When a steel part is specified to be bent to a short radius, it is important to know how to mandrel bend pipe and other sections. A short radius is usually somewhere below 6-7 x the diameter or depth of the steel member being curved.
The image below shows 6in outside diameter, Schedule 40 pipe with two 90 degree 12in center-line-radius bends. The two bends are 2D, i.e. the center-line radius (12in) is two times the outside diameter of the pipe (6in).
The method that is best suited to complete this job is called rotary draw bending or mandrel bending. Successful bending of these parts leaves little or no distortion in the bend area. Without the proper method, the tighter the radius the more likely that unacceptable distortion will occur. This is due to the nature of steel and how the material gets displaced and actually flows within itself when cold working or bending is being done. The saying is that when working steel the material must go somewhere, and when we cannot control where we want the material to move, that is when we usually see distortion.
Rotary draw/mandrel bending is the best method to use on short radius bends because its tooling prevents the material from moving in an undesirable way.
Proper tooling completely supports all sides of the steel member being curved, both inside and out. Supporting the outside of the steel shape being bent is done through specialty bending and clamping dies that are machine-cut out of billet steel to the specified profile of the member and to the radius that the parts are to be bent to. For this reason most dies are specific to the parts they make and unless you encounter similar parts, they usually cannot be used for other jobs. The inside of steel sections, like rectangular and square tubes or round tubing and pipe, are supported during bending by using a tool called a mandrel. This tool is shaped to fit perfectly to the inside of these members and prevents any crushing or concaving of the parts. With the proper tooling, short radii are achievable on many steel sections through the rotary draw/ mandrel bending method.
