MEC560_Chapter 3_Metal Forming and Shaping Processes

MEM560

EMD5MPL

Chapter 3: Metal Forming and Shaping Processes

Introduction to Nature of Plastics Deformation

• The permanent change in shape of a metallic body as the result of forces acting on its surface

• The strength of metal depend on level/degree of plastic deformation and grain sizes and distribution

• The deformed metal had higher strength because the entanglement of dislocation with grain boundaries and each other

• The higher the deformation, the higher the strength. • Metal with smaller grain size had higher strength. • Effect of plastic deformation Grain be come deformed and elongated Grain become intact but mass continuity is maintained.

Introduction to Nature of Plastics Deformation

Hot and cold working process

•Hot working Plastic deformation that occurs above recrystallization temperature Process involved:Extrusion , welding, Forging, Drawing, and Rotary piercing •Cold working Plastic deformation that is carried out at room temperature Process involved : Bending , rolling, extrusion, forging, Shearing, Drawing, etc •Warm working Plastic deformation that is carried out at intermediate temperature, compromise of cold and hot working

Advantages of Hot working process

• Eliminate the effect of strain hardening - new grain of metal

grow from the just deformed grains • High strength material- small grain is obtained after hot

working which provide better strength • Reduced defect- Blow hole sand pores disappear by welding

action under high pressure and temperature or when they are reduced in size.

• There is increase in ductility- hence larger deformation can be carried out at single stage

• During hot working, metal strength is low, hence, less force is adequate for causing deformation - Large component can be deformed using machines for reasonable size.

Disadvantages of Hot working

• Undesirable reactions between the metal and the surrounding atmosphere - scaling or rapid oxidation of the work piece

• Less precise tolerances- due to thermal contraction and warping from uneven cooling

• Uneven grain structure- Grain structure may vary throughout the metal for various reasons

• Expensive process- Requires a heating unit of some kind such as a gas or diesel furnace or an induction heater

Advantages of cold working

• More economical process- no additional features (heater) required

• Good surface finish and dimensional control – no oxidation or scaling occur

• Reduces waste as compared to machining- eliminates with near net shape methods

• Low production time- economical for large production

Disadvantages of cold working

• Greater force is required – metal is formed at room temperature and solid state

• Expensive tooling- Heavier and more powerful equipment and stronger tooling are required

• Complex process - Metal surfaces must be clean and scale-free

• Only suitable for large volume productions- Due to the large capital costs required to set up a cold working process

• Metal is less ductile - Intermediate anneals may be required to compensate for loss of ductility that accompanies strain hardening

Example of cold working

Products

• What type of process???

• Forging

Forging

Forging is a method of shaping metals and alloys into parts of useful shape.

Using compressive force applied through various die and tooling.

May be carried out at room temperature or elevated temperature

Forged parts had good strength and toughness as the metal flow in a die and material’s grain structure can be controlled.

Forged parts are suitable for highly stressed and critical applications

May be carried out at room temperature, warm or elevated temperature

Subjected to additional finishing operation such as machining and heat treatment

Characteristics of forged parts

Forged parts had good strength and toughness

as the metal flow in a die and material’s grain structure can be controlled.

Schematic illustration of a part made by three different processes showing grain flow. (a) Casting (b) Machining form a blank (c) Forging.

Each process has its own advantages and limitations regarding external and internal characteristics, material properties, dimensional accuracy, surface finish, and the economics of production.

Types of Forging

• Open die forging

• Close die forging

• Precision forging

• Upset die forging

• Coining

Open die forging

Also called Upsetting or flat die forging The simplest forging operation Very versatile- from small to very large sizes parts (275

metric tons) The process start by placing a solid work piece in

between two flat dies and reduced the height by compressing it

Open die forging

(a) Schematic illustration of a cogging operation on a rectangular bar. Blacksmiths use this process to reduce the thickness of bars by hammering the part on an anvil.

(b) Reducing the diameter of a bar by open-die forging

(c) The thickness of a ring being reduced by open-die forging.

Closed-die forging

Also called as Impression-die and drop forging The process start as a work piece is placed in between two

shaped dies. The work piece will takes the shape of die cavity while

being forged between two separate dies. Process usually done at elevated temperature to lower the

force and accomplish ductility

Standard terminology for various features a forging die

Preforming operation in closed die

forging

• Used to distribute the material properly into various region of blank using simple shape dies with various contour

• Fullering – material is distribute away from an area • Edging- material is gathered into localized area • Blocking – parts is formed into rough shape using blocker die • Finishing – give the forged parts the final shape • Trimming – removal of flash

Precision forging

Also known as flashless forging Use to reduced the number of finishing process

required and to diminish flash occurrences Typical products- gear, connecting rods, turbine

blades This type of forging produce net shape product but

require high capacity equipment

Precision forging

• Material is placed on top of lower punch and in between two horizontal flat dies.

