Roll forming basics
Roll forming, often called open-section forming, uses successive sets of roller dies to bend a strip of steel progressively until the desired shape is achieved. This process is very similar to traditional tube- and pipemaking, but differs in that it can form more complicated sections.
Both roll forming and tube and pipe forming involve bending steel with roller dies, with each pair of rolls working the strip progressively until the desired shape is achieved. How roll forming differs is that it lends itself well to prepunching, midpiercing, and postpunching all inline, as well as sweeping before cutoff.
Application examples include impact bars, chassis sections, and window guides in automobiles, which may be prepunched, roll formed, midpierced, roll formed more, then welded, swept, and cut off in a continuous operation. Some of these sections are butt welded, like a tube or pipe, and others can be seam or lap welded.
Although the processes are similar and some of the equipment may be the same, there are differences in the design of a roll formed section and the tooling needed and in the layout and setup of the machine.
Producing a Tube the Traditional Way
Let's look at producing a 2-in.-diameter tube on a 2-in.-shaft tube mill. The mill might have three breakdown passes with idlers between one and two, two and three, and two pairs of idlers between the breakdown passes and three fin passes. Between each fin pass is an idler pass and perhaps a seam guide before the welding station. Welding 0.030 in. oversize on the 2-in. diameter tube increases the diameter to 2.030 in. For this reason, welding requires reducing the diameter.
After the welding station is usually a scarfing stand, with or without an accompanying ironing pass, followed by an ovaling guide or similar device to guide the tube into the first sizing pass with three driven passes and idlers between each. After the sizing is a single or double Turk's head stand for straightening.
In this example, the total number of driven passes before the welding station is six, with three sizing after the welding. Each of the driven sizing passes and idler passes works the tube down in diameter until the desired size is attained. During sizing the tube is elongated, its wall thickens, and often the location of the weld cannot be guaranteed.
Roll Forming a Tube
Now let's roll form the same 2-in.-diameter tube with prepunching. The process requires 10 passes before the welding station, because a typical roll former doesn't have idler stands. Tube and pipe mills use idlers between almost every pass to assist in forming the round. These idlers help form the strip, feed the steel into the next pass, and stabilize the section as it goes through the mill. Most roll forming does not require idlers but typically uses many more passes to make a tube. Also, a tube mill may be required to run several hundred feet a minute, but a roll former, especially one with prepunching, can never attain those speeds. With a few exceptions, most roll formers, especially those prepunching steel, run much slower than tube mills.
Additionally, strip widths are calculated differently in roll forming. On a tube and pipe mill, a fin allowance and a welding allowance are calculated and added to the strip width. If you're prepunching, the pressure in the fin passes distorts the holes.
Roll forming the tube is described best as a progressive edge form, high oval without idlers. The strip is formed from the outside using an overformed radius on the first pass of about 15 degrees. The second pass forms the next 15 degrees, allowing the first pass work to relax, and then continuing using this type of forming method, allowing the high oval to stay in the sides. Roll forming supplies pressure in the vertical plane only, and the tooling must be designed with this in mind.
Like tube and pipe mills, roll forming works with all metals, both ferrous and nonferrous. Stainless steels, martensitic steels, high-strength low-alloy steels, and aluminum coils can be roll formed. Steel and aluminum coils sometimes are embossed with wood grain finishes, such as for garage doors. When embossing, the depth of the embossment must be considered so that the roller dies do not roll the embossment back out.
Prepainted and even vinyl-laminated steels can be roll formed but not welded. Tubing from prefinished materials is usually formed with a lock seam. A lock seam requires knurling the edge of the strip, either in the flat or elsewhere in the forming process, then making the opposing hooks and forming the tube, which is run over a mandrel to lock the hooks and then cut off.
Roll forming is a versatile process that allows design innovation. Setting up a roll forming line requires the expertise of a skilled roll form designer and cooperation between the vendor and manufacturer in developing a successful product. But with the right preparation, the transition can be painless. Carefully choosing the proper equipment, skillfully designing the product and tooling, properly training the operators, and adequately maintaining the production line make roll forming easier.