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Compressing and Stretching 101

Compressing and Stretching 101

There are very few, if any, people that bend tubes and nail a perfect bend on the first tube after a new setup. Because it seems that no matter how many times you have set up a machine to run a given tube size, material type, wall thickness, etc., most of the time you find yourself adjusting to get it just right. A new batch of material may have a slightly different metal content or wall thickness, the tip of your wiper may have become shorter from normal wear, or even the wear on the mandrel balls can cause an issue where there wasn’t any the last time you ran that part. With each bend set up it seems that you have to chase the settings and make fine adjustments, but that’s just the nature of the process.

Let me step back for a minute and give you a brief description of how mandrel tube bending is done. The tube bending is done with a mandrel inside. The mandrel helps support the tube from the inside while it is being stretched on the outside of the bend and compressed on the inside of the bend. Basically, the clamp grips the end of the tube that is being bent and pulls it over the mandrel while at the same time dragging it along a wiper until it meets the bend die and starts the process of bending. At the same time a pressure die is applying side pressure on the tube keeping it seated against the wiper.

It sounds simple but there is a lot going on within the wall thickness of the tube itself. As the outside of the bend is being stretched, the wall of the outside of the bend gets thinner. Meanwhile the inside of the tube is being compressed so the wall of the inside of the bend becomes thicker. The neutral area between the stretching and compressing is trying to keep the tube straight. This neutral area force is what causes spring back after bending has taken place. An example of spring back is where you bend a tube to 92.4 degrees; once the machine pressures have released the tube, it springs back to 90 degrees which was your final target. If you stretch the outside of the tube too much it will break apart. If you compress the inside of the bend and the wiper is not set just right, with the correct amount of die pressure, you get wrinkles or waves along the inside of the bend or even after the bend.

Learning to read the results of a bent tube is something that takes time. And just when you think you know what to do to correct an issue or a bending problem, along comes Mr. Humble making you feel as if you’ve never bent a single tube. In the long run, Mr. Persistence pays off.