An Introduction to Methods of forming and cutting

An Introduction to 
Methods of forming and cutting 

Introduction 

manufacturing processes are classified into six main groups: primary shaping, material forming, dividing, joining, modifying material property and coating figure 1
Primary shaping is the creation of an initial shape from the molten, gaseous or formless solid state. Dividing is the local separation of material. Joining is the assembly of individual workpieces to create sub assemblies and also the filling and saturation of porous workpieces.
Coating means the application of thin layers on components, for example by galvanization, painting and foil wrapping. The purpose of modifying material property is to alter material characteristics of a workpiece to achieve certain useful properties. Such processes include heat treatment processes such as hardening or re crystallization annealing.

FIG 1

Forming – as the technology forming is defined  as manufacturing through the three dimensional
or plastic modification of a shape while retaining its mass and material cohesion. In contrast to deformation, forming is the modification of a shape with controlled geometry. Forming processes are
categorized as chipless or non-material removal processes.
the field of “forming technology” includes not only the main category of forming but also subtopics, the most important of which are dividing and joining through forming figure 2
Combinations with other manufacturing processes such as laser machining or casting are also used.

FIG 2

1- Forming
Forming techniques are classified in accordance with DIN 8582 depending on the main direction of applied stress figure 3

– forming under compressive conditions,
– forming under combined tensile and compressive conditions,
– forming under tensile conditions,
– forming by bending,
– forming under shear conditions.

FIG 3

1-1 Forming under compressive conditions
Rolling
Cast slabs, rods and billets are further processed to semi-finished products by rolling. In order to keep the required rolling forces to a minimum, forming is performed initially at hot forming temperature. At these temperatures, the material has a malleable, paste-like and easily formable consistency which permits a high degree of deformation without permanent work hardening of the material. Hot forming can be used to produce flat material of the type required for the production of sheet or plate, but also for the production of pipe, wire or profiles. If the thickness of rolled material is below a certain minimum value, and where particularly stringent demands are imposed on dimensional accuracy and surface quality, processing is performed at room temperature by cold rolling. In addition to rolling semi-finished products, such as sheet and plate, gears and threads on discrete parts are also rolled
under compressive stress conditions. figure 4

FIG 4 

Open die forming
Open die forming is the term used for compressive forming using tools which move towards each other and which conform either not at all or only partially to the shape of the workpiece. The shape of the workpiece is created by the execution of a free or defined relative movement between the workpiece and tool similar to that used in the hammer forging process (figure 5)

FIG 5

Closed die forming
Closed die forming is a compressive forming process, where shaped tools (dies) move towards each other, whereby the die contains the workpiece either completely or to a considerable extent to create the final shape (figure 6 )

FIG 6

Coining 
Coining is compressive forming using a die which locally penetrates a workpiece. A major application where the coining process is used is in manufacturing of coins and medallions (figure 7 )

FIG 7

Forming by forcing through an orifice 
Forming by forcing through an orifice is a forming technique which involves the complete or partial pressing of a material through a forming die orifice to obtain a reduced cross-section or diameter. This technique includes the subcategories free extrusion, extrusion of semi-finished products and extrusion of components
During free extrusion, a billet is partially reduced without upsetting or bulging of the non-formed portion of the workpiece (figure 8)

FIG 8

Free extrusion of hollow bodies or tapering by free extrusion involves partial reduction of the diameter of a hollow body, for example a cup, a can or pipe, whereby an extrusion container may be
required depending on the wall thickness. (figure 9 )

FIG  9

In extrusion of semi-finished products a heated billet is placed in a container and pushed through a die opening to produce solid or hollow extrusions of desired cross-section. (figure 10)

FIG 10

Cold extrusion of discrete parts involves forming a workpiece located between sections of a die, for example a billet or sheet blank . In contrast to free extrusion, larger deformations are possible using the extrusion method.
Extrusion is used for the manufacture of semi-finished items such as long profiles with constant cross sections. Cold extrusion is used to produce individual components, e. g. gears or shafts. In both methods, forming takes place using either rigid dies or active media. In addition, a difference is drawn depending on the direction of material flow relative to the punch movement – i.e. forwards, backwards or lateral –  and the manufacture of solid or hollow shapes (figure 11)

FIG 11

1-2 Forming under combination of tensile and compressive conditions
Drawing is carried out under tensile and compressive conditions and involves drawing a long workpiece through a reduced die opening.The most significant subcategory of drawing is strip drawing.
stripping
This involves drawing the workpiece through a closed drawing tool (drawing die, lower die) which is fixed in drawing direction. This allows the manufacture of both solid and hollow shapes. In addition to the manufacture of semi-finished products such as wires and pipes, this method also
permits the production of discrete components. This process involves reducing the wall thickness of deep-drawn or extruded hollow cups by ironing, and has the effect of minimizing the material input, particularly for pressure containers, without altering the dimensions of the can bottom (figure 12)

FIG 12

Deep drawing

Deep drawing is a method of forming under compressive and tensile conditions whereby a sheet metal blank is transformed into a hollow cup, or a hollow cup is transformed into a similar part of smaller dimensions without any intention of altering the sheet thickness . Using the single-draw deep drawing technique it is possible to produce a drawn part from a blank with a single working stroke of the press(figure 13)

