BEND TOOLING INC.: Rotary-Draw Tube-Bending Tools ~ Die Sets ~ Mandrels ~ Wipers ~ Mandrel-Bending Tools
radial growth — The increase in the length of the centerline radius that occurs as a result of springback, which see. Radial growth increases with rigidity of the material and "D" of bend. If specification of the centerline radius is critical, radial growth is offset by decreasing the radius of the bend form.
rake — The angle at which the wiper die is set relative to the tube, as illustrated in the diagram to the right. See our technical article for a full treatment of this topic. Compare zero-rake. Also see, offset geometry and simple-sweep geometry.
reach -- The width of a rotary-draw bending clamp die or pressure die; the dimension of such a die from the center of its cavity to its mounting face; the dimension lying in the plane of bend and perpendicular to the cavity and corresponding to the X-axis of the tube-bending machine. Most models of rotary-draw tube-benders have a preferred dimension for reach, but it can vary. This is often the case when clamp and pressure dies must have extended reach in a multi-radius stack of two or three or more die sets so that the cavity faces of these dies can reach the different centerline radiuses of their matching bend dies while having a common location for their mounting faces. See height and length.
rectangular coordinates — A Cartesian coordinate system for specifying tubular components, especially automotive components, in three-dimensional space. Bends are located by defining the X, Y, and Z coordinates of the three points on the tube's centerline: Each line of tangency and the theoretical intersection of both end tangents. This manner of specifying the bends is precise, of course, but causes difficulties in translating rectangular coordinates into bend data for machine programming. Use our Bend Data Calculator under the Black Art section of the website to quickly perform this translation. Compare bend data.
reduction — The flattening the extrados of the tube during the bending process. Reduction is in tension with wall thinning. Generally, to the extent that reduction is decreased, wall thinning is increased. Using assist pressure (or in extreme situations, boost pressure) mitigates both reduction and wall thinning by causing material to flow from the back tangent into the arc of the bend. Excessive direct pressure and other causes of drag will exacerbate reduction.
region — A physically active section of the tube during the bending process as opposed to an idealized geometrical element. The point of bend, intrados, extrados, and neutral axis are the primary regions of the tube. Compare geometry. (See diagram under that entry for illustration of regions.)
reverse-mount — For inserted wiper dies, a manner of attaching the insert into the holder by a flat-head cap screw which is threaded into the holder through a countersunk hole in the bore of the insert; for this reason, sometimes called "bore-mount". This is in contrast to the original type of attachment in which the screw is threaded into the insert through a hole in holder. The advantage of a reverse-mount inserted wiper die is the easy access to the screw while the wiper is fixtured on the bending machine. However, because of the countersunk hole in the bore of the insert, reverse-mount may not be suitable if the wiper must be fixtured at zero-rake.
roll bending — A method of bending in which material is fed through a triangular arrangement of rollers, the positioning of which determines the radius of the bend. A quick and effective method of forming extremely large radius bends in material with relatively large wall thicknesses.
rotary-draw bending — The principal method of high-quality tube-bending today in which the bend is formed by drawing the material around a rotating bend form. Specifically, the forward tangent of the tube is clamped to the bend die and is drawn around the die as it rotates while the back tangent is held in place against the rotating bend die by the pressure die. Thus, the point of contact between the rotating bend die and the pressure die is the line of tangency. What distinguishes rotary-draw bending from other methods is that this line of tangency is fixed in space; therefore, mandrel and wiper tooling can be fixtured at the point of bend to fully control the flow of material. This control helps to produce tube bends of superior quality.
schedule — A wall thickness designator used in certain pipe specification systems, such as IPS or EMT, in which a schedule number for a given nominal pipe diameter designates a wall thickness in decimal inches. Typically the same schedule number will designate different wall thicknesses for different nominal pipe diameters. For example, Schedule 40 for 1.5" IPS pipe is a wall thickness of .145 inches, while Schedule 40 for 2.5" IPS pipe is a wall thickness of .203 inches. Note that in the IPS system, Schedule 40 is sometimes referred to as "standard" and Schedule 80 as "XS" or "extra-strong". Schedule is commonly abbreviated "sch." or "sched."
seam — The joint along the axis of tube at which the two edges of the original coiled stock are rolled together to create the tube. Usually the seam is not a matter of concern in tube-bending, unless it consists of a weld that protrudes excessively into the inside diameter of the tube and causes interference with the mandrel. The problem of an excessive seam, short of replacing the tubing material, can be addressed in two ways:  Machine a groove into the mandrel that is sufficiently deep and wide to clear the seam; or  reduce the diameter of the mandrel to minimize interference with the seam.
