Nontraditional Machining Techniques Unit - 2

7/29/2019 Nontraditional Machining Techniques Unit - 2

1/39

Click to edit Master subtitle style

3/5/13

ME0028 NON TRADITIONALMACHINING TECHNIQUES

UNIT 2 MECHANICAL ENERGY TECHNIQUES


2/39

3/5/13

Unit 2 - MECHANICAL ENERGYTECHNIQUES

Abrasive Jet Machining (AJM): Operating principles Equipment Parameters influencing metal removal Benefits Applications Advantages and Limitations.

Water Jet Machining (WJM): Operating principles Equipment Parameters influencing metal removal Benefits Applications Advantages and Limitations.

Ultra Sonic Machining (USM): Operating principles Equipment and sub systems Parameters influencing metalremoval Benefits and Applications Advantages andLimitations


3/39

3/5/13

ABRASIVE JET MACHINING (AJM)


4/39

3/5/13

In Abrasive Jet Machining (AJM), abrasive particles are made toimpinge on the work material at a high velocity.

The jet of abrasive particles is carried by carrier gas or air.

The high velocity stream of abrasive is generated by convertingthe pressure energy of the carrier gas or air to its kineticenergy and hence high velocity jet.

The nozzle directs the abrasive jet in a controlled manner ontothe work material, so that the distance between the nozzle andthe work piece and the impingement angle can be setdesirably.

The high velocity abrasive particles remove the material bybrittle fracture of the work material.

In AJM, generally, the abrasive particles of around 50 m grit

size would impinge on the work material at velocity of 200 m/sfrom a nozzle of I.D. of 0.5 mm with a stand off distance of


5/39

3/5/13

Equipment:

compressed air at a pressure of around 5 bar is used as thecarrier gas.

Gases like CO2, N2 can also be used as carrier gas which maydirectly be issued from a gas cylinder.

Generally oxygen is not used as a carrier gas.

The carrier gas is first passed through a pressure regulator toobtain the desired working pressure.

The gas is then passed through an air dryer to remove anyresidual water vapour.

To remove any oil vapour or particulate contaminant the sameis passed through a series of filters.


6/39

3/5/13


7/393/5/13

The abrasive particles enter the chamber from a hopperthrough a metallic sieve.

The sieve is constantly vibrated by an electromagnetic shaker.

The mass flow rate of abrasive (15 gm/min) entering thechamber depends on the amplitude of vibration of the sieveand its frequency.

The abrasive particles are then carried by the carrier gas to themachining chamber via an electro-magnetic on-off valve.

The machining enclosure is essential to contain the abrasive

and machined particles in a safe and eco-friendly manner.

The machining is carried out as high velocity (200 m/s)abrasive particles are issued from the nozzle onto a work piece.


8/393/5/13

Process Parameters:Abrasive

Material Al2O3 / SiC / glass beads Shape irregular / spherical Size 10 ~ 50 m

Mass flow rate 2 ~ 20 gm/min

Carrier gas

Composition Air, CO2, N2 Density Air ~ 1.3 kg/m3 Velocity 500 ~ 700 m/s

Pressure 2 ~ 10 bar Flow rate 5 ~ 30 lpm

Abrasive Jet

Velocity 100 ~ 300 m/s

Mixing ratio mass flow ratio of abrasive to gas - ~


9/393/5/13

The important machining characteristics in AJM are The material removal rate (MRR) mm3/min or gm/min The machining accuracy

The life of the nozzle


10/39

3/5/13


11/39

3/5/13

MODELLING OF MATERIAL REMOVAL:

Material removal in AJM takes place due to brittle fracture ofthe work material due to impact of high velocity abrasiveparticles.Modelling has been done with the following assumptions:

(i) Abrasives are spherical in shape and rigid. The particles are

characterised by the mean grit diameter.

(ii) The kinetic energy of the abrasives are fully utilised inremoving material.

(iii) Brittle materials are considered to fail due to brittle fractureand the fracture volume is considered to be hemispherical withdiameter equal to chordal length of the indentation.

(iv) For ductile material, removal volume is assumed to be equal

to the indentation volume due to particulate impact.


12/39

3/5/13


13/39

3/5/13


14/39

3/5/13


15/39

3/5/13

Applications

For drilling holes of intricate shapes in hard and brittle materials

For machining fragile, brittle and heat sensitive materials AJM can be used for drilling, cutting, deburring, cleaning andetching. Micro-machining of brittle materials

Limitations

MRR is rather low (around ~ 15 mm3/min for machining glass) Abrasive particles tend to get embedded particularly if the workmaterial is ductile

Tapering occurs due to flaring of the jet Environmental load is rather high.


16/39

3/5/13

ULTRA SONIC MACHINING (USM)


17/39

3/5/13

USM is grouped under the mechanical NTM processes.

A tool of desired shape vibrates at an ultrasonic frequency (19~ 25 kHz) with an amplitude of around 15 50 m over thework piece.

