Altan T. Metal Forming Handbook

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Wedge-action cutting of workpieces is generally performed using a

wedge-shaped cutting edge. The workpiece is divided between the blade

and a supporting surface. Bite cutting is a method used to divide a

workpiece using two wedge-shaped blades moving towards each other.

This cutting method is employed by cutting nippers or bolt cutters

(Fig. 2.1.31).

The processes tearingand breakingsubject the workpiece either to ten-

sile stress or bending or rotary stress beyond its ultimate breaking or

tensile strength.

Methods of forming and cutting technology

die

counterpunch

punch

serrated ring

annular serrated

blank holder

Fig. 2.1.30Fine blanking

workpiece

tool

Fig. 2.1.31Bite cutting

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

2.1.4Combinations of processes in manufacturing

Various combinations of different forming processes or combinations

of forming, cutting and joining processes have been found to be suc-

cessful over many years.

Stretch drawing and deep drawing,for example, assume an important

role in the sheet metal processing industry (cf.Sect.4.2.1). During

stretch drawing, the blank is prevented from sliding into the die under

the blank holder by means of a locking bead and beading rods or by

applying a sufficiently high blank holder force (Fig. 2.1.32). As a result,

the blank is subjected to tensile stress during penetration of the punch.

So the sheet metal thickness is reduced.

Deep drawing, in contrast, is a process of forming under combined

tensile and compression conditions in which the sheet is formed under

tangential compressive stress and radial tensile stress without any

intention to alter the thickness of the sheet metal (cf. Fig. 4.2.1).

For example when drawing complex body panels for a passenger car,

stretch drawing and deep drawing may be conducted simultaneously.

The tool comprises a punch, die and blank holder (Fig. 2.1.32). The

blank holder is used during stretch drawing to act as a brake on the met-

al, and during deep drawing to prevent the formation of wrinkles.

Modern pressing techniques today permit the desired modification

of the blank holder force during the drawing stroke. The blank holder

forces can be changed independently at various locations of the blank

holder during the drawing stroke. The blank is inserted in the die and

clamped by the blank holder. The forming process begins with penetra-

Basic principles of metal forming

drawingwithblankholder

stretchforming deepdrawing

punch

blankholder

blank

die

Fig. 2.1.32 Overlapping deep drawing and stretch forming in the sheet metal forming process

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

tion of the punch to perform a stretch drawing process in which the

wall thickness of the stretched blank is reduced. The bottom of the

drawn part is subsequently formed.

The deep drawing process begins once the required blank holding

force has been reduced to the extent that the blank material is able to

flow without generating wrinkles over the rounded sections of the die.

At the end of the drawing process, the blank holder force is frequently

increased again in order to obtain a reproducible final geometry by

respecting the stretching portion of the drawing stroke.

In addition to deep drawing, body panels are additionally processed

in the stamping plant by forming under bending, compressive and

shearing conditions. A characteristic of the bendingprocess is that a

camber is forced on the workpiece involving angular changes and swiv-

el motions but without any change in the sheet thickness. The spring-

back of the material resulting from its elastic properties is compensated

for by overbending (cf.Sect.4.8.1). Another possibility for obtaining

dimensionally precise workpieces is to combine compressive stresses

with integrated restriking of the workpiece in the area of the bottom

dead center of the slide movement.

Formingis almost always combined with cutting. The blank for a sheet

metal part is cut out of coil stock prior to forming. The forming process

is followed by trimming, piercing or cut-out of parts (cf.Sect.4.1.1).

If neither the cutting nor the forming process dominates the pro-

cessing of a sheet metal part, this combination of methods is known as

blanking. Where greater piece numbers are produced, for most small

and medium-sized punched parts a progressive tool is used, for example

in the case of fine-edge blanking (cf.Sect.4.7.3). However, solid form-

ing processes often also combine a number of different techniques in a

single set of dies (cf.Sect.6.1).

