How to Bend Mandrel Tubing

by Ian Kelly

Mandrel tube bending is used where the inside radius of the tube must be bent without wrinkling or deformation. This is achieved by inserting a flexible or solid fixed "mandrel" to support the interior walls of a tube during the bending process. The walls are supported internally with either a flexible mandrel that bends with the tube, or a fixed "bullet" mandrel set internally at the pivot point of the radius die. Mandrel bending is commonly used in the manufacture of automobile exhaust pipes, turbocharger intake and outlet tubing, and custom exhaust systems.

Anneal the steel tube by marking the length of the bend with four evenly spaced lines drawn across the tube with a sharpened piece of ordinary household soap. Heat the tube evenly with a propane torch adjusted to a soft flame until the soap lines turn black. Do not bring the flame into contact with the soap mark during the heating process. Allow the steel pipe to cool. This will soften the metal so the tube can be bent without deformation.

Locate the tubular shaped mandrel bullet screwed onto the end of a long, round shaft mounted to the frame of the tube-bending jig. The mandrel bullet is machined to fit snugly into the tube to prevent the walls from buckling during the bending process. Adjust the bullet by aligning a mark near its end with a mark on the top surface of the radius die mounted on top of the frame. This will place the bullet precisely at the point where the bend will start.

Adjust the die by moving the degree disc mounted below its underside to the desired angle of the bend. A movable stop will prevent the die from turning past the preset point.

Lubricate the bullet and the inside of the tube with an aerosol can charged with compression-resistant lubricant. Slide the tube over the bullet, and slip the tube "keeper" collar over the end of the tube and onto the steel peg on the side of the die. This collar is designed to grip the tube and draw it around the radius die during the bending process.

Place the sliding back plate with its lengthwise half-round slot over the tube. Push the sliding back plate between the adjustable rollers opposite the radius die and the tubing. Tighten the adjusting knob so that the length of the back plate half-round slot is clamped firmly over the tube. The back plate will slide forward with the tube, applying support and squeezing the walls of the tube between the die and the internal mandrel bullet during the bending process.

Insert the round pivot lever into the matching hole on top of the radius die. Grip the end of the lever with both hands, and apply enough rotational force to draw the tube around the die until the preset stop prevents further movement. The tube will now be bent to the desired angle without a trace of deformation on the inner radius of the bend.


  • close Wear protective gloves and other protection as needed, especially when using the torch.

Items you will need

About the Author

After graduating from the University of the Witwatersrand and qualifying as an aircraft engineer, Ian Kelly joined a Kitchen remodeling company and qualified as a Certified Kitchen Designer (CKD). Kelly then established an organization specializing in home improvement, including repair and maintenance of household appliances, garden equipment and lawn mowers.

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