Chapter 20: Sheet-Metal Forming

The text categorizes the vast array of formed parts—from singly curved sections to deep-recessed shapes—and delineates the machinery employed, including single-action, double-action, and triple-action presses that utilize punches and dies to manipulate material. Fundamental shearing operations such as blanking, punching, and fine blanking are analyzed, with a specific focus on how tool clearance influences fracture mechanisms, edge quality, and burr formation. The discussion progresses to bending mechanics, examining the shift of the neutral axis, the distribution of tension and compression across the sheet thickness, and the critical issue of springback, where elastic recovery necessitates compensation strategies like overbending or stretch forming to achieve dimensional accuracy. The text further explores stretch forming and the limits of formability caused by plastic instability, specifically distinguishing between diffuse necking and local necking, and introducing the Forming Limit Diagram (FLD) as a predictive tool for failure based on major and minor strain distributions. A significant portion of the chapter is dedicated to deep drawing, detailing the complex stress states—biaxial tension under the punch and circumferential compression in the flange—that govern the process, while establishing the Limiting Draw Ratio (LDR) as a key measure of drawability. The influence of material properties is emphasized, particularly the plastic strain ratio (R-value) relating to normal anisotropy, and the crystallographic texture which determines resistance to thinning. Finally, the summary addresses common defects in formed parts, such as flange wrinkling, orange peeling caused by coarse grains, stretcher strains associated with the yield-point phenomenon, and earing resulting from planar anisotropy in the rolled sheet.