Tube Bending Terms

Centerline Radius (CLR)

The radius of the bend measured to the centerline of the tube. CLR is the primary dimension that defines a bend die.

D of Bend

Centerline radius divided by tube OD. A 2″ OD tube bent on a 4″ CLR is a “2D” bend. Lower D of bend values are tighter bends and usually require a mandrel and wiper die.

Wall Factor

Tube OD divided by wall thickness. A higher wall factor means a thinner wall relative to the diameter, which makes the tube more likely to wrinkle or collapse.

Tangent Point

The point where the straight section of the tube meets the start of the bend. Wiper die tips and mandrel positions are set relative to the tangent point.

Ovality

The amount a round tube flattens out of round in the bend, usually expressed as a percentage of OD. Mandrels are the primary tool for controlling ovality.

Wrinkling

Rippling of material on the inside radius of a bend, caused by compression. Wiper dies, mandrels and correct pressure die settings control wrinkling.

Wall Thinning

Stretching of the outside wall of the bend. Pressure die assist and proper tooling setup reduce wall thinning.

Springback

The amount a tube opens back up after it is released from the tooling. Springback depends on material, wall thickness and radius, and is compensated for by overbending.

Rotary Draw Bending

A bending method where the tube is clamped to a rotating bend die and drawn around it. Rotary draw bending uses a bend die, clamp die, pressure die and, when needed, a wiper die and mandrel. It is the standard method for precision tube bending.

Compression Bending

A bending method where the tube is held stationary against a fixed die and a roller or shoe wraps it around the die. Compression bending is simpler than rotary draw bending but offers less control on thin-wall tube.