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Hot and Cold Metal Working ProcessLO#3a, EMC 2033

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Hot and cold Metal Working

1. Rolling2. Forging3. Extrusion4. Drawing5. Bending6. Material removal

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Classification of Metal Forming Metal forming includes a large group of

manufacturing processes in which plastic deformation is used to change the shape of metal work piece.

Metal Forming classified into: Bulk Deformation. Sheet Metal working.

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Reasons for metal forming:The cast ingots produced by the

raw material producers are of little use for manufacturing processes until they have been formed to a suitable shape, i.e. sheet, plate, strip, bar sections.

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Basic Bulk Deformation In these processes significant deformation and

massive shapes happened to the starting work piece, and its surface area-to-volume ratio is small.

The starting work piece usually cylindrical billets or rectangular bars, and the processes includes:

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Rolling In each case, (hot and cold rolling), the metal is

worked while in a solid state and is shaped by plastic deformation.

Working metals in its solid state will help to produce shapes which would be difficult or expensive to produce by any other method, example: Long length of sheet, sections, rod, etc.,

and to improve mechanical properties.

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ROLLING PROCESS1. Rolling: this is a compressive deformation process in which

the thickness of a slab or plate is reduced by two opposing cylindrical tools called rolls. The rolls rotates so as to draw the work into the gap between them and squeeze it.

http://www.doitpoms.ac.uk/tlplib/metal-forming-2/rolling.php

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Hot rolling advantages Most ingots when cast contain many small

holes- a condition known as porosity. During hot rolling, these holes are pressed together and eliminated.

Any impurities contained in the ingot are broken

up and dispersed throughout the metal.

The internal grain structure of the metal is refined, resulting in an improvement of the mechanical properties, e.g. ductility and strength.

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Hot Rolling Disadvantages Due to high temperatures, the surface oxidizes,

producing a scale which result in poor surface finish, making it difficult to maintain dimensional accuracy.

Where close dimensional accuracy and good surface finish are not of great importance, e.g. structural shapes for construction work, a descaling operation is carried out and the product is used as-rolled. Alternately further work can be carried out by cold rolling.

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Cold Rolling When metal is cold rolled, greater forces are

required ,necessitating a large number of stages before reaching the required shape. The strength of the material is greatly improved, but this is accompanied by decrease in ductility.

Depending upon number of stages required in producing the shape, annealing may have to be carried out between stages.Beside improving mechanical properties, cold rolling produces a good surface finish with high dimensional accuracy.

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EXTRUSION PROCESS3. Extrusion: similar to plastics extrusion process, but here a compression process used in which the work metal is forced to flow though a die opening. Thereby taking the shape of the opening as its own cross section.

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Extrusion The extrusion produced has a constant cross

section along its entire length. The die may contain a number of openings,

simultaneously producing a number of extrusions.

There are number of variations of extrusion process, the two most common methods are

Direct extrusion and indirect extrusion.

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Important Features of the extrusion process The complexity of the shape possible is

practically unlimited, and finished products can be produced directly.

A good surface finish can be maintained. Good dimensional accuracy can be obtained. Large reduction in cross sectional area can be

achieved. Since the metal is in compression during the

process, so relatively brittle materials can be extruded.

The mechanical properties of the material are improved.

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Extrusion Limitation The extrusion process is however, limited to

products which have a constant cross section. Any slots, holes etc. not parallel to the longitudinal axis have to be machined.

Due to extrusion-press power capabilities, the size of shape which can produced is limited.

The process is normally limited to long runs, due to die costs, but short runs can be economical with simple die shapes.

Typical materials used are copper and aluminum alloys.

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DRAWING PROCESS Drawing :The process of drawing is a cold working process, i.e. carried out at room temperature. It is mainly used in the production of wires, rods and bar.

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Drawing In continuous wire drawing , the wire is pulled

through the series of progressively smaller dies until the final-size section is reached.

The strength of the material will limit the force which can be applied in pulling the wire through the die, while the ductility of the material limits the amount of reduction possible through each die.

Typical materials used are steel, copper, aluminum and their alloys.

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FORGING RPOCESS2. Forging: In forging a work piece is compressed between two opposing dies, so that the die shapes are imparted to the work. Forging is traditionally a hot working process, but many types of forging are performed cold.

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Forging The oldest form of forging is Hand forging as

carried out by the black smith. Hand tools are used to manipulate the hot metal to give changes in section and changes in shape by bending, twisting, etc.

Due to hand operation, it is not possible to achieve high degree of accuracy or extreme complexity of shape.

This method is limited to one -off or small quantities and requires a high degree of skill.

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Forging When forging is large, some form of power is

employed. Steam or compressed air hammers or a forging press is used , the process is know as Open-sided forging.

This method is used to produce large forgings such ads propeller shaft for ships.

When large quantities of accurately shaped product are required, these are produces by a process known as closed-die forging or Drop forging.

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Forging application Forging is used in the production of parts which

have to withstand heavy or unpredictable loads, such as levers, cams, gears, connecting rods and axel shafts and spanners.

Mechanical properties are improved by forging due to controlled grain flow.

For Example: A spanner cut from the rolled sheet would be much weaker compared to one produced from forging.

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Selection of manufacturing process A number of factors have to be considered

before a choice of process for a given component can be made.

For example: consideration would have to be given to the type of material used, the mechanical properties required, shape, accuracy, degree of surface finish and the quantity to be produced.

Table on the next slide gives the idea.

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Comparison of different processes

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Sheet Metalworking: sheet metalworking processes are forming and

cutting operations performed on metal sheets, strips, and coils.

The surface area-to-volume ratio of the starting metal is high, this is an important point to distinguish these processes from the bulk deformation processes.

Pressworking is the term often used to sheet metal operations because the machines used in these operations are presses.

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Sheet Metalworking Sheet metal operations are always performed as

cold working processes, using a set of tools called a punch and die.

The punch: is the positive portion of the tool set.

The Die: is the negative portion of the tool set. basic sheet metalworking operations includes:

1. Bending.2. Drawing.3. Shearing.

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Sheet Metalworking

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Bending operation Bending involves straining of a metal sheet or

plate to take an angle along a straight axis.

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Sheet metal drawing and Shearing Drawing in sheet metalworking refers to the

forming of a flat metal sheet into a hollow or concave shape, such as a cup, buy stretching the sheet metal. A blank holder must be used to hold down the

blank while the punch pushes throw the sheet metal.

to distinguish sheet metalworking drawing from wire drawing the term deep drawing is used here.

Shearing : in this process involves cutting rather than forming.

A shearing operation cuts the work using a punch and die.