In daily sheet metal processing, most workpieces are bent into right-angle shapes, while some are non-right-angle bent parts. There is also a type of workpiece with large rounded bends—namely, sheet metal parts bent into arcs. Common bending dies are typically 90° bending dies, so how is sheet metal bending performed on arc-shaped steel plates?
Take the following example: a 2mm-thick sheet needs to be bent at a 120° angle, with an outer bending radius of 30mm and a neutral axis radius of 29mm. For arc bending, the developed length (flat pattern length) is calculated based on the arc length of the neutral axis. Therefore, when determining the number of bending passes and the bending angle per pass, calculations are also based on the arc length of the neutral axis. The distance from the neutral axis to the edge of the sheet is equal to half the sheet thickness.
How many passes are required for the arc bending in the figure above, and what is the bending angle per pass?
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Calculation of Bending Passes
Bending passes = Arc length ÷ Feed per pass = 30.37 ÷ 2 = 15 passes
Total bending passes: 15
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Calculation of Bending Angle per Pass
Bending angle per pass = 180° – [(Feed per pass ÷ Arc length) × (180° – Target bending angle)]
= 180° – [(2 ÷ 30.37) × 60°] = 176°
Bending angle per pass: 176°
If the arc length cannot be measured during bending, the following formula can be used instead:
Bending angle per pass = 180° – [(Feed per pass ÷ (3.14 × Neutral axis radius)) × 180°]
= 180° – [(2 ÷ (3.14 × 29)) × 180°] = 176°
Note: If the feed per pass is 3mm, simply replace "2" with "3" in the formulas above.
The formulas above are theoretical calculation formulas; adjustments should be made based on actual processing conditions. This method benefits from the development of modern CNC technology—most current CNC bending machines are equipped with an arc bending function. Using standard 90° bending dies, the machine can complete large arc processing through program-controlled multi-pass bending (each pass with a small angle).
The arc length of a large arc is relatively long, far exceeding the precision range of a single bending pass. CNC bending machines achieve the overall bending of the large arc by performing multi-pass positioning and bending. Similarly, the angle control of CNC bending machines is sufficiently accurate to complete large arc bending through multiple small-angle passes. While this description may not be intuitively clear, observing on-site bending will make it much easier to understand.
This method effectively solves the problem of large arc bending in sheet metal processing, but it also has limitations:
- Few bending passes: Will result in obvious bend marks on the workpiece.
- Many bending passes: Will reduce processing efficiency.
- Machine precision requirement: If the CNC bending machine has low precision, deviations may occur in bending angle or dimension. Therefore, a high-precision CNC bending machine is required.
In conclusion, to process sheet metal parts with large arcs, it is advisable to select a high-quality CNC bending machine.
