Tube Bending Made Simple - Making Use of the Compression Bending Process

Bent tubing is everywhere. Just look under the hood of a car, in the back of a pool, or inside a plane. Tube benders are used everyday to support all kinds of equipment. Knowing how to design and then bend the tubing can often translate into products that are easy to build and service. A few of the basics of tube bending are described below. 

The Bend Radius 
In general, it is good practice to design a part with a minimum centerline bend radius of 2 1/2 times the tube's diameter. If the design constraints dictate a tighter bend radius, there is a good chance your part will require a rotary draw bending process. A rotary draw bending process, with the right die geometry, will tend to prevent the tube from experiencing too much ovality. This is often the case when the bend radius is less than 2 times the tube's outside diameter. Centerline bend radii of 2 1/2+ times the diameter can often be formed using a compression bending process without excessive flattening of the tube in the bend region. This is good because tooling for a compression bending process is less expensive than that for a rotary draw process. 

Straight Length Between Bends 
If possible, leave at least 2 tube diameters of length between bends. Also, leave at least 2 tube diameters from the end of a tube to the start of the first bend. Short bend tangents can lead to out-of-tolerance parts when using the rotary draw process. However, short bend tangents can be accommodated with dedicated tooling designed for short tangents. Dedicated tooling designed for short tangents is often less expensive for the compression bending process when compared to the rotary draw process. 

Ovality 
Industry standards suggest a maximum ovality of 10 percent on most formed parts that operate below 500 psi. By designing parts with a generious bend radius, the ovality will tend to stay below the 10 percent mark. The following table outlines a few experimental results for the compression bending process expressed in inches. The bend radius is the centerline bend radius. 
 

Percent Ovality Based On Actual Lab Tests

 

Tube O.D. WallThickness Material CenterlineBend Radius Ovality (%) BendAngle (Degrees)
.125 - s.s. - - 90
.188 .016 s.s. .281 11 60
.188 .020 s.s. .281 10 60
.250 .035 s.s. .375 11 45
.250 .035 s.s. .50 4.7 90
.250 .035 s.s. .56 3 90
.250 .035 steel .56 3 90
.250 .035 brass .56 3 90
.375 .035 s.s. 1.00 - 90
.375 .035 s.s. 1.25 5 90
.375 .035 brass 1.25 5 90
.375 .035 steel 1.25 5 90
.500 .035 s.s. 1.25 9.5 90
.625 .035 s.s. 1.25 10+ 90

About the Author

George Winton, P.E. designs and builds CNC tube fabrication equipment for Winton Machine in Suwanee, GA. He can be reached at gwinton@wintonmachine.com or 888.321.1499

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All of our semi-rigid coax and tube fabrication machines at Winton are designed, manufactured, and tested in-house.  We have a large line of standard products as well as the ability to engineer the best solution for our customer’s needs.  Our experienced sales staff makes sure that our customers can justify their capital equipment investment by offering a solution that is exactly what they need in order to manufacture their parts.  Please contact us today to discuss your project.