BIZOMAT MAKEOS · Fabrication
Tube Bending Wall Thinning
How much will the tube wall thin on the outside of this bend, and thicken on the inside?
Approximate outer wall thinning and inner wall thickening of a tube bent to a centreline radius, with the bend ratio and wall factor used to judge bendability.
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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 | Nominal wall thickness. |
| Centreline bend radius | mm | Centreline radius of the bend die. |
| Minimum wall required (0 = not set) | mm | Minimum wall thickness allowed after bending (from the pressure or strength calculation). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Tube outside diameter
- What it is
- The outside diameter of the tube being bent, measured across the tube before it goes into the bender.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- For the same bend radius, a larger diameter thins the outer wall more.
What this means: Wall thickness
- What it is
- The nominal wall thickness of the tube before bending.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The outer and inner wall thicknesses after bending scale in proportion to the starting wall.
What this means: Centreline bend radius
- What it is
- The radius of the bend measured to the tube centreline, normally the centreline radius of the bend die.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger bend radius reduces both outer thinning and inner thickening.
- Get it right
- Use the centreline radius, not the inside radius of the bend; it must exceed half the tube diameter.
What this means: Minimum wall required (0 = not set)
- What it is
- The smallest wall thickness the bent tube is allowed to have, from its pressure or strength calculation; zero skips the check.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.6 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Zero means not set, not a zero-thickness limit.
Method and formulas
- Outer wall ≈ t × R ÷ (R + D/2); inner wall ≈ t × R ÷ (R − D/2) (constant volume along the arc).
- Thinning % = (t − outer) ÷ t.
- Bend ratio = R ÷ D; wall factor = D ÷ t.
- Example: a 50 × 2 mm tube on a 100 mm radius thins to about 1.6 mm (20%).
Limitations
- Simple geometric estimate; actual thinning depends on the process (rotary draw, mandrel, pressure die boost).
- Ovality and wrinkling are not predicted.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check tube wall after bending against a minimum thickness
- Choose a centreline radius
- Decide whether a mandrel is needed
Who it is for
- Fabrication supervisors
- Manufacturing engineers
- Estimators
What you get
- Outer wall thickness and thinning
- Inner wall thickness
- Bend ratio R/D and wall factor D/t
Interpreting the result
- Small bend ratios with large D/t usually need a mandrel and wiper die.
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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