Mandrels come in all sizes, but they’re all used to shore up the ID of tubes so that minimum deformation occurs on the ID and OD as the tube deforms during bending. Images: Lincoln Brunner
I often provide training for bender operators who have never seen a tubing bender or even a tube being bent. One of the biggest mysteries to a newbie is the mandrel: What is it? What does it do? How do you set it up for a quality part?
The Mandrel Bending Process
To explain how a mandrel works in tube bending applications, I use a plastic drinking straw as an example.
Using your thumb as the bend die, your fingers as a pressure die, and your opposite hand as the clamp and bend arm, try bending the straw. You may be able to get 5 to 10 degrees or so around your thumb, then suddenly the outside part will collapse. You may get another degree or so, then the inside radius will start to fold instead of bending. On a steel tubing part, this will look like a series of wrinkles.
So why does this happen?
Before bending, the tube is all the same length. During bending, the end of the tube will have traveled an infinite number of distances, but it is helpful to think in terms of three specific ones.
Think of three different size bicycle tires—small, medium, and large. When each tire has made a 360-degree rotation, each has traveled a different distance. The small tire will have traveled a shorter distance than the medium tire, and the medium tire a shorter distance than the large one. Now apply that logic to a tube in a bend die:
- The part of the tube against the bend die (inside radius) is the small tire.
- The center of the tube (centerline radius) is the medium tire.
- The outer part of the tube farthest from the bend die (outside radius) is the large tire.
Once bending is complete, the outer part of the tube will have traveled a greater distance than the center, and the center more than the inside. If you assume that the center of the tube stays the same length, this means that the outside of the material will either have to stretch and finish the bend longer than it started, or it will have to take a shortcut and collapse toward center of the tube. That’s why with the drinking straw, the outside of the straw flattens toward the center of the straw.
The opposite is true of the inside radius. Assuming that the center of the tube stays the same length, the inside radius will have to compress and finish the bend shorter than it started or it will fold inward toward the center of the tube.
A Mandrel Makes the Difference
To solve this problem of flattening on the outside radius—and, to a lesser degree, wrinkling on the inside radius—a mandrel can support the tube from the inside.
For round tubing, a mandrel is a solid piece of material that is slightly smaller than the inside diameter of the tube being formed. It is held stationary near the point of bending (the tangent) while the tube is drawn over it. As the outside part of the tube tries to collapse toward the center, the mandrel holds the outside radius back in its original shape, causing it to stretch to accommodate the longer distance it has to travel.
One of the chief benefits of properly sized and deployed mandrels is the minimal deformation suffered by the tube during bending, even in high-strength steels or other difficult-to-bend materials.
As the material being bent gets thinner and the centerline radius gets smaller, more and more support is required to hold that shape. This can be accomplished by making the end of the mandrel more of a round shape so that there is more surface area supporting the outside of the tube. This forms a longer section of the mandrel to the actual inside part of the outside radius of the tube, supporting even more of the outside part of the tube.
Mandrel balls can be added to support the tube through a much larger degree of bend. Mandrel balls are designed as sections of a ball, usually the same diameter as the mandrel itself, that are then linked to the mandrel and each other, essentially making the mandrel flexible so it can support the inside of the tube throughout the bend.
Making the Fit
For a mandrel to fit inside a tube, it must be slightly smaller than the ID. How much smaller depends on the application requirements and the materials being bent.
“For aerospace, aesthetic, and high-difficulty applications, a mandrel will be designed with 0.003 to 0.005 in. [about a sheet of paper’s worth] of clearance to the inside of the tube,” said Scott Mitchell of Omni-X, a bend tooling design and manufacturing company. “For automotive and high-volume use where minor deformation is acceptable and expected, 0.010- to 0.015-in. clearance is more common. On shipyard, pipe, or other heavy-wall applications, 0.020– to 0.080-in. clearance is typical due to the ability of the heavy wall thickness of the material being bent to support itself during the bending process.”
Because the inside of the tube is drawn over the end of the mandrel body and the mandrel balls during bending, mandrels are generally considered “wear items” that must be replaced from time to time. Knowing how much clearance mandrels should have from the inside of the tube will help you determine when they should be replaced.
“Mandrel style and clearance is determined through review of the difficulty of the bend,” Mitchell said. “Wall thickness, bend radius, tube outside diameter, and material being bent all factor into the difficulty evaluation. As the mandrel wears, it will allow more space on the inside diameter of the tube, reducing support and allowing material to flow into that space, affecting the quality.”
Setting Up a Mandrel
We tend to think of a mandrel doing all of its work at the point of bending, which is the tangent line perpendicular to the centerline of the tube while it is still straight. However, because the mandrel is always slightly smaller than the ID of the tube, the tube would have to slightly collapse before it comes into contact with the mandrel.
Because of this, a mandrel is usually set so when it is fully forward during bending, the edge of the radius on the nose is slightly forward of the tangent line. How much further forward depends on the application and mandrel design. The math can be worked out to know exactly how far forward from tangent a mandrel can go, but I personally tend to think of this as a maximum position rather than a set-in-stone placement.
I typically rely on a series of test bends to determine where the mandrel should be set for the best quality results. If the mandrel is too far forward, the tube may tear or break in the outside radius, or a slight “hump” may form on the outside of the tube at the end of the bend. If it is too far back, the outside radius will flatten.
Lubrication
The act of bending tubing over a mandrel entails two materials being moved against each other at very high pressure. This can generate tremendous heat, degrading the quality of the material being bent and increasing the amount of wear on the mandrel itself.
No matter what size your tubing, the mandrel must be made slightly smaller than the ID. How much clearance you have typically depends on the stringency of the end user’s standards.
There are several different types of lubrication, from thin oils to heavy gels, designed specifically for forming metal tubes over a mandrel. The manufacturers can recommend the most suitable products for specific applications.
Mandrels can be designed to allow the application of lubrication through the body of the mandrel during the bending process. While there is no specific formula to determine how much lubricant to use, it should be mostly consumed in the act of bending. Too much lubricant can actually cause quality issues with the parts being bent.
