BEND TOOLING INC.: Rotary-Draw Tube-Bending Tools ~ Die Sets ~ Mandrels ~ Wipers ~ Mandrel-Bending Tools
offset geometry - A style of cut for the feathered edge of a wiper die, in which the radius face is substantially offset from its natural point of intersection with the tube cavity of the wiper. Also called aero-cut, aerospace cut, or aircraft cut, which see. The purpose of offset geometry is to produce a feathered edge that can be set at zero-rake and withstand the forces of high-pressure bending. Compare simple-sweep geometry.
orientation — 1. The bend specification for whether a non-round tube bend is "easy way" or "hard way"; more generally, the position of a tube's cross-section relative to the plane of bend. This specification is commonly relevant to rectangular and oval tubing because it is necessary to identify whether the major (or long) axis of the tube's shape is perpendicular to the plane of bend — i.e., easy way — or lies in it — i.e., hard way.
2. In a multiple-bend part, the location of the plane of one bend to that of another.
outside diameter — Tube outside diameter; the standard manner of specifying the size of a tube or pipe as measured from its outside surface; the bore diameter plus twice the wall thickness of a tube.
outside radius — A specification for the arc of a tube bend, most commonly used with non-round tubing. (The centerline radius, which see, is almost always used to specify the bend of a round tube.) Physically, it is the outermost edge of bend; geometrically, it is the continuation of the outboard line of the tube into the arc. Compare extrados. See "OSR" under geometry for illustration.
ovality — The deformation of round tube into an elliptical shape; symmetrical flattening of a round tube; a measure of the difference between the maximum and minimum outside diameters of a round tube. Ovality is not a form of bend reduction, which is flattening of a tube's outside radius as result of the bending process. In contrast, ovality is a deviation of a tube from an ideal round shape caused by imperfection during milling or flattening from storage.
pilot — A washer-shaped fixturing device that centers the bend die on the bend head's axis of rotation. The outside diameter of the pilot mates with the pilot counterbore of the bend die with a slip fit. The only purpose of the pilot is to locate the bend die and is not designed bear the load of the bending process.
Occasionally the pilot is an integral part of the bend die as a low-profile boss, especially in the case of platform bend dies. The disadvantage of this approach is that the pilot's edge becomes damaged over time, and if it is integral to the bend die, it must be re-machined rather than replaced when worn out. Therefore, the integral pilot is recommended only if the cross-section of the bend die does not accommodate a pilot counterbore.
pilot counterbore — A primary feature of most key-driven rotary-draw bend dies; it is the shallow counterbore surrounding the post hole which mates with the pilot to center the bend die on the bend head's axis of rotation.
pipe — 1. Tubing, the outside diameter and wall thickness of which are identified by one of the pipe specification systems, such as IPS or EMT; otherwise, pipe is indistinguishable from other tubular materials. Because pipe specification systems use a nominal value to specify the outside diameter instead of the actual value — e.g., the outside diameter of 1 1/2" IPS pipe is 1.900" — one should be alert to this fact when tubing is referred to as "pipe" to ensure the diameter is correctly identified.
2. Jargon for tube in the automobile industry, especially when referring to exhaust systems. In this case, the outside diameter and wall thickness specifications of automotive "pipe" are actual, not nominal, values.
pitch — Distance between the crowns of assembled mandrel balls. (Note the different pitches in the photograph to the right.) Ideally, the higher the wall factor — i.e., the ratio of tube diameter to wall thickness — the closer the pitch, because thinner walls of a given tube diameter require more points of support, as provided by the crown of ball, over a given distance than do thicker walls of the same tube diameter. (This is the same reason why the mandrel nose radius should decrease as the wall factor increases.) However, H-type link design, which is the most common type of link used in mandrel assemblies, does not allow pitch to be precisely adjusted to a particular wall factor. Instead, pitch must change in intervals, because H-type link length, which determines the distance between mandrel ball crowns, is fixed. These intervals are commonly termed "regular pitch", "close pitch", and "double-close pitch".
Each standard H-type link length, or size, is designed to provide optimal support for a certain range of tube diameters at wall factors less than 40-45. The pitch resulting from this is known as regular pitch. A close-pitch mandrel assembly is typically recommended for uses links applications ranging from 40 to 70 wall factor and uses links one size smaller than would be specified for a regular-pitch mandrel. A double-close pitch mandrel, 60 to 90 wall factors, uses links two sizes smaller than regular pitch.
plane of bend — The plane perpendicular to the bend die axis of rotation which intersects the tube at full diameter. In the rotary-draw process, the plane of bend is that determined by the machine's X- and Y-axes. Because in the rotary-draw process the plane of bend is fixed in space at the bend die's centerline height, it does not require any particular notice in single-bend applications once all of the tools are properly fixtured relative to it.
However, in multiple-bend applications the raw tube material acquires an orientation in terms of plane of bend upon completion of the first bend. All subsequent bends must now be made relative to that orientation. This means rotating the tubing material on the B-axis — i.e., rotary motion centered on the Y-axis — a certain number of degree relative to the plane of the previous bend. On CNC bending machines, the collet grips the tubing material and rotates it as needed. On manual machines, stops must be set for the bent section of the tube to rest against at the correct orientation relative to the plane of the next bend.
