92
Various Forming Processes
7.3 EXPANDING
Expanding is a process that is used to enlarge the diameter of the drawn shell or tube in one or more sec- tions by a different kind of punch, such as a flexible plug rubber or polyurethane, by hydraulic pressure,
or by a segmented mechanical die. Fig. 7.9 shows the characteristic shapes of shells formed by expanding methods.
Sizing is a term used to describe the flattening or improvement of a selected surface on previ-
ously drawn parts to a closer limit of accuracy than is possible by conventional drawing methods. Sizing consists
of squeezing the metal in a desired direction. Bulging basically involves placing a tubular, conical, or drawn workpiece in a split female die and
expanding it, such as by means of a flexible plug rubber or polyurethane. The punch
is then retracted and the part is removed by opening the die. The major advantage of using polyurethane plugs is that they are
resistant to abrasion, wear, and lubricants. Hydraulic pressure can also be used in this operation but will require sealing and hydraulic controls.
--
Fig.
Characteristic shapes
of shells
formed
by
expanding.
Segmented tools are usually used for forming cone rings, sizing cylindrical rings, and expanding tubular and drawn shells. These tools do not have a female die; the workpiece pulls on the punch, which
has the shape of the final part. Expansion is thus carried out by expanding the punch mechanically. Fig. 7.10 shows the action of force in segmented punches.
The expanding force for this type of tool is given by the formula:
1. lk
+
-
where: a
=
half angle of cone,
=
coefficient of friction between cone and segmented punch,
=
coefficient of friction between segments and lower supporting plate of tool, D,d
=
outer and inner diameters of workpiece,
=
specific deformation impedance,
b
=
width of workpiece. 7.1
Various Forming
Processes
93
Fig. 7.10
Schematic illustration of segmented punch.
A
method of enlarging portions of a drawn shell in a press is shown in Fig. 7.1 1. The value of the
expanding ratio
=
Dd is dependent on the kind and heat-treatment condition of the material and the rel- ative thickness of the material
Some values of are given in Table 7.2. The thickness of the material at the deformation zone
of the workpiece is reduced by the expansion operation and is given by the formula:
=
e
where:
T
=
thickness of the material before expanding T
=
thickness of the material in the deformation zone D
=
main diameter of workpiece before and after expanding.
Punch
\
F Die
7.12
Fig.
Expanding a drawn shell with a punch.
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