The large impact of small things
June 13, 2006
The mechanical aspects of rotary draw tube bending haven't changed since modern tooling was developed 50 years ago. Likewise, the role of the tooling (mandrel, pressure die, bend die, and wiper die) hasn't changed. However, tube fabricators these days have many choices in regard to the tooling, especially wiper dies. Choices include material, rake angle, and whether the wiper die's feathered edge is fully machined or honed by hand.
In rotary draw tube bending, the tools make the bend. Indeed, this is why the process often is called mandrel bending.
Despite the many advances in tube bending machinery, the rotary draw process itself has not changed since modern tube bending tools appeared a half century ago. Likewise, the role of the tooling—to make repeatable, high-quality bends—hasn't changed, either. It is just as important today as it was back then. Then, as now, small variations or errors in the design, manufacture, or setup of rotary draw tools can lead to loss of process control, poor bend quality, shortened tool life, and other headaches.
This includes the wiper die. At first blush it is a simple tool. Often the wiper is a solid block machined to fit the gap between the bend die and the back tangent of the tube to be bent. Frequently the wiper die is an uncomplicated, two-piece assembly in which the leading edge of the tool is a disposable insert. Other than size and material, it appears that little else is required in specifying a wiper die. However, looks can be deceiving.
The essential element of wiper die design is the feathered edge—the knifelike edge formed by the convergence of the tube cavity with the sweep of the radius face. The rest of the tool is nothing but mass to support the feathered edge and provide sufficient surface area for mountings to the tube bending machine.
The geometry of the feathered edge is not as simple or obvious as it may appear to be. Precisely how this intersection of the tube cavity and the radius face is machined into a wiper has a significant impact on the tool's performance.
Before examining that issue, let's review the wiper die's purpose. It serves two functions in the rotary draw process. The first is to prevent a hump from forming at the trailing end of the inside radius of the tube bend. As the tube is drawn into the bend, it becomes plasticized at the point of bend. The plasticized material behind the line of tangency flows into the curve of the bend die cavity sweeping away from the back tangent of the tube.
Upon completion of the draw, if this deformation exceeds the elasticity of the tubing material, it will set as a hump, or a series of small humps, at the end of the bend. Fixturing a wiper die in the gap between the bend die and the tube stops the deformation by blocking the flow of the material.
Raking, or angling, a wiper die can extend its useful life, but some applications are not suitable for a raked wiper die.
Because all tubing materials have some elasticity—that is, the property of resuming its original shape when stress is relieved—it is not necessary to fixture a wiper so that it fills the entire gap to prevent a terminal hump from forming. A wiper can be raked, or fixtured, so the feathered edge is angled away from the line of tangency, to block the flow of material only before it passes the point of elasticity (see Figure 1). The advantage of doing so is longer tool life. On a worn wiper die that was set at little or no rake, the tube cavity immediately behind the feathered edge is dished out from blocking all flow of material. This dishing shortens the life of a wiper die.
However, raking a wiper die to extend its life can be at odds with its second function: full containment of the tubing material at the point of bend when bending under high pressure. High radial pressure as applied by the pressure die is not necessary in most draw bending applications if the mandrel nose is used aggressively.
Higher pressures cannot be avoided, however, for bending materials such as 304 stainless steel or titanium or even mild steel on an extremely tight centerline radius. These materials resist the compression that occurs as the intrados thickens during the draw. If the flow of material is not completely contained by tooling at the point of bend (the mandrel, the pressure die over the outside radius of the bend, the bend die over the inside radius ahead of the line of tangency, and the wiper also over the inside radius behind it), compression will buckle the tube.
Therefore, the tubing material (and to a lesser extent, the bend specifications) dictates whether a wiper can be raked. These factors also dictate the geometry—a feathered edge designed to be raked for low-pressure bending will not perform well at zero rake under high pressure, and vice versa.
Most wipers are manufactured with some variation of simple feathered-edge geometry of the sweep formed by the convergence of the tube cavity and the radius face. The exact geometry usually is determined not by how the wiper die should be fixtured on the tube bending machine, but rather by ease of manufacture.
Even though the technology now exists to precision-machine a feathered edge to completion, most simple-sweep feathered edges are made, at least partly, by manual honing or sanding. This creates variations in the finished tool.
Usually the end user of such wipers compensates for variations in the geometry in two ways. First is by adjusting the rake. Second is by using aluminum-bronze as the material for the wiper to wear-in the desired feathered-edge geometry. However, both of these remedies work against process control in rotary draw bending, because they are inherently variable or, worse, unstable and defeat attempts to standardize setup parameters for a given tube bending application.
Moreover, the simple-sweep feathered-edge geometry that most wipers have is entirely unsuited for using the wiper at zero rake for high-pressure bending. Fortunately, the user can overcome these problems by taking into account the design and manufacture of the feathered edge when specifying a wiper die.
The first consideration is the material of the wiper. The non-nickel aluminum-bronzes remain the superior choice for steel, stainless steel, and titanium tubing. Untreated leaded steels are best for nonferrous tubing. Harder materials, heat treatment, and coatings tend to make the feathered edge like an eggshell and lead to premature failure as it fractures.
The second consideration is the geometry of the feathered edge. If the wiper will be raked, then simple-sweep geometry is required for the feathered edge. If the wiper will be set at zero rake to accommodate high-pressure bending of stainless steel, titanium, INCONEL® alloys, or similar tubing materials, then the feathered edge must have offset geometry.
With the proper offset machined into the wiper die between the tube cavity and the radius face, the feathered edge will maintain its integrity at the point of bend and provide sufficient containment of the material without deeply marking it.
The final consideration is to ensure that the feathered edge is completely manufactured by machine. This is a precision attribute of a wiper. If it is finished by manual methods, the soundness of its design will be compromised by the unavoidably wide variations. These considerations apply to both solid-body wiper dies and inserted ones.
William Tingley is vice president and general manager of Bend Tooling Inc., 1009 Ottawa Ave. N.W., Grand Rapids, MI 49503, 616-454-9965, fax 616-454-9958, firstname.lastname@example.org, www.bendtooling.com.
INCONEL is a registered trademark of Huntington Alloys Corp.