Both of these solutions have significant drawbacks. If the mandrel is grooved, then it acquires an orientation that must be maintained relative to the tube. This complicates the loading of material, especially if the grooved mandrel balls are not fixed in same manner to prevent their rotation. If the mandrel is reduced in diameter, then its effectiveness in controlling wrinkling and reduction is decreased — although this side-effect can be partially offset by setting the mandrel nose deeper into the bend. However, once the mandrel is compromised in its design to accommodate a weld seam, bend quality will be limited.
serpentine bend — A long tubular part, especially as a component of a heat exchanger, that is characterized by a pattern of 180-degree bends alternating in two planes of bend that are 180 degrees apart in orientation. While the bends of a serpentine application are typically not intrinsically difficult to form — indeed, the "D" of bend is usually generous — they often must be formed without a mandrel, because the great length of the tube needed for the finished part exceeds the length of the bed of the bending machine. Setting the wiper at zero-rake and increasing the assist pressure can compensate for the absence of a mandrel if the bend specifications are not otherwise extreme.
serrations — The threading of clamp die and clamp insert cavities to improve the grip of those tools on the tube. The drawback of serrations is that they can mark, and even cut into, the outside surface of the tube. Serrations of a fine pitch will reduce marking, but have less of a grip. Moreover, their grooves more readily fill with dirt, oil, and other debris of the bending process, further reducing their effectiveness if not cleaned regularly. Nevertheless, if the part does not require an unmarked surface, serrations are an effective solution for applications demanding clamps as short as one times the tube diameter.
shoulder — A feature of interlock system for bending dies; the relief machined into a clamp or pressure die block, above and below the cavity, to clear the interlocking tabs of the bend die. See interlock (bend die).
simple-sweep geometry - The standard cut for the feathered edge of a wiper die. Ideally it is the intersection of the the radius face with the tube cavity of the wiper to fill the gap formed by the trailing sweep of the bend die cavity behind the line of tangency and the back tangent of the tube. Because this results in a feathered of zero thickness, which cannot be reliably machined, in practice the radius face is slightly offset from the tube cavity. The simple-sweep cut is satisfactory for most tube-bending applications which allow the wiper to be raked. However, for high-pressure bends an offset geometry for the feathered edge is preferred, so that the wiper will perform better at zero-rake.
springback — The release of stress from a bent tube upon completion of the bending process. To the extent that the tube's intrados has not been completely compressed or its extrados elongated while under the stress of the bending process, the bend will open up afterwards with two effects. One is radial growth, which see. The other is an increase in the degree of bend. Because radial growth is seldom critical, it is the second springback effect that garners the most attention. This second effect is readily offset by "over-bending" the tube — that is, exceeding the specified degree of bend by three to eight percent — so that it will open up to the desired degree of bend once off the machine.
Material rigidity, "D" of bend, and wall factor are the key factors in springback. However, there are other factors, such as machine set-up, that can intervene, and there exists no comprehensive guide as to how any of these factors combine to produce a predictable measure of springback. Therefore, with each new application, trial-and-error remains the quickest way to determine the amount of springback that will occur with that application.
square-back — The traditional style of wiper die, especially solid-bodied, in which the body is a rectangular block. The square-back style is recommended when the bending under high direct pressure, otherwise the half-round style is suitable for most other applications. Compare half-round; see wiper.
steel — The role of steel in tube-bending is, of course, fundamental. It is the material most tubing and most tube-bending tools are made of.
strain — The deformation of a material from its original shape while under stress.
stress — A general term for a group of forces encountered in metalworking: tensile (stretching), compressive (squeezing), and shear (cutting). Tensile and compressive are the stresses encountered in the normal bending process. Shearing occurs only under circumstances of process failure.
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