Page 46 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

24 General Introduction
level of complexity, the parts may be precision cast, fabricated from individual
pieces, or produced by powder-metallurgy techniques.
° Dimensional tolerances and surface finish in hot-working operations are not as
fine as those obtained in operations performed at room temperature (called cola'
working), because of the dimensional changes, distortion, warping, and surface
oxidation that occur at the elevated temperatures involved.

The size of manufactured products, and the machinery and
equipment involved in processing them, vary widely, ranging from
microscopic gears and machanisms of micrometer size, as illustrated
in Fig. 1.8, to (a) the main landing gear for the Boeing 777 aircraft,
which is 4.3 m high and includes three axles and six wheels; (b) the
runner for the turbine for a hydroelectric power plant, which is 4.6 m
in diameter and weighs 50,000 kg; and (c) a large steam turbine rotor
weighing 300,000 kg.
Process Substitution. It is common practice in industry that, for
a variety of reasons and after a review of all appropriate and appli-
cable processes, a particular production method (that may have
been employed in the past) may well have to be substituted with
another. Consider, for example, the following products that can be
produced by any of the sets of processes indicated: (a) cast vs.
forged crankshaft, (b) stamped sheet-metal vs. forged or cast auto-
mobile wheels, (c) cast vs. stamped sheet-metal frying pan, (d) in-
jection molded vs. extruded or cast polymer bracket, and (e)
welded vs. riveted sheet-metal safety hood for a machine.
Many varieties of such products are widely available in the
FIGURE l.7 Cutting sheet metal with a laser marketplace. However, a customer’s preference will depend on his
beam. Source: Courtesy of Rofin-Sinar, Inc., or her particular needs, which include factors such as the product’s
and Society of Manufacturing Engineers. appeal to the customer, its cost, whether maintenance is required,

diameter \'fl!'f0 Comb Teeth
Torsion
W//

Fixed Comb Teeth

Ano h

(8) (D)

FIGURE l.8 (a) Microscopic gears with dust mite. Source: Courtesy Sandia National
Laboratory; (b) A movable micromirror component of a light sensor. Note the 100-/,tm scale
at the bottom of the figure. Source: Courtesy of R. Mueller, University of California at
Berkeley.