• The upper die will compress the material to take the required shapes and the dies will prevent any flash from forming

Precision forging

• Requirement of precision forging : Special and complex die Precise control of blank volume and shape Accurate positioning of blank in die cavity

Advantages

Near net

shape parts

Save material

Simple

process

Reduce process

Reduce scrap

Save money

Increase

productivity

Stronger and

tougher

Versatile

Coining

• Use to make coin, jewelry and medallion

• The process starts as the blank or slug is coined (compress in high pressure) in a completely close die cavity.

• The pressure required is 5-6 times the strength of metal to produce fine details

• Marking- engraving the coining parts with letter and number

• Sizing- impart the desired dimensional accuracy with little or no change in part size

(a) Schematic illustration of the coining process. The earliest coins were made by open-die forging and lacked precision and sharp details. (b) An example of a modern coining operation, showing the workpiece and tooling. Note the detail and superior finish that can be achieve in this process.

Upset forging

• An upset forging operation to form a head on a bolt or similar hardware item The cycle consists of: (1) wire stock is fed to the stop, (2) gripping dies close on the stock and the stop is retracted, (3) punch moves forward, (4) bottoms to form the head.

Extrusion

• A process of pushing a material through a die for the purpose of reducing or changing it’s cross section area

• type of extrude material- plastic, metal, alloy.

• Typical product – railing for sliding doors, window frames, tubing with various cross section, aluminum ladder frame, structural and architectural shape parts, gear, bracket, coat hanger.

• Advantages – economical for large production, low tooling cost, can be done both cold and hot extrusion

• There are three type of extrusion – (i) direct extrusion (ii) indirect extrusion and (iii) hydrostatic extrusion

Direct extrusion

• Also call forward extrusion

• A billet is place in a chamber and force through a die opening by hydraulically driven stem

• The dummy block protect the tips of pressing stem.

• This process usually done in hot working

Indirect extrusion

• Also called reverse , inverted or backward extrusion

• The dies move forward to unextruded billet

• Had no billet-container friction, thus normally used for high friction material (e.g : high strength steel)

Hydrostatic extrusion

• The billet is smaller in diameter than the chamber filled with fluid

• The pressure is transmitted to the fluid by the ram

• The fluid pressure help improved formability and thus extruded the parts with less friction

Impact Extrusion

Impact extrusion process involve inserting a metal blank inside a vertical/ horizontal dies and the punch quickly impacted the metal blank .

The reasons are : Impact extrusion is perform at higher speed and

shorter strokes compared to conventional extrusion making it a much faster process

Very thin parts are possible to be produced on impact extruded parts as the punches impart the work part rather than simply applying pressure to it.

High speed characteristic of impacting, permit larger reduction and high production rate, making it an important commercial process.

Impact Extrusion

Drawing

• A process of reducing the cross section of a

long rod or wire by pulling it through a die (draw die)

• Typical parts- wire, rod , shaft for power transmission machine, blank for bolt and rivet, electrical wiring cables,welding electrodes

Tube drawing

Tube sinking- Process of reducing the diameter or wall thickness of seamless tube or pipe after the internal tube had been produced by some other process such as extrusion. No mandrel is use in this operation. The limitation is lack of control over inside diameter and wall thickness of tube.

Using fixed mandrel – Using a fixed long support bar to establish inside diameter and wall thickness of tube/pipe. The limitation of this process is it restrict the length of tube that can be drawn

Using floating plug- using a removable plug in which shape is design so that it finds a natural position in the reduction zone of die.

Comparison of extrusion and drawing

Extrusion Drawing

Raw Material Billet Rod, wire or tube

Process The process can be

done both hot work

and cold work

There is a container

(chamber) to place the

raw material

Process of forcing billet

through a die

Drawing can only done cold

The raw material passes

through the die only.

In drawing, rod, wire or tube is

pulled through the die or set of

dies in tandem

Product Solid and hollow products

can be produced

Section, channel, curtain

rail, architecture parts

solid cross-sectional is produced in

drawing

If tube – reduce diameter or

thickness using internal

mandrels

Wire, tube

Rolling process

• Process of reducing the thickness or changing the cross section of long workpiece by compressive force applied through a set of rolls.

