Roll formed sections have an advantage over extrusions of a similar shapes. Roll formed parts are generally much lighter and stronger, having been work hardened in a cold state. Another advantage is that the part can be made having a finish or already painted. Labor is greatly reduced since volume is a major consideration for choosing the roll forming process.
Roll forming lines can be set up with multiple configurations to punch and cut off parts in a continuous operation. For cutting a part to length, the lines can be set up to use a pre-cut die where a single blank runs through the roll mill, or a post-cut die where the profile is cutoff after the roll forming process. Features may be added in a hole, notch, embossment, or shear form by punching in a roll forming line.
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.
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.
The process of roll forming is one of the simpler manufacturing processes. It begins with a large spool of metal strips, usually between 1 in. and 20in. in width, and 0.004 in. and 0.125 in. thick. This is held by a device called a dispenser. The metal strip is then unrolled and fed into a machine starting with the stock feeder which is connected to the cutoff attachment. After the cutoff attachment, the metal strip is fed into the forming rolls. These mating die-set rolls are constructed to form the desired shape in stages sequentially by means of various shaped rolls. The layout of these rolls can be flower shaped as mentioned previously, progressive upper/lower rolls, side rolls, or as overhung spindle rolls.
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.
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.
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 over formed 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.
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