Page 395 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 15 8 Drawing Practice 7

reduction sequence per pass require considerable experience to ensure proper mate-
rial flow in the die, reduce internal or external defects, and improve surface quality.
The wall thickness, diameter, or shape of tubes that have been produced by
extrusion or by other processes described in this book can be reduced further by
tube-drawing processes (Fig. 15.20). Tubes as large as 0.3 m in diameter can be
drawn by these techniques. Mandrels of various profiles are available for tube-
drawing operations.
Wedge-shaped dies are used for the drawing of flat strips and are used only in
specific applications. However, the principle behind this process is the fundamental
deformation mechanism in ironing, used extensively in making aluminum beverage
cans, as shown in Fig. 16.30.

I5.8 Drawing Practice

As in all metalworking processes, successful drawing requires careful selection of
process parameters. In drawing, reductions in the cross-sectional area per pass range
up to about 45 %. Usually, the smaller the initial cross section, the smaller the reduction
per pass. Fine wires usually are drawn at 15 to 25% reduction per pass and larger sizes
at 20 to 45%. Reductions of higher than 45% may result in lubricant breakdown,
leading to surface-finish deterioration. Although most drawing is done at room tem-
perature, drawing large solid or hollow sections can be done at elevated temperatures
in order to reduce forces.
A light reduction (sizing pass) also may be taken on rods to improve their sur-
face finish and dimensional accuracy. However, because they basically deform only
the surface layers, light reductions usually produce highly nonuniform deformation
of the material and its microstructure. Consequently, the properties of the material
will vary with location within the cross section.
Note in Fig. 15.19 that a rod or wire has to have its tip reduced in cross section
in order to be fed through the die opening and be pulled. This typically is done by
swaging the tip of the rod or wire in a manner similar to that shown in Figs. 14.15a
and b; this operation is called pointing. Drawing speeds depend on the material and
on the reduction in cross-sectional area. They may range from 1 to 2.5 m/s for heavy
sections to as much as 50 m/s for very fine wire, such as that used for electromag-
nets. Because the product does not have sufficient time to dissipate the heat generat-
ed, temperatures can rise substantially at high drawing speeds and can have
detrimental effects on product quality.
Drawn copper and brass wires are designated by their temper (such as 1/4 hard,
1/2 hard, etc.) because of work hardening. Intermediate annealing between passes
may be necessary to maintain sufficient ductility of the material during cold drawing.
High-carbon steel wires for springs and for musical instruments are made by heat
treating (patenting) the drawn wire; the microstructure obtained is fine pearlite (see
Fig. 4.11). These wires have ultimate tensile strengths as high as 5 GPa and a tensile
reduction of area of about 20%.

Bundle Drawing. Although very fine wire can be produced by drawing, the cost
can be high. One method employed to increase productivity is to draw many wires
(a hundred or more) simultaneously as a bundle. The wires are separated from one
another by a suitable metallic material with similar properties, but lower chemical
resistance (so that it subsequently can be leached out from the drawn-wire surfaces).
Bundle drawing produces wires that are somewhat polygonal, rather than
round, in cross section. In addition to producing continuous lengths, techniques have