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What is D of bend?

Tube fabricators sometimes indicate the centerline radius using D of bend.

We say it by speaking three words: "D of bend".

Some tube fabricators also say "D factor" to mean the same thing.

It is a ratio calculated like this: D of bend = Centerline Radius / OD

How Is It Used?

FABRICATORS: The D of bend value helps fabricators know if the radius is tight or large compared to the OD. It serves as a general guide for how difficult it will be to create a quality bend. It helps fabricators know what kind of tooling will be necessary to achieve the bend. It is useful, therefore, in building quotations for customers.

CUSTOMER DESIGNERS: It is also helpful for designers as a limit for what is practical.

It is common for fabricators will tell customers the lower limit for their D of bend capability.

Examples

In most applications, a lower D of bend valued radii is less complex and therefore less expensive.

D of bend = 2.00 is generally considered not difficult*
D of bend = 1.50 is moderately difficult.
D of bend = 1.00 is difficult.
D of bend = 0.75 is very difficult and much more expensive.

D of bend values at 1 and below are possible - but are rare and require special benders that can press material into the bender's dies. (Most call this "carriage boost".)

'*'Other factors, like wall factor, impact what is difficult. Wall factor = OD / Wall Thickness