How to specify a tube bending machine

The bend radius is measured to the tube's centerline (CLR), and what matters is not the absolute radius but its ratio to the diameter: the so-called D of bend. A loose radius (high D) can be achieved with simple methods; a tight radius, on the order of 1.5 to 2 times the diameter or less, requires mandrel bending and fine control so the tube does not collapse. Defining the tightest radius the part needs is what determines the bending method and the level of tooling.

The difficulty of bending a tube is given not by the diameter alone, but by its ratio to the wall thickness: the wall factor (diameter over thickness). A large-diameter, thin-wall tube is the hardest case, because it has a lot of hollow section and little wall to hold it: it is the one most prone to ovalizing and wrinkling. That factor defines whether a simple bend is enough or whether a mandrel and a wiper die are needed to support the wall inside and out.

In tube bending there are three defects to watch: ovalization of the section, thinning of the outer wall and wrinkling of the inner one. The mandrel supports the tube from inside so it does not ovalize, and the wiper die prevents wrinkles on the inside of the bend; how much tooling is needed is decided by the section tolerance the part requires. A structural tube tolerates more ovalization than a tube that must carry fluid or mate with another: the demand of the application defines the tooling.

The material's yield strength, tensile strength and elongation define how much force is needed, how far it can be bent without cracking and how much it springs back on release. A high-strength tube springs back more and forces over-bending to compensate, and a low-ductility material limits the minimum radius before the outer wall cracks. Catalog capacities are stated for a reference material and are worth verifying against the real working tube.

Unlike a plate roll, a tube bender works with a die set specific to each diameter-and-radius combination: bend die, clamp die, pressure die, mandrel and wiper. A machine's real range is not just its maximum capacity, but which diameters and radii the available tooling covers; changing diameter or radius usually means changing tooling. On CNC machines, tooling and programming also resolve parts with multiple bends in different planes. Models and configurations are detailed on the product page.

Specifying a tube bending machine means describing the most demanding tube you will bend: its diameter, its wall thickness, the tightest radius and the material, along with the section quality the part tolerates. With that defined, the choice stops being a matter of force and becomes which machine and which tooling deliver that radius on that tube without ovalizing or wrinkling the section. As the representative of AKYAPAK tube bending machines, CYM Materiales supports that definition so the equipment matches the real work.