Chap4 Advanced Machining

7/23/2019 Chap4 Advanced Machining

1/33

Chapter 4:

Advanced machining

Ref: Manufacturing Engineering &

Technology,S. Kalpakjian, Pearson


2/33

Chapter Outline

1. Introduction

2. Electrochemical Machining (ECM)

3. Electrical discharge Machining (EDM)

4. Electron-beam Machining (EBM)

5. Ultrasonic Machining (USM)


3/33

Machining processes that involve chip

formation have a number of limitations:

Large amounts of energy

Unwanted distortion

Residual stresses

Burrs

Delicate or complex geometries may be difficult orimpossible

4.1 Introduction


4/33

4.1 Introduction

Non-traditional machining (NTM) processes have

several advantages

Complex geometries are possible

Extreme surface finish Tight tolerances

Delicate components

Little or no burring or residual stresses

Brittle materials with high hardness can be machined Microelectronic or integrated circuits are possible to mass

produce


5/33

NTM Processes

Four basic groups of material removal using

NTM processes

Mechanical

Chemical

Electrochemical

Thermal


6/33

Conventional End Milling vs. NTM

Typical machining parameters

Feed rate

Surface finish

Dimensional accuracy

Workpiece/feature size

NTM processes typically have lower feed

rates and require more power consumption The feed rate in NTM is independent of the

material being processed


7/33

4.1 Electrochemical Machining (ECM)

ECM removes material by

an electrochemical

process

The tool is the cathode

and the workpiece is the

anode

Shape of workpiece is the

mirror image of tool

Figure Schematic diagram of

electrochemical machining process

(ECM).


8/33

cathode (tool): electrolyte & water molecules ionize:

H2OH++ OH-

NAClNa++ Cl-

+ve ions move towards the tool & negative ions movetowards the workpiece:

2H++ 2eH2

Anode (workpiece): metal molecules ionize break freefrom the workpiece & travel to the tool

FeFe 2++ 2e

FeCl2and Fe(OH)2precipated in the form of sludge

Electrochemical Machining

Process


9/33

4.2 Electrochemical Machining

Process

p - density


10/33


Application Used to machine complex cavities and

shapes in high-strength materials

ECM process leaves a burr-free (deburring

process)

Excellent surface finish, thermal free &

stress free


11/33


Application


12/33


Burr free edges & smooth finishes

in medical devices

Application


13/33

Advantages and Disadvantages of


Advantages

ECM is well suited for the

machining of complex

two-dimensional shapes

Poorly machinable

materials may be

processed

Little or no tool wear

No burr & no distortion ofthe holes

Disadvantages

Initial tooling can be

timely and costly

Environmentally harmful

by-products


14/33



15/33

4.3 Electrical Discharge Machining (EDM)

Electrical energy is used to generate electrical spark

Materials removal occurred due to the thermal energy ofspark

Two different types of EDM exist based on the shape ofthe tool electrode

Ram EDM/sinker EDM Wire EDM


16/33

EDM Processes

Examples of EDM workpieces Graphite electrode & product


17/33

Electrical-discharge Machining

Process

Figure 4.2EDM or spark erosion machining of metal, using pulse

DC. The table can make X-Y movements.


18/33


Working Principle


19/33

EDM system consists of a electrodeand the workpiece,

connected to a power supply (pulse DC) and placed in

a dielectric fluid.

The gap is filled by a die-electric fluid, which ionized

after collision with accelerated electron from tool at

inter electrode gap.

The ionization of die electric fluid creates a conduction

path (plasma channel) and produced spark.

The spark produces tiny crater in the workpiece by

melting & vaporization

High temperature also melt & vaporize the tool (wear)


Working Principle


20/33

Fig. Formation of plasma channel

& spark at inter electrode gapFig. Plasma channel collapse &

die electric fluid flushes the chips


Principle of Operation


21/33

Electrical-discharge MachiningPrinciple of Operation

Electrical energy is dissipated as the thermal energy of

the spark.

