Considerations for square, rectangular, and oval tubing
August 1, 2009
While bending round tube and pipe involves many variables and challenges, the difficulties in bending nonround shapes are more numerous and complicated. Among the most common shapes are square, rectangular, and oval (elliptical and flat-sided). None react to the bending force in the same way that round shapes do, so understanding how the material reacts is the first step in learning about bending nonrounds.
Although round is the most common tube shape, opportunities abound for nonround tubing. Square and rectangular tubes, which have more strength compared with round tube, commonly are used in architectural and structural applications. Automotive frames and furniture also are good candidates for the strength-to-weight advantages of nonrounds.
While it is most common for nonrounds to be used when the design is secondary to strength, excellent aesthetics are possible. A strong knowledge of the basic principles of forming nonrounds is necessary to achieve a balance between strength and visual impact. For example, in furniture, substituting oval tubing for round or square can yield the best combination of strength and appearance.
The fabricating processes and variables associated with bending tubular sections generally make nonround components more expensive than rounds. Therefore, it is critical to plan properly to develop a cost-effective process.
The most common nonround shapes, and the focus of this article, are square, rectangular, and oval. The two types of oval are elliptical (a true oval) and flat-sided. Two bend designations for these shapes are E plane, or easy way, and H plane, or hard way (see Figure 1). These terms are universal. It should be noted that the bend radius of a nonround tube generally is stated as an inside radius (ISR), while on a round tube the bend radius usually is stated as a centerline radius (CLR). Be sure to check the prints indicating the bend radii on nonround tube assemblies closely.
These cross sections can be bent using the same methods as those used for round tubing.
The part's design, the desired finished appearance, and target production rates determine the optimal bending method, each of which has advantages and limitations. The most common methods are press bending, compression bending, roll bending, and rotary draw bending (see Figure 2).
Press Bending. This is a good choice for applications in which production volume is more important than appearance. This method generally is not used with internal support to the tube.
Compression Bending. Like press bending, this method favors speed over looks. However, compared with press bending, it is slower and, when fitted with proper tooling, can produce better-looking parts. This method does not often utilize an internal support for the tube, except in the case of dedicated, custom-built, high-speed equipment that can produce two bends on the tube simultaneously.
Roll Bending. This process is good for large-radius forming of all noted cross sections. It can even form spirals and parabolas.
Rotary Draw Bending. This is the most versatile bending platform. It provides many possibilities for die design and implementation for all noted shapes.
This bender type can be used with or without an internal supporting mandrel; with or without a wiper die to prevent wrinkling on the inside wall on a tight bend; and with more than one die set, which is especially handy for bending parts with two or more bend radii or a short, straight length of tube between bends.
Some rotary machines incorporate two methods of forming. For example, some can perform both push bending and rotary bending with a single tooling setup. This is useful when a single part has two bends with vastly different radii. This circumvents the need for a second operation (and a second machine).
Round tubing tends to support itself during the bending process. A nonround tube does not support itself; it is more likely than a round to flatten or buckle in the bend area (see Figure 3). The walls that are parallel to the bend plane (in Figure 3 these are the top and bottom walls) provide more resistance to the bending force than do the side walls. To prevent buckling or flattening, the tube needs internal and external support.
Tube bender tooling designs are as varied as tubing applications themselves. Like the bender selection, tooling design is driven by several considerations, including part criteria (cosmetics and flattening), bent part configuration, and production requirements.
Crush Style. In this tooling style, the bend die has a projection in the tube groove. The projection drives the tube's inner wall halfway through its cross section (see Figure 4). This prevents the tube from buckling in the bend by forcing the tube to support itself. It doesn't use a mandrel.
Collapse Style. This is similar to crush style, but it drives the inner tube wall all the way to the outer wall. This reduces the tendency for the outer wall to go concave in the bend area. The tube can get stuck in the tooling after the bend is complete.
Split Actuated Style. The bend die is machined in two (or more) plates to be split vertically. This provides downward pressure during the bending process, preventing the top and bottom tube walls from becoming convex. Because it is split, it releases pressure after the tube is formed, easing removal from the bend die.The pressure can be provided by something as simple as a large nut on top of the bend die that is tightened prior to the bend and loosened afterward. Or it can be accomplished by means of hydraulic cylinders. These can be incorporated on top of the bend die and plumbed to the machine's hydraulic circuit. Some bending machines can be supplied with a hydraulic split actuator built in "under nose" so that all the tooling can be the split actuated style.
Single Leaf Style. This design achieves the same goal as the actuated method, but it doesn't use a cylinder to provide the downward pressure. The bend die is machined from several plates and bolted together. The tube groove in the bend die that holds the tube captive is machined so that the opening is larger than the tube is. The clamp die and pressure die have a hardened plate, or leaf, bolted to them. When closed, the leaf fills the gap between the tube groove in the die and the tube. Filling this void provides downward pressure during the bend cycle and releases the tube for removal when opened.
This method has advantages—the design is simple, it requires no extra hydraulics, and it's fast—but it has limitations as well. The leaf develops minimal downward force, and as the tooling wears, the force diminishes. If the tubing's outside dimensions are smaller than specified, the leaf cannot exert any pressure at all, and the operator likely will have difficulty clamping the tube tight enough to avoid slippage during the bend cycle.
In some cases, the gap is above the tube; gravity holds the tube in position even when the dies are open, and prevents the tube from being caught in the clamp dies when they close. In other cases, the gap is beneath the tube. In this situation, an indexing carriage (collet) is necessary to support the tube and maintain this gap. Otherwise gravity will cause the tube to sit too low in the bend die, leaving little or no gap beneath it; the result is that the tube tends to get caught in the clamp dies when they close.
Wedge Leaf Style. The function is the same as the single leaf style, except the wedge leaf style's bend die has an angle at either the top or bottom side wall of the tube groove (see Figure 5). The clamp and pressure dies have a corresponding angle. When the dies are closed, the angles force the clamp and pressure dies to squeeze the tube. Unclamping releases the pressure, making it easy to remove the tube from the tooling.
The tooling designer has to clear some substantial hurdles before this tooling type can work properly. All benders have an envelope that the tooling has to work within. This is defined and limited by the capacity of the bender. Critical parameters include tube size, minimum and maximum centerline radius, centerline height that the tooling runs, and clamping style (direct or drop-away). A machine that has stacked-die capability multiplies these variables exponentially. Getting the width (reach) of the clamp and pressure die as well as the wedge angles correct can be a daunting task. A machine that has several die stacks yet does not provide any fine-tuning capability for independent clamp and pressure die positions leaves no margin for error in the design and use of this type of tooling.
The details concerning bend tooling design—the types or styles of dies, mandrels, and wipers; physical sizes and clearances; and materials, finishes, and coatings—would fill volumes. Suffice it to say that each application is unique, and the tool design must match it (see Figure 6 and Figure 7).
Successful bending boils down to proper material containment in concert with adequate drag reduction, and the ability to implement them correctly. The variables include:
While the variables are too numerous to count, it's important to stay focused so they don't overwhelm the project. The goal is to produce the parts at a rate that achieves the target profit margin. With that in mind, one good strategy for dealing with the many variables is a simple one: Eliminate the variables that can be eliminated, then manipulate the rest.
The best way to reduce the number and severity of the variables is to design them out of the product as early as possible in the process, preferably when the project is still in the development stage. Establishing this sort of partnership with the customer early in the process would be ideal. If this is possible, it would be just as well to try to expand this partnership to include trusted sources for the machinery, material, and tooling, thereby establishing some level of control over the entire project.