• Suitable for almost 90% of metal in the form of slab, billet and bloom.

• Also suitable in rolling plastic, metal powder, ceramic slurry and also hot glass.

Hot rolling and Cold rolling

Hot rolling

• Carried out at elevated (high) temperature.

• The coarse grain, brittle and porous structure of ingot is transform into wrought structure having finer grain which resulted in higher strength and toughness

Cold rolling

• Carried out at room temperature

• Compressed grain be come deformed and elongated, resulted in higher strength and hardness

• Better surface finish as no scaling and oxidation occur

General application of rolling

Rolling plates

• Thickness > 6mm, used for structural applications such as structural support (300mm), boiler and nuclear vessel (150mm), bridges and machinery frames (100-125mm).

Rolling sheet

• Thickness < 6mm, used for coil, automobile and aircraft bodies, appliances, food and beverage container.

Flat rolling

• In flat rolling, a bulk material with thickness H1 is reduced to a thinner thickness of Ho through the force of the rolls mills.

Flat rolling

Shape rolling

• To produce straight and long structural shapes (chanel, I beam, solid bar)

• Formed at high temperature

• Stock of material is fed into a specially designed rolls.

• Might undergo different series of rolls to obtain desired shapes.

Roll forging

• Also known as cross rolling

• To produce tapered shaft, table knife and hand tools.

• The cross section of a round bar is shape by passing it through a pair of rolls with profiles groove.

Skew rolling and Upset forging

• Use to produce steel ball and ball bearing

• Round wire of rod is fed into the roll gap and roughly spherical blank are formed continuously by the action of rotating rolls.

• A shear pieces from round bar (blank) is upset in the headers between 2 dies with hemisphere cavities.

• The balls is then ground and polished in special machinery

Ring Rolling • Use to produce large rings for rocket turbine, jet engine cases, gear wheel

rims and flanges. • A thick ring is expanded into a large diameter thinner ring • The ring is place in between two rolls, one is idle (stationery) and one is

driven (moving). • The thickness is reduced by bringing the rolls closer together as they rotate. • Since the material volume is constant during the plastic deformation

process, the ring reduction resulted in increase in it’s diameter. • The advantages of ring rolling process- short production time, material

saving, close dimensional tolerance, enhance strength part

Thread rolling

Thread rolling is used to form external thread on cylindrical parts.

The parts is rolled in between two dies which is one fixed die and another one is moving die until threading surface are formed all around the rolled parts.

It is usually done in cold working condition and the machine is equipped with special dies that determine the size and form of the thread.

Rolling force

Higher rolls force is undesirable as it can cause deflection and flattening of the roll mill, thus damaging it and effect rolling operation. Roll force can be reduce by : •Reducing the friction on roll workpiece interface •Using smaller diameter roll to reduce contact area •Taking smaller diameter reduction per pass to reduce contact area •Rolling at elevated temperature to lower the strength of material

Part B: Sheet-Metalworking Processes

SHEET METALWORKING

1. Cutting Operations

2. Bending Operations

3. Drawing

4. Other Sheet Metal Forming Operations

5. Dies and Presses for Sheet Metal Processes

6. Sheet Metal Operations Not Performed on Presses

7. Bending of Tube Stock

Sheet Metalworking Defined

Cutting and forming operations performed on

relatively thin sheets of metal

Thickness of sheet metal = 0.4 mm (1/64 in) to 6

mm (1/4 in)

Thickness of plate stock > 6 mm

Operations usually performed as cold working

Sheet and Plate Metal Products

Sheet and plate metal parts for consumer and industrial products such as

Automobiles and trucks

Airplanes

Railway cars and locomotives

Farm and construction equipment

Small and large appliances

Office furniture

Computers and office equipment

Advantages of Sheet Metal Parts

High strength

Good dimensional accuracy

Good surface finish

Relatively low cost

Economical mass production for large quantities

Sheet Metalworking Terminology

Punch-and-die - tooling to perform cutting,

bending, and drawing

Stamping press - machine tool that performs

most sheet metal operations

Stampings - sheet metal products

Basic Types of

Sheet Metal Processes

1. Cutting

Shearing to separate large sheets

Blanking to cut part perimeters out of sheet metal

Punching to make holes in sheet metal

2. Bending

Straining sheet around a straight axis

3. Drawing

Forming of sheet into convex or concave shapes

(1) Just before punch contacts work; (2) punch pushes into

work, causing plastic deformation; (3) punch penetrates into

work causing a smooth cut surface; and (4) fracture is

initiated at opposing cutting edges to separate the sheet

Sheet Metal Cutting

Shearing, Blanking, and

Punching

Three principal operations in pressworking that cut

sheet metal:

