KEVOS® MAKEOS · Fabrication
Tube Bending Plan (Length, Wall Thinning, Strain and Overbend)
How long a tube do I cut for this bent part, how thin will the outside wall get, is the bend radius tight enough to crack the tube, and how far do I overbend?
Plan for rotary-draw or press bending of a round tube: developed length from straights and bend arcs (as the tube bend length tool), outer and inner wall thickness after bending and the thinning share (as the tube bend thinning tool), outer-fibre strain against the tube’s elongation, the centreline radius ÷ diameter and diameter ÷ wall ratios, and the angle to bend to with your springback allowance.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Tube outside diameter | mm | Outside diameter of the tube. |
| Wall thickness | mm | Wall thickness before bending. |
| Centreline bend radius | mm | Radius of the bend measured to the tube centreline. |
| Bend angle | ° | Angle of each bend. |
| Number of bends | Number of bends of this radius and angle. | |
| Total of straight lengths | mm | Sum of all the straight lengths between and beyond the bends. |
| Least wall thickness allowed | mm | Least wall the part may have after bending (pressure rating, code). |
| Smallest bend radius ÷ diameter allowed | Tightest radius as a multiple of diameter your tooling can bend without mandrel problems (your value). | |
| Elongation of the tube material | % | Tensile elongation of the tube material (from the certificate). |
| Springback allowance | % | Extra angle to bend beyond the target, as a share of the angle, for springback (your value from trial bends). |
Method and formulas
- Developed length = straights + bends × π R θ ÷ 180 (as the tube bend length tool).
- Outer wall ≈ t R ÷ (R + D ÷ 2); inner wall ≈ t R ÷ (R − D ÷ 2) (as the tube bend thinning tool).
- Outer-fibre strain ≈ (D ÷ 2) ÷ R.
- Bend-to angle = angle × (1 + springback share).
- Example: cut 776.99 mm; the outside wall thins to 1.6552 mm (17.241%) with 20.833% outer strain; bend to 92.7°.
Limitations
- Simple thinning estimate; mandrels, wipers and pressure dies change the real wall.
- Ovality is not calculated; thin-walled tubes at tight radii need a mandrel.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Cut tube to length for a bent part
- Check whether a tight bend will thin or crack the tube
- Set the bend angle on the bender
Who it is for
- Fabricators
- Tube bender operators
- Design engineers
What you get
- Developed (cut) length
- Wall thickness after bending and the thinning share
- Strain, ratios and the angle to bend to
Interpreting the result
- Thinning and strain fall quickly as the bend radius grows: one size larger die is often the easiest fix.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1382–1386. Topic reference; no source tables or recommended settings are reproduced.
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Open the Tube Bending Plan (Length, Wall Thinning, Strain and Overbend) calculator