The call for greater cost reductions during part manufacture has

brought about the integration of additional production techniques in

the forming process. Stacking and assembly of punched parts, for exam-

ple, combines not only the classical blanking and forming processes

but also joining for the manufacture of finished stator and rotor assem-

blies for the electric motor industry (Fig. 2.1.33, cf. Fig. 4.6.22and

4.6.23). Sheet metal parts can also be joined by means of forming, by

the so-called hemming or flanging (Fig. 2.1.34).

Methods of forming and cutting technology

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

Dividing, coating and modifying material property technologies will sub-

stantially expand the field of application covered by forming technolo-

gy in the future. This will allow finish processing in only a small num-

ber of stations, where possible in a single line, and will reduce costs for

handling and logistics throughout the production sequence.

Basic principles of metal forming

Fig. 2.1.33

Joining by parting

Fig. 2.1.34

Joining by forming: hemming

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

2 Basic principles of metal forming

2.2 Basic terms

2.2.1 Flow condition and flow curve

Metallic materials may be shaped by applying external forces to them

without reducing their structural cohesion. This property is known as

the formability of metal. Deformation or flow occurs when the rows of

atoms within the individual crystalline grains are able, when stressed

beyond a certain limit, to slide against one another and cohesion

between the rows of atoms takes place at the following atomic lattice.

This sliding occurs along planes and directions determined by the crys-

talline structure and is only made possible by, for example, dislocations

(faults in the arrangement of the atomic lattice). Other flow mecha-

nisms such as twin crystal formation, in which a permanent deforma-

tion is caused by a rotation of the lattice from one position to another,

play only a minor role in metal forming technology.

Flow commences at the moment when the principle stress difference

max

– s

min

) reaches the value of the flow stress k

, or when the shear

strain caused by a purely shearing stress is equal to half the flow stress,

given by:

By neglecting the principle stress s

, this mathematical expression rep-

resents an approximate solution of the shearing stress hypothesis with

the greatest principle stress s

and the smallest principle stress s

=σ σ

max min

–

=σ σ

–

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

The value of the flow stress depends on the material, the temperature,

the deformation or strain, w, and the speed at which deformation or

strain rate is carried out, w. Below the recrystallisation temperature, the

flow stress generally rises with increasing deformation, while the tem-

perature and deformation rate exert only a minimal influence. Excep-

tions to this rule are forming techniques such as rolling and forging, in

which extremely high deformation rates are used. Above the recrys-

tallisation temperature, the flow stress is generally subject to the tem-

perature and deformation rate, while a previous deformation history

has only minimal influence. The flow stress generally drops with in-

creasing temperature and decreasing deformation rate.

Accordingly, DIN 8582 differentiates between metal forming process-

es involving a lasting change in strength properties and those involving

no appreciable change in strength properties, previously designated as

cold and hot forming. In the temperature range between, deformation

involves only a temporary change in the strength properties of the

material. In this case, the deformation speed is higher than the recov-

ery or recrystallisation rate. Recrystallisation starts only after comple-

tion of the forming process. The rules of metal forming with lasting

change in the strength properties apply in this case.

The DIN 8582 standard also breaks down the process according to

forming without heating (cold forming) and forming after the applica-

tion of heat (hot forming). These terms simply specify whether heating

devices are necessary. Unlike their former meaning, these terms are not

physically related to the material concerned. The flow stress of the indi-

vidual materials is determined by experiments in function of deforma-

tion (or strain) and deformation rate (or strain rate) at the various tem-

perature ranges, and described in flow curves. One of the uses of flow

curves is to aid the calculation of possible deformation, force, energy

and performance.

