Introduction to Sheet Metalworking

Sheet Metalworking Defined

• Sheet metalworking involves cutting and forming relatively thin sheets of metal.
• Sheet metal thickness ranges from 0.4 mm (1/64 in) to 6 mm (1/4 in).
• Plate stock thickness is greater than 6 mm.
• Operations are typically performed as cold working.

Sheet and Plate Metal Products

• Sheet and plate metal parts are used in various consumer and industrial products.
• Examples include automobiles, airplanes, railway cars, farm and construction equipment, appliances, and office furniture (including computers and office equipment).

Advantages of Sheet Metal Parts

• Sheet metal parts offer high strength.
• They have good dimensional accuracy and finish.
• They are relatively low cost.
• For mass production, economical operations are available.

Sheet Metalworking Terminology

• Punch-and-die: Tooling used for cutting, bending, and drawing sheet metal.
• Stamping press: Machine tool performing most sheet metal operations.
• Stampings: Sheet metal products.

Three Major Categories of Sheet Metal Processes

• Cutting: Shearing large sheets, cutting perimeters, or creating holes.
• Bending: Straining sheet metal around a straight axis to create a bend.
• Drawing: Forming sheet metal into convex or concave shapes.

Cutting (Shearing, Blanking, Punching)

• Cutting involves shearing between two sharp cutting edges.

• Shearing: Used to cut large sheets into smaller sections, typically for further processing.

• Blanking: Cutting a piece from the surrounding stock, leaving the cut piece (blank) as the desired part.

• Punching: Cutting a piece from the surrounding stock, leaving the remaining stock (slug) as the desired part.

• Clearance between the punch and die is crucial and should be between 4% and 8% of stock thickness. A too small clearance can lead to double burnishing and higher forces, while too large a clearance leads to metal pinching and excessive burrs.

• Clearance can be calculated as c = at, where c = clearance, a = allowance, and t = stock thickness.

• Allowance depends on the metal type, and values are available for different metal groups.

Bending

• Bending involves straining sheet metal around a straight axis to create a permanent bend.
• Metal on the inside of the neutral plane is compressed, while metal on the outside is stretched.
• Bend allowance formula is given as BA = 2πA/360 (R + Kba t), where BA= bend allowance; A= bend angle; R= bend radius; t= stock thickness; and Kba= factor to estimate stretching.
• Springback is the increase in the included angle of a bent part after the bending tool is removed. Springback is caused by the elastic energy that remains in the bent piece when the pressure is removed.
• Bending forces are estimated as F = KbfTSwt^2/D, where F = bending force; TS= tensile strength of sheet metal; w = part width; t= stock thickness; and Kbf depends on the type of bending.

Drawing

• Drawing involves forming sheet metal into cup, box, or complex shapes.
• The sheet metal blank is positioned over a die cavity and a punch pushes the metal into the opening.
• Products include beverage cans, ammunition shells, and automobile body panels.
• Clearance in drawing is about 10% greater than stock thickness (c = 1.1t).
• Drawing ratio (DR) is defined as Db/Dp and indicates the severity of the operation (upper limit = 2.0).
• Reduction (r) is defined as (Db - Dp)/Db and should be less than 0.50.
• Thickness-to-diameter ratio (t/Db) should be greater than 1% to avoid wrinkling.
• Blank size determination involves calculating the starting blank diameter (Db) to achieve the desired dimensions of the drawn part. This often involves equalizing the starting sheet metal blank volume with the final product volume.

Other Sheet Metal Forming Operations on Presses

• Various operations are performed on conventional presses using metal or flexible rubber tooling.
• Examples include ironing (uniforming the wall thickness), embossing (creating indentations or raised features), and the Guerin process (for low-volume tooling).

Other Sheet Metal Operations Not Performed on Presses

• Other processes such as stretch forming, roll bending and forming, spinning, and high-energy-rate forming are not typically performed on presses.

Power and Drive Systems

• Hydraulic presses use a large piston/cylinder to drive the ram and are suited for deep drawing, but are slower than mechanical presses.
• Mechanical presses convert motor rotation to linear ram motion. They are suited for blanking and punching operations.

Dies for Sheet Metal Processes

• Dies are custom-designed for a particular part.
• Stamping dies are used for high-production processes.
• Pressworking operations generally use conventional punch-and-die tooling.

Types of Stamping Press Frames

• Gap frames (often called C-frames) have a gap in the frame and are suited for lower tonnage presses.
• Straight-sided presses are typically box-style and suitable for higher-tonnage presses

Additional Notes

• Numerous figures illustrating various processes and components are included for visual understanding.
• Specific figures and formula are provided for each operation.