Inserted wiper die for high-pressure tube-bending and method of using same

The present invention is a wiper die assembly that combines the performance of a solid-body wiper die with the economy of a inserted wiper die for high-pressure rotary-draw tube-bending. The key features of the present invention are: [1] A wiper insert attached to the wiper holder by a mechanical means that retains the bore of the insert as a smooth, uninterrupted working surface, and [2] radius face support shoe attached to the wiper holder that can be adjusted to support and stabilize the entire wiper assembly against the cavity of the bend die.

FIELD OF THE INVENTION

The present invention relates generally to tube bending and, more particularly, to a method and apparatus for high-pressure rotary-draw tube-bending.

BACKGROUND

In rotary-draw tube-bending the tools make the bend. Indeed, that is why the process is often called “mandrel bending.” Despite the many advances in tube-bending machinery, the rotary-draw process is mechanically identical to what it was a half-century ago when modern tube-bending tools made their first appearance. Therefore, the central importance of the tools in making repeatable high-quality tube bends back then remains just as important today. Small variations or errors in the design, manufacture, or set-up of rotary-draw tools can lead to loss of process control, poor bend quality, abbreviated tool life, and other headaches.

FIG. 1illustrates a cross-sectional view, through the plane of bend, of the operation of a wiper die assembly100that is known in the prior art. The wiper die101is a block machined to fit the gap between a bend die103and the back tangent105of the tube107to be bent. The essential element of the wiper die101is the knife-like feathered edge109, which is formed by the intersection of two curved surfaces of the wiper die101: [1] The bore into which the back tangent105of the tube107nests, and [2] the radius face against which the bend die103bears. Typically, the wiper die101is a two-piece assembly comprised of a wiper holder111and a wiper insert113, which features the feathered edge109. The wiper insert113is disposable. Alternatively the wiper die101can be a solid block.

The bore of a wiper die will vary with the shape and size of the tube to be bent. The radius face will vary for the same reasons plus the radius of the tube bend. The material of the wiper die is usually metal, although plastic and wood have been used. Occasionally the bore, and less often the radius face, is plated or coated to reduce friction during bending. The material of the wiper die is determined by the material of the tube with durability and friction the primary considerations. Other than size and material, there are no rigid specifications for a wiper die. Most of the tool is nothing but mass to support the feathered edge, to provide sufficient surface area for mounting the wiper die to the tube-bending machine, and in the case of high-pressure tube-bending to act as a backstop for the pressure the tube-bending machine applies to the tube during the rotary-draw tube-bending process. Although the feathered edge is the essential element of a wiper die, no design standard has emerged for it.

The wiper die101serves two functions in the rotary-draw tube-bending process. The first is to prevent a hump from forming at the inward half of the back tangent105immediately behind the line of tangency117when the tube107comes to rest at the end of the bending process. As the tube107is being drawn around the bend die103to form the bend115, it becomes plasticized at the point of bend119, which is the region of the tube107both ahead of and behind the line tangency117being formed into the bend115. The plasticized material behind the line of tangency117continuously flows into the curve of the bend die cavity103that is sweeping away from the back tangent105of the tube107, thus forming a hump. As this humped material is draw through the line of tangency117it is flattened out. However, if this deformation exceeds the elasticity of the tubing material, the hump or a series of small humps will set in the back tangent105. Fixturing a wiper die101in the gap between the bend die103and the back tangent105of the tube107stops the deformation from reaching that point by blocking the flow of the material. Because all tubing materials have some elasticity, i.e., the property of resuming its original shape when stress is relieved, it is not necessary to fixture a wiper die101so that it fills the entire gap to prevent the formation of this hump. As seen inFIG. 1, a wiper can be “raked” as shown by angling the back end of the wiper die101from point Ra(i.e., “zero-rake”) and point Rbso that it blocks only that amount of the flow that would exceed the elasticity of the material. The advantage of this technique is longer tool life. In instances where a worn wiper die was set at little or zero-rake, the bore immediately behind the feathered edge109is “dished” out from blocking the entire flow of material. This dishing reduces the usable life of a wiper die.

