Calculator Inputs
Example Data Table
| Tube OD | Wall | Angle | CLR | Radius Ratio | Arc Length | Typical Use |
|---|---|---|---|---|---|---|
| 25.4 mm | 2 mm | 90° | 76.2 mm | 3D | 119.69 mm | Frame tube |
| 38.1 mm | 2.5 mm | 45° | 114.3 mm | 3D | 89.77 mm | Roll cage member |
| 12.7 mm | 1 mm | 90° | 25.4 mm | 2D | 39.9 mm | Small fluid line |
Formula Used
Inside radius: IBR = CLR − OD / 2
Outside radius: OBR = CLR + OD / 2
Angle in radians: A = degrees × π / 180
Centerline arc length: Arc = A × CLR
Neutral axis radius: NAR = IBR + K × wall thickness
Bend allowance: BA = A × NAR
Setback: SB = CLR × tan(A / 2)
Chord: Chord = 2 × CLR × sin(A / 2)
Rise or sagitta: Rise = CLR × (1 − cos(A / 2))
Outer strain estimate: Strain % = ((OD / 2) / CLR) × 100
Corrected angle: Corrected angle = desired angle × (1 + springback % / 100)
Estimated cut length: straight before + straight after + total centerline arc + waste allowance
How to Use This Calculator
- Select the unit used for all length fields.
- Choose a material preset or keep custom values.
- Enter outside diameter, wall thickness, bend angle, and centerline radius.
- Leave centerline radius blank if you want to use OD multiplied by the radius rule.
- Enter K factor, springback, bend count, straight lengths, and waste allowance.
- Press calculate to view the result above the form.
- Use the CSV or PDF button to save the output.
Understanding Tube Bend Radius
Tube bending looks simple, yet small radius errors can ruin a part. A tube bends around a centerline. That centerline radius controls fit, flow, strength, and appearance. A tight bend saves space, but it also raises wall thinning and ovality. A wider bend is safer for many materials. It needs more room and more developed length.
Why the Centerline Matters
Most shop drawings use centerline radius, often called CLR. It is measured from the bend center to the tube centerline. The inside bend radius is smaller by half the outside diameter. The outside radius is larger by half the outside diameter. These three paths have different lengths. The calculator separates them, so the fabricator can see the real shape.
Setback, Arc Length, and Gain
Setback is the tangent distance from the bend point to the tangent point. It helps mark where straight tube begins to curve. Arc length is the length along the bend. Bend gain compares tangent travel with curved travel. It helps estimate cut length when several bends are used. These values are important for frames, roll cages, handrails, hydraulic lines, and instrument tubing.
Wall Thickness and Strain
Wall thickness changes how a tube behaves. The outer wall stretches during bending. The inner wall compresses. A small centerline radius gives higher outer fiber strain. Excess strain can cause cracking, wrinkling, or flattening. The radius ratio, shown as CLR divided by outside diameter, is a quick quality check. Many jobs start with a 2D, 3D, or 4D rule. Harder materials often need a larger ratio.
Using Results in Fabrication
Use this tool before cutting material. Enter the actual tube size, not the nominal trade name. Add springback when the material opens after bending. Review the correction angle before setting a machine. Keep notes from real test bends, because dies, lubrication, temper, and tooling support change the final result. A calculator gives a strong estimate. A short trial bend confirms the production setup.
Recording Shop Data
Record every trial result. Note die size, clamp pressure, tube lot, and lube type. Compare the measured angle with the corrected angle. Save the best setup. Future bends become faster, cleaner, and easier to repeat across similar real production projects.
FAQs
What is centerline radius?
Centerline radius is the distance from the bend center to the tube centerline. It is the most common radius used on tube drawings and bending setup sheets.
What does 3D bend radius mean?
A 3D bend radius means the centerline radius is three times the tube outside diameter. For example, a 25 mm tube has a 75 mm 3D radius.
Can I leave centerline radius blank?
Yes. The calculator will estimate centerline radius by multiplying tube outside diameter by the radius rule multiplier entered in the form.
What is bend allowance?
Bend allowance is the developed length along the neutral axis of the bend. It helps estimate how much tube length is consumed by the curved section.
Why is springback included?
Springback is the amount a tube opens after bending force is released. The corrected angle helps you overbend slightly to reach the desired final angle.
What is a safe tube bend radius?
Safe radius depends on material, wall thickness, temper, tooling, and support. Many jobs start near 2D or 3D, then confirm with a trial bend.
Does this replace a test bend?
No. It gives a strong planning estimate. A test bend is still important because actual machines, dies, lubrication, and tube lots can change results.
What units should I use?
Use one unit system for every length field. Do not mix millimeters and inches in the same calculation, or the result will be wrong.