98
Various Forming Processes Roller
Roller
Fig. 7.16
Shear spinning method.
Type spinning
Type spinning is known as tube spinning and it consists of reducing the thickness of a cylindrical
work piece while it is spinning on a cylindrical form block, using rollers Fig. 7.17. There are two meth-
ods: forward and backward. In either example, the reduction in wall thickness results in a longer tube. The ideal tangential force in forward tube spinning may be calculated by the following formula:
where: AT
=
T,
=
average
flow
stress of the material,
=
feed. Because of friction and another influencing factors, the force exerted is about twice that of the ideal force.
Roller
Fig. 7.17
Tube spinning method.
Various Forming Processes
99
7.6 FLEXIBLE DIE FORMING
In flexible die forming, one of the dies in a set punch or die is replaced with a flexible material such as a rubber or polyurethane membrane. Polyurethane is widely used because of its resistance to abrasion and
long fatigue life. Note that the solid female or male die has been eliminated, thus reducing the cost of the operation. Furthermore, the surface of the workpiece is protected from damage or scorching during the
forming operation. There are four basic types of flexible forming:
Verson-Wheelon, Guerin, and hydroform processes. Fig. 7.18 shows the Guerin process.
A typical setup is illustrated 7.18, which shows the ram containing a flat, thick pad of rubber or
polyurethane. The form block can be made from epoxy, resin, wood, steel, aluminum, or cast iron. In oper- ation, a metal blank is placed on the form block and is formed to the desired shape under continuous pres-
sure during a complete stroke of the press. The hardness of the rubber pad is 60 to70 Shore and the rela- tive elongation is 600 to 700 percent. Used extensively in the aircraft industry, this process is particularly
useful in making parts with shallow flanges.
Ram Air
space channel
Flexible die
Blank
Fig. 7.18
Schematic illustration
of
forming in a flexible die.
7.6.1 Force in Die Forming
The press force for flexible die forming
is
given by the following formula:
F
=
where:
A
=
area of the pad of rubber
p
=
specific pressure Table 7.5.
7.20
A specific pressure
for drawing a workpiece of aluminum alloy is given in Table 7.5.
100
Ratio rn
=
0.6
Various
Forming Processes
Relative thickness of material Tsr
x 100
1.3
1
0.66 0.44
25.5 27.5
32.0
I
35.5 0.5
0.44 27.5
29.5 34.0
37.0 29.5
32.0 34.4
39.3
In Table 7.6 are given maximum values for the drawing ratio m, the drawing height in one operation
and the minimum drawing radius for different kinds of materials.
MATERIAL Aluminum
Low-carbon steel Stainless steel
h Rrn
0.45
1
.Od 0.50
0.50 0.65
The maximum drawing height h of rectangular and square-drawn shells in one drawing operation is given by the following:
h -
for
steel h
-
for aluminum.
where:
=
drawing corner radius.
PART
DIE DESIGN
It is the intention of this part to present as complete a picture as space will permit of the knowledge and skills needed for the effective design of dies for sheet metal forming. The text is presented in six chapters,
with detailed discussions of die design for each of the manufacturing processes covered in Part 11. Chapter
Thirteen gives data on basic tool and die materials, their properties and applications. Special pay attention is given to the basic functions of work and die components, their design and necessary calculations.
Although many examples are included, it should be evident that it is not possible to present all the data, tables, and other information needed to design complicated tools, dies, etc. in one text. The tool and die
designer must have tables, handbooks, and literature available to be completely effective as a die designer. This part supplies most of the practical information needed by a designer.
8.1 Die
Classifications
BASIC DIE CLASSIFICATIONS AND COMPONENTS
8.1 DIE CLASSIFICATIONS
Dies can be classified according to a variety of elements and in keeping with the diversity of designs of dies. It is possible to classify dies according to:
Manufacturing processes: blanking, punching, bending, deep drawing, etc. Number of operations: single-operation simple, and multi-operation combination dies.
Number of stations: single station and progressive dies.
Single station dies may be: stroke, or
a Combination a die in which both cutting and noncutting operations are accomplished at one press b compound a die in which two or more cutting operations are accomplished at every press stroke.
Progressive dies are made with two or more stations. Each station performs an operation on the work- piece or provides an idler station
so that the workpiece is completed when the last operation has been accomplished. After the first part has traveled through all the stations, each subsequent stroke of the press
produces another finished part.
103
104
Basic Die Classifications and Components Depending on the production quantities of pieces
-
high, medium, or low
-
press dies may be graded as follows:
Class A-These dies are used for high production only. The best of materials are used, and all easily
worn items or delicate sections are carefully designed for easy replacement. A combination of long die life, constant required accuracy throughout the life, and ease in maintenance are prime considerations, regard-
less of tool cost.
Class B-These dies are applicable to medium production quantities and are designed to produce the designated quantity only. Die cost as related to total production becomes an important consideration.
Cheaper materials may be used, provided they are capable of producing the full quantity, and less consid- eration is given the problem of ease of maintenance.
Class C-These dies represent the cheapest usable tools that can be built and are suitable for ume production.
Temporary dies.-These dies are used for small production and represent the lowest cost tools that will produce the part.
Fig. 8.1 shows die classifications schematically according to the guide methods used in the upper and lower die sets:
Plain dies Fig. dies are very simple and are used for low and individual production,
Plain
dies with movable elastic stripper dies are mostly used for the punching and
Plain dies with solid stripper Fig. dies are used for the punching and blanking of sim-
blanking of simply shaped parts of thinner sheet metals. ply shaped parts of thicker materials.
I
Fig. 8.1
Schematic illustration of die classifications according to the guide methods
of upper and lower die sets: a plain dies, b plain dies with movable elastic stripper, c plain dies with solid stripper, d dies with
guide plate, e dies with guide posts.
