Putting a Curb on End DistortionComments Off on Putting a Curb on End Distortion
Roll forming is a time-honored manufacturing process, performed by bending a flat metal strip continuously and progressively by tandem sets of roll tooling. Many customers use roll forming in place of press braking, stamping, and extruding because it allows for longer lengths, higher tolerances, sweeping, hole punching, and the use of high strength materials. It is safe to say that customers who want high quantity parts will save money, production, time, and will have more options when roll forming is used.
There is a problem that occurs occasionally with customers moving from other fabrication techniques. Having not dealt with this problem, they don’t understand one of the primary complications of roll forming. The problem is end distortion. End distortion, better known as end flare, is the deformation at the ends of a roll formed part. Roll formed parts with end flare may have one end bent slightly inward and the other end bent a little more than that, but outward (Type A in image), or a more likely result when both ends are bent outward (Type B), with one end slightly less than the other. Each type is caused by the release of residual forming stresses after the part is cut.
In order to reduce end flare, it is important to consider the causes and what steps can be taken to prevent and correct them as much as can be possible.
The forces exerted on the metal strip during roll forming are not the only sources that can cause distortion. The distances between the forming regions is also instrumental in this process. A greater distance between the ends is likely to have less end distortion than a shorter distance. Pre-notched materials can be another source of distortion. The back edges of the notches can hit the rolls during the rolling process, causing them to toe in, which results in more distortion than a normal end, unless the distortion is corrected within the die set up and design. The cutoff process is the main potential source for flaring, with the die potentially causing distortion at both the leading and the trailing ends of a roll formed profile. The cutoff die will produce more distortion from its leaving side, as more clearance is necessary to allow the part to move out of the die and the entry end to move through without getting snagged. Another consideration of distortion from the cutoff die is when the orders are in very high footage quantities. On high quantities, if tolerances are allowed to be greater, adding more clearance so parts can run through the die faster, providing lower prices and helping to prevent any potential downtime.
Crop or slugless cutoff dies generally produce more end distortion, but they allow for less downtime than slug type cutoff dies. The slug type cutoff dies typically require more downtime because they wear more quickly than the crop or slugless dies. With additional wear, the slug type cutoff dies require more sharpening to prevent burs. Slug cutoff dies do allow for less flare or end distortion and can even be designed to provide the least flare possible or to spank the end of the parts on the sides to remove most of the flare. The more complicated the die, the more maintenance and downtime that will increase the part cost.
Fortunately, other steps can be taken to troubleshoot this problem. A downhill roll die design helps minimize end distortion, with less longitudinal elongation and forming than conventional uphill designs. Another method during the design stage is using a longer length profile region during manufacturing, or by spreading the forming out over more stations. When using more stations, there is less angle increment per length, less forming movement, and less end distortion than on less end stations used to make the same part. Mounting a pair of side rolls before a forming pass is another common method for preventing end distortion, as these reduce the surface stresses in the forming zone, and are particularly effective when dealing with notched rolls, but can be much less exact and can form with more variance or be less accurate between stations. Sleds or solid fixtures from one roll station to the next will also help keep away distortion, especially when pre-punching, but these can wear quicker than roll dies do, creating more downtime.
When distortion has already occurred, a number of post-treatment options are available, including counter-forming, straightening, and a combination of the two. With proper design and post treatment options, the problem of end flaring can be dealt with safely and effectively. One example of post-treatment options is a secondary conveyor-fed operation may be used to remove the end flare by spanking the sides of the part at the ends. This secondary operation typically allows faster production with a less complicated die design, and can be added if the current flare is not acceptable. With proper design and post-treatment options, the problem of end flaring can be dealt with safely and effectively.
Roll forming has many advantages over the other fabrication techniques, but if end distortion is overlooked, especially during the design phase, it will be very costly. It is imperative that any potential roll forming customers know about end flare and work with a roll form manufacturer who understands how to avoid it.