Page 452 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

432 Chapter 16 Sheet-Metal Forming Processes and Equipment
lead times to make the dies are rather low. On the other hand,
8
these costs for large-scale operations (such as stretch forming of
2 7 1 T aircraft panels and boat hulls) are very high. Furthermore, because
T

E 6 1 0-‘gm the number of such parts needed is rather low, the cost per piece
can be very high (see Fig. 14.18).
3 4 \ |.¢/0.3 m/rl Similar considerations are involved in other sheet-forming
c 3 \ diameter
g \ Spinning operations. Deep drawing requires expensive dies and tooling, but
Drawing
5 2 X """"""""""' a very high number of parts, such as containers, cans, and similar
8 1 products, are produced with the same setup. These costs for other
0 processes, such as punching, blanking, bending, and spinning, vary
0 1 2 3 4 5 considerably, depending on part size and thickness.
Number of parts (>< 103) Equipment costs vary widely and depend largely on the com-
plexity of the forming operation, part loading and unloading fea-
FIGURE l6.6l Cost comparison for manufac- tures, part size and shape, and level of automation and computer
turing a round sheet-metal container either by control required. Automation, in turn, directly affects the amount of
conventional spinning or by deep drawing. labor required and the skill level. The higher the extent of automa-
Note that for small quantities, spinning is more tion, the lower the skill level required. Furthermore, many sheet-
economical.
metal parts generally require some finishing operations-one of the
most common being deburring of the edges of the part, which gen-
erall is labor intensive althou h some advances have been made in automated debur-
g
.
,
Y
ring (which itself requires computer-controlled equipment; hence, it can be costly).
As an example of the versatility of sheet-forming operations and the costs in-
volved, note that a cup-shaped part can be formed by deep drawing, spinning, rub-
ber forming, or explosive forming. Moreover, it also can be formed by impact
extrusion, casting, or fabrication from different pieces. The part shown in Fig. 16.61
can be made either by deep drawing or by conventional spinning, but the die costs
for the two processes are significantly different.
Deep-drawing dies have many components, and they cost much more than the
relatively simple mandrels and tools employed in spinning. Consequently, the die
cost per part in drawing will be high if only a few parts are needed. However, this
part can be formed by deep drawing in a much shorter time than by spinning, even
if the latter operation is automated and computer controlled. Furthermore, spinning
generally requires more skilled labor. Considering these factors, the break-even
point can be seen as around 700 parts, and deep drawing is more economical for
quantities greater than that. Chapter 40 describes further details of the economics of
manufacturing.

SUMMARY

° Sheet-metal-forming processes are among the most versatile of all operations.
They generally are used on Workpieces having high ratios of surface area to thick-
ness. Unlike bulk deformation processes such as forging and extrusion, sheet
forming often prevents the thickness of the material from being reduced (to avoid
necking and tearing).
° Important material parameters are the quality of the sheared edge of the sheet
metal prior to forming, the capability of the sheet to stretch uniformly, the mate-
rial’s resistance to thinning, its normal and planar anisotropy, its grain size, and
its yield-point elongation (for low-carbon steels).
° The forces and energy required in sheet-metal-forming processes are transmitted
to the workpiece through solid tools and dies, by flexible rubber or polyurethane
members, or by electrical, chemical, magnetic, and gaseous means.