FIG 13

Flanging

Flanging is a method of forming under combined compressive and tensile conditions using a punch and die to raise closed rims (flanges or collars) on pierced holes (. The holes can be on flat or on

curved surfaces. Flanges are often provided with female threads for the purpose of assembly. (figure 14

FIG 14

Spinning

Spinning is a combined compressive and tensile forming method used to transform a sheet metal blank into a hollow body or to change the periphery of a hollow body. One tool component (spinning mandrel, spinning bush) contains the shape of the workpiece and turns with the workpiece, while the mating tool (roll head) engages only locally . In contrast to shear forming, the intention of this process is not to alter the sheet metal thickness. (figure 15)

FIG 15

Wrinkle bulging

Wrinkle bulging or upset bulging is a method of combined tensile and compressive forming for the local expansion or reduction of a generally tubular shaped part. The pressure forces exerted in the longitudinal direction result in bulging of the workpiece towards outside, inside or in lateral direction (figure 16)

FIG 16

1-3 Forming under tensile conditions

Extending by stretching

Extending by stretching is a method of tensile forming by means of a tensile force applied along the longitudinal axis of the workpiece. Stretch forming is used to increase the workpiece dimension in the direction of force application, for example to adjust to a prescribed length. Tensile test is also a pure stretching process. Straightening by stretching is the process of extending for straightening rods and pipes, as well as eliminating dents in sheet metal parts. (figure 17)

FIG 17

Expanding

Expanding is tensile forming to enlarge the periphery of a hollow body. As in case of deep drawing, rigid  as well as soft tools, active media and active energies are also used.

FIG 18 

Stretch forming

Stretch forming is a method of tensile forming used to impart impressions or cavities in a flat or convex sheet metal part, whereby surface enlargement – in contrast to deep drawing – is achieved by reducing the thickness of the metal.(figure 19)

FIG 19

1-4 Forming by bending

bending with a linear die movement

bending with a linear die movement the die components move in a straight line . The most important process in this subcategory is die bending, in which the shape of the part is impacted by the die geometry and the elastic recovery . Die bending can be combined with die coining in a single stroke. Die coining is the re striking of bent workpieces to relieve stresses, for example in order to reduce

the magnitude of spring back. (figure 20)

FIG 20

Bending with rotary

Bending with rotary die movement includes roll bending, swivel bending and circular bending. During roll bending, the bending moment is applied by means of rolling. Using the roll bending process, it is possible to manufacture cylindrical or tapered workpieces (Figure 21).

FIG  21

Swivel bending is bending using a tool which forms the part around the bending edge (Figure 22). Circular bending is a continuous process of bending which progresses in the direction of the shank using strip, profile, rod, wire or tubes (Figure 23). Circular bending at an angle greater than 360°, for example is used in the production of springs and is called coiling.

FIG 22

FIG 23

1-5 Forming under shear conditions

Displacement 

Displacement is a method of forming whereby adjacent cross-sections of the workpiece are displaced parallel to each other in the forming zone by a linear die movement (Figure 24). Displacement along a closed die edge can be used for example for the manufacture of welding bosses and centering indentations in sheet metal components. 

FIG 24

Twisting 

Twisting is a method of forming under shearing conditions in which adjacent cross-sectional surfaces of the workpieces are displaced relative to each other by a rotary movement (Figure 25).

FIG 25

2- Dividing

Dividing is the first subgroup under the heading of parting, but is generally categorized as a “forming technique” since it is often used with other complementary production processes . According to the definition of the term, dividing is taken to mean the mechanical separation of workpieces without the creation of chips (non-cutting).
the dividing category includes the subcategories shear cutting, wedge-action cutting, tearing and breaking (Fig. 26). Of these, the shear cutting is the most important in industrial application.

FIG 26

2-1 Shear cutting
Shear cutting – known in practice as shearing for short – is the separation of workpieces between two cutting edges moving past each other

FIG 26

2-2 Wedge-action cutting
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(figure 27)

FIG 27

2-3 tearing and breaking
The processes tearing and breaking subject the workpiece either to tensile stress or bending or rotary stress beyond its ultimate breaking or tensile strength.

3- joining through forming
Various combinations of different forming processes or combinations of forming, cutting and joining processes have been found to be successful over many years.
3-1 Stretch drawing and deep drawing,
an important role in the sheet metal processing industry. 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 (Figure 28). As a result, the blank is subjected to tensile stress during penetration of the punch. So the sheet metal thickness is reduced.

FIG 28

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

FIG 29

Forming is 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 (figure 30)

FIG 30

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 example, combines not only the classical blanking and forming processes but also joining for the manufacture of finished stator and rotor assemblies for the electric motor industry (Figure 31).

FIG 31

 Sheet metal parts can also be joined by means of forming, by the so-called hemming or flanging (Figure 32).

FIG 32

Dividing, coating and modifying material property technologies will substantially expand the field of application covered by forming technology in the future. This will allow finish processing in only a small number of stations, where possible in a single line, and will reduce costs for handling and logistics throughout the production sequence.