Heat flux leads to extreme rise in temp. (>10,000deg C)

Material removal occurs due to instant vaporization ofthe material & melting

The material-removal rate can be estimated from

23.14104

w

ITMRR I = current in amperes

Tw= melting temp of work piece


22/33


Dielectric Fluids

The functions of the dielectric fluid are to:

1. Act as an insulator until the potential is sufficiently high

2. Provide a cooling medium3. Act as a flushing medium and carry away the debris in

the gap

i.e. kerosene, distilled and deionized water


23/33


Electrodes

Electrodes are made of graphite, brass, copper or

coppertungsten alloys

Can be shaped by forming, casting, powder metallurgy,

or CNC machining techniques

Tool wear is related to the melting points of the

materials involved

Lower the melting point of the electrode, the higher is

the wear rate


24/33

Electrical-discharge Machining:

Wire EDM Similar to contour cutting with a band saw

A slowly moving wire travels along a prescribed path

will cut the workpiece

Wire is made of brass, copper, tungsten, molybdenum,

zinc- or brass-coated or multicoated


25/33

Advantages and Disadvantages of EDM

Advantages

Applicable to allmaterials that are fairlygood electrical

conductors Hardness, toughness,

or brittleness of thematerial imposes no

limitations Fragile and delicate

parts

Disadvantages

Produces a hard recast(resolidify) surface

Surface may containfine cracks caused bythermal stress

Fumes can be toxic


26/33



27/33

4.4 Electron-beam Machining The energy source is high-velocity electrons, which strike the

workpiece surface and generate heat

Used for very accurate cutting of a wide variety of metals

Surface finish is better and kerf width is narrower than in other

thermal cutting processes

Requires vacuum condition


28/33

4.5 Laser-beam Machining

The source of energy is a laser which focuses optical

energy on the surface of the workpiece

The highly focused, high-density energy source melts

and evaporates portions of the workpiece in a

controlled manner


29/33

Laser-beam Machining

Process Capabilities

It is used for drilling and cutting metals, nonmetallic

materials, ceramics, and composite materials

Laser-beam machining is being used increasingly in

the electronics and automotive industries

Also used for welding, small-scale and localized heat

treating of metals and ceramics, and marking of parts


30/33

4.6 Ultrasonic Machining (USM) In ultrasonic machining (USM), also called ultrasonic grinding, high-frequency

vibrations delivered to a tool tip, embedded in an abrasive slurry, by a booster,create accurate cavities of virtually any shape; that are, negatives of the tool.

Since this method is non-thermal, non-electrical, and non-chemical, it

produces virtually stress-free shapes even in hard and brittle work-pieces.

Ultrasonic drilling is most effective for hard and brittle materials; soft materials

absorb too much sound energy and make the process less efficient.


31/33

Ultrasonic Machining (USM) The tool, typically vibrating at a low

amplitude of 0.025 mm at a frequency

of 20 to 100 kHz, is gradually fed intothe work-piece to form a cavitycorresponding to the tool shape.

The vibration transmits a high velocityforce to fine abrasive grains betweenthe tool and the surface of the work-

piece. In the process material isremoved by micro-chipping or erosionwith the abrasive particles.

The grains are in a water slurry whichalso serves to remove debris from thecutting area. The high-frequency

power supply for the magneto-strictiveor piezoelectric transducer stack thatdrives the tool is typically ratedbetween 0.1 and 40 kW.

Channels and holes ultrasonically machined

in a polycrystalline silicon wafer.


32/33

Ultrasonic Machining (USM)

The abrasive particles (SiC, Al2O3) are

suspended in water or oil. The particle size determines the roughness or

surface finish and the speed of the cut.

Material removal rates are quite low, usuallyless than 50 mm3/min.

Coin with grooving carried out with USM

graphite electrodes with intricate shape machined by USM


33/33

Ultrasonic Machining (USM)

Advantages and disadvantages of ultrasonic

machining.

Advantages Disadvantages

Machining of any material regardless of conductivity Low material removal rate

Precision machining of brittle hard materials Tool wears fast

Does not produce electric, thermal or chemical defe cts at

the surfa ce

Machining area and depth are quite restricted

Can drill circular or non-circular holes in very hard

materials

Less stress because of its non-thermal nature

Chap4 Advanced Machining

Documents

Transcript of Chap4 Advanced Machining