Shearing

Blanking

Punching

Shearing Operation

(a) Side view of the operation; (b) front view of power

shears equipped with inclined upper cutting blade

Blanking and Punching

(a) Blanking - sheet metal cutting to separate piece (called a

blank) from surrounding stock, (b) punching - similar to

blanking except cut piece is scrap, called a slug

Clearance in Sheet Metal Cutting

Distance between punch cutting edge and die cutting

Typical values range between 4% and 8% of stock

thickness

If too small, fracture lines pass each other,

causing double burnishing and larger force

If too large, metal is pinched between cutting

edges and excessive burr results

Clearance in Sheet Metal Cutting

Recommended clearance is calculated by:

c = at

where c = clearance (mm); a = allowance (no unit);

and t = stock thickness, (mm)

Allowance a is determined according to type of metal

Example: Given thickness of 5052S Alloy is = 10 mm calculate the

clearance?.

Solution: c = at

c = (0.045) x (10)

c = 0.45 mm WRONG!

Sheet Metal Groups Allowances

Metal group a

1100S and 5052S aluminum alloys, all

tempers

2024ST and 6061ST aluminum alloys; brass,

soft cold rolled steel, soft stainless steel

Cold rolled steel, half hard; stainless steel,

half hard and full hard

Punch and Die Sizes

For a round blank of diameter Db:

Blanking punch diameter = Db - 2c

Blanking die diameter = Db

where c = clearance

For a round hole of diameter Dh:

Hole punch diameter = Dh

Hole die diameter = Dh + 2c

where c = clearance

Die size determines

blank size Db

Punch size

determines hole

size Dh

c = clearance

Punch and Die Sizes

Purpose: allows slug or blank to drop through die

Typical values: 0.25 to 1.5 on each side

Angular Clearance

Cutting Forces

Important for determining press size (tonnage)

F = S t L

where S = shear strength of metal (Mpa); t = stock

thickness (mm), and L = length of cut edge (mm)

Example: Determine the tonnage requirement for the blanking operation given

thickness = 4mm, Diameter = 150mm and that the stainless steel has a yield

strength = 275 MPa, shear strength = 450 MPa, and tensile strength = 650 MPa.

Solution: F = StL

t = 4 mm

L = 150mm x = 471 mm

F = 450(4.0)(471) = 847,800 N 86 Tonnes

(a) Straining of sheet metal around a straight axis to take a

permanent bend, (b) metal on inside of neutral plane is

compressed, metal on outside of neutral plane is stretched

Sheet Metal Bending

Types of Sheet Metal Bending

V-bending - performed with a V-shaped die

Edge bending - performed with a wiping die

(1) Before bending,

(2) after bending

Application notes:

Low production

Performed on a

press brake

V-dies are

simple and

inexpensive

V-Bending

(1) Before bending, (2) After bending

Application notes:

High production

Pressure pad required

Dies are more complicated and costly

Edge Bending

Stretching during Bending

If bend radius is small relative to stock thickness,

metal tends to stretch during bending

Important to estimate amount of stretching, so final

part length = specified dimension

Problem: to determine the length of neutral axis of

the part before bending

Springback

Increase in included angle of bent part relative to

included angle of forming tool after tool is removed

Reason for springback:

When bending pressure is removed, elastic

energy remains in bent part, causing it to

recover partially toward its original shape

Springback results in a decrease in bend angle and an

increase in bend radius: (1) during bending, work is forced to

take radius Rt and angle b' of the bending tool, (2) after

punch is removed, work springs back to R and ‘

Springback

Die opening dimension D for (a) V-die, (b) wiping die

Die Opening Dimension

Drawing

Sheet metal forming to make cup-shaped, box-shaped,

or other complex-curved, hollow-shaped parts

Sheet metal blank is positioned over die cavity and

then punch pushes metal into opening

Products: beverage cans, ammunition shells,

automobile body panels

Also known as deep drawing (to distinguish it from

wire and bar drawing)

(a) Drawing of

cup-shaped part: (1)

before punch contacts

work, (2) near end of

stroke

(b) Starting blank and

drawn part

Deep Drawing of Cup

Clearance in Drawing

Sides of punch and die separated by a clearance c

given by:

c = 1.1 t

where t = stock thickness

In other words, clearance is about 10% greater than

stock thickness

Shapes other than Cylindrical

Each of the following shapes presents its own unique

technical problems in drawing

Square or rectangular boxes (as in sinks)