However, raking a wiper die to extend its life can be at odds with its second function, which is full containment of the tubing material at the point of bend when bending under high pressure. Normally, high pressure as radially applied by the pressure die (not shown) against the back tangent105of the tube107opposite of the wiper die101is not necessary in most rotary-draw tube-bending applications. However, higher pressures often cannot be avoided for bending materials such as stainless steel or titanium or even mild steel on an extremely tight bend radius115. These materials resist the compression that occurs as the intrados of the tube bend115(approximately the wall of the inward half of the tube107) thickens during the bending process. If the flow of material is not completely contained by tooling at the point of bend117—i.e., the mandrel (not shown) inside in the tube107, the pressure die (not shown) over the outward half of the back tangent105, the bend die over the inward half of the tube bend115, and the wiper over the inward half of the back tangent105—the compression will buckle the intrados.

FIGS. 2A and 2Billustrate a side view and end view of a traditional solid-body wiper die200as used in the prior art. The solid-body wiper die is a solid block201with one or more tapped holes203at its back end for mounting it to a post or other fixturing device of the tube-bending machine (not shown). The front end is a compound curved face formed by the arc of the bore205, into which the back tangent of the tube nest, swept along the arc of the radius face207, which mates with the bend die cavity, starting from the feathered edge209to the top of the wiper block201. Typically the tapped holes203do not extend into the bore205, thus there are no interruptions to the smooth surface of the bore. Similarly, the radius face207is an uninterrupted smooth surface. Therefore, therefore are no discontinuities in these two surfaces which act as bearings—to wit, the bore205against the radial pressure of the pressure die as applied through the back tangent of the tube and the radius face207against the bend die cavity—when a solid-body wiper die200is mounted at zero-rake for high-pressure tube-bending. For this reason, along with its generally large mass, the solid-body wiper die200is preferred for high-pressure tube-bending.

FIGS. 3A and 3Billustrate a side view and end view of a standard inserted wiper die300as used in the prior art.FIG. 3Cis a side view of the wiper insert component303of that assembly300. In contrast to the solid-body wiper die200, the inserted wiper die300is a two-piece unit consisting of a wiper holder301and a wiper insert303. The insert303is attached to the holder by means of a screw305. The wiper holder301functions in a manner similar to the block201of the solid-body wiper die200. The back end of it has one or more tapped holes307to mount the entire wiper assembly300to the tube-bending machine. It otherwise does no work during the rotary-draw tube-bending process. The only working surfaces of the inserted wiper die are those of the bore309and the radius face311of the wiper insert303. Together these surfaces form the feathered edge313of the wiper insert303. When one of these features wears out, the wiper insert303is detached from the wiper holder and replaced with a new one. Replacing an insert is less costly than either re-machining the worn bore205, radius face207, or feathered edge209of a solid-body wiper200or disposing of it. For this reason, the inserted wiper die300is preferred for all rotary-draw tube-bending except high-pressure tube-bending.

The inserted wiper die300lacks the mass and uninterrupted working surfaces of the solid-body wiper die200. Because the only function of the wiper holder301is to act as a fixture for the wiper insert303, its bore315and radius face317are offset from the corresponding surfaces309,311of the insert303so as to not incur unnecessary wear. Also because of this limited function the holder301has only the mass necessary to support the insert303. A consequence of this reduced bulk of the holder301relative to the block201of a solid-body wiper die200is that its tapped holes307go through its body into the bore315, which are additional interruptions to the overall continuity of the bore309,315of the inserted wiper die300. Because of the lack of mass and interrupted working surfaces of the inserted wiper die300, it is not suited for high-pressure tube-bending.

Therefore, the need exists for a wiper die that incorporates both the performance of the solid-body wiper200and the economy of the inserted wiper die300. The present invention overcomes the shortcomings of all other wiper dies and methods of use as an inserted wiper die with continuous working surfaces and sufficient mass necessary for effective performance in high-pressure tube-bending.

