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Tube Bending Information

Terminology

Mandrel Bending – tube bending process that utilizes a mandrel inside the tube to keep the tube from collapsing. Used mainly on thin walls and tight bend radii. With this type of bending you need as many as 5 parts to the tooling:

Bend Die – the center part that the tube bends around.

Clamp Die – clamps the tube to the bend die.

Pressure Die – holds the tube against the bend die at point of bend (counters the bending forces.

Wiper Die – fills the void where the tube enters the dies to keep it from bulging at end of bend.

Mandrel – fits snugly inside tube to keep it from collapsing. May be a simple plug mandrel or a multi-segmented die the bends with the tube.

Empty Bending – tube bending process that does not utilize a mandrel or wiper die. Used on thicker wall tubes and larger bend radii. This is how most roll cage work is done.

Center Line Radius (CLR) – the most common way to define how tight a bend is. Measured through the exact center of the resulting bend.

Bend Tangents – the starts and stops of the bends. In other words, the point where the straight part of the tube stops and the bend starts.

"D" - Diameter of the tubing. It is very common to refer to the CLR of the bend in terms of D. For instance, a 3" OD tube bent on a 6" CLR is commonly called a 2D bend.

Tooling Materials

The bend die, clamp die, and pressure die are made from a heat treatable steel; usually D2 tool steel or at least 8620. These tools must be machined to an extremely smooth finish or they will "print" the machining marks onto the surface of the tube.

The wiper die and the mandrel must both be made of a material that is frictionally compatible with the tubing being bent since the tube slides over both. There are 3 commonly available materials - aluminum-bronze, hard steel, and 4130 (cromoly). The hard steel in some cases may be chrome plated. These tools need to be opposite of the tubing - i.e a soft tube like aluminum needs hard tooling like hardened steel. Here is a rough compatibility chart:

Tubing

Tooling

mild steel or aluminized (exhaust pipe)

Aluminum-bronze is best, hard steel also works

stainless steel

Aluminum-bronze

aluminum

hard steel

Other Important Facts:

One thing that is crucial to designing affordable parts is the distance between bends. This is often overlooked, but we need enough straight distance between each bend to clamp on the tube. Most of our tooling has the shortest possible clamp lengths at approximately 2 times the tube diameter. For instance, you would need to have a minimum of 5" between the bend tangents on a 2.5" OD tube. Anything shorter requires very expensive custom tooling or cutting and welding the tube after forming.

Some of the things that increase the difficulty of the bend are thinner walls, smaller bend radii, and harder materials.