Therefore, the terms plane of bend and orientation are closely related, and are sometimes used interchangeably in reference to programming CNC machines. However, there is a useful distinction to be maintained between them. Plane of bend refers to geometry of a tube bend, whereas orientation refers to the motion of the machine to locate to the tube bend.
platform bend die — A relatively uncommon type of bend die in which the die block — i.e., that portion of the tool containing the tube cavity — is mounted upon or integrated with a platform. The purpose of the platform is either:  To provide a stable base upon the machine's bend head for a relatively small die block; or  to form a flange that is drilled and counterbored with a bolt-hole circle in order to screw onto the bend head a die block too small to accommodate a mounting post. As indicated by these purposes, a platform bend die is required when the centerline radius of an application is much smaller than would be typical of a machine of given size or if the combination of tube diameter and centerline radius creates a bend die cross-section too small to allow the die block to be drilled out for a mounting post. Because platform bend dies are expensive, they should be avoided when possible. Therefore, in the former instance, serious consideration should be given to using a smaller machine, and in the case of the latter, the design of the bend should be reviewed to determine if a larger centerline radius is acceptable. Often only a small increase in the centerline radius will obviate the need for a platform bend die.
point of bend — That region of the tube surrounding the line of tangency which is in a plastic state during the bending process. Therefore, the point of bend is not synonymous with line of tangency. The point of bend refers to a part of the tube, whereas the line of tangency defines a fixed plane in space through which the tube passes as it is bent. It is important to understand the point of bend as region rather than a geometrical element, because good bend quality requires fixturing the mandrel and the wiper die properly to control the flow of material in that plasticized region. For example, the reason why the mandrel nose must be set past the line of tangency and into the arc of the bend is because the point of bend exists both behind and ahead of the line of tangency and needs to be supported by the mandrel nose to prevent it from buckling or collapsing in its plastic state. See entries under line of tangency and forward mandrel set-up for details. See geometry for illustration.
poppet link — One of the two varieties of H-type links in which the body of the link is solid instead of split into two multiple pieces. The base of the poppet link is serrated to allow it to flex open as the head of another link is inserted into it. This means that poppet links require a press for assembly, unlike split links which can usually be assembled by hand. However, there single-piece design makes them more durable than the split variety and also makes them less expensive to manufacture. See H-type link; compare split link.
post — Part of the bend die mounting system of a rotary-draw machine. The post is a large threaded stud that is screwed into the center of the bend head, around which the bend die mounted (and centered upon by the pilot), and to which the bend die secured by a large nut.
pre-bending — Jargon in hydroforming operations for the rotary-draw bending process. The "pre" prefix refers to the fact that the bending process occurs before hydroforming and does not otherwise denote any aspect of the bending process that is particular to hydroforming.
press bending — One of the primary methods of tube bending along with rotary-draw and compression. Technically press bending, sometimes called vertical bending, is distinguished from these other two methods by the fact that when the ram die (analogous to the bend die in rotary-draw bending) pushes the tube through a pair of wing dies (analogous to the pressure die in rotary-draw bending), two lines of tangency form following the points of contact between the circumference of the ram die and each face of the two wing dies.
Press bending is less expensive and faster than rotary-draw bending. Its primary disadvantage is reduced bend quality. Because the lines of tangency in press bending move through space, as opposed to the fixed single line in rotary-draw bending, it is not possible to fixture mandrel tooling inside the tube nor wiper tooling outside of it to control the flow of material at the two resulting points of bend. Therefore, press bending is only suitable for those applications with relatively heavy tube walls and large centerline radiuses.
One exception is if the application allows for a crush bend. If so, then the ram die can be equipped with a "crush knob" which will force the intrados of the tube to stretch around it to mitigate wrinkling. However, this will not reduce flattening of the extrados, especially as the diverging lines of tangency move further out along the circumference of the ram die. See crush bending.
pressure die — The tool that holds the tube against the bend die under pressure applied by the bending machine at the line of tangency during the rotary-draw bending process, thus creating the point of bend. Originally the pressure die was a static die block that once clamped into position at the line of tangency did not move forward as the bend die rotated. This, of course, created considerable drag on the tube at the point of bend, which being in a plastic state, caused excessive flattening in the extrados of the bend. The follower type of pressure die, sometimes called the follower die, corrected this problem by traveling forward during the bending process and so reducing drag. Almost all modern rotary-draw bending machines are equipped with this type of pressure die. (Some lower-end machines use rollers instead of a static die block to function as a pressure die. This greatly reduces drag but raises the problem of effective containment of the tubing material at the line of tangency.)
The primary specification of the follower-type pressure die is its length, which is a function of travel and material rigidity. A formula for pressure die length is: L = (r x 3.14 x (b/180)) + (t x k) where "L" is length, "r" is centerline radius, "b" is degree of bend, "t" is tube diameter, and "k" is a constant for rigidity. For most round tubing, 2 is a good value for "k". Increase "k" to 2.5 to 3 for rigid materials such as nickel stainless, titanium, and superalloys in round tubing, 3 to 4 for easy-way bends in non-round tubing, and 4 to 5 for hard-way bends. The purpose of this rigidity factor is to ensure that there is a sufficient length of pressure die behind the line of tangency at the end of the bend to hold the tube true to the Y-axis of the bending machine.
On most bending machines there are two settings controlling the operation of the pressure die: direct pressure and assist pressure. To reduce drag on the point of bend direct pressure must be kept to the minimum necessary to hold the tube without it separating from the bend die cavity. A low direct pressure setting also improves the effect of assist pressure on the extrados of the tube. See the entries under direct pressure and assist pressure more information on these settings.
Pythagorean theorem — An equation defining the length of each side of a right triangle: a2 + b2 = c2 where "c" equals the length of the hypotenuse. The Pythagorean theorem has many applications in rotary-draw tube-bending, of which one of the most useful is calculating the position of the mandrel nose relative to the line of tangency.
quick-change tooling - A die set that has mountings that allow for rapid installation and removal. Also applies to inserted grips for bend dies that are attached to the radius block by a T-key instead of screws. Stacked tooling is a common alternative to quick-change tooling.
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