What is the D of bend in tube bending?

The D of bend is the centerline radius divided by the tube's outside diameter. A tube bent to 1D has a radius equal to its diameter —a very tight curve—; at 3D or 4D, a much more open and easy curve. By expressing it as a ratio rather than an absolute measure, it allows comparing the difficulty of curves on tubes of different sizes with a single number.

The D of bend says how tight the curve is, but not the whole story. The real difficulty also depends on the wall factor —diameter over thickness—: a thin-wall tube at 2D is far harder than a thick-wall one at the same 2D. That is why the D of bend is always read together with wall thickness and material; alone, it can mislead.

Knowing the tightest D of bend the part requires is what organizes the technical conversation: it defines whether a mandrel is needed, how many balls, and what tooling. A low D with a thin wall is the sign that the job is demanding and needs a machine and tooling to match. How it combines with the rest of the variables is developed in the article on how to specify a tube bending machine.

The D of bend translates «tight radius» into a comparable, objective figure, and it is the gateway to specifying a tube bend. Read together with the wall factor and the material, it anticipates whether a mandrel is needed and how demanding the machine will be. Defining the part's tightest D of bend is the first step to not falling short.