Sheet-metal forming

Forrning processes

The considerable success and continuous expansion of the sheet-forming industry are due to a number of unrelated but
important developments.

The introduction of the continuous tandem rolling mill has made it possible to produce wide strip which can either be
coiled or cut to suitable operational sizes. At the same time, cold forming of sheet ensures greater resistance of the formed
section to corrosion, since the preformed material has good surface finish due to treatments such as pickling. Surfaces
cleaned in this way form suitable bases for the application of protective layers of, for instance, paint or plastic coating and,
furthermore, they can be galvanized and then rolled or drawn. The good dimensional qualities of preformed sheet make it
a good starting material for operations such as bending,

piercing, dimpling and drawing, by ensuring general uniformity of thickness. The exception here is the change in sheet
thickness on transition sections where thinning is likely to occur. However, in a cold operation, strain hardening accompanied
by changes in thickness will tend to counteract the effect of the actual variation in thickness. The five basic processes effecting a change in shape are:

The five basic processes effecting a change in shape are:

1. shearing,
2. bending,
3. spinning,
4. stretching, and
5. drawing.

A wide range of sections and shapes, and a varied range of products for industrial, domestic or general use can be produced.
Industrial applications are found in the following areas:
1.
2. the chemical industry;
3. the aircraft industry;
4. the car industry; and
5. the food industry.

Cold-formed sections are used industrially as components, plant, equipment, containers, panels, radiators, body parts,
chassis, frames, racks, stiffeners, etc.

1. components,
2. cookers,
3. panels,
4. refrigerator bodies,
5. washing machines, and
6. shower cabinets.

General applications range from building constructions and components through structural elements, to rail and road
transport, civil and highway engineering, agricultural machinery and equipment, and architectural and shop fittings. A
major part of the output is intended for the electrical heating and ventilating applications represented by conduits, casings,
cable supports, electrical appliances, heating panels, and dust-extraction equipment.

Shearing operations

The shearing operation constitutes the first stage of any forming process by producing either the starting material
(cutting out of a sheet) or preparing an existing work piece by punching a hole or a series of holes before forming.
Basically, the operation involves placing the material between the edges of a shearing tool, which serve as supports, and separating a part of it by the action of a punch. Purely shearing stresses are generated, but the quality of the separating
cut depends on the clearance between the tool and the specimen. If the clearance is too small, the cracks produced by
the tool do not coalesce and tearing occurs. At the other extreme, too large a clearance results in considerable plastic
flow which inhibits formation of the cracks and produces a burr at the upper edge.

Bending is executed either by pushing the strip into a die of the desired profile or, more often, by cold-roll bending (see
Section 16.2.4.7). Either method can be used to produce a variety of shapes, a selection of which is shown in Figure
16.53.

Since the final quality depends on the material used, it may well be that the phenomenon limiting the degree of bending
allowable in any given case will not be simply that of fracture or instability, but also one that produces an unsatisfactory
finish. A selection of possible limiting factors, including the basic fracture and necking, is given in Figure 16.54. The
maximum deformation may be limited by indentation caused by the die edges, galling, wrinkling and crease forming.
In addition to actual sheet bending, the operation is often extended to tube or blank flanging. Severe compressive
stresses are imposed in these processes and may lead to buckling in a tube.

Spinning

Sheet forming by spinning is applicable to axisymmetric shapes only. The operation involves a driven, rotating system
to which the male forming die is attached. The sheet to be formed is held against the die. Either manual or power, cold
or hot spinning operations are used. The limit of formability is imposed by the ductility of the material and can be assessed
from a simple tensile test.

Stretch forming

Stretch forming is often used to produce either simple components, comparable to cylindrical cups, or more complicated
pressing-type shapes. Basically, stretch-forming operations depend on the use of conventional punch and die systems or a
dieless-punch system. In either case, the rim of the blank is clamped (Figure 16.55) and the stretching, with consequent
thinning of the material, can proceed on loading the punch. The thinning is particularly pronounced on the radiused rim of
the supporting die or anvil. Instability can set in and lead to failure through localized necking.

Dynamic or explosive stretch forming is also used (see Section 16.2.9.3).

