The purpose of the pressure die is to keep the tube against the bend die through the duration of bending. Unbalanced Pressures The pressure die should be adjusted for a light pressure against the tube. There also are certain limitations for tubes larger than 3/8” diameter. Exceptions to this are thin wall tubing or a centerline radius that is less than 2 x tube O.D. A plug mandrel can be used to produce relatively good quality bends for tubing 3/8” diameter and smaller. This force, acting on the mandrel tip, supports the inner radius of the bend, holding it firmly into the bend die groove. The material stretching is done on the forward tip of the mandrel (Figure 5). The tube stretching process is localized on the outer radius of the bend and the material is workhardened to retain its shape and not flatten. The mandrel is held in a fixed position while the tube is pulled over it. A plug mandrel (Figure 5) is indicated if the tube buckles and is still within the wall factor and the diameter of the bend.īENDING TUBE WITH A PLUG MANDREL Balanced Pressures The purpose of a plug mandrel is to prevent the tube from flattening and to bend without wrinkles or kinks. This condition can be corrected (provided the tube is not too hard) by proper set up of the tooling. The material, not being able to compress, pushes in toward the centerline of the tube. This may be due to hard material which will not compress on the inside radius of the bend. Kinked or Buckled Bends The tube may kink or buckle as shown in Figure 4. The smaller the radiuses of bend the smaller the springback. Springback will cause the tube to unbend from two to ten degrees depending on the radius of bend, and may increase the bend radius of the tube. “Springback” is the term used to describe the tendency of metal that has been formed to return to its original shape. This should be considered when selecting a bend die. The pressure die forces the tube into the bend die.įigure 3 Control of Springback Springback is excessive when a mandrel is not used. The clamp die holds the tube in position while bending. The bend die helps to prevent the tube from flattening and forms a given radius of bend. Basic Primary Tooling A bend die, clamp die, and pressure die are the minimum essentials for bending tube. The forces acting on the tube also becomes greater as the radius of the bend becomes smaller. If the wall thickness of the tube is decreased, it also becomes weaker. As the size of the tube diameter is increased, the tube becomes weaker. Two factors that help prevent this from happening are a grooved bend die, which supports the tube along the centerline and the natural strength of the tube round or square (Figure 2).įigure 2 Little or no support is needed within the tube when the tube diameter is small and the wall is thick. The outside and inside of the bend tend to pull towards the centerline of the tube (flattening). Excessive flattening or wrinkling of the bend should not occur. Compressedįunction of Bend Die When the ratio of the tube diameter to wall thickness is small enough, the tube can be bent on a relatively small radius (Centerline Radius or CLR = 4 x Tube O.D.). The material that forms the outside of the bend has further to travel and therefore is stretched the inside of the bend has less distance to travel and is compressed. The material actually is formed approximately about the centerline of the tube. The outside wall is reduced in thickness due to the stretching of the material and the inside wall becomes thicker due to the compressing of the material (Figure 1). Stretching and Compression Principles When a tube is bent, two things happen to metal (Figure 1-a). TROUBLESHOOTING Tube breakage Tube wrinkling Scratches/Marking on Tube Centerline Tube Collapses Either With or Without Wrinkling Through Bend Hump at the End of a Bend and Mandrel Ball Humps Tool Marks/Scratches Excessive Springback Possible Reasons for Link Failure Final Tips V. BENDING WITH BALL MANDREL AND WIPER DIE Bending Thin Wall Tubing IV. BENDING TUBE WITH A PLUG MANDREL Balanced Pressures Unbalanced Pressures Mandrel Too Far Back Mandrel Too Far Forward III. BENDING TUBE WITHOUT A MANDREL Stretching and Compression Principles Function of Bend Die Basic Primary Tooling Control of Springback Kinked or Buckled Bends II. BASIC TUBE BENDING GUIDE 2710 Swamp Cabbage Court ٠ Fort Myers, FL 33901 23 ٠ 23 fax ٠ 80 US & Canada ٠email:
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