2.2.2 Deformation and material flow

Actual deformation w, also called logarithmic or true strain, is given by:

Basic principles of metal forming

∫

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

in which w

is deformation in one principle axis and w

and w

in the

other two principle axes. This equation will give, for example, the

amount of compression in a body with height h (Fig. 2.2.1). wis calcu-

lated from the compression relative to the starting measurement eor

from the relative deformation

in which h

stands for the height of the body before compression and

the final height of the body after compression:

In accordance with the law of volume constancy, according to which

the volume is not altered by the deformation process (Fig. 2.2.1), the

sum of all deformation values is always equal to zero:

Basic terms

–

∆

ϕε

1==+

()

ln ln

ϕϕϕ

123

0++=

V= l b h = V= l b h

0000 1 111

Fig. 2.2.1 Dimensional changes in frictionless upsetting of a cube

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

The greatest deformation, which is equal to the sum of the two other

deformations, is designated the principle deformation j

The principle deformation must be a known quantity, as it forms the

basis for every calculation, for example of deformation force. It is easy

to determine, as it carries a different sign to the other two. In the com-

pression of a cubic body, for example, the increase of width (b

> b

)

and length (l

> l

) results in a positive sign, while the decrease of height

< h

) produces a negative sign (Fig. 2.2.1). Accordingly, the absolute

greatest deformation is along the vertical axis j

Similar to the sum of deformations, the sum of deformation rates j

must always be equal to zero:

The flow law applies approximately:

with the mean stress s

given by

The material flow along the direction of the stress which lies between

the largest stress s

max

and the smallest stress s

min

will therefore be

small and will be zero in cases of plane strain material flow, where

deformation is only in one plane.

2.2.3Force and work

In calculating the forces required for forming operations, a distinction

must be made between operations in which forces are applied directly

and those in which they are applied indirectly. Direct application of

force means that the material is induced to flow under the direct appli-

Basic principles of metal forming

ϕϕϕϕ

123

=− +

()

ϕϕϕσσσσσσ

123 1 2 3

:: : : ,=−

()

−

()

−

()

mmm

σσσ

123

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

cation of an exterior force. This requires surfaces to move directly against

one another under pressure, for example when upsetting and rolling.

Indirect application of force, in contrast, involves the exertion of a force

some distance from the actual forming zone, as for example when the

material is drawn or forced through a nozzle or a clearance. Additional

stresses are generated during this process which induce the material to

flow. Examples of this method include wire drawing or deep drawing.

In the direct application of force, the force F is given by:

where A is the area under compression and k

is the deformation resis-

tance. The deformation resistance is calculated from the flow stress k

after taking into account the losses arising, usually through friction.

The losses are combined in the forming efficiency factor η

The force applied in indirect forming operations is given by:

where A represents the transverse section area through which the force is

transmitted to the forming zone, k

is the mean deformation resistance

and k

the mean stability factor, both of which are given by the integral

mean of the flow stress at the entry and exit of the deformation zone.

The arithmetic mean can usually be used in place of the integral value.

The referenced deformation work w

is the work necessary to deform a

volume element of 1mm

by a certain volume of displacement:

The specific forming work can be obtained by graphic or numerical

integration using available flow curves, and in exactly the same way

as the flow stress, specified as a function of the deformation j

Figure 2.2.2illustrates the flow curves and related work curves for dif-

ferent materials.

Basic terms

FAk A

wm g

=⋅⋅=⋅ ⋅=⋅ϕ

wkdk

id f fm g

=⋅≅⋅

∫

ϕϕ

FAk

=⋅

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

If there is no flow curve available for a particular material, it can be

determined by experimentation. A tensile, compressive or hydraulic

indentation test would be a conceivable method for this. If the specific

deformation work w

and the entire volume V or the displaced volume

are known quantities, the total deformation work W is calculated on

the following basis:

2.2.4 Formability

The identification of formability should only be based on those cases of

material failure caused as a result of displacement or cleavage fractures

where no further deformation is possible without failure. If, therefore, a

material breaks before reaching maximum force as a result of a cleavage

Basic principles of metal forming

200

400

600

800

1000

100

300

500

700

specific deformation work w

200

400

600

800

1000

100

300

500

700

flow stress k

Nmm

0,2 0,4

0,6

0,8

1,0

1,2

log. principle deformation

St 14

St 37

C35

(normally annealed)

Fig. 2.2.2

Flow curves

and curves

showing the

specific defor-

mation work

for different

materials

=⋅ ≅⋅ ⋅ = ⋅

ηη

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998