Bending tube is harder than bending sheet because the section wants to collapse. The outside of the bend stretches and thins while the inside compresses and wrinkles, and the whole cross section flattens unless something holds it round. Every tube bending method is a different answer to that problem.
The methods
Rotary draw bending clamps the tube to a die and draws it round, giving the most accurate and tightest bends and needing tooling per diameter and radius. Mandrel bending adds an internal support inside the bend to stop collapse, which is what allows tight radii and thin walls. Roll bending passes the tube through three rollers for large sweeping radii. Compression bending is simple and limited to gentle bends in thick wall.
Centreline radius and wall factor
The two numbers that decide feasibility are the centreline radius expressed as a multiple of the tube diameter, and the wall factor, which is diameter divided by wall thickness. A large radius and a thick wall bend easily with no mandrel. A tight radius on a thin wall needs a mandrel with several supporting balls and possibly a wiper die, and each of those is tooling.
What the design should do
Use one bend radius throughout so one tool set does the job, exactly as with sheet metal. Keep straight lengths between bends long enough for the machine to grip, which is a real constraint that catches people out. Avoid the tightest radius the process can reach unless the design needs it. And allow for springback, which the bender compensates by overbending.
What happens to the tube
The outside wall thins in proportion to how tight the bend is, so a tight bend in a pressure carrying tube may fail its wall thickness requirement even though it looks fine. The section also ovalises slightly. Both are predictable and both should be checked against the requirement rather than assumed acceptable because the part came out of the machine looking correct.
Questions people ask about tube bending services
What is mandrel bending?
Bending with an internal support inside the tube at the bend, which stops the section collapsing and allows tighter radii and thinner walls than bending without one.
How tight can a tube be bent?
It depends on the centreline radius as a multiple of diameter and on the wall factor. Tighter radii on thinner walls need more mandrel support and more tooling.
Does the wall get thinner at a bend?
Yes, on the outside of the bend, in proportion to how tight it is. On pressure carrying tube this must be checked against the minimum wall requirement.
How do I keep tube bending cheap?
One bend radius throughout, straight lengths long enough for the machine to grip, and avoiding the tightest radius the process can manage unless it is needed.