Tube and pipe basics: How to achieve the perfect bend
Tube Bending Basics
When tubing is bent to a specific shape or geometry, force must be applied so as to exceed the materials 'yield point', which is the material's capability to resist permanent deformation, while staying below the material’s 'ultimate tensile strength', which is the limit of the material's ability to resist
tearing. As the tube is bent over a specific radius the outside wall will stretch in tension while the inside wall bends under compressive forces. The boundary line through the center of the tube, between the tension and compressed zones is called the 'neutral axis'. The 'neutral axis' is the location from
which all dimensional calculations are made.
Bending Operations
Tube bending can be done manually, semi-automatically, with CNC equipment, or
with dedicated machines.
There are several different bending methods, the use of which depends upon the
tube diameter, wall thickness, minimum bend radius required, and part
complexity. These bending methods include:
• Rotary draw bending
• Compression bending (wrap bending)
• Ram bending
• Roll bending
Rotary draw bending uses a movable bending form or die, along with a clamping
die, a pressure die, and a wiper die, and is the most common tube bending
method. Rotary draw bending can produce bends up to 180 degrees with standard
tooling.
Compression bending is similar to rotary bending except that the bending form
remains stationary rather than rotating with the workpiece. This method is used
where there is minimum clamping space between bends.
Ram bending uses either a horizontal or vertical press to force a die or forming
shoe to against a workpiece supported by supporting dies. These dies are also
contoured like the die or forming shoe to match the profile of the tube. As
force is applied the supporting dies rotate or swivel, following and supporting
the workpiece during the forming cycle.
Roll bending is used to produce large radius bends on heavy walled tubing. Roll
bending is not typically used to bend thin wall tubing due to the high degree of
wall stretching and thinning that occurs with the process. Roll bending uses
three forming rolls arranged in a pyramid configuration, either in a horizontal
or vertical position. Each roll has approximately the same diameter and all are
contoured to match the cross-sectional shape of the workpiece. Two of the rolls
are fixed while the third is adjusted to determine the finished bend radius.
Roll bending can produce a multiple radius part, full circles, and helixes on
tube, pipe, solid, as well as extruded material.
Compare Compression bending and Rotary draw bending
Compression bending is a rather simple method that is often used on
construction sites for bending electrical conduit. However this type of
bending is limited as it can’t make tight (small radius) bends because
the pipe will buckle or break.
Rotary draw bending on the other hand allows for more control during
the tube bending process as you use mandrels and dies inside the tube.
This control allows us to make a very tight bend and can be used to make
small dimensional tolerances.