Page 402 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

3 Chapter 16 Sheet-Metal Forming Processes and Equipment w>»= =;;,
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FIGURE 6.l Examples of sheet-metal parts, (a) Stamped parts. (b) Parts produced by spinning.
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Source: (a) Courtesy of Williamsburg Metal Spinning 86 Stamping Corp.

set of dies. A sheet-metal part produced in presses is called a stamping (after the word
stamp, first used around 1200 A.D. and meaning “to force downward” or “to
pound”). Note that this is a term similar to forging or casting, commonly used to de-
scribe parts made by those individual processes using dies or molds, respectively.
Low-carbon steel is the most commonly used sheet metal because of its low
cost and generally good strength and formability characteristics. More recently de-
veloped alloys, such as TRIP and TWIP steels (see Section 5.5.7), have become pop-
ular for automotive applications because of their high strength; they are well suited
for providing good crash protection in a lightweight design. Aluminum is the most
common material for such sheet-metal applications as beverage cans, packaging,
kitchen utensils, and applications where corrosion resistance is a concern. The com-
mon metallic materials for aircraft and aerospace applications are aluminum and
titanium, although they are being replaced increasingly with composite materials, as
described in Chapters 9 and 19.
Most manufacturing processes involving sheet metal are performed at room
temperature. Hot stamping is occasionally performed in order to increase formabil-
ity and decrease forming loads on machinery. Typical materials in hot~stamping
operations are titanium alloys and various high-strength steels.
This chapter first describes the methods by which blanks are cut from large
rolled sheets then processed further into desired shapes by a wide variety of meth-
ods. The chapter also includes discussions on the characteristic features of sheet
metals, the techniques employed to determine their formability, and the construction
of forming-limit diagrams. All of the major processes of sheet forming and the
equipment used to make sheet-metal products (as outlined in Table 16.1) are also
described.

l6.2 Shearing

Before a sheet-metal part is made, a blank of suitable dimensions first is removed
from a large sheet (usually from a coil) by shearing. This sheet is cut by subjecting it
to shear stresses, generally using a punch and a die (Fig. 16.2a). The typical features
of the sheared edges of the sheet and of the slug are shown in Fig. 16.2b and c,
respectively. Note that the edges are not smooth nor are they perpendicular to the
plane of the sheet.
Shearing generally starts with the formation of cracks on both the top and bot-
tom edges ofthe workpiece (at points A and B, and C and D, in Fig. 16.2a). These