Tube-Bending Basics Equipment changes but the basics don't Here's a refresher for the veterans and new information for the newbies. By Ronald R. Stange, President, Tools for Bending Inc., Denver, CO To order a hardcopy reproduction of this article, click here. To purchase digital reprints or reproduction licenses, please contact the resource center at
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There are several methods for bending tube, pipe, and extruded shapes. A bending facility’s economic productivity depends not only on selecting the most effective method but also proper tooling and proven techniques. The operator is, of course, a factor, but the right equipment and tooling minimize the degree of craftsmanship and expertise required. The three primary tube bending methods are compression (Figure 1), press (Figure 2), and rotary (Figure 3). Two principles apply to all three methods. First, material on the inside of the bend is compressed. Second, material on the outside of the neutral axis is stretched (Figure 4). A fourth method, crush bending, uses press bending. Figure 1. Compression bending Little or no support is needed inside the tube when its diameter is small and the wall is relatively thick. Tubes become weaker as diameter increases or wall thickness decreases. In addition, when bend radius decreases, forces acting on the tube are effectively increased. Figure 2. Press bending Bend Die Functions When the ratio of tube diameter to wall thickness is small enough, the tube can be bent to a relatively small radius (e.g., where Centerline Radius [CLR] = 4 × tube OD, Figure 5) without excessive bend flattening or wrinkling. The bend’s outside tends to pull towards the centerline, flattening the tube. A conventionally grooved bend die supports the tube along the centerline, and round or square tube’s inherent strength helps prevent flattening. Compression Bending: There are three basic steps to compression bending: 1. The workpiece is clamped to a bend die, or radius block. 2. The wipe shoe, or slide block, contacts the workpiece. 3. As the wipe shoe rotates around the static bend die, it bends the workpiece to the bend die radius. Figure 3. Rotary draw bending
Depending on tube and bend specifications, compression bending can be a simple or complex procedure. It is relatively simple when the bend radius is generous (e.g., 4 × OD) and the wallto-tube ratio is low. Compression bending is more complex when bend radius is 1.5 or 2 × OD and the wall factor is as thin as 20 (wall factor is the ratio of tube OD to wall thickness). In the latter situation, the bend die’s tube groove and/or the tube wipe shoe should be altered to enhance compression and stretching. Rather than a true radius in the tube groove, it may be heart-shaped or contain multiple radii to prevent tool marks and minimize collapse. In addition, depending on tubing material, wipe shoe tool material should be hardened tool steel or a hard aluminum bronze surface. Bend dies are generally made from tool steel and hardened. Press Bending: This method (Figure 2) consists of three steps: 1. A ram die with the desired bend radius is fitted to the press arm. 2. The ram die forces the tubing down against two opposing wing dies. 3. The wing dies, with resisting pressure, pivot up, forcing the tubing to bend around the ram.
Figure 4. Tube bending reaction Tooling design is based on the desired production rate and part design. When limited production is required, a half-round (R-1) ram (Figure 6) with just a case-hardened, tube groove can be used. For high production rates of parts having bends in a single plane, multiple tube-groove tooling can be used (R-3). High-carbon, high-chrome tool steel should be used.
Figure 5. Tube factor definitions Because of its high rate of bending, press bending probably bends more tubing than any other method. However, considerable distortion can occur since the tubing is not supported internally.
The tube may flatten on the outside of the bend and wrinkle or hump on the inside. A washouttype ram die (RW-4) helps eliminate humps at tangencies on the inside of the bend. Washouttype ram dies are designed for a specific bend angle. Split ram and wing dies (R2 and W2) can also be used to increase tool life and improve bend quality. The split construction varies “squeeze” pressure and permits adjustments for workpiece OD variations.
Figure 6. Ram and wing dies Crush Bending: In this method, as the name implies, the ram crushes the inner wall to bend the tube. Wing dies can also be designed to produce a “double crush” on the outside of the bend. This can produce tighter bends but with weaker bend sections. Tooling must provide tubing material flow, minimal tool marks, acceptable bend cosmetics, and easy removal from the tool.
