BEND TOOLING INC.: Rotary-Draw Tube-Bending Tools ~ Die Sets ~ Mandrels ~ Wipers ~ Mandrel-Bending Tools
camber - A measure of warp in tubing material; the deviation from straightness.
carbide impregnation — A process of electrically embedding carbon onto a steel surface in order to increase its roughness. Sometimes used on the cavities of clamp dies and clamp inserts to improve their grip on the tube without leaving the marks serrations or knurling do. The drawback is that this surface treatment degrades faster than serrations and is expensive to apply. “Surfalloy” and “Rocklinizing” are trade names for this process. Compare serrations.
case-hardening — A heat treatment process which cases a steel component with a very hard skin, usually with carbon. Carburizing, nitriding, and flame hardening are good examples of case-hardening. Case-hardening is a good process for hardening bend dies, clamp dies, and pressure dies because it combines a hard wear surface with toughness — i.e., resistance to chipping and breakage. Compare through-hardening.
cavity — also known as the “ tube groove” that part of the bend die, clamp die, pressure die, and wiper which contains the tubing material during the bending process. Generally the cavity is machined to the true shape and size of the tubing material with a small allowance to ensure that the tube moves smoothly through the cavity. However, there are variations of this “true cavity” design:
captive lip — A design for bend dies only; this is a true cavity for round tubing with flats (i.e., the lip) at the top and bottom of the cavity extending past the vertical centerline of the cavity. These flats have the effect of “capturing” the tube inside the bend die and mildly compresses it towards the plane of bend. This compression helps the tube resist flattening of the outside radius of the bend and wrinkling of the inside radius. This design is generally to be preferred, but it is not recommended for very thin-walled tubes or tubing materials with a high content of chromium, nickel, or other alloys with poor plasticity at the point of bend.
no-lip or plain — A true cavity design without the flats described above. The face of the bend die block containing a no-lip cavity is usually offset one percent of the tube outside diameter from the centerline of the cavity, and the faces of clamp and pressure die blocks are usually offset one to three percent. This type of cavity should be specified if maximum containment of the tube at the point of bend is required, as is the case with extremely thin-walled and rigid materials.
The two most common “nominal cavity” designs are:
heart-shaped — Also known as the non-mandrel, elliptical, or empty-bend cavity. The heart-shaped cavity, like the captive lip, compresses the tube towards the plane of bend but more so. The heart-shaped cavity approximates the ellipse a round tube of a given diameter becomes if flattened. The further a tube is flattened, the more elliptical its shape, and so the greater is its resistance to deformation at the point of bend. The objective of the heart-shaped design is to create sufficient resistance to eliminate the need for mandrel and wiper tooling. The heart-shaped cavity is most effective for bend applications in which the wall factor is 20 or less, the “D” of bend is 2 or greater, and the depth of bend is less than 90 degrees.
tapered leaf cavity — This is a cavity design for square and rectangular tubing. True cavity designs for these shapes have the significant disadvantage of causing the tube to stick in the die after forming. Increasing the relief allowance does not help because the tube is then not sufficiently contained to form properly. The tapered leaf cavity eliminates this problem by tapering the top leaves of the mating dies so that when closed a true cavity is created, but when open the cavity size is enlarged so that the tubing material is freely removed from the die.
centerline radius — The most common specification for the arc of a tube bend. Physically, it is the location of the crown of bend; geometrically, it is the continuation of the vertical centerline of the tube into the arc. Compare neutral axis. See geometry for illustration.
clamp die — The tool that clamps the tubing material against the bend die as it rotates to form the bend. There are two specifications of primary importance in a clamp die: Length and cavity texture, which are related to each other. The shorter the clamp, the rougher the cavity surface must be to maintain the force of the grip on the tube.
A formula for clamp die length is: L = t x k - r where "L" is length, "t" is tube diameter, "k" is a constant for rigidity, and "r" is centerline radius. For most round tubing, 5 is a good value for "k". Increase "k" for non-round tubing, hard-way bends, and rigid materials such as nickel stainless, titanium, and superalloys. A minimum value for "L", if the clamp die cavity is smooth, is around two times the tube diameter. Increase this minimum as you would increase "k".
Serrations, knurling, and carbide impregnation roughen the cavity surface, therefore improve the clamp die's grip upon the tube. Therefore, the rougher the cavity surface, the shorter the clamp die can be. Serrations and knurling generally permit "L" to be about half of what a smooth-cavity clamp die length would be. With carbide impregnation, about two-thirds the smooth length. Consequently, these cavity textures are a good solution for mid-tangents (i.e., a tangent between bends) that are shorter than the ideal "L" value for a smooth-cavity clamp. However, if a mid-tangent is significantly shorter than the ideal "L" value for textured cavity clamp, then a compound cavity may be the best solution.
close pitch — A shorter than standard pitch for a given tube diameter, pitch being the distance between the crowns of each ball (note: not the gap between the faces of the balls). When the tubing material requires more than normal support after the point of bend because it is thin-wall, soft, or some combination of factors, a close-pitch mandrel assembly is usually the solution. This is because there are a larger number of ball crowns supporting the tube bend over a given length of the arc than with a regular-pitch mandrel assembly. See pitch.
compound clamp — 1. A clamp die or clamp insert with a cavity shaped to grip on a bend rather than a tangent. This type of clamp is used when a mid-tangent (i.e., a tangent between bends) is too short for a conventional straight-cavity clamp. See clamp die.
