General Tech Document #1:
Do you have any program which can calculate the minimum bending radii for a steel tube? All necessary material parameters and attributes are available.

Document For: Tube Fabricators


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A tube fabricator in Finland asked us this question. The answer may be useful to designers and manufacturers who are new to tube fabrication, so its included here.

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Variables Beyond The Tube

We have no program that will calculate the exact minimum radius because this type of calculation requires more than tube parameters. The method of bending, the bending machine, the tooling, and entire setup also have a dramatic effect on the final answer.

Enterprising Tube Fabricators

If I were to write a program that said that tube fabricators could use a minimum 1.5D (Centerline Radius = 1.5 x the OD) for a given process, then it wouldn't surprise me if some enterprising fabricator would figure out a way to get a smaller CLR in that process. It happens all the time.

For example, I know of tube fabricators who change the shape of the dies, push the tube into the bend tools (pressure die assist), and use mandrels in various configurations to achieve what would seem impossible to some.

Rule Of Thumb

There are general "rules of thumb", however. If your process allows mandrel bending, then the bend radius can be much smaller than without a mandrel. Manufacuturers can obtain 1D bends with mandrels, but they often avoid this low ratio because bending success also depends on other factors often outside their control, like fluctuating material properties.

Open bending (without a mandrel) typically requires a much larger D value because the tube has no inside support. 2.5D is generally accepted as the safe minimum CLR for tube bending without mandrels. Tube fabricators tell me that it is possible to use a smaller CLR in open bending, but the risk of collapse or splitting increases quite a bit beyond this point.

In open bending, the D value can be decreased by using special techniques. For example, a regular practice is to make oval-shaped grooves in the forming area (bend die) to obtain better material flow in the bend.

Written by Michael Cone
Advanced Tubular Technologies

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Copyright © 1997 Advanced Tubular Technologies, Inc.