According to dictionary.com, a mandrel is a "shaft or bar the end of which is inserted into a workpiece to hold it during machining." This strict definition includes rotating clamps, such as those found on lathes and circular saws, as well as the mount on a grinder or extension housing used for die grinders.
However, where automobiles are concerned, the term "mandrel" is used in several different ways. The different contexts generally include any machining tool used to steady a metal form for shaping or tooling, such as shaping dollies, die presses, and internal exhaust tubing shapers.
The first recorded use of mandrels was in ancient Egypt, where metal-workers used internally clamping mandrels to shape gold jewelry and decorations. The first recorded usage of the modern rotating, screw-in type mandrels was in the 1785 Encyclopédie Française (French Encyclopedia.) This mandrel used an arbor that was inserted into the piece to be worked on, which was then secured by an expanding screw clamp.
In the modern context, a mandrel is a tool around which a piece of metal is to be bent. These tools are differentiated from anvils, which are flat or rounded, as they are specifically shaped to the final form of the desired metal piece. The small hand-held anvils used to shape metal during bodywork (dollies) could be considered mandrels, as they are often very specialized. However, what most people mean when they speak of mandrels are those specifically made for tube bending.
Tubing mandrels function like an internal anvil during the bending process. If a piece of tubing is bent without an anvil, the excess metal on the inside of the bend will fold up on itself along the pipe's lateral lines. This creates a great deal of flow inefficiency, as the fluid inside is disrupted by the deep ridges in the metal. Inserting a mandrel into the tubing forces the metal to stretch and to shrink where it needs to, instead of folding up, making more a much more smooth-flowing tube.
Mandrel bending a tube starts in the same way as standard bending, in that both ends of the tube are clamped into either side of an angle setter. For this technique, the tools are generally hydraulic-powered, since it takes much more pressure to mandrel bend a pipe than to crimp-bend it. The mandrel (which resembles a large metal spinal column and is sized to the exact inside diameter of the tube) Is lubricated with grease, and inserted into the section to be bent. While the tube is bending, the mandrel is worked back and forth inside, to ensure that all sections are smooth.
According to Borla, a mandrel bent exhaust pipe can have as much as 10% greater flow capacity than a crimp-bent pipe. This allows the exhaust installer to use a smaller diameter tubing for easier routing and greater ground clearance, while maintaining flow comparable or superior to larger pipes.
