June 29, 2021

Unison Ltd Blog – An introduction to tube bending

An introduction to tube bending

In this blog we give you an introduction to the art of tube bending without a mandrel, helping you to achieve the perfect bend on your tube or pipe.


Stretching and Compression Principles 

When a tube is bent, two things happen to metal. The outside wall is reduced in thickness due to the stretching of the material and the inside wall becomes thicker due to the compressing of the metal.

The material is actually formed approximately about the centerline of the tube, with the material forming the outside of the bend having further to travel and therefore being stretched; the inside of the bend has less distance to travel so it is compressed.


Basic Bend Tooling

A Bend Die, Clamp Die, and Pressure Die are the minimum essentials for bending tube.

The Bend Die helps define the radius of bend, by the material being drawn around it hence the term “Rotary Draw Bending”.


Bending Limitations

When the ratio of the tube diameter to wall thickness is small enough, the tube can be bent around a relatively small Radius Bend Die and excessive flattening or wrinkling of the bend should not occur, the tighter the radius the more difficult it is.

The Radius is typically called Centerline Radius or CLR and is quite often defined in relation to the Outside Diameter (OD) of the Tube (e.g. 3D meaning 3 x the OD).


Check out some of the tubes our machines have bent outside Unison HQ


During Bending the outside and inside of the bend tend to pull towards the centerline of the tube (flattening), there are two ways of helping to reduce this either by supporting the inside of the tube with a Mandrel or by using Heart Shaped Bend Die grooves, which supports the tube along the centerline and the natural strength of the tube.


Elongation Properties

For a material to be able to be bent, it has to have the right properties, these are called Elongation Properties and are usually defined as a percentage.

The tighter the radius is, the more elongation is required to the point when you are being at 1D you need something approaching 50%.


Kinking or buckled bends

Kinking or buckling may be due to hard material which will not compress on the inside radius of the bend.

Some tubes that our Unison machines have bent
Some tubes that our Unison machines have bent

The material, not being able to compress, pushes in toward the centerline of the tube.

This condition can be corrected (provided the tube is not too hard) by the use of a Mandrel.

A Plug Mandrel is indicated if the tube buckles and is still within the wall factor and the diameter of the bend.


Please use our Web App to help understand what subtle differences cause the requirement for different tooling.



Springback is the term used to describe the tendency of metal that has been bent to return to its original shape, when it is released from the Bend Die.

The extent of springback can be anything from 2-15 degrees depending on the material, radius of bend, and tooling used.

The resultant springback will manifest itself in a smaller bend angle and a larger radius than the original Bend Die.

To overcome this, Unison’s All-Electric Bending machines offer Laser Springback Correction a feature which allows on machine corrections of under bent parts.

This is especially useful when material is supplied with varying tolerances.

For applications where the Radius is critical, it is sometimes required for a tooling manufacturer to use a Radius Compensation Calculation.


At Unison Ltd, we have been specialising in tube bending since being established in 1973.

Contact our experts today to find out more about our tube and pipe bending machines, our tube and pipe bending software and any other support you need to achieve your goal.


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