Page 412 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

392 Chapter 16 Sheet-Metal Forming Processes and Equipment

l6.2.4 Miscellaneous Methods of Cutting Sheet Metal
There are several other methods of cutting sheets and, particularly, plates:
° Laser-beam cutting is an important process (Section 26.7) typically used with
computer-controlled equipment to cut a variety of shapes consistently, in vari-
ous thicknesses, and without the use of any dies. Laser-beam cutting also can
be combined with punching and shearing. These processes cover different and
complementary ranges. Parts with certain features can be produced best by
one process; some with other features can be produced best by the other
process. Combination machines incorporating both capabilities have been
designed and built. (See also Example 27.1.)
° Water-jet cutting is effective on many metallic as well as nonmetallic materials
(Section 27.8).
° Cutting with a band saw; this method is a chip-removal process.
° Friction sawing involves a disk or blade that rubs against the sheet or plate at
high surface speeds (Section 24.5).
° Flame cutting is another common method, particularly for thick plates; it is
used widely in shipbuilding and on heavy structural component (Section 30.8).

l6.3 Sheet-metal Characteristics and Formability

After a blank is cut from a larger sheet or coil, it is formed into various shapes by
several processes described in the rest of this chapter. We will now briefly review
those characteristics of sheet metals that have important effects on these forming
operations, as outlined in Table 16.2.
TABLE |6.2

Important Metal Characteristics for Sheet-forming Operations
Characteristic Importance
Elongation Determines the capability of the sheet metal to stretch without necking and failure; high
strain-hardening exponent (n) and strain-rate sensitivity exponent (m) are desirable
Yield-point elongation Typically observed with mild-steel sheets (also called Luder’s bands or stretcher strains);
results in depressions on the sheet surface; can be eliminated by temper rolling, but sheet must
be formed within a certain time after rolling
Anisotropy (planar) Exhibits different behavior in different planar directions, present in cold-rolled sheets because
of preferred orientation or mechanical fibering, causes earing in deep drawing, can be reduced
or eliminated by annealing but at lowered strength
Anisotropy (normal) Determines thinning behavior of sheet metals during stretching, important in deep drawing
Grain size Determines surface roughness on stretched sheet metal; the coarser the grain, the rougher is
the appearance (like an orange peel); also affects material strength and ductility
Residual stresses Typically caused by nonuniform deformation during forming, results in part distortion when
sectioned, can lead to stress-corrosion cracking, reduced or eliminated by stress relieving
Springback Due to elastic recovery of the plastically deformed sheet after unloading, causes distortion of
part and loss of dimensional accuracy, can be controlled by techniques such as overbending
and bottoming of the punch
Wrinkling Caused by compressive stresses in the plane of the sheet; can be objectionable; depending on
its extent, can be useful in imparting stiffness to parts by increasing their section modulus; can
be controlled by proper tool and die design
Quality of sheared edges Depends on process used; edges can be rough, not square, and contain cracks, residual
stresses, and a work-hardened layer, which are all detrimental to the formability of the sheet;
edge quality can be improved by fine blanking, reducing the clearance, shaving, and
improvements in tool and die design and lubrication
Surface condition of sheet Depends on sheet-rolling practice; important in sheet forming, as it can cause tearing and poor
surface quality _