Sheet Metal Flatness Tolerance
My steel bowed – what happened, and how can I prevent it?
There you are at the back of the line. The line just shut down because the steel has a bow, preventing the manufacturing of a good part. Where did things go wrong? You know it is not the line, as the last three coils ran without an issue. The steel must be bad. Right?
The above is a frustrating and common situation. This article will look at the common causes of shape issues, such as coil set, edge wave, center buckle, crossbow, camber and explain how Worthington can help you mitigate them. While no one likes the extra cost of having a flatness-critical specification, we will examine when this specification is vital for manufacturing success and will save you a lot of money.
Coil Set/Reverse Coil Set
What is Coil Set?
When a coil of steel arrives from your flat roll provider, it will always have coil set. You can think of coil set as “the memory” of the steel.” When you uncoil it, the steel remembers that it was in coil, and it will hold onto that shape until you reset its memory with your in-house leveler or straightener.
How often do you adjust your leveler/ straightener to remove coil set?
Whenever the grade or gauge of steel changes, the leveling/straightening equipment must also be adjusted. The more the thickness and strength vary from one changeover to the next, the greater the adjustment may be to remove the coil set. If the leveler or straightener does not have enough power or is not set up properly, the coil set will remain. At Worthington, our metallurgists are available to help our customers with recommended leveler adjustments – we know that this equipment can be a bit of a black box, and we want to ensure it is doing its job effectively.
Edge Wave, Center Buckle, Cross Bow, and Camber
What is Edge Wave?
Edge wave is an edge deformation that occurs when the edges of a coil are longer than the center of the coil. This type of shape deformation is usually addressed with tension leveling, which will hold the length of the edges, with stretching the length of the center to match.
What is Center Buckle?
Center buckle is a deformation in the center of the sheet that occurs when the center of coil is longer than the edge of the coil. This shape deformation can also be fixed with tension leveling, but in this case the length of the center is held, while the edges are stretched to match.
What is Cross Bow?
Cross bow occurs when the coil edges bow up or down, creating a frown or smiley face. Cross bow can be caused by several varied factors in processing and requires a metallurgist to come on site to troubleshoot.
What is Camber?
Camber is a deformation that occurs when one side of the sheet is elongated more than the other. In wide material the longer edge may show up as a wave because the coil is too wide to curve sideways. In narrow material, camber will visibly curve off to one side, or snake back and forth. To address camber a metallurgist will come on site to determine the root cause, and optimal solution.
When and why is a flatness-critical specification needed?
The Natural Variation in Steel
While extreme examples of shape issues are rare, it is important to realize that there is an expected and allowable amount of shape variation in steel. To make flat roll steel, you turn raw earth, specifically iron ore into metal by heating it, removing its impurities, and pouring it into a large slab. This slab is further reduced into a hot roll sheet product. The rudimentary nature of this foundational process produces the natural variation.
When is the Natural Variation in Steel Problematic?
For many applications, like deep-drawn or heavily stamped parts, the natural amount of variation in steel shape within a coil or between coil to coil will not present an issue. However, it is a deal breaker for a shape or flatness-critical application like laser welding. Many manufacturers who need a flatness-critical specification, are surprised by the amount of natural variation in steel when a flatness-critical tolerance hasn’t been specified.
How do I know if my steel application is Flatness-critical?
The amount of allowable variation in shape changes with the steel’s gauge and tensile strength. The thicker the gauges and the higher the tensile strength, the more prone the steel is to shape variations. Generally, any tolerance of 12mm (1/2 inch) or less should be considered flatness-critical and require extra processing.
If you are uncertain about the flatness requirements for your specific application, consulting with a metallurgist can be immensely beneficial. A metallurgist is an expert in the properties and behavior of metals, and they can provide valuable insights into the flatness standards and tolerances that are relevant to your application.
Do I Really Need to Specify That My Steel is Flatness-Critical?
Flatness-critical steel applications demand the steel to exhibit extremely low levels of surface imperfections and variations in thickness, as even minor deviations can adversely affect the successful manufacturing and performance of the final product.
Without additional processing, such as tension leveling, temper passing, or stretcher leveling, to reduce shape variations inherent in steel, the material will not achieve the amount of uniformity required by a flatness-critical steel application. For this reason, it is essential to communicate your shape requirements to your flat-roll steel processor, if your application requires it. It is important to note that the additional processing required will increase the overall cost of the material.
Why is a flatness-critical specification worth the extra cost?
While the additional processing required for flatness critical applications may come at an increased cost, it is worth the investment. By ensuring the flatness of the steel, you minimize the risk of downstream issues such as buckling, distortion, or fitment problems. This can save you time, money, and potential rework in the long run, making the extra processing cost well worth it.
What does Worthington do to eliminate shape issues?
At the time of order, Worthington metallurgists conduct a full discovery to understand the customer’s manufacturing processes and how their parts are used in real life. After we have a comprehensive needs assessment, we can ensure proper ordering and processing of the steel. If a shape problem still arises in the manufacturing process, Worthington metallurgists will visit the customer’s plant to troubleshoot the possible root cause(s) of the issue. This may result in customer equipment adjustments or changes to how the steel is ordered.