TPT July 2018

T E CHNOLOG Y

Making tubular profile straight – even when there’s a curve By George Winton, president, Winton Machines

THE need for straightening a tube is throughout the tube fabrication industry. You may have a coil in bulk form that feeds a forming process. You may have purchased a straight tube, as defined by your supplier, and you need it straighter. In both cases, a tube straightener may be of interest.

Tube straighteners have been around for years. The US Patent Office documents various straightening devices going back decades. The problem is simple: take a bent metal tube and straighten it. This sounds straightforward, and sometimes it is, but other times you may get a curve. When forming a tube, fabricators have to take into account spring back. For example, if you take a straight tube and bend it in a rotary draw bender to 90°, after unclamping the tube may spring back to 86°. To bend a tube to 90°, you may have to bend it first to about 90° + 4°, then when you release the tube from the tooling the tube will relax. In this case, the metal that forms

Figure 1: The runout being measured between two V-blocks

Figure 2: A tube being forced through a series of roll dies

the tube has memory, and tries to return to its previous relaxed shape. This is the spring back. Taking this concept forward, to straighten a tube that is bent requires that the tube be bent in a direction past the point of straightness, in order to get the tube to spring back to a straight condition. There are several ways to define straightness. One involves rotating a fixed length of tubing between two V-blocks and noting the runout between the V-blocks (figure 1). A straightening operation is a bending operation, but trying to get the tube to take a set in a new position. The ‘2D problem’ is often found when trying to straighten a tube that starts out in a bulk spool. As the tube is predominantly bent in a single plane, the problem is considered 2D. For example, a dead soft bulk spool of copper that feeds into a cut- to-length line; in this case, as the copper is pulled from the bulk spool it has a consistent bow in it from being formed around the bulk spool itself. Running this bowed tube through a series of precision rollers goes a long way towards straightening the tube (figure 2). A bonus here is having the copper dead soft. This means that the copper has very little spring back, and sending it through a series of roll dies can quickly remove a large amount of runout. While this is considered a 2D problem, in truth as the tube is pulled from a bulk spool, the tubing has a tendency to untwist about its long axis: now it is a 3D problem. The twist is a result of the stored energy in the bulk spool. Think of the bulk spool as one big torsion spring. When the untwisting condition exists, it can be gradual; say 5° over a 15-foot span. This gradual twist can propagate through the straightener and can cause a fluctuation in the output. This is because the straightening dies are set as the tube twists through the straightener dies. The output fluctuation is spring- back dependent. The straightening process above works well when the system is used to remove a bow in a long length of tubing. However, if the tube is ten feet long and looks like a piece of spaghetti, then another straightening process should be considered. The process involves a couple of V-blocks and a manual press. Where the bows in the tube are many, the straightening process may involve straightening the tube one section at a time.

Winton Machine Company – USA Website: www.wintonmachine.com

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JULY 2018