BEND TOOLING INC.: Rotary-Draw Tube-Bending Tools ~ Die Sets ~ Mandrels ~ Wipers ~ Mandrel-Bending Tools
tangency — In bending, the point of contact between the face of the die block applying force against the material being bent and the circumference of the bend form. In rotary-draw and compression bending, this would be the contact between the pressure die and the bend die. In press bending, this would be the contact between the wing die and the ram die. The ray passing through this point is known as the line of tangency, which see, despite the fact that geometrically the tangent is actually perpendicular to that ray.
tangent — The unbent section of a bent tube, as opposed to the arc of the bend. Each bent part consists of at least two tangents and a bend. The end tangents of a bent part are those tangents at each end of the part, and the mid-tangents are the remaining tangents between the bends of the part (assuming the part has more than one bend in it). The forward or lead tangent is the tangent that lies ahead of the line of tangency during the bending process, and the back or trailing tangent is that which lies behind. Compare arc or bend.
tangent length — The distance between the lines of tangency (in the case of a mid-tangent) or between the end of the part and the line of tangency (in the case of an end tangent). This dimension is important in rotary-draw bending, because clamp length is limited by tangent length, especially that of mid-tangents. If a mid-tangent is too short for a recommended clamp length, then the grip of the clamp must be increased by either machining serrations, which see, into its tube cavity or by using a compound cavity instead of a straight one. See clamp die.
teardrop bend die — A single-piece bend die design for rotary-draw bending that accommodates overbending of 180-degree bends to compensate for springback, hence the die's teardrop shape. See bend die.
tensile — The type of stress the extrados of a tube is under as it stretches, or elongates, during the bending process.
terminal hump — The deformation in the form of wrinkle that occurs when the material exceeds its elastic limit and sets in the curve of the bend die cavity immediately behind the line of tangency at the end of the bending process. In rotary-draw bending, the terminal hump is eliminated by use of a wiper die.
through-hardening — A heat treatment process that employs the existing carbon content in a steel part to increase its hardness from its surface down to its core. Through-hardening is effective on steels with 30 points or more of carbon. Low-carbon steels cannot through-hardened, but can be case-hardened instead. Through-hardened materials tend to be harder but more brittle than case-hardened ones, and this can be a drawback in bending dies which require toughness rather than extreme hardness. Compare case-hardening.
through-keyway — A keyway in rotary-draw bend dies which extends from one edge of the die to the other. A through-keyway bend die can be mounted on a machine equipped with drive keys on both sides of the pilot.
TiN — Titanium nitriding, a chemical coating applied to carbon and tool steel cutting and forming tools to reduce friction and increase hardness, thereby extending tool life. TiN (pronounced "tin") coating, a relatively expensive process, has limited utility for tube-bending tools primarily because improvements in tool set-up (along the lines of the four-step forward-mandrel, low-pressure procedure) have a greater impact in extending tool life at no additional cost.
T-key — A t-shaped key commonly used to mount clamp dies on modern bending machines, in contrast with the previously ubiquitous pin-type mounting used on older models of Pines bending machines.
toughness — Durability; hardness not compromised by brittleness. A prized quality in bending dies often optimally acquired by case-hardening a die machined from an alloy steel.
true cavity — A die cavity machined to the exact size and shape of the tube with only sufficient allowance for a slip fit with the tube. This allowance may be as little as only a couple thousandths of an inch on the diameter; therefore, tubing that significantly varies in its outside diameter will require the ideal true cavity specification to be compromised with additional allowance in order to ensure at least a slip fit. Compare nominal cavity.
tube, tubing — A tube is a single piece of tubing, which is a general term for a wide range of materials that are hollow in form. Most tubing is circular in cross-section, though square and rectangular tubing is common, and is made of either plastic or metal. Of primary interest in this article is metal tubing, because it must be formed — as in the rotary-draw bending process, for example — to produce a finished tubular component. (Of course, almost all plastic tubular components are manufactured from raw material into their finished shape. Thus, plastic tubing is seldom "formed" in the same sense as metal tubing and occupies a different universe in industry.)
Metal tubing is typically manufactured by roll-forming a strip of sheet metal, although it can also be extruded or drawn. (Extruded and drawn tubing typically feature very heavy walls or complex cross-sections and are not frequently encountered in tube-bending.) Once manufactured metal tubing can be formed into a finished part by a variety of processes including flaring, swaging, beading, hydroforming, and, of course, bending.
The fundamental specifications of a tube are outside diameter, wall thickness, and material. Occasionally tubing, especially if it is extruded or drawn, will be specified without a wall thickness and instead by outside and inside diameter. All pipe is tubing and is mostly distinguished by the manner of its specification, in which nominal values are used instead of true ones to identify the outside diameter and wall thickness. (See pipe.)
tube fabricating - Narrowly, the manufacture of tubing. More generally, all metal-forming of tubular material.
tube forming - Bending, end-forming, beading, and all other metal-forming of tubular material.
