Understanding tube forming's not-so-distant cousin
March 27, 2003
You can successfully roll-form a profile more than one way. In fact, many roll form designers take different approaches to design and development. Also, because the roll forming process has a seeming limitless capacity to produce complex profiles with just as many variables, many roll form designs are one-of-a-kind; therefore, theory can go only so far.
However, you must understand and apply common practices to the design process, and couple them with a certain degree of "magic" gained from experience.
For each new tooling requirement in a roll form design, take the following five steps:
Develop a cross-sectional drawing.
Calculate the estimated strip width.
Produce a bend progression, or "flower" pattern.
Lay out and design the roll tooling around the flower.
Incorporate tooling accessories.
|Figure 1On the cross-sectional drawing, you'll need to determine the part, material, and mill specifications and tolerances.|
The process begins with a cross-sectional drawing that serves as a kind of blueprint of what your profile's shape and dimensions should be. On the drawing, you should note each bend's radius and arc length, straight lengths, and material thicknesses. You'll also need to document any prenotching, precut lengths, and multiple gauges and combination sets (see Figure 1).
After you've finalized the cross-sectional drawing, you'll need to determine the part, material, and mill specifications and tolerances, including steel type and grade. At this point, it's best to plan for secondary processes, which can limit roll design options.
Probably the most common problem associated with designing roll tooling is predicting how a forming bend will react during the rolling process. When calculating a strip width, using the proper K factor, or bend allowance, should be your main concern. Various factors contribute to the bend allowance, such as material type and yield strength, profile characteristics such as large radii or 180-degree bends, and roll design techniques.
Generally, you should use the maximum thickness (+) within the gauge range. This will eliminate interference between the male and female rolls when the material is passing through. However, running the minimum-thickness material within the gauge range leads to a lower-quality cross section with poor dimensional and bend characteristics.
First, consider the material to be formed. For steels with yield strengths of 30 to 55 kilopounds per square inch (KSI), forming angles of about 90 degrees, and a 1t to 5t inside bend radius, a K factor between 35 and 40 percent of the material thickness is most common.
Higher-yield materials between 60 and 85 KSI (with a low elongation percentage) may require a K factor between 40 and 55 percent. For bend angles of more than 120 degrees and inside bend radii less than 1t, the K factor should be about 50 percent. These numbers are only rules of thumb for typical roll formed bends.
Note that the strip widths always should be considered estimates, and it is important not to order large quantities of raw material before proving the tooling. In some instances, you'll need to modify the width during testing.
|Figure 2Air, blind, or box forming is defined as forming a bend that does not have a roll holding the inside corner.|
Before you can design the rolls, you must determine the proper number of passes and the rolling mill. After calculating the estimated strip width, you can develop the flower by using the arc and straight lengths.
On a typical C channel, the return leg (bend No. 2) is the first forming to be done (see Figure 2). Some designers prefer to form this leg to about 70 to 80 degrees in three passes, depending on the length of the leg.
This angle is important because if you form it all the way to its finished angle of 90 degrees, your access to the inside corner of bend No. 1 will be limited in the later passes. By keeping bend No. 2 open 10 to 20 degrees, you'll ensure that one additional pass will have contact with bend No. 1. The final 10 to 20 degrees of forming would take place after the last pass that contacts bend No. 1.
|Figure 3The third step of designing roll forming tooling is to produce a bend progression, or "flower," that describes where the forming takes place. The angles show how much forming is being done at each pass.|
Forming a bend that does not have a roll holding the inside corner is called air, blind, or box forming. When this occurs, forming the remaining angle from bend No. 2 helps to reduce distortion in the radius of bend No. 1.
If the bend corner of No. 2 has a prepunched slot or hole, it is better to finish the bend while No. 1 is flat (0-degree forming), because any box forming in the later passes will increase distortion to and around the notch. However, you should still hold the return leg after contact with the inside corner is no longer possible to prevent bend No. 1 from lifting while minimizing bend No. 2 distortion.
Other forming methods also can be used for bends, such as constant radius, arc in, and arc out. The constant-length method is the most widely used. One approach is to calculate all the passes using this method to develop the progression and then sharpen the radii in the early passes slightly to help the section track properly.
