How do you eliminate coil defects in sheets, plates, and parts?
June 28, 2013
Not all metal forming jobs are done off the coil, so what can stampers do to ensure that the metal is level? They can use a stand-alone hydraulic roller leveler to ensure parts meet original specs.
Metal stampers are aware that material is leveled at coil processing facilities (see Figure 1)—typically using technology such as inline roller levelers, stretch levelers, and temper mills—to remove residual stress and flatness defects in the material before it is rolled into coils. However, they also know that sometimes they work with sheets and plate, which originally came off a coil, but still demonstrate coil-related defects that make further processing difficult (see Figure 2). Material with edge wave, when the material edges are rippled because they are longer than the center, and center buckle, when the ripples are in the middle of the material because the center is longer than the edges, can pose a significant threat to metal forming equipment.
For example, metal that still retains internal strains and coil defects can damage a die when the press comes down, as well as affect the part quality. Uneven strips actually can cause the whole press to become deadlocked, resulting in tremendous repair costs for both the machinery and tooling.
Additionally, material that hasn’t been properly leveled threatens other equipment that might be found in a metal forming shop. Thermal cutting processes can unleash internal stresses that remain hidden to the naked eye and result in deformations that could cause a collision with the cutting torch. Such a collision can result in the need to replace a cutting head, which can be expensive on equipment like a laser cutting machine, and to take the cutting equipment out of service as the repair is being made.
Sometimes the parts cut from the material that was once coiled leave the thermal cutting equipment in what appears to be good shape, but they still require postleveling because the laser, plasma, or oxyacetylene torch has created distortion in the material. Parts such as these simply don’t meet customers’ quality criteria, or they aren’t suitable for further processing. For instance, a blank that has obvious edge deformation can’t be processed further in a robotic welding cell because the automated equipment requires precise edges and consistent gaps. Manual welders may be able to overcome such material deficiencies with extra time and filler metal, but robotic welding equipment can’t cope with such variables.
That’s why leveling beyond coil processing may be needed for truly flat parts.
Before the advent of modern leveling technologies for coil processing lines and for stand-alone leveling machines, metal formers had to rely on the skills of their employees. This was never more evident than when a hammer and a heat source were used to take the shape defect out of the metal for better flatness. Unfortunately, such an approach was very time-consuming and truly provided no stress relief in the material. Additionally, the excessive banging on the material and exposure to a heat source could damage and discolor the material surface.
A straightening press might have been used, but such a device, again, didn’t provide any stress relief in the material. Even a skilled operator needed a significant amount of time to do the leveling job correctly using this equipment. Also, the press often left an imprint of the die on the material.
Some companies to this day rely on a press brake to achieve flatness, but this approach is not efficient for high-throughput jobs and takes a skilled operator to make it work.
Roller levelers have emerged as a suitable tool to eliminate residual stress that may be found in sheet, strip, plate, and even parts. These leveling units have both large- and smaller-diameter rollers that are located close to each other, so when material is fed into them, they are able to stretch the tension and flatness errors out of the metal (see Figure 3).
More specifically, when the material is fed into the leveling unit, the unleveled portion is subjected to a series of up and down bends over the smaller rollers, which are backed up by the much larger rollers that sandwich the leveling rollers. As a result, the sheet metal takes a wave-like path through the leveling machine. This up and down movement helps to stretch the material’s unlevel sections to its yield point and helps to create a uniform fiber length throughout the blank.
The bending force is greatest at the machine’s entry. As the material proceeds toward the machine’s exit, the depth of the up and down bends is gradually reduced. This is comparable to a decaying sinusoidal curve. When the material fibers are nearly the same length, the residual stress is gone, and the metal is level.
As a rule, the more leveling rollers, the greater the number of alternating bends, and the better the leveling result. At least 13 rollers are needed on a leveling machine to achieve acceptable tolerances, but 17 to 21 leveling rollers are recommended (see Figure 4).
Servo-hydraulic technology helps to provide the support necessary to accommodate this dramatic alternating bending. For example, to level a 1.3-in.-thick steel plate, a technician would enter an entry value that is smaller than 1.3 in. and an exit value that roughly equals the sheet thickness. That dramatic up and down bending is enhanced by the servo-hydraulics that provide the necessary stretching. (Stampers or fabricators using roller leveling technology for the first time can rely on a user guide and a values database to determine the specific setting values that make sense for the proc-ess material.) Gap control prevents damage to the leveler: Nothing gets stuck in case of an incorrect setting or if the operator feeds the wrong part. It helps the shop floor meet the tightest tolerance requirements.
Roller leveling technology has proven to be a suitable fit for high-production environments. For example, one operator can send as many as seven metal parts through a servo-hydraulic roller leveler in a minute—the time it might take an operator to set up one part on a press brake to try and level it.
Those shops that don’t want just one operator having to feed material into the leveler and then retrieve it as it exits the roller leveler can use a reverse mode. Here, the operator sets the part length and size, feeds the material into the leveler, and waits for it to return to the same spot when the leveling is done. Upon entry, the workpiece runs through the machine until it reaches a stop, which signals the upper leveling unit to lift up while the leveled part is returned to the operator at the entry.
The machine’s design depends on the materials to be leveled. For example, it is believed that if the metal has a fracture strain of at least 5 percent and a pronounced yield strength, it can be leveled. However, material that still maintains a fracture strain after hardening can be leveled only within certain limits. The machine for this type of application would have to deliver strong forces to prevent deflection and very small roller diameters to work the metal intensely.
Once the material is identified, the metal former can decide on the diameter of the rollers, the size of the entry pitch, and the number of leveling rollers needed. In general, the smaller the pitch and the leveling roller diameter, the better the result.
For high-volume manufacturing environments, the leveling machines should be equipped with a quick-change system for the leveling rollers. This allows the operator to change out the rollers easily and provides ample room for a thorough cleaning of the unit. Dirt or any other residue that collects in the rolling chamber not only impairs the leveling results, but also can damage the leveling machine.