Stepped cups

Cups with spherical rather than flat bases

Irregular curved forms (as in automobile body

panels)

Other Sheet Metal Forming on

Presses

Other sheet metal forming operations performed on

conventional presses can be classified as

Operations performed with metal tooling

Operations performed with flexible rubber tooling

Ironing

Achieves thinning and elongation of wall in a drawn cup:

(1) start of process; (2) during process

Creates indentations in sheet, such as raised (or

indented) lettering or strengthening ribs: (a) Punch and

die configuration during pressing; (b) finished part with

embossed ribs

Embossing

(1) Before

and (2) after

Guerin Process

Advantages of Guerin Process

Low tooling cost

Form block can be made of wood, plastic, or other

materials that are easy to shape

The same rubber pad can be used with different form

blocks

Process attractive in small quantity production

Dies for Sheet Metal Processes

Most pressworking operations are performed with

conventional punch-and-die tooling

Custom-designed for the particular part

The term stamping die is sometimes used for high

production dies

Components of a punch and die for a blanking operation

Punch and Die Components

Components of a

typical mechanical

drive stamping

Stamping Press

Types of Stamping Press Frame

Gap frame

Configuration of the letter C and often referred to

as a C-frame

Straight-sided frame

Box-like construction for higher tonnage

Gap Frame

Gap frame press for

sheet metalworking

(photo courtesy of BCN

Technology Services)

Capacity = 1350 kN

(150 tons)

Press Brake

Press brake

(photo courtesy

of Strippit, Inc.)

CNC Turret Press

Computer

numerical

control turret

press (photo

courtesy of

Strippit, Inc.)

Straight-sided frame

press for sheet

metalworking (photo

courtesy of BCN

Technology Services)

CNC Turret Press Parts

Collection of

sheet metal

parts produced

on a turret press,

showing variety

of hole shapes

possible (photo

courtesy of

Strippit Inc.)

Power and Drive Systems

Hydraulic presses - use a large piston and cylinder to drive the ram

Longer ram stroke than mechanical types

Suited to deep drawing

Slower than mechanical drives

Mechanical presses – convert rotation of motor to linear motion of ram

High forces at bottom of stroke

Suited to blanking and punching

Operations Not Performed on

Presses

Stretch forming

Roll bending and forming

Spinning

High-energy-rate forming processes

Sheet metal is stretched and simultaneously bent

to achieve shape change

Stretch Forming

Large metal sheets

and plates are

formed into curved

sections using rolls

Roll Bending

Continuous bending process in which opposing rolls

produce long sections of formed shapes from coil or

strip stock

Roll Forming

Spinning

Metal forming process in which an axially symmetric

part is gradually shaped over a rotating mandrel

using a rounded tool or roller

Three types:

1. Conventional spinning

2. Shear spinning

3. Tube spinning

(1) Setup at start of process; (2) during spinning; and

(3) completion of process

Conventional Spinning

High-Energy-Rate Forming

(HERF)

Processes to form metals using large amounts of

energy over a very short time

HERF processes include:

Explosive forming

Electrohydraulic forming

Electromagnetic forming

Explosive Forming

Use of explosive charge to form sheet (or plate)

metal into a die cavity

Explosive charge causes a shock wave whose

energy is transmitted to force part into cavity

Applications: large parts, typical of aerospace

industry

(1) Setup, (2) explosive is detonated, and (3) shock

wave forms part and plume escapes water surface

Explosive Forming

Electromagnetic Forming

Sheet metal is deformed by mechanical force of an

electromagnetic field induced in the workpart by an

energized coil

Presently the most widely used HERF process

Applications: tubular parts

(1) Setup in which coil is inserted into tubular workpart

surrounded by die; (2) formed part

Electromagnetic Forming

Bending of Tube Stock

Dimensions and terms for a bent tube: D = outside

diameter, R = bend radius, t = wall thickness

Stretch bending of tube: (1) start of process and (2)

during bending

Draw bending of tube: (1) start of process and (2)

during bending

TEST Information

TEST 1 • 21/10/2015 (Exam Hall Level 8) • 9.00 PM– 10 PM • Chapter 1,2 and 3

TEST 2 • 02/12/2015 (Exam Hall Level 8) • 9.00 PM– 10 PM • Chapter 4,5 and 6

MEC560_Chapter 3_Metal Forming and Shaping Processes

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