Page 73 - Shigley's Mechanical Engineering Design

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48 Mechanical Engineering Design
Figure 2–11

Common shapes available
through hot rolling. Round Square Half oval Flat Hexagon
(a) Bar shapes

Wide flange Channel Angle Tee Zee
(b) Structural shapes

Hot rolling is usually used to create a bar of material of a particular shape and
dimension. Figure 2–11 shows some of the various shapes that are commonly produced
by the hot-rolling process. All of them are available in many different sizes as well as
in different materials. The materials most available in the hot-rolled bar sizes are steel,
aluminum, magnesium, and copper alloys.
Tubing can be manufactured by hot-rolling strip or plate. The edges of the strip are
rolled together, creating seams that are either butt-welded or lap-welded. Seamless tub-
ing is manufactured by roll-piercing a solid heated rod with a piercing mandrel.
Extrusion is the process by which great pressure is applied to a heated metal billet
or blank, causing it to flow through a restricted orifice. This process is more common
with materials of low melting point, such as aluminum, copper, magnesium, lead, tin,
and zinc. Stainless steel extrusions are available on a more limited basis.
Forging is the hot working of metal by hammers, presses, or forging machines. In
common with other hot-working processes, forging produces a refined grain structure
that results in increased strength and ductility. Compared with castings, forgings have
greater strength for the same weight. In addition, drop forgings can be made smoother
and more accurate than sand castings, so that less machining is necessary. However, the
initial cost of the forging dies is usually greater than the cost of patterns for castings,
although the greater unit strength rather than the cost is usually the deciding factor
between these two processes.
2–13 Cold-Working Processes

By cold working is meant the forming of the metal while at a low temperature (usually
room temperature). In contrast to parts produced by hot working, cold-worked parts
have a bright new finish, are more accurate, and require less machining.
Cold-finished bars and shafts are produced by rolling, drawing, turning, grinding,
and polishing. Of these methods, by far the largest percentage of products are made by
the cold-rolling and cold-drawing processes. Cold rolling is now used mostly for the
production of wide flats and sheets. Practically all cold-finished bars are made by cold
drawing but even so are sometimes mistakenly called “cold-rolled bars.” In the drawing
process, the hot-rolled bars are first cleaned of scale and then drawn by pulling them
through a die that reduces the size about 1 to 1 in. This process does not remove
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material from the bar but reduces, or “draws” down, the size. Many different shapes of
hot-rolled bars may be used for cold drawing.
Cold rolling and cold drawing have the same effect upon the mechanical proper-
ties. The cold-working process does not change the grain size but merely distorts it.
Cold working results in a large increase in yield strength, an increase in ultimate