2. A clamp die or clamp insert with multiple cavities.
compression — 1. The thickening of the intrados of a tube while under the force of the bend forming process. If a compressive force exceeds the elastic limit of the tubing material, the tube will wrinkle or buckle. Compare elongation.
2. The effect created by captive lip and heart-shaped cavities of slight flattening of the tube towards the plane of bend at the line of tangency. See cavity.
compressive — The type of stress the intrados of a tube is under as it compresses during the bending process.
2. An older method of bending in which the tube is clamped against a stationary bend die and the pressure die sweeps the tube around the bend die to form the bend. This differs importantly from rotary-draw bending in that the point of bend is the point of contact between the pressure die and bend die. Therefore the point of bend moves through space, which makes the use of a mandrel impossible. Compare rotary-draw bending.
crown — The "high" point of the ball; more precisely, the line of intersection between the spherical surface of the ball and its vertical center plane. The distance between the crowns of assembled balls is called the pitch. See ball; see pitch.
crush bending — A non-mandrel method of bending in which the compression of the intrados is controlled by stretching it over a "crush knob" seated in the cavity of the bend die. This eliminates the wrinkling or buckling that might occur if the tube were bend without a mandrel. Commonly used on non-round tube bends.
crush knob — A raised section of a bend die cavity over which the intrados is stretch to alleviate the compression, thus eliminate wrinkling and buckling.
"D" — A unit of radial measurement peculiar to tube-bending. "D" is the ratio of the centerline radius to the nominal, not the true, outside diameter of the material: "D" = CLR / nominal TOD. This distinction between nominal and true diameters is critical when "D" of bend specifies the centerline radius of a pipe (as opposed to a tube) bend, because pipe diameters by definition are identified by nominal values. For example, the centerline radius of a 3-"D" bend of a 2" IPS pipe (which is 2.375" true diameter) is 6 inches instead of 7.125 inches.
degree of bend — The depth of bend; the sweep of the arc. The minimum degree of bend is about five degrees; the maximum degree of bend in rotary-draw bending is 180 degrees. See "DOB" under geometry for illustration.
direct pressure — The pressure a bending machine applies radially (in the X-axis) to a tube at the point of bend in order to hold the tube against the bend die during the bending process. Direct pressure, unlike assist or boost pressures, is a fundamental element of the bending process, because without direct it, the back tangent of the tube would swing away from the bend die, thus no bend would be formed. Direct pressure is applied by means of the "pressure die", which see. Compare assist pressure and boost pressure.
Direct pressure should be increased as the rigidity of the tubing material increases. Direct pressure unavoidable causes drag on the tube, which works against the free flow of material at the point of bend. Therefore, no more direct pressure should be applied to the tube than is necessary to prevent it from pulling away from the bend die during the bending process. Excessive direct pressure has the effect of clamping on the back tangent of the tube, just as the clamp die clamps on the forward tangent. Thus the tubing material excessively stretches and flattens between these two points of die contact along the outside radius of the bend.
double-wall tubing — Also called laminated tubing, this type of tubing consists of two strips of coiled steel, on atop the other, rolled together in a tube mill producing, in effect, one tube inside another. Double-wall tubing is most commonly seen in the automotive industry for exhaust systems. The tooling and set-up for rotary-draw bending of double-wall tubing is the same as regular tubing except for the specification of the mandrel assembly. Because, obviously, material cannot flow from one wall to the other as it becomes plastic at the point of bend, the total thickness of the double-wall is not relevant for specifying how much mandrel support the tube requires. Instead, the thickness of the interior wall should determine the number of balls, nose and ball diameters, and the nose placement of the mandrel assembly. In other words, for purposes of mandrel specification, the inside wall should be treated as a thin-walled tube.
drag — The friction between the tube and the tools at the point of bend. Drag has the undesirable effect of retarding the free flow of the tubing material as it drawn forward around the bend die and so becomes plastic at the point of bend. This retardation of the flow exacerbates the inherent compression of the intrados and elongation of the extrados, and so must be minimized. Drag, however, is also inherent to the rotary-draw bending process and cannot be completely eliminated, but low direct pressure and a forward mandrel setting do keep drag to a minimum.
ductility — A measure of metal's capacity to flow; the plasticity of a material as force is applied to it. In the context of tube-bending, ductility is the ability of the tubing material to compress or elongate at the point of bend. The greater a tube capacity to flow at the point of bend, the greater its ductility; in this sense, ductility is roughly the opposite of rigidity.
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