Figure 7. Rotary draw bending tools Rotary Draw Bending: This is probably the most versatile and precise bending method (Figure 7). It consistently produces high-quality bends, even with tight radii and thin tube walls, as follows: 1. The workpiece is locked to the bend die by the clamp die. 2. As the bend rotates, a follower-type pressure die advances with the tube. 3. As the tube wall becomes thinner and/or bend radius is reduced, a mandrel and/or wiper are required. Talk to your bending tool provider for guidance as to whether a mandrel and/or wiper are required. Controlling Springback Springback describes formed metal’s tendency to return to its original shape. There is excessive springback when a mandrel is not used. This should be a consideration when selecting a bend die. Springback causes the tube to unbend from 2 to 10%, depending on bend radius, and increases bend radius after bending. The smaller the bend radius, the less the springback. Springback can be affected by pressure die location and the pressure it exerts (Figure 8). This method can allow a range of radii to be formed from one bend die. Tool design and manufacture is influenced by several factors. Wall factor and “D of bend” (“D of bend” is the ratio of centerline radius to tube OD) are the two most critical considerations, followed by desired production rate, tubing shape and material, and required bend quality. Thin-Wall Tube Bending Because of the critical nature of tubing used in aerospace and aircraft applications, a few words about bending thin-wall tubing are in order. Tubing used in these applications is characterized by several factors:
1. 2. 3. 4. 5. 6.
Tight radius bends, some less than one diameter (1D) Wall factors of 100 or more Too little or no straight tubing between bends Minimal allowable wall thinning (12 to 28%) Tube material with poor elongation characteristics Low production with frequent tool changes
Consider acquiring all thin-wall tubing from one supplier, preferably from the same lot or heat number. Premium-priced, close-tolerance tubing may save money in the long run. It is often cost effective to size tubing batches that best fit several mandrels for the same OD and wall thickness before bending. When inside finish is important, the tube’s ID can be electro-polished before bending. Care should be taken to protect the finish during bending. Guidelines for Thin-Wall Bending The tubing should have a firm slip fit on the mandrel. Clearance should not exceed 15% of wall thickness. This clearance also applies to the four pieces of outside tools. Tube material specifications and characteristics found in tubing catalogs are very helpful. For example, rated ductility and material elongation may indicate the need for special bending methods such as heated tools, positive boost bender, or even mandrel oscillation. Few tube-bending machines are capable of bending thin-wall, 1D tubing. Even machines designed for this special bending must be in excellent condition and be large enough to assure tooling rigidity. Sources of appreciable loss of tooling rigidity should be eliminated. The machine spindle should have less than 0.0005" (0.01 mm) total indicated run-out. The mandrel rod should be as large as possible to eliminate stretching. Wiper dies and their holders must be solid. Clamp and pressure die slides and tool holders must be tight. A full complement of controls is essential for bending thin-wall tubing. The machine must be capable of retracting and advancing the mandrel with clamp and pressure dies closed. A directacting, hydraulic-actuated pressure die is desirable because it provides consistent pressure on the tube regardless of wall variation. A pressure die advance should also be available. It counteracts the drag of the mandrel and pressure and wiper die. It also pushes the tube into the bending area to prevent excessive wall thin-out. Without a pressure die advance, the normally expected thinning is about three-quarters of the elongation of the outer wall. Therefore, a 2" (51 mm) tube bent to a 3" (76 mm) center line radius will thin about 25%. Since the comprehensive yield stress is generally greater than tensile, the neutral axis will be inside the tube’s geometric axis. Since distortion is proportional to the distance from the neutral axis, thinning at the outside of the bend may be greater than thickening on the inside. Moving the neutral axis toward the outside of the bend will reduce thinning. Theoretically, when the neutral axis coincides with the outer wall, thinning is eliminated but heavy thickening occurs on the inside. There are two methods of moving the neutral axis toward the outside of the bend. First, put the tube in compression before bending. The second method is to reduce the material’s compressive yield stress by heating the side of the tube that will be the inside of the bend. Thinning can be substantially reduced when both methods are employed simultaneously. It is not necessary to reduce wall thinning much more than 10% so the extra advantage can be gained in terms of closer bend radii or lighter-walled tubing. The pressure-die assist can push the pressure die and tube separately or simultaneously.
Figure 8. Springback adjustment Use a clamping plug when the wall is so thin it is distorted by the clamp die or collapses under clamp-die pressure. It also helps eliminate slippage with very short clamp dies and with less clamping pressure. The clamping plug should have a press fit. It is placed in the clamping area prior to closing the clamp die and removed before the pressure die is opened. Expanding clamping plugs are also available to make insertion and removal easier. They are designed to accommodate wall variations, as well as different walls for tubing with the same OD.
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