Deep drawing

Deep drawing is normally associated with the manufacture of cups, cans and similar containers. The operation is usually
divided into two main groups: first-stage drawing, in which a flat circular metal blank is made into a cup; and a redrawing

stage (or stages) in which the cup reaches its final size. The latter operation is necessary because first-stage drawing cannot
normadly produce a higher degree of deformation than that defined by the ratio of the diameter of the blank and the
die throat (drawing ratio) of about 2.2, or a cup height/ diameter ratio of about 1.
The sequence of operation is as follows. Initially, the specimen held in position by a blank holder, is partly in
contact with the die, partly with either the die or punch, and partly with the punch only. The downward movement of the
punch initiates drawing. The outer rim of the blank is then subjected to pure radial drawing (i.e. drawing towards the
vertical axis of the system) between the die and blank holder. A part of the material bends and slides over the die and is
further stretched between the punch and the die, whereas the material initially in the vicinity of the punch head and actually
in contact with it bends and slides over the radiused part of the punch and stretches over the punch head.
The redlrawing systems often used are shown in Figure 16.56. Parts (a) and (b) in the figure show direct redrawing
systems with and without blank holders, respectively, while a reverse sy:stem is shown in (c). In (a), the wall of the cup

stage (or stages) in which the cup reaches its final size. The latter operation is necessary because first-stage drawing cannot
normadly produce a higher degree of deformation than that defined by the ratio of the diameter of the blank and the
die throat (drawing ratio) of about 2.2, or a cup height/ diameter ratio of about

 1. The sequence of operation is as follows. Initially, the specimen held in position by a blank holder, is partly in
contact with the die, partly with either the die or punch, and partly with the punch only. The downward movement of the
punch initiates drawing. The outer rim of the blank is then subjected to pure radial drawing (i.e. drawing towards the
vertical axis of the system) between the die and blank holder. A part of the material bends and slides over the die and is
further stretched between the punch and the die, whereas the material initially in the vicinity of the punch head and actually
in contact with it bends and slides over the radiused part of the punch and stretches over the punch head.
The redlrawing systems often used are shown in Figure 16.56. Parts (a) and (b) in the figure show direct redrawing
systems with and without blank holders, respectively, while a reverse sy:stem is shown in (c). In (a), the wall of the cup

undergoes double bending and unbending, the severity of which is expected to be high because the respective directions
of deformation are at right angles to each other. System (b) shows less severity because of the tapered wall support,
although double bending is involved. This system can be used only for relatively low cup diameter/wall thickness ratios
which do not require the use of a blank holder. In comparison with the direct methods, system (c), having a generously
radiused die profile, tends to reduce the degree of (or with a semicircular profile to eliminate completely) one bending and
unbending effect. Whether there is significant advantage to using any system depends on the balance between the reduction
in redundancy and practical production considerations.

The definition of ‘redundancy’ in deep drawing is not easy since redundancy is not necessarily associated with the effects
of macroshear. The nature of the processes is such that portions of the blank material undergo some phases of deformation
which in themselves induce redundant effects and yet are physically unavoidable if the process is to be completed. It
is therefore the degree of severity imposed rather than the avoidance of a certain phase of the operation that matters. In
this respect, the process differs significantly from the bulk forming operations discussed previously.The three main sources of unnecessary strain in and/or distortion of the blank or cup material are flange wrinkling, the already discussed bending and unbending, and, partly, ironing. The latter is used to eliminate the increase in cup wall thickness which can be as much as 30% in the first stage of drawing. If this is followed by a further substantial rise in successive processing stages and is accompanied by wrinkling, an additional drawing operation becomes necessary. As far as redundancy is concerned, ironing is the only operation that brings back the ’standard’ features of shearing.

The formability of a material depends on the blank-holder pressure and, consequently, the deep drawing ratio R = Q/d
may be limited either by wrinkling of the flange, tearing of the cup bottom, or by galling. Figure 16.57 shows diagrammatically
the boundaries of these conditions and indicates the presence of a ‘safe window’ within which deep drawing is
likely to be successful.

In determining the drawability, the criterion to be adopted is that relating to the first incidence of any fault.

Hydroforming and hydromechanical forming

To increase the depth of the draw, while reducing local stress concentrations, the techniques of fluid-backed or fluidmechanically
augmented processing have been introduced.

In a pure hydroforming operation (Figure 16.58(a)), a rubber diaphragm pressurized by the fluid acts as both a blank
holder and a flexible tool. The only rigid tool is the punch. Since hydraulic pressure is exerted on the blank, the stressing
is uniform and is accurately controlled. With reduced sheet thinning and considerable suppresion of cracks, high draw ratios (up to R = 3) are possible. The method is suitable for the forming of both jet-engine components (some 1 m X 1 m
in size) and a variety of car components, including side and roof panels. A selection of parts so formed is shown in Figure
16.58( (b)-(e)).

In the Hydromec, or mechanically augmented system (Figure 16.59). the sealing diaphragm is omitted and the blank
comes into direct contact with the fluid. The sealing of the fluid is effected through the use of sealing rings. The blank is
pressed firmly against the punch and thus possible bulging between the die ring and punch is avoided; a feature of the
process that is of particular importance in the deep drawing of parabolic components. Complex parts can be drawn in a single
operation, thus reducing operational costs. Sizing tools are claimed to be unnecessary.