tube bending machine

What is tube bending process?

Tube bending is a metal forming process that shapes the tube to achieve the desired angle or radius through applying tension and stress forces on the opposite side of the tube. Generally, most common metals can be bent, providing they have sufficient elongation to achieve the desired angle and radius before reaching their ultimate strength.

Material suitable for bending

The tube material is normally hollow and round section profile, but it can also be square or irregular extruded or drawn profile, or even solid rod or wire.

In general, all materials with different section profile can be bent, but there exist bending principle that much harder the material is, much smaller the plastic elongation is. For hard materials, the bending angle needs to be smaller, and the bending radius needs to be larger, so reversed is it for softer material.

Materials have different temper status that represents the ductile degree of material. For example, a steel tube may be annealed to make it much softer in a mill plant, and heat-treated or cold worked aluminum alloy has their temper code to indicate its ductile degree.

Design consideration of tube bending

The proper design of tubular parts incorporating bends can contribute greatly to production efficiency and low unit costs.

#1 Selection of bend centerline radius

Usually, a reasonable bend radius is an even multiple of the outer diameter of the tube, this reduces the amount of money invested in tooling by avoiding having a number of die sets for the same tube diameter, also it will reduce the lead time required to make or buy the proper tooling to keep the design consistent.

#2 Specify bend radius to tube centerline

It is a general practice to indicate bend radius on prints or drawings to the center of round tubing or pipe, or round solid stock. The radius for square or rectangular tubing or solid bar should be shown to the inside of the bend, or to a major face line of extrusion, molding, or other shaped section.

tube bending design requirement
tube bending design requirement

#3 make all bends in one workpiece to the same radius

If it is possible, it is a basic design principle to make all bends in one workpiece to the same radius. This reduces tooling cost and bending process setup time, and increase production efficiency.

#4 Allow sufficient clamp length between bends

When at all possible, avoid compound bends. A compound bend is one bend designed so close to adjacent bends that it does not allow sufficient straight material for clamping between bends. Consequently, material slips in the clamp during the second bend. This almost always results in material wrinkles.

Guide to bending clamp length

Guide to bending clamp length

The amount of clamp length required to distribute the pressure over a sufficient area to prevent distortion or collapse of the tube depends on material type and grade, diameter, wall thickness, surface condition, and radius of the required bend. Other factors that help determine clamp length include:

  • Marking of the work due to high clamping pressure distributed over a minimum are
  • Amount and rate of tool wear
  • Type of mandrel and use
  • Number of balls used on multi-ball mandrels; and
  • Scrap loss due to extreme clamp lengths

#5 minimum centerline radius of the bend

The minimum radius to which a tube can be bent is a function of the elongation of the material. If the outside of the bend is tensioned beyond the maximum elongation, it breaks. The equation to determine the minimum radius of a bend is:



R=minimum centerline radius of the bend

D=outside diameter of the tube

E=elongation in 2 inches, %

The formula that follows should be used only as a guide to the minimum bend radius possible. It does not take into consideration of friction between tube and tools.

Tube bending Method

Basic bending methods are classified as follows:

    • Rotary-draw bending
    • Compression bending
    • Roll bending
    • Stretch forming

Each type has certain applications and limitations with regards to the kinds of bends it produces and the maximum angle of bend it achieves as indicated in the below table:

Tube bending process capability

Tube bending process capability

Selection of a bending process for tubing depends on

  • Quality of bend and production rate required
  • Diameter, wall thickness, and minimum bend radius desired

#1 rotary-drawing bending

Drawing bending is the most common method used on rotary-type bending machines, which can be powered by hydraulic, pneumatic, electrical/mechanical, manual, or numerically controlled. The essential tooling for draw bending consists of the rotating bending form, clamping die, and pressure die as shown in the below picture.

rotary draw bending

rotary draw bending

#2 Compression Bending

Compression bending uses a group of tools very similar to those used in draw bending (refer beside figure), except the bending form is stationary and a movable wiping shoe replaces the clamp die. In compression bending, the workpiece is clamped to the stationary die and the wiping shoe rotates around the form, wrapping the material against the bending form.

compression bending

compression bending

This method is not practical for producing bends requiring a mandrel when there is more than one bend in a workpiece. In some cases where two or more bends in the same workpiece are closely adjacent, this method may avoid compound dies that would be required if the draw-bending method were used

#3 Roll bending

Roll benders provide a simple means of bending a wide range of cross-sections. The principle of roll bending tubing is shown in the figure, three forming rolls of approximately the same diameter are used. They are arranged in a pyramid in either vertical or horizontal planes.

roll bending
roll bending

#4 Stretch forming

Single-axis stretch forming is used to meet forming requirements for metals such as high-temperature, high-strength nickel, and cobalt alloys, which are not readily formed by conventional methods.

stretch forming

stretch forming process

Apart to be stretch formed should normally have a constant cross-sectional area throughout its entire length, and be free from holes, notches, or cutbacks. All stretch forming operations stress the part up to or beyond the yield point of the material, placing the material into its plastic range.

Stretch forming is used primarily in the aircraft and missile industries to form intricate and accurate parts without spring back from difficult to work materials.

So far, the tube bending process is introduced in general. To form a tube into pipes that are applicable in multiple industries, there still needs other forming processes, such as tube end forming, branching, and fabrication process.

Case study for further learning

The theory is always grey and empty, but the practice is always evergreen. Sipaitech mfg has developed wide range tubes and pipes that are used in multi-sector industries, and our effort meets customer ultimate demands. Learn more about our successfully developed pipe projects as a case study!

Tube bending-Engine vacuum pipe

Engine Pipe Case Study

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