Mandrels are a commonly used piece of tooling when bending pipe and tube. Used correctly, mandrels can help prevent some of the most common problems and issues you can run into when pipe bending.
However, if not positioned correctly, mandrels can also cause problems of their own. It is vital that you understand how to mandrel bend a pipe before you begin bending.
Need the OD and wall for your pipe size? See our pipe schedule chart for NPS 1/8" to 24" in every common schedule.
First, however, it is important to understand why we use a mandrel when bending pipe.
Why Do We Use a Mandrel?
The most basic reason to use a mandrel when bending a pipe is for support. The mandrel offers support throughout the bend radius and holds the pipe firmly against the bend die groove.
Without that support, the pipe can flatten (go oval), wrinkle on the inside of the bend or collapse completely. The thinner the wall and the tighter the radius, the more support the pipe needs. A mandrel does not prevent springback, the tendency of the metal to spring back toward its original shape after bending. Springback is corrected by overbending slightly in the bender program.
How to Mandrel Bend a Pipe
The first step in mandrel bending is the same as any bend, you must set your tooling. Correct tooling setting is essential to performing the bend correctly. Improperly set tooling can result in wrinkles, kinks, buckling, bulging, and tube collapse. In fact, the first step in troubleshooting any pipe bending problem should be checking the tooling setup.
Once your machine is ready and your tooling set, you can now begin. When using a mandrel, the tube is first pulled over the mandrel, which is held in a fixed position. That position, however, is very important.
For example, when the mandrel is placed too far back in the pipe, it is not far enough forward to generate the necessary pressure on the inside of the bend to compress the material. Your bend may begin well enough, but once the bend has progressed past 20 degrees or so, the material begins to push back. This, in turn, forms a ripple or a wave.
The ripple is forming and being flattened continually between the mandrel and the bend die. When the pipe is removed from the bend die, you will observe a large buckle (point A in the diagram below).
Moving the mandrel too far forward can also result in serious problems with the bend. Mainly, when the mandrel is too far forward, it can cause a bump to form at the end of the bend.
These types of issues can become worse when making tight bends or when bending thin-walled tubing. It is essential that your mandrel, as well as your other tooling, is set correctly.
Choosing a Mandrel for Pipe
Pipe is ordered by nominal size and schedule, so start with the true OD and wall from our pipe schedule chart. Then work out the degree of bend (CLR divided by OD) and the wall factor (OD divided by wall):
- Heavy wall on a generous radius: often bends without a mandrel, or with a plug mandrel.
- Medium wall or tighter radius: a ball mandrel with one or two balls.
- Thin wall on a tight radius: a multi-ball mandrel and a wiper die.
Our free tube bending calculator gives a mandrel and wiper starting point from these numbers.
Mandrel Bending Pipe vs. Tube
The process is the same. Schedule 40 and 80 pipe have heavier walls than most tube of the same OD, so they often need fewer mandrel balls but more bending force. Larger pipe sizes need a bender with enough capacity and torque. See our CNC tube and pipe benders.
Frequently Asked Questions
Why use a mandrel when bending pipe?
A mandrel supports the inside of the pipe through the bend, so it doesn’t flatten, wrinkle or collapse. It is most important on thin walls and tight radii.
Does a mandrel prevent springback?
No. Springback comes from the material and is corrected by overbending. A mandrel controls the shape of the pipe through the bend, preventing flattening and wrinkling.
What happens if the mandrel is too far back?
The inside of the bend is not supported, so ripples form and you get wrinkles or a buckle once the bend passes about 20 degrees.
What happens if the mandrel is too far forward?
The mandrel pushes into the bend, causing a hump on the outside of the bend, excess wall thinning or a broken mandrel.
Learn More
Click here or follow the link below to download a free introduction to tube bending to learn more, or contact us at any time with any questions or comments you may have.
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