Overforming a bend beyond the finished angle is necessary to account for material springback. For the section in Figure 3, which is made of 0.060-in. cold-rolled steel with a yield strength of 35 KSI, a 2- or 3-degree overform is required to overcome the springback effect. Materials that have high elongation rates, such as aluminum, may need only 1/2 or 1 degree, while high-strength and high-yield materials may require 5 to 15 degrees of overform.
Try to avoid overforming more than one bend at the same pass, because you may need to control one more than the other using roll pressure. In Figure 3, bend No. 2 is overformed at pass No. 9 and then allowed to spring back to 90 degrees at pass No. 10. Bend No. 1 is overformed in pass No. 10.
Take care to limit the material's lateral and vertical movement from pass to pass, especially lateral movement. Note how the Figure 3 flower progression forms more at first and less toward the end. The material flows more laterally once the forming angle approaches 90 degrees. This increases load against the forming rolls and creates many problems, such as marking or scuffing, end flare, twist with asymmetrical profiles, and camber or bow.
When developing the flower, you need to consider the correct number of passes. The 10-pass example should be fine if you're running non-notched, postcut mild steel. If the section is notched, precut, or high-strength material, you'll need more passes to achieve desired results. Additional passes allow the material to flow through the mill with less strain.
After completing the flower, you need to choose drive diameters and check for maximum flange roll sizes and possible interference with the rolling mill.
During the design phase, you'll need to consider increasing the pitch diameters of the rolls from pass to pass (also called step-up). In many situations, overfeeding or buckling between passes, particularly in the early stages, will occur. This problem is caused when the material (especially lighter gauges) from a previous pass is driving faster than the next.
The amounts of forming torque and surface contact on the rolls are the key factors as to why this problem exists. However, after the profile has ample column strength, you can reduce or eliminate step-up. With prenotched applications, take care not to increase drive diameters too much, because this may lead to increased part lengths and distorted notches.
Guidelines for step-up are as follows:
0.015- to 0.030-in.-ga. material, prenotched--Use about 0.010-in.-diameter step-up for all passes.
0.015- to 0.030-in.-ga. material, not prenotched--Use 0.050-in.-diameter step-up for passes No. 1 through No. 3, 0.030-in.-diameter step-up for passes No. 4 and up.
0.060-in-ga. C channel, not prenotched--Use 0.030-in-diameter step-up for all passes.
Other ways to counteract overfeeding are idling key spindles on the mill or idling key forming rolls, clearance angles on rolls that appear to be driving too hard, or reducing roll diameters in a previous pass.
|Figure 5Today most companies use some sort of computer-aided design software to calculate pass layouts.|
With the drive diameters chosen, the vertical centers are 7.060 in. The maximum roll diameter is based on the required flange needed for smooth material transition from the previous pass. It is important to check that this maximum diameter does not interfere with the mill base or any other maximum diameter from an adjacent pass. The location of the profile on the roll space, roll widths, and spacer lengths also is determined in this layout.
Today most companies use some sort of computer-aided design software. A visual description of pass layouts is shown in Figure 5.
You might want to incorporate fixtures, guides, side rolls, and straightening devices where applicable. Most sections require a straightening device after forming to remove unwanted twist, camber, and bow. These straighteners usually are a block type made from aluminum, bronze, or nylon, but rollers also can be used. The key to straightening is to have the device fairly close to the last pass or between the last two passes if you're using precut strips.
Side rolls, or rolls that are mounted on a vertical axis and located between passes, are excellent for helping a profile enter the main roll passes. In addition to smoothing out the forming transition between passes, side rolls also reduce scuffing of the vertical legs because they spin on the same axis that the profile travels.
Chuck Summerhill is engineering manager with Roll-Kraft Inc., 8901 Tyler Blvd., Mentor, OH 44060, 888-953-9400, fax 440-205-3110, email@example.com, www.roll-kraft.com. SolidWorks® drawings by Chaz Rau, Roll-Kraft Inc. Roll-Kraft Inc. provides products and services for tube, pipe, and roll forming industries.