Shapes and Distortions - Metal Forming ABCs

The fabrication and metal manufacturing world can be difficult to understand if you’re not a veteran on shop floor. Not only that, but when you want to look something up, clear explanations can be hard to come by. In this post, we'll give you a handy ABC guide to metal forming terminology dealing with shapes and distortions.

Shapes & Distortions

• Bending - Developing a profile out of metal strip by changing the shape using successive pairs of forming rolls in a continuous operation. The thickness of the metal is generally maintained except at the bend radius where some thinning occurs.
  
  
• Back Bend (or Overbend) - An element of roll forming design where a feature is bent more than the desired dimension in its finished state. This can be used to control the dimension of a feature, such as bend angle, flatness, or twist. Back bend and overbend can be used early in the roll forming progression, to help set blind corners. It is commonly used near the end of the forming process when approaching the finish pass.
  
  
• Bow - Bow is the variation from a straight line in the vertical plane of a coiled strip or roll formed shape. It can be either a cross bow or a longitudinal bow. Cross bow is a common result of the strip rolling and slitting process. Longitudinal bow occurs in roll formed shapes and welded tube.
• Camber - Camber is the variation of a metal strip edge from a straight line. This gradual deviation from straightness of the edge of metal sheet or coil stock is typically caused by the fan out of recoiling after the slitting operation. Camber is the edgewise curvature, a lateral departure of a side edge of sheet or strip metal from a straight line. Extreme camber in the metal strip can contribute to curve, bow, and/or twist in the roll formed profile.
  
  
• Coil breaks or Crossbreak - Coil breaks are a defective condition composed of ridges or marks across the width of sheet or coil caused by improper coiling or insufficient leveling. They appear as:
  • Creases or ridges which appear as parallel lines, transverse to the direction of rolling, and which generally extend across the width of the sheet
  • A discontinuous curvature in the strip in the direction in which the material was rolled or uncoiled
  • Coil breaks are generally found in uncoiled, hot rolled strip. They can occur in light gauge material on a roll former, stamping press, slitter, cut-to-length line, or tube mill if the mandrel of the uncoiler is over-expanded, due to stretching of the ID at the 3 or 4 contact points of the mandrel.
    
    
• Coil Set - A lengthwise curve or set found in coiled strip metals following the coiling process. A departure from longitudinal flatness, it can be removed by roller or stretcher leveling from metals in the softer temper ranges. Coil set occurs in material when it is coiled below its critical diameter, which can be calculated by the thickness and mechanical strength of the material.
  
  
• Cold work - Plastic deformation at such temperatures and rates that substantial increases occur in the strength and hardness of the metal. Visible structural changes include changes in grain shape.
  
  
• Cross section - This term describes the geometric shape around the outer perimeter of the product being roll formed. The cross section is commonly referred to as the profile.
  
  
• Cross section depth - The distance between the peak and valley of a profile with respect to the orientation in which the profile or cross section is formed.
  
  
• Curve (or Sweep) - Curve is the formed variation from a straight line in the horizontal plane measured after the part has been roll formed. Causes of curve include incorrect horizontal roll alignment and uneven forming pressure. Parts may be intentionally curved for their application, such as roof bows or bumper beams.
• Curving - Curving is the process of adding curvature to a flat sheet by passing it through a set of rolls or blocks using multiple passes to add curvature and/or radial bending in a continuous process without kinking or creasing the sheet.
  
  
• Embossing - A process used for both decorative and functional purposes and is typically performed prior to the roll forming operation. The decorative appearance is often a stucco or grained look.
  
  
• End Flare - End flare is the deformation at the ends of a roll formed part. End flare can be eliminated or at least reduced through using proper roll forming tool design. End flare is normally apparent after cut off and is caused by the residual forming stresses in the material being roll formed, where one longitudinal end springs open and the other springs closed.  In high strength materials, both ends may spring open.
  
  
• Flower - A drawing or layout showing the cross-section progression of the strip at each forming station. Generally used during the design stage to determine the number of passes required to form the product, the drawing shows all the cross-sections layered together to review and revise the forming flow, prior to design of the tooling.
  
  
• Form - A bend in a material, or the actual process of bending a metalformed part.
• Hem - This refers to a formed feature when material is roll formed back onto itself. Typically a hem formed flush unto itself is known as a flat hem or a zero-T-hem.  If a hem has a gap into it, it may be referred to as a teardrop hem or an open hem.
  
  
• Inside radius - Bend radius formed by the male roll.
  
  
• Lock Seam - Refers to the mechanical locking of the outer edges of the material being roll formed together.
  
  
• Marking - Marking is a process that stamps information or features into the strip as it is roll formed.  Marking could include:
• Knurling
• Company logo
• Name
• Production information, such as material type or date

• Metal thinning - The thickness reduction that occurs during any metal forming operation. Metal thinning generally occurs at the bend where permanent deformation occurs.
  
  
• Net shape - Net shape refers to the final dimensions of a product that results after the roll forming or tube forming operation is complete. Typically, a roll formed shape is progressively formed from the flat strip into the final, "net shape."  For tubing and hollow structural sections, the Net Shape is commonly formed from a welded round tube then reshaped or sized to the Net Shape.
  
  
• Oil canning - Colloquial term for cross bow and shows up in flat areas between bends.
  
  
• Outside radius - The formed outside radius of a bend. In roll forming, the outside radius is typically the inside radius, formed by the male roll tooling, plus the material thickness, less some thinning that occurs from the forming process. Precisely controlling outside radius is more of a feature of extruding or tube reshaping.

• Plastic deformation - The permanent deformation that occurs when metals are formed beyond their elastic limits.  Plastic deformation occurs when a material has been stretched or compressed beyond its yield strength.
  
  
• Roll formed shape, closed - A roll formed profile that is closed by welding or mechanically (e.g. lockseam).
  
  
• Roll formed shape, open - Any roll formed profile in which the two opposing edges of the strip are not welded or mechanically fastened together.
  
  
• Section shape - The final profile or desired roll formed part. Depending on the complexity of the shape, a requisite number of roll forming passes will be used to form the profile.
  
  
• Section width - The width of a roll-formed section.
  
  
• Springback - A mechanical phenomenon when a formed piece of metal relaxes, and does not maintain the set from the force applied to it through the metal forming process. Springback can occur because the metal formed feature has not completely entered plastic deformation, so some extent of the material does not reach its yield.
  
  
• Straightness - Term used to define the flatness of the metal formed part after being finished by the roll forming machine, tube mill, or stamping operation.
  
  
• Twist - Term used to define parts when they resemble a corkscrew shape. Twist may occur in asymmetrical profiles. It can also occur by excessive or uneven forming. Twist should be less than 0.120" in 10" (3mm in 1000mm).