Manufacturing Process

Tube Bending

管材弯曲

Controlled CNC bending of tubes and pipes into 3D shapes while managing wall thinning and ovality.

Process Steps

Tube loaded
CNC bend sequence
Mandrel support as needed
Angle and springback verified

What is tube bending?

Direct answer: tube bending plastically deforms a tube around a curved path without collapsing it. Rotary draw bending is the method used for most precision work: the tube is clamped to a rotating bend die, a mandrel supports the inside of the wall and a wiper die supports the outside.

The three numbers that decide whether a bend is possible

Ratio	What it means	Why it matters
Wall factor = D / t	Outside diameter divided by wall thickness	High wall factor means a thin wall that collapses easily
Bend factor = R / D	Centreline bend radius divided by outside diameter	Below about 1 it needs a mandrel; below 0.7 it may need a special set-up
Length between bends	Straight length between two bends	Too short for the clamps and there is nowhere to hold the tube

Those ratios explain most failures. A Ø10 mm tube with a 1 mm wall (D/t = 10) bends easily at R/D = 2. The same tube bent at R/D = 0.8 needs a mandrel, precise tooling and a slower cycle.

What a bent tube holds

Feature	Typical tolerance
Bend angle	±0.5° – ±1° (springback compensated in the program)
Centreline radius	±0.1–0.3 mm
Distance between bend tangents	±0.2–0.5 mm
Ovality in the bend	Typically under 5 % of the original diameter
Wall thinning on the outside of the bend	Typically under 10–15 %

Materials

Mild steel, stainless steel, aluminium, copper and brass all bend, but they fail differently. Stainless work-hardens and springs back more, so it needs more over-bend and a properly sized mandrel. Aluminium bends easily but marks easily, which is why a soft bend die or a protective film matters on visible parts. Copper and brass bend well but are soft enough that handling damage, not forming, is the quality risk.

Cut and bend in one route

A bent tube usually also needs its ends profiled and its wall pierced — holes, slots or sensor ports. Balford runs laser tube cutting and tube bending as one route, so the tube is cut, bent and cut again without a second supplier and a second set of fixtures. That matters most on parts with a tight bend-to-hole relationship, where a hole that drifts in the bend moves out of position.

Common problems

Flattening at the bend. The wall collapses on the inside. Fixed with a mandrel, a tighter bend die fit or a thicker wall.
Wrinkles on the inside of the bend. Usually insufficient mandrel support or too aggressive a bend rate.
Cracking on the outside. The outer fibre ran out of ductility: increase the bend radius, anneal, or use a formable temper.
Springback. The angle opens after the tool releases. Compensated in the program, and re-checked when a new material batch arrives.
Tool marks on visible surfaces. Managed by polished dies and protective handling rather than by bending harder.

Frequently asked

What is the minimum bend radius for tube?

It depends on the bend factor. R/D down to about 1 is routine with a mandrel; below 0.7 the tooling becomes special and the wall has to be thick enough to survive. A DFM review answers it for a specific tube.

When is a mandrel needed?

When the wall is thin relative to the diameter, or the bend radius is tight — roughly R/D below 1.5 for a typical D/t of 20. Without one, the tube flattens on the inside of the bend.

Can you bend a tube and then cut holes in it?

That is usually the wrong order. Holes near a bend distort, so the robust route is to cut the holes and profiles first and bend afterwards, or to keep the holes clear of the bend tangents. Laser tube cutting makes the cut-first route practical.

Why does the angle change between batches?

Springback depends on the material's yield strength, which varies with the heat and the supplier. Small adjustments between batches are normal; a large drift usually means the material specification changed.

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