In the ideal world, the combination of job modelling technology and worker skill results in the perfect component every time. In reality, mistakes happen and in more complex cases, distortion cannot be avoided during the sheet metal fabrication process. To remedy these issues, a number of corrective techniques have developed to restore the component with a minimum amount of damage. In this blog, we look at the four most common sheet metal fabrication issues wrinkles, splits, springbacks and general distortion, and run through the best practice solutions for correcting these issues.
Problem: Wrinkles – minor or severe
Cause: Wrinkles are a result of compressive strains in the sheet metal stamping process that pushes the metal together. This effect is usually caused by using the wrong manufacturing process or an incorrect parameter.
Solution: Stretching or drawing the material back out with the use of pads and binders is the best way to fix wrinkled sheet metal. For more complex shapes, draw beads may also be required. Removing wrinkles usually involves extra flat material to clamp and stretch the wrinkles out, producing waste.
Cause: Splits are caused when the materials is extended past its ultimate tensile strength
Solution: Splits can usually be avoided through software modelling, which should be able to use the forming limit diagram to predict all splitting defects.
Fixing a split also usually requires stamping simulation software which can consider variables such as material type, thickness, number of forming stages, blank shape and size, form depth and form radius.
Cause: Springbacks occur when the material is strained in processing and then relaxes due to it’s elastic characteristics. This is a common problem with high strength metals as the difference between yield and tensile strength is much narrower in these types of materials.
Solution: Compensating or ‘over bending’ to accommodate the metal’s elasticity is the most common way to manage a springback but it is not necessarily effective. Positive stretching may be used to increase part strength and therefore make the material stiffer and stronger to reduce springback.
Cause: When the material is unable to handle the stretching and compressing of the fabrication process, deformation occurs. This can be a result of bending across the grain, working too fast and reducing work hardening, variations in hardness and thickness, the position of holes and the size of the piece of sheet metal.
Solution: Avoiding deformation is usually a matter of good project planning and material knowledge. Different types of distortion need to be managed in different ways:
- Mechanical straightening
Hammering and pressing are the two most common techniques for deformation correction. However, as hammering can produce both surface damage and work hardening, pressing is the preferred management method.
- Bowing or angular distortion can usually be straightened on a press through over correction
- Bowing in a flanged plate will require incremental pressings from a press brake to correct the distortion. In these cases, the load needs to act on the flange to prevent web damage at the load points. A former is the best tool to use in order to achieve a straight component or smooth curvature as incremental point loading will only produce an approximately straight component
- Thermal correction
Thermal correction is achieved by heating a local area of the metal to such high stresses that when it cools, it pulls the rest of the component back into shape. There are three key thermal correction techniques which are spot, line and wedge-shaped heating.
- Spot heating
Ideal for removing buckling in thin sheet metal when it is welded to a stiff frame. The convex side is heated in spots which are arranged depending on the shape of the buckling.
- Line heating
Used for angular distortion, the component is headed along the welded join (on the opposite side), pulling the flange flat.
- Wedge shaped heating
Used to correct distortion in more complex components, wedge heating is used in conjunction with line heating to achieve the desired shape. This technique can be used on thicker metals; however, two torches may be needed to heat the component from opposite sites in order to achieve uniform heating.
Acra are sheet metal machinery specialists offering a wide range of quality new and used machinery for sale as well as expert servicing solutions. If you have an ongoing sheet metal problem that you are struggling to diagnose, our experts are on hand to help you manage the issue so you can finish your project on time and within budget. To learn more about our capabilities, please don’t hesitate to get in touch with us by calling 03 9794 6675.