Ever bent a stick of tube, taken it out of the bender, and found it wasn’t the angle you bend to? Relax. Your bender isn’t broken. The die isn’t machined improperly. This is supposed to happen. That phenomenon is called springback. The cause of springback is uneven density of molecules resulting from stretching and shrinking of a material during a bend. The centerline material is trying to return to its original shape, yet is constrained by the unyielding material on either side. The effect is noticed when the material is taken out of the bender. See here:
We set up the bender for normal bending:
Bent to 90-degrees in the bender:
Took the tubing out (it’s not 90 degrees):
The degree of bend is 88 degrees:
Springback must be compensated for by adding the springback factor (the number of degrees that a material springs back) to the desired degree of bend. You can easily figure out the springback factor by performing test bends. You can see that for this particular batch of material in this centerline radius, the springback factor is 2 degrees. So whenever we want to make a bend in the future with this batch of material, we will have to “overbend” by 2 degrees.
Springback is not a constant factor for all materials and can even change within materials of the same OD and wall thickness based on its quality resulting from different manufacturing processes. It's extremely important to perform test bends on each batch of material you buy. Keep a log book of batch numbers (you can find it on the mill certificate) and the results of your test bends with each batch to keep things straight. Happy fabricating!
Tools Used in This Demonstration:
-Hydraulic 105 Heavy Duty Tube and Pipe Bender
-180-Degree, 1-1/2” Round Tube Die, 4-1/2” Centerline Radius
-Lithium Grease
Pro-Tools
Patrick,
Awesome question! It is not a constant value, but it’s not a fixed percentage either. The best way to determine the springback angle for all your bends is to do a variety of test bends and plot them in a chart or graph. In most cases, you can almost treat it as a constant (unless your tolerances are extremely tight) or if you are bending material which exhibits more than average springback. With the Bend-Tech software we use, you can plug a couple of angles in based on testing and it will figure everything else for you. If you want me to go into more detail, please feel free to reach out!
Pat
Is the springback a constant value regardless of degree of bend, or is it better represented as a percentage?
With the example given in the write-up above, if you bend the same piece of tube to 180*, would it spring back to 178, or 176 (doubling the 2* springback given that you’re doubling the angle) or roughly 2.22% of the total bend?
Lets take the example to a more complicated place, lets say that we need a 135* bend, do we bend to 137 (constant 2* spring back) or do we bend to 138* (plus 2.22%)? Sure, they aren’t that far apart, but if you have a long straight section on either end it’ll be drastically different measurement from wherever you’re trying to land.