Have you ever made two bends in a stick of tubing and when you put it on the floor, one leg is higher than the other? The first time it happened, you thought maybe you looked at the level wrong. The second time it happened, you thought you might need a new pair of glasses, but you invited your buddy over so he could take a look himself. You two swore you got it perfectly level, but it happens again. The third time, you disassembly your bender, put it back together, level it off, put a fresh stick of tube in, and it still happens. Around the fourth or fifth time, you call us. We get it; you feel like you’ve wasted a mile of tubing. Rest assured: that is supposed to happen; it’s part of the mechanical properties of bending material. Twist occurs to varying degrees whenever you bend material. Luckily for you, it’s a predictable phenomenon and we made a step-by-step tutorial to help you figure it out.
Just like springback, there are many variables that can change just how many degrees your material will twist. Outside diameter or the material, wall thickness of the material, centerline radius of the bend, degree of bend, and the distance between bends can affect how many degrees the material will rotate when you bend it. It’s important to do test bends whenever you are trying to bend a new part.
For this example, we’re going to make two 90-degree bends in a four foot stick of 1-1/2” tubing on a 4-1/2” centerline radius with a distance between bends of about 2 feet.
-Magnetic Bubble Level (optional)
Step 1: Set up your bender for bending normally. Don’t forget to lubricate the backing block and pins with a coat of lithium grease.
Step 2: Attach a plane of bend bracket as close to the next bend as possible. It’s not crucial that it’s level, since you will have to calibrate the angle gauge anyways.
Step 3: Attach a magnetic angle gauge and calibrate it to read zero degrees.
Step 4: Make your first bend.
Step 5: Check how many degrees the tube rotated during the bend. In this case, it rotated counter-clockwise 1-1/2 degrees. Make a note of that measurement.
Step 6: Detach your plane of bend bracket and magnetic angle gauge and get ready to make your next bend. Check to make sure your material is level.
Step 7: Attach your plane of bend bracket and magnetic angle gauge again and calibrate it to read zero.
Step 8: To compensate for the bend, rotate it the number of the degrees you calculated in Step 5 the opposite way it rotated in that step. So in Step 5, our material rotated 1-1/2 degrees counter-clockwise, so, to compensate for that twist, we will rotate our material 1-1/2 degrees clockwise before we make our next bend.
Step 9: When you finish your second bend, the other leg will level out and your part will be on plane.
See the difference? On the left is an example of what happens when you don’t account for twist; on the right is an example of what happens when you do account for twist.