Lecture- 8

Cold Working of metals

Altering the shape or size of a metal by plastic

deformation.

Mechanical working of metals below the

recrystallisation point usually at room

temperature.

Cold working is employed

Improved mechanical properties

Better machinability

Special size accuracy

Bright surface

The major cold-working operations are classified asA.Squeezing - Rolling

SwagingRiveting

Cold forgingExtrusion CoiningHobbing

B. Bending - Roll bending- Seaming- Straightening

C. ShearingD. BlankingE. Drawing - Stamping/Embossing

- Ironing

Cold rolling is a metal working process in which metal is deformed by passing it through rollers at a temperature below its recrystallization temperature. Passing unheated, previously hot-rolled bars, sheets or strip (cleaned of scale) through a set of rolls, often many times, utilize the final size is obtained.

Cold rolling

Cold rolling mill

Rolling of stainless steel into thin sheet.

The term swage can apply to the process of swaging, a die used for swaging, or a tool used to swage.

It is basically a process for reducing the diameter, tapering or pointing round bars or tubes by external hammers.

It is done by means of a rotary swaging machine consisting mainly a hollow spindle, which carries the die sections and rollers.

The die is inserted in the slot in a spindle rotated and forced together repeatedly by the rollers around the periphery forming taper on the work.

Cold forging or Swaging

Swaging is a forming method of reducing a regular or irregular cross section area of tube, bar or wire using two or more dies which surround the outside of the workpiece completely or partially, to a round predetermined diameter.

1 - Head

2 - Rollers

3 - Backers

4 - Motorized spindle

5 - Dies

The dies rotate and apply a radial force around the workpiece toward its center.

The radial forces are applied with short strokes and high frequency.

The forming by swaging converts kinetic energy into impact energy and then into forming forces, applied by the die to the workpiece.

Operation of swaging

Riveting

It is a permanent fastening operation in which the end of the metal pin is pressed over or spread out by blow of the hammer.

Coining is the squeezing of metal while it is confined in a closed set of dies.

A workpiece is placed a confined die.

A movable punch is located within the die.

The actions of this punch cold works the material and can form intricate features.

Coining

Material is displaced to form specific shapes in the part.

Coining typically should not exceed a depth of 30 percent of the material thickness.

Requiring more force than normal deep draw, this operation also requires strong, durable tooling.

This process is used to produce coins, medallions and other similar products on flat stock with relief features.

It is a cold working process that is used to form cavities in the various types of dies, such as those used for moulding plastics.

A hob is a punch having an outside contour similar to that of the parts to be moulded.

Like the punch, the hob is pressed into the block of steel.

Hobbing

It is the plastic deformation of metal about a linear axis with little or no change in the surface area.

When two or more bends are made simultaneously with the use of a die, the process sometimes called forming.

Bending - 1.Roll bending2.Seaming3.Straightening4.Stamping

Bending

Plates, heavy sheets and rolled shapes can be bent to a desired curvature on forming rolls.

These usually have three rolls in the form of a pyramid, lower roll being driven and the upper roll adjustable to control the degree of curvature.

Roll bending

A line of junction formed by sewing together two pieces of material along their margins.

It is used to join ends of sheet metal to form containers such as cans and drums.

The seams are formed by a series of small rollers on seaming machine.

Seaming

After certain processes, such as rolling, extruding and drawing, bars, tubes, plates and sheets usually require some straightening or flattering.

Straightening and flattering machines may be classified as either roller or stretching types.

The rollers bend the work piece in all appropriate directions with a gradually increasing radius so that at the exit end straightness is achieved.

Straightening

Tube straightening

The cutting and separating of material without the formation of chips.

Punching, piercing, lancing, parting, notching, and shaving are all shearing operations.

It is the mechanical cutting of materials in sheet or plate form without the formation of chips or use of burning or melting.

When the two cutting blades are straight, the process is called shearing.

Shearing

During the shearing process, an upper blade and a lower blade are forced past each other with the space between them determined by a required offset.

Normally, one of the blades remains stationary.

Blanking is the operation of punching, cutting, or shearing a piece out of stock to a predetermined shape by die cutting the outside shape of a part.

Blanking is a metal stamping operation by which the sheet metal is punched to get the required outer profile of the sheet metal component. During the blanking process the blanking punch penetrates into the sheet metal and forces the material into the blanking die. The portion of the sheet Metal which comes out through the blanking die opening is the component with the required profile.

Blanking

Blanking is a metal fabricating process, during which a metal workpiece is removed from the primary metal strip or sheet when it is punched.

The material that is removed is the new metal workpiece or blank.

It is similar to hot drawing but the higher deformation forces, thinner metal and limited ductility are the distinc tive features.

When the depth of the drawn part is less than the diameter or minimum surface dimension, of the blank, the process is called as shallow drawing.

If the depth is great er than the diameter, it is called deep drawing.

Drawing

Drawing is the pulling of a metal piece through a die by means of a tensile force applied to the exit side.

A reduction in cross-sectional area results, with a corresponding increase in length.

Letters, numbers, or other figures can be produced in the surface of sheet metal and other metal parts by a process called "Stamping" which consists of forging a hardened steel punch against the metal surface.

Stamping/Embossing

Stamping is used to make high volume parts such as aviation or car panels or electronic components.

Mechanical or hydraulic powered presses stamp out parts from continuous sheets of metal or individual blanks.

The upper die is attached to the ram and the lower die is fixed.

Mechanical machinery transfers all energy as a rapid punch, Whereas hydraulic machinery delivers a constant, controlled force.

Ironing is the name given to the process of thinning the walls of the drawn cylinder by passing it between a punch and die where the separation is less than the original wall thickness.

Mechanical working is necessary in the manufacture of all forms of metals except that, which is cast directly into its finished shapes.

Many of the shapes or forms produced in a particular process will be reshaped to some other forms in a later process.

The final shape or form will frequently be the result of se quence of operations.

Ironing

Shot peening• Shot peening is a cold working process in which the

surface of a part is bombarded with small spherical media called shot. The shot peening is done by air blast. As the shot strikes, small indentations are produced causing a plastic flow of surface metal to a depth of few hundreds of a centimetre. The surface is slightly hardened and strengthened.

• Compressive stresses are beneficial in increasing resistance to fatigue failures, corrosion fatigue, stress corrosion cracking.