Managing "Tail Off" in Tube Bending
When the End of a Tube is Not Exactly Right
Bent tubes don’t exist in a vacuum. What I mean by that is that a bent tubing has to work with other devices in a system. Very few people just need a simple piece of tub. With this in mind, sometimes a bent tube has a bend very close to the end of the tube. If it happens to be the last bend on a rotary draw process, the possibility of tail off exists. The question becomes can the tail off coincide with the rest of the system?
What Causes "Tail Off"?
Tail off occurs when the stresses in the last leg of a tube cause the end of the tube to go out of square with respect to the axis of the tube itself. From Figure 1 below, the tube on the left shows a square bent end. However the tube on the right depicts a tube with tail off.
Figure 1 above shows a square end (after bending) on the left and a non-square end on the right. The non-square end can occur when the last leg length L is too short.
Most often tail off can happen when the end of the tube is in the pressure die and it is closing in on bend tangent. When this happens the stresses on the outside of the tube tend to pull more in a direction causing the end of the tube to go out of square. If the vertical leg were longer (Figure 1), the tail off effect would not be as prevalent.
Figuretivily speaking, tail off occurs when the moons are aligned. Such factors as the material, temper, wall thickness, OD, bend radius, and the length of the last L are all factors that can product a non-square end. For example, if the last L is short enough, a solid copper rod of ½” diameter can experience tail off, but this problem is not limited to just copper tubing by any means. However, softer metals such as copper and aluminum are more susceptible to this effect.
How to Minimize "Tail Off"?
Tail off can be controlled or minimized. For example, increasing the following conditions will lessen the chances of tail off and minimize the effects:
- Wall thickness
- Hardness of the material
- Centerline bend radius
- Length of the last leg
As a general rule, if the last leg length bent on a rotary draw bender is < 2 x tube OD, tail off is common. Nevertheless, this may not be a problem and the negative effects of tail off are application dependent. For example, if the bent tube has a fitting brazed onto its non-squared end, it is possible that the brazing process can tolerate a little tail off. In an application where tail off cannot be tolerated, a secondary sawing operation is often needed to produce the short final leg length.
One final approach to counteract tail off is to use an internal plug that expands into the inside diameter of the tube. The plug can provide additional structural support to the end of the tube during the bending process, thus reducing the tail off. However, wall thinning on the outside of the tube in the area of the last leg length may occur. The expanding plug technique is often used in conjunction with an outside diameter steel ring. When a ring is used a modified pressure die is required to accommodate the external ring.
About the Author
All of our semi-rigid coax and tube fabrication machines at Winton are designed, manufactured, and tested in-house. We have a large line of standard products as well as the ability to engineer the best solution for our customer’s needs. Our experienced sales staff makes sure that our customers can justify their capital equipment investment by offering a solution that is exactly what they need in order to manufacture their parts. Please contact us today to discuss your project.