modulus of steel by 1.2%. In high-temperature environments, the material becomes softer, increasing springback; in low temperatures, it becomes harder, requiring higher bending force to avoid under-bending.

Four Core Parameters Affecting Deviations and Quantitative Standards

1. Pressure Parameters

Insufficient bending force will cause springback to exceed 2°, while excessive force will trigger plastic deformation of the sheet. For example, when the pressure on a 3mm steel plate exceeds 600kN, the dimensional deviation increases by 0.15mm. The reasonable pressure range is 1.1-1.2 times the theoretically calculated value. This range ensures full bending without causing irreversible deformation.

2. Positioning Accuracy

The repeat positioning error of the slider must be ≤±0.01mm. Otherwise, every 0.01mm error will translate into a 0.02mm dimensional deviation. For a 500mm long workpiece, the cumulative deviation can reach 1mm, which is far beyond the acceptable range for most precision parts.

3. Die Parameters

The opening width of the female die should be 6-8 times the sheet thickness. For a 2mm sheet, the corresponding opening is 12-16mm. An opening that is too small easily leads to a larger angle deviation (+1.5°), as the material is subjected to excessive lateral pressure; an opening that is too large increases springback because the material cannot be fully constrained during bending.

4. Pressure Holding Time

For thick plates (≥3mm), insufficient pressure holding time (less than 3 seconds) increases springback by 0.8°. This is because thick plates need more time to release internal stress. For thin plates (≤1mm), pressure holding time exceeding 2 seconds causes dimensional shrinkage (deviation of -0.05mm) due to excessive compression.

To ensure these parameters work together, it is necessary to establish a parameter database based on material types and thicknesses. This allows quick call-up of optimal parameters during production, reducing the probability of deviations.

Seven Solutions to Avoid Deviations

1. Adjust Upper and Lower Die Alignment

Misalignment of the upper and lower dies during tool setting causes dimensional errors. Before bending, use an alignment instrument to adjust the concentricity of the upper and lower dies. The deviation between the edge of the upper die and the center line of the lower die’s V-groove should be controlled within 0.03mm. For example, in the production of 1mm stainless steel sheets, even a 0.05mm misalignment can lead to about 0.3mm dimensional difference between the left and right sides of the workpiece.

2. Re-determine Back Gauge Position

After moving the left and right positions of the CNC sheet bending machine’s back gauge, the relative position between the sheet and the lower die may change, affecting the bending size. For servo bending machines, re-measure the back gauge position with a laser rangefinder (accuracy ±0.02mm) before bending. Record the data in the CNC system and lock it to prevent position drift during batch production. A case study shows that rechecking the back gauge position can reduce dimensional deviations by 60%.

3. Adjust Parallelism Between Workpiece and Lower Die

Insufficient parallelism between the workpiece and the lower die causes bending springback and affects the bending angle. Before bending, measure the parallelism with a level meter. The allowable error is ≤0.03mm/m. If the error exceeds this value, adjust the support screws under the workbench. For example, when bending a 2m long aluminum alloy sheet, a parallelism error of 0.05mm/m will result in a 1° angle difference between the two ends of the workpiece after bending.

4. Ensure Accuracy of Single Bending

Insufficient angle in the first bending affects the second bending, and the accumulation of bending errors enlarges the dimensional and angle errors of the formed workpiece. Therefore, ensuring the accuracy of single-sided bending is particularly important. The angle error of the first bending should be controlled within ±0.3°. Use a digital angle ruler (accuracy ±0.1°) to check the first piece. If the error exceeds the standard, adjust the pressure or die parameters in time. This can reduce the cumulative error of secondary bending by more than 80%.

5. Select Appropriate Lower Die V-groove

The size of the lower die’s V-groove is inversely proportional to the bending pressure during bending. When processing metal sheets of different thicknesses, select the appropriate lower die V-groove according to regulations. Generally, it is suitable to choose 6-8 times the sheet thickness. For a 4mm sheet, a 24-32mm V-groove is appropriate. Using a smaller V-groove (such as 16mm) will increase the pressure by 30%, causing the sheet to deform; a larger V-groove (such as 40mm) will reduce the pressure, resulting in insufficient bending and increased springback.