DETAILED DESCRIPTION

FIGS. 4A,4B and4C illustrate various views of a high-pressure wiper die assembly400in accordance with an embodiment of the present invention.FIGS. 4A and 4Bare side and back views, respectively, of the high-pressure wiper die assembly400. The assembly400includes three major components: [1] A wiper insert401, [2] a wiper holder403, and [3] a radius face support shoe405.FIG. 4Cis a side of the wiper insert401. Unlike the pocket-type wiper insert303depicted inFIGS. 3A,3B, and3C, the high-pressure wiper insert401is in the form of a sleeve with a smooth, continuous, uninterrupted surface for the bore407. This allows the full length of the wiper die's bore407to be used as a linear bearing for tube-bending (not shown). Those skilled in the art will recognize that this is necessary for high-pressure bending. The wiper insert401is mounted to the wiper holder403by a pair of button-head screws409at small recesses411located approximately mid-length of the bore407, as shown inFIGS. 4A and 4B. The wiper insert401is further secured to the wiper holder403by a lip413extending from its outside diameter and overlapping the back end of the holder403. These mounting features411413, by exploiting the direction of the forces being applied to the wiper die assembly400during the tube-bending process, obviate the need to interrupt the surface of the bore407with tapped holes, like the mounting305shown inFIG. 3C. The wiper insert401is a disposable component of the high-pressure wiper die assembly and is intended to be discarded when worn out. It will generally be made of either alloy steel or aluminum-bronze depending upon the material of the tube, although it can be made of any machinable material including plastic.

The wiper holder403provides most of the mass of the high-pressure wiper die assembly400. To it are attached the wiper insert401and the radius face support shoe405, as shown inFIGS. 4A and 4B. In turn, the wiper holder403has one or more tapped holes415towards the back of it by means of which the entire wiper die assembly400can be attached to the rotary-draw tube-bending machine (not shown). The length and width of the wiper holder403will vary with the size of the tube to be bent. Its height will vary with the radius of the tube bend so that the radius face of the entire assembly400will cover a sweep of approximately ninety degrees from the feather edge417of the wiper insert401to the top of the support shoe405. Because the radius face419of the wiper insert401and the nose421of the support shoe405provide the critical bearing surfaces against the bend die cavity (not shown) for the wiper die assembly400, the radius face423of the wiper holder403is recessed in a manner similar to the wiper holder301depicted inFIG. 3Ato prevent unnecessary wear. The wiper holder403is permanent component of the high-pressure wiper die assembly400and generally will not wear out under normal use. It is made of alloy steel to enhance its durability.

The radius face support shoe405extends beyond the radius face423of the wiper holder403into direct contact with the bend die cavity to support and stabilize the entire wiper die assembly while under high pressure during the tube-bending process. As a consequence the nose421of the support shoe405will wear over time. Therefore, the shoe405can be advanced forward relative to the radius face423of the holder403so that its421continues to make direct contact with the bend die cavity. To this end, the shoe405is mounted to the holder403by a pair of socket-head cap screws425through a counterbored slot, as shown inFIG. 4A, to allow this forward adjustment. Furthermore, this forward adjustment is precisely controlled by a pair of pusher screws427at the back end of the shoe405, as shown inFIGS. 4A and 4B. The support shoe405is, like the wiper insert401, a disposable component of the high-pressure wiper die assembly400and is intended to be discarded when worn out. It is generally made of bronze because of its function as a bearing, although it can also be made of other types of material that do not deform under high-pressure tube-bending yet minimize friction.

This assembly400of wiper insert401, wiper holder403, and radius face support shoe405effectively combines those characteristics of a solid-body wiper die200that facilitate high-pressure rotary-draw tube-bending with those characteristics of a standard inserted wiper die300that lower the cost of the tube-bending process. This is because: [1] The sleeve-type wiper insert401has a bore407with a continuous uninterrupted working surface like the bore205of the solid-body wiper die200, yet is a replaceable component like the wiper insert303of the standard inserted wiper die300; [2] the wiper holder403has a mass similar to the block201of the solid-body wiper die200, yet no working surfaces subject to intensive wear like the wiper holder301of the standard inserted wiper die300; and [3] the radius face support shoe405provides the support and stability for the entire assembly400during high-pressure tube-bending as does the radius face207of the solid-body wiper die200, yet is a replaceable component that obviates the need to re-machine or discard the wiper die assembly400when it is worn out, unlike the solid-body wiper die200when its radius face207is worn out. It is the support shoe405that makes the high-pressure wiper die assembly400effective as an inserted tool for high-pressure tube-bending and can be used for any current tube-bending application in place of a solid-body wiper die200.