Type K copper tubing and Type L copper tubing — A specification system for tube outside diameter and wall thickness peculiar to pressure tubing made of copper. In these systems, each nominal outside diameter (.125 of an inch smaller than the true value) is associated with a particular wall thickness. Type K tubing is heavier walled than Type L, and was originally intended to specify copper tubing for use in underground plumbing and heating systems. Fabricators of this type of tubing will frequently specify the diameter by the nominal value without reference to the fact that it is Type K or Type L tubing; therefore it is always prudent to double-check the diameter given for a copper tube.
wall — The material substance of a tube contained between its outside and inside surfaces. The wall is specified in terms of its thickness.
wall factor — The ratio between tube outside diameter and wall thickness. It is a rule of thumb for assessing the difficulty of a tube bend: The higher the wall factor, the more difficult the bend. The rationale behind this rule is that a wall that is thin relative to the tube outside diameter requires more support at the point of bend to prevent wrinkling or collapse. As a practical matter, the higher the wall factor, the more likely a mandrel and a wiper are needed to achieve good bend quality in rotary-draw bending. The wall factor needs to be considered in conjunction with other factors, such as the "D" of bend, to fully gauge the difficulty of a bend.
wall thinning — The reduction in wall thickness that occurs as the extrados is elongated at the point of bend. The reason the tube wall thins out is simple: The fixed amount of material contained in the wall must spread out to cover the extrados as its area increases at the point of bend. Conversely, the wall of the intrados thickens at the same time as the area of that region decreases. (See geometry for an illustration of the regions mentioned in this entry.)
Wall thinning, as opposed to wall thickening, is an issue tube-benders take particular notice of, because many bend applications require a minimum wall thickness in the finished product. Many factors play a role in determining how much a wall will thin, not the least of which is the set-up. One of the key objectives of a good set-up is to minimize drag at the point of bend, which in turn minimizes the amount of wall thinning.
To determine a realistic target for wall thickness of the extrados after thinning, use the following formula: WT - (( OSR - CLR ) / OSR x WT ) = RWT where "WT" is the starting wall thickness, "OSR" is the outside radius, "CLR" is the centerline radius, and "RWT" is the thickness of the wall after being reduced by bending. For example, in a 2" TOD x .065" WT x 4" CLR tube bend, the calculation for the target post-bend wall thickness is .065 - (( 5 - 4 ) / 4 x .065) = .04875. Therefore, a realistic target for wall thickness in this application is about .049".
Achieving this target value assumes a "forward-mandrel, low-pressure" set up that reduces the drag of direct pressure at the point of bend and maximizes the effect of assist pressure. Increasing the assist pressure will account for variations in material plasticity, but will not significantly exceed the target value. To substantially increase this value will normally require starting with a heavier wall. However, boost pressure, which is a special feature most machines lack, is a mechanical means of measurably exceeding the target value, although at the expense of excessive thickening of the wall of the intrados. (See the articles under assist pressure and boost pressure for additional details.)
wing die — One of a pair of dies that create the point of bend on a press-bending machine as the ram die (which see) pushes the tubing through them. Similar in function to the pressure die in rotary-draw tube-bending.
wiper — Short for wiper die; the tool in a rotary-draw bending set-up which prevents the formation of a terminal hump or wrinkle (which see) on the inside radius of the bend. The wiper is fixtured between the cavity of the bend die and the tube immediately behind the line of tangency. The critical feature of a wiper is the feathered edge, which prevents the terminal hump from exceeding its yield point and setting into a wrinkle. Precision machining of this edge is paramount for good performance.
The key setting for the wiper is rake. Rake is the maximum angle relative to the tube centerline at which the wiper can be set before the terminal hump forms. More rather than less rake is normally desirable because increasing rake increases the life of the wiper; however, certain applications require zero-rake. (See entries under rake and zero-rake. See our tech article for a full discussion of rake.)
Aluminum-bronze and alloy steel are the most common materials for wipers. While steel offers good durability under the right circumstances, it galls when used on steel tubing, especially stainless steel. Aluminum-bronze has the advantage of resisting galling, so that it tends to wear out instead of fail. Consequently, aluminum-bronze is preferred for steel tubing and steel for non-ferrous tubing.
Like mandrels, wipers are available in solid-body and inserted versions. Inserted wiper tooling has been widely used for about two decades and have completely supplanted solid-body wipers in a number of industries, especially automotive. An inserted wiper consists of a disposable feathered edge, the wiper insert, which is pocketed into a steel fixture, the wiper holder. The inserted wiper offers ease of use and reduced cost over the traditional solid-body wiper and can be used in all applications except those requiring zero-rake.
wiper holder — The fixture into which a wiper insert fits to form an inserted wiper assembly. While the wiper insert tends to be a standardized component to ensure low cost and ready availability, the wiper holder — which has relatively long tool life — is customized to suit the particular requirements of machine and set-up. Therefore, the wiper holder can be extremely short to clear collets holding short end-tangents, or extremely long to clear the backside of a small-radius teardrop bend die making a 180-degree bend. Wiper holders can also be half-round (most common) or square-back (to simulate the traditional square-back solid-body wiper).
wiper overall length — The length of an assembled inserted wiper.
wrinkle control cavity - A corrugated cavity featured in some heart-shaped bend dies to force the wrinkling of the inside radius of the tube bend into a regular pattern. In the past the wrinkle control was used in the production of aftermarket automotive exhaust systems. However, that has declined because of the increased rate of corrosion failure and impeded exhaust flow that resulted from products with controlled wrinkles instead of the smooth surfaces of mandrel-bent products.
X-axis — On a CNC rotary-draw bending machine, the linear axis in the direction of which the clamp and pressure dies move to capture and release the tube. This motion is radial with respect to the bend die. There is no related rotary axis.
Y-axis — On a CNC rotary-draw bending machine, the linear axis along which the collet "transports" the tube to locate the bend at the line of tangency. The related rotary axis, which orients the tube to the plane of bend, is the B-axis.
yield point — The level stress at which a material will permanently deform during a forming process, such as bending.
yield strength — A material's resistance to permanent deformation while under stress.
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