Tube Bending: When You Must Use a Mandrel

Two numbers to work out first

D/t ratio — outside diameter divided by wall thickness.

D/t > 20 wall is thin relative to diameter
 high risk of flattening; use a mandrel

R/D ratio — bend radius divided by outside diameter.

R/D < 1.5 radius is small relative to the tube
 re-check the tooling set and mandrel

A Φ38 tube with a 1.2 mm wall has D/t ≈ 32 — quite thin. The same diameter with a 3 mm wall is D/t ≈ 13 and far easier to bend.

How much the wall thins after bending

On the outer radius the material stretches, so the wall thins. As an estimate:

t' ≈ t × 2R / (2R + D)

A Φ50 tube with a 2 mm wall, bent at R75: `t' ≈ 2 × 150 / (150 + 50) = 1.5 mm` — around 25% thinner.

This is an estimate. Actual thinning also depends on material, bending speed, tooling, and whether a mandrel is used. If the tube carries pressure, this figure must be confirmed on a physical sample, not taken from the formula.

On the shop floor

• Decorative tube, frames, handrails: usually no mandrel needed when D/t is low.
• Thin-wall tube, or a tight radius: a mandrel is close to mandatory, otherwise it will flatten.
• Pressure tubing: build the wall thinning into the design, and run a test bend first.