6. Align Key Positions for V-grooved Workpieces

When bending a workpiece with a V-groove, ensure that the edge of the upper die, the bottom of the workpiece’s V-groove, and the bottom of the lower die’s V-groove are on the same vertical plane. Use a dial indicator to check the alignment, with an allowable deviation of ≤0.05mm. If they are not aligned, the workpiece will twist during bending. For example, in the production of V-grooved door frames, a 0.1mm misalignment will lead to a 0.5mm twist in the final product, affecting the installation.

7. Control Upper Die Angle for Grooved Workpieces

To prevent tool jamming when bending grooved workpieces, the upper die angle of the bending machine should be controlled at around 84°. If the angle is larger than 85°, the workpiece is prone to jamming between the upper and lower dies; if it is smaller than 83°, it will leave indentations on the workpiece surface. Practically, setting the angle to 84°±1° balances both anti-jamming and surface quality requirements.

Daily Maintenance: Key Measures to Extend Accuracy Lifespan

1. Daily Inspection

• Clean oil stains on the die surface before starting the machine to avoid dimensional deviations caused by sheet slippage. Oil stains reduce the friction between the sheet and the die, leading to unstable positioning.
• Check the hydraulic system pressure, which should fluctuate within ±2MPa. Excessive pressure fluctuations (such as ±5MPa) will cause uneven bending force, resulting in angle deviations.

2. Weekly Calibration

•Use standard gauge blocks to detect the slider stroke, with an allowable error of ≤0.02mm. If the error is too large, adjust the stroke parameters in the CNC system.
•Adjust the limit switch position to ensure consistent repeat positioning. A misaligned limit switch can cause a about 0.1mm deviation in the slider’s stopping position.

3. Monthly Lubrication

Apply special grease (viscosity 320cSt) to the guide rails and lead screws. This reduces the friction coefficient from 0.1 to 0.05, lowering positioning errors. Insufficient lubrication increases wear, leading to increased guide rail clearance over time.

4. Quarterly In-depth Inspection

• Use a coordinate measuring machine to detect die wear. The allowable change in fillet radius is ≤0.05mm. Replace severely worn dies in time to avoid worsening angle deviations.
• Replace bearings with excessive wear (clearance ≤0.01mm). Worn bearings cause shaking during slider movement, affecting bending stability.

5. Semi-annual Accuracy Recovery

Use the “accuracy compensation” function of the CNC system to correct positioning deviations caused by mechanical wear. This function can recover up to 0.5° of angle error. For example, after 6 months of use, a bending machine with an accumulated angle deviation of 0.8° can be adjusted to within 0.3° through compensation.

Parameter Optimization Cases for Different Materials

Conclusion

Avoiding deviations in CNC sheet bending machines requires comprehensive control from understanding causes and parameters to implementing solutions and maintenance. By strictly following the above methods, enterprises can significantly reduce rework rates, improve production efficiency, and reduce costs. Remember that accuracy control is a continuous process, and regular inspection and adjustment are key to maintaining long-term stability. Whether processing low carbon steel, stainless steel, or other materials, matching parameters to material characteristics and maintaining equipment in good condition are the foundations of ensuring bending quality.

DXTECH’s CNC sheet bending machines are engineered to minimize these deviations from the start. Equipped with high-precision servo systems (positioning error ≤0.01mm/pulse) and intelligent parameter databases, they simplify accuracy control for operators. Our machines also feature automated die alignment and real-time parallelism monitoring, reducing manual intervention by 40%. Backed by a global service network, DXTECH ensures your equipment maintains peak performance, helping you achieve consistent bending quality with less effort. Choose DXTECH for reliable precision that drives productivity.