Roll forming custom welded tubes with complex features, all in one operation
August 3, 2012
Roll forming isn’t just for simple gutters or panels, but also for custom welded tubes. Components begin as metal coil and are formed in the rolling mill; prenotching adds the desired slots and holes; then, they are finally seam-welded. All this can be accomplished in one production process.
Managers at Meggitt Defense Systems needed a way to reduce the cost of its indoor and outdoor gun ranges and simulators. Every time a user pushes the button to retrieve a target, the motor pulls the cable, an action that induces significant stress. After repeated use, the tension of the cables needs to be adjusted. Eventually, the cables fray and break.
That’s why the Irvine, Calif., company wanted to develop a one-piece retrieval system with a design that eliminated the cables altogether. The new design entails a motor-driven caster assembly that rides along a rail. When a user pushes the button, the motor immediately drives the casters, with no cables required.
For the new rail, the firm evaluated many options, most of which included multiple pieces that had to be assembled, as well as hardware. All this added complexity and cost. What this application needed was a process that could combine many fabrication processes into a “done-in-one” operation. In the end Meggitt managers went with a technology that has done this for years. They chose roll forming.
Stories like this have become more common as more manufacturers take a new look at the century-old metal forming process. Not only can roll tooling form seriously complex profiles, but the process also can eliminate time-consuming and costly secondary processes. Roll forming isn’t just for simple gutters or panels, its also for custom welded tubes. Components begin as metal coil and are formed in the rolling mill; prenotching adds the desired slots and holes; then, they are finally seam-welded. All this can be accomplished in one production process: roll forming.
In roll forming, a continuous strip of metal, typically coiled, passes through a consecutive set of rolls, or forming stands. Each stand gradually forms the strip into the desired cross-sectional shape. The nature of roll forming allows secondary operations—be it welding, countersinking, or another process—to be integrated into one production line. Pre-, mid-, or postnotching (that is, notching before, in the middle of, or after forming and welding) can form holes and slots easily (see Figure 1). The process excels in applications that require high-volume production runs of metal parts with tight tolerances and consistent cross sections, whether simple or complex. It typically handles material thicknesses from 0.010 to 0.375 inch.
The only significant limitation is a material’s formability. Material in custom tubes is not limited to cold- or hot-rolled carbon steels. In fact, many are made from high-strength, low-alloy (HSLA) steels, aluminum, stainless steel, and other weldable material.
Roll formed custom shapes aren’t limited to square, rectangular, or round tubing; they can also have multiple sides and radii. For instance, a portion of the material can be bent 180 degrees, creating hems that strengthen certain areas of a cross section.
In fact, manufacturing managers at Meggitt Defense Systems used this concept to its full advantage for the company’s “wireless” shooting range rail—that is, a welded tube design that effectively eliminates the need for a cable to move the shooting target back and forth.
The concept features a hollow square tube constructed of durable galvanized steel to withstand the rigors of outdoor and indoor use. The cross section has a T shape at one corner, a form that requires several 180-degree bends. This design creates a track guide for the motor-driven casters (see Figure 2).
Between the roll stands is an inline countersinking process that eliminates the need for secondary operations, or the need for Meggitt technicians to drill holes when installing the rails at customer sites. Each 8-foot section has four self-tapping, countersunk holes. These allow installers to connect these sections using specially designed inserts, for quick on-site assembly. The modular system can be built to suit any size shooting range.
Once installed, the interlocked tubes create a one-piece track that allows the target to glide back and forth. This is the industry’s first truly wireless, 360-degree-turning (meaning it can rotate the target all the way around) target retrieval system—and without roll forming, the design wouldn’t have been cost-effective.
West Hartford, Conn.-based A.H. Harris, a construction supply company, needed a formwork system that was light and easy to assemble. Used throughout the construction industry, formwork is a series of custom rails, assembled in Erector® set or Lego® block fashion, built to form the structural framework for concrete walls. The stronger and easier to assemble these formwork systems are, the better.
Producing the tubular sections of this formwork is a roll forming system with inline high-frequency welding. Contact HF welding uses squeeze rolls to heat the workpieces to produce a forge weld. The process uses a high-frequency power source. This forge weld can force impurities onto the surface in the form of slag, which can be removed (or scarfed) with an inline cutter.
Inline HF welding is used for the roll formed, rectangular tubing that comprises perimeter sections of A.H. Harris’ formwork panel sections. Separate hat sections, each with very complex geometry requirements, span the formwork cross sections (see Figures 3 and 4).
A robotic welding system connects those interior hat sections to the exterior rectangular tubing. This requires postnotching in the roll forming process to create the tab at the end of each hat section. The tabs, which have especially critical dimensions, slide into slots in the rectangular tubing (also cut on the roll former) to create a self-fixturing, repeatable joint setup for robotic welding.
Still, regardless of how precise the roll forming or robotic welding is, it won’t matter if the formwork cross sections are too heavy for construction workers to assemble quickly. That’s why the design uses galvanized, high-strength steel, which allows the use of lighter-gauge metal while improving overall strength. Reducing the weight also reduces the cost of the roll formed components.
A.H. Harris uses its patented clamp (see Figure 3) that allows construction workers to connect various formwork sections quickly on the job site. Making this clamp concept work, though, requires various holes, slots, and other cutout geometries and forms—and every one of these is integrated inline on the roll former.
When you marry innovative design with modern roll forming, efficiencies abound, especially when integrating secondary operations. Thinking outside the box, designers and roll forming engineers can work together to create a truly competitive—and profitable—product.
Gas tungsten arc welding (GTAW) has been integrated on roll forming lines for years. Still, faster welding can mean a faster line speed, which is why higher-speed processes like high-frequency resistance welding have been so popular. Another extremely fast process growing in popularity is laser beam welding, which these days can often replace GTAW.
Producing a small heat-affected zone (HAZ), the laser can weld very close to features in a cross section with less distortion than other welding processes. The laser allows the designer to create cross sections with various attributes and fewer restrictions. The small HAZ means that features such as holes or slots can be placed near the weld seam with minimal distortion.
The laser also lets the product designer get creative. The ability to use multiple laser beams and change beam locations enables the designer to weld multiple strips and, thanks to the laser weld’s small HAZ, put various welds in different linear locations on a cross section. This can help add strength and reduce part weight.