Various
Forming
Processes
87
Fig. 7.3
Stretch draw forming with mating dies: a work material is held in tension; b punch moves to form workpiece.
Fig. 7.4
Typical shapes of part formed with mating dies.
7.2 NOSING
Nosing is a die reduction method whereby the top of a cup or a tubular shape may be made closer or small- er in diameter than its body.
There are three types of end profiles after nosing: Frustum of cone
Neck Segment of sphere.
It is possible to reduce the top of the cup if the material is not too thin, to about 20 of its diameter in
one operation. Nosing compresses the work metal, resulting in an increase in length and wall thickness. Fig. 7.5 shows schematic illustrations of three types of die reduction methods. The calculation of the height
of a drawn shell or tubing by nosing its end is different for each type.
88
Various
Forming Processes
Type Type
Type
Fig. 7.5
Schematic illustration of different types of nosing.
a Type I reductions
nosing operation may be calculated by the following formula: Fig.
7.6 shows a nosing cup in the shape of a frustum of a cone. The height of a shell before the
1
where:
D
=
mean diameter of a drawn cup before nosing
d
=
main diameter of a workpiece after nosing a
=
angle of cone. Force
-
The nosing force for Type I reductions may be calculated by the formula below:
Various
Forming Processes
89
where:
=
k 0 for
E
=
-
specific deformation impedanc e non- nosing part of shell
d
k
=
E
for
E
=
1
-
- -
specific deformation impedance nosing part of shell
D T
=
material thickness.
b Type reductions be calculated by the formula:
For a necking type of nosing, as shown in Fig. 7.7, the height of the workpiece before necking may
Force
-
The necking force for Type reductions is given by the following formula:
=
d
7.3
7.4
90
Various Forming Processes where:
=
from equation 7.2
,
T’
=
material thickness of necked part of shell
=
die ring corner radius. c Type
reductions operation may be calculated by the following formula:
For a segment of sphere type nosing, as shown in Fig. 7.8, the height of the shell before the nosing
H
=
+
Note: This formula is applicable for R ,
=
Force
-
The nosing force for Type reductions is given by the following formula:
d
Various Forming Processes
91
MATERIAL THICKNESS
mm 0.5
to
1.0 1.0
where:
Steel
Brass
=
coefficient of friction.
0.75
0.70 to 0.65 0.70
to 0.65 0.80 to 0.70
The thickness of the material in the deformation zone of the workpiece becomes greater as a result of nosing. The maximum thickness T’ at the end of the workpiece, after the nosing operation, may be cal-
culated by the following formula:
The median nosing ratio is given by the formula:
Values of for steel and brass are given in Table 7.1.
For the first operation, the value of the nosing ratio is:
m,
=
For subsequent operations, the value of the nosing ratio is 5 to
10 greater than ms from Table 7.1. The nosing diameter for the final phase of nosing is given by this formula:
From formula the necessary number of nosing operations is:
-
D
Table 7.1 Median values of nosing ratio ms for different materials.
7.10
MATERIAL
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
