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March 15, 2017 | by Dean Linders, Red Bud Industries

How to Produce Flat Material—That Stays Flat

Using stretcher leveling to produce flat, low-stress material

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Over the years, much has been written about the principles of leveling and how to produce flat material. In addition to simply getting the material flat, there is a greater emphasis placed on developing technology to assure the material stays flat after subsequent processes, such as laser or plasma cutting, welding, and/or other various forming and fabricating operations. For these systems to work properly, they require material that is flat and will remain flat throughout the entire manufacturing process. 

 

If you are a manufacturer or you sell to one, you have likely experienced the issue of “spring back,” that is, once a previously flat piece of material is cut, it can warp or distort, sometimes to the degree that it will cause damage to the manufacturing equipment. Inevitably, the question is asked, “How do I get the material flat and how do I make sure it stays flat?” These are actually two separate issues, and consequently, each requires its own specific solution. In addition, with new grades of high strength and ultra-high strength steels being used throughout the industry, processors have found that in many instances these materials cannot be leveled using traditional methods.

 

Shape Correction − How do I get the material flat?

If the strip has edge wave or center buckle, some sections of the strip across its width are longer than others. This is referred to as a “side-to-side length differential”. Since we cannot make the longer portion of the strip (the wavy parts) shorter, the only way to get the material to lay flat is to elongate the short portions lengthwise to “dimensionally equalize” the material. The goal is to make the strip dimensionally the same length across its entire width, which in turn will result in a visually flat strip, sheet or part. So how do you do that? Over the years, there have been several methods and machines developed to level strip. And while there is a general understanding and acceptance regarding the common principles of leveling, there is still considerable debate as to how or if one type of leveler or leveling process is really better than another. To answer this question, we must first take a hard look at the basics of leveling. 


The Basics of Leveling and “Yield Strains” − Understanding the leveling process

Basically all levelers/leveling processes, whether they use rollers, compressive force or stretch the material, elongate the strip. When it comes to leveling, it is all about elongation. Regardless of how it is done, all levelers are designed to make portions of the strip longer than others. The main difference between one system and another comes down to the amount of elongation each can produce, how they do it and how much of the material, through its cross section, is taken past its yield point. Once you take the material past the yield point, the material goes into a “plastic” state where as everything before that is forgotten. For example, if you ever bent a piece of metal or wire and it stays bent, you have exceeded its yield strength. If it returns to its original shape, you have not. If your car has been caught in a hail storm and the hail stones leave dimples, the material in these dimpled areas has been elongated past its yield point and has taken a new shape.

One Yield Strain (YS) is the amount of elongation required to take the material to its yield point without exceeding it. The term yield strain is frequently used to describe the amount of elongation required to eliminate a shape defect. One yield strain is also the amount the material will “spring back” after an applied force is removed. As an example, if you were using a break press to put a 90-degree bend into a piece of material, you actually need to bend it to an angle somewhat less than 90 degrees due to spring back. It is important to note the amount of elongation required to exceed a material’s yield point is a function of the modulus of elasticity (MOE), that is, how springy the material is. As an example, aluminum with a given yield strength would require three times the amount of elongation as its equivalent steel counterpart.

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