The tool is pressed downward with a feed force, F.

Between the tool and work piece, the machining zone isflooded with hard abrasive (a water based slurry).

As the tool vibrates over the work piece, the abrasive particlesindent both the work material and the tool.

if the work material is brittle, material removal is due to crackinitiation, propagation and brittle fracture of the material.

USM is mainly used for machining brittle materials {which are

poor conductors of electricity and thus cannot be processed byECM and EDM}.


18/39

3/5/13

MACHINE:

work piece is mounted on a vice, which can be located at thedesired position under the tool using a 2 axis table.

The table can further be lowered or raised.

The typical elements of an USM are

1. Slurry delivery and return system.2. Feed mechanism to provide a downward feed force on the

tool during machining.3. The transducer, which generates the ultrasonic vibration.4. The horn or concentrator, which mechanically amplifies the

vibration to the required amplitude of 15 50 m andaccommodates the tool at its tip.


19/39

3/5/13


20/39

3/5/13

Transducer:

The transducer is driven by suitable signal generator followed

by power amplifier.

The transducer for USM works on the following principle1. Piezoelectric effect2. Magnetostrictive effect (most popular, shown below)

3. Electrostrictive effect

.The horn or concentrator is a wave-guide, which amplifies andconcentrates the vibration to the tool from the transducer.


21/39

3/5/13

Mechanisms of Material Removal in USM

USM is generally used for machining brittle work material.

As the tool vibrates, material removal primarily occurs due to theindentation of the hard abrasive grits on the brittle workmaterial.

During indentation, due to Hertzian contact stresses, crackswould develop just below the contact site.

As indentation progresses, the cracks would propagate due toincrease in stress and ultimately lead to brittle fracture of the

work material.

The tool material should be such that indentation by the abrasivegrits does not lead to brittle failure.

Thus the tools are made of tough, strong and ductile materialslike steel, stainless steel and other ductile metallic alloys.


22/39

3/5/13

Other than this brittle failure due to indentation, some materialremoval may occur due to free flowing impact of the abrasivesagainst the work material, but it is estimated to be ratherinsignificant.

Thus, in the current model, material removal would beassumed to take place only due to impact of abrasives betweentool and workpiece.

In the current model, all the abrasives are considered to beidentical in shape and size.

An abrasive particle is considered to be spherical but with localspherical bulges as shown in Fig.

The abrasive particles are characterised by the average gritdiameter, dg.

that the local spherical bulges have a uniform diameter, db andwhich is related to the grit diameter by db = dg2


23/39

3/5/13


24/39

3/5/13

dg - Diameter of grit.f - Frequency of the tool vibration.C - Concentration of abrasive grit.A - total surface area of the tool facing

the workpiece.

MRR


25/39

3/5/13


26/39

3/5/13

Assumption of cooks model:

This model is based on the brittle fracture of the work materials and underfollowing assumptions.

The abrasive grits are spherical in nature.

Material removal is based on hemispherical fracture mechanism due tothe indentation.

Tool and abrasive are rigid.


27/39

3/5/13

PROCESS PARAMETERS AND THEIR EFFECTS:

Amplitude of vibration (ao) 15 50 m

Frequency of vibration (f) 19 25 kHz Feed force (F) related to tool dimensions Feed pressure (p) Abrasive size 15 m 150 m Abrasive material Al2O3

- SiC- B4C- Boronsilicarbide- Diamond

Flow strength of work material

Flow strength of the tool material Contact area of the tool A Volume concentration of abrasive in water slurry C


28/39

3/5/13

Applications

Used for machining hard and brittle metallic alloys,semiconductors, glass, ceramics, carbides etc.

Used for machining round, square, irregular shaped holes andsurface impressions.

Machining, wire drawing, punching or small blanking dies.

Limitations

Low MRR Rather high tool wear Low depth of hole


29/39

3/5/13

WATER JET MACHINING (WJM):


30/39

3/5/13

It utilizes a high velocity stream of water as cutting agent.

The mechanism of metal removal is erosion.

Water is pumped at a sufficiently high pressure, 100-1000MPa using intensifier technology.

An intensifier works on the simple principle of pressure

amplification using hydraulic cylinders of different cross-sections.

From the intensifier /pump the water goes to an accumulator.

The accumulator helps in eliminating pulsation and also actsas an energy reservoir.

From the accumulator the water is lead to the nozzle through ahigh pressure thick stainless steel tubes.


31/39

3/5/13

water at such pressure is issued through a suitablenozzle/orifice (generally of0.075- 0.4 mm dia).

potential energy of water is converted into kinetic energy,yielding a high velocity jet (300-1000 m/min) and inducinghigh stress in the work material.

When the induced stress exceeds the ultimate shear stress ofthe material, rupture takes place.

Such high velocity water jet can machine thin sheets/foils ofaluminium, leather, textile, frozen food, plastics, wood,ceramics etc.


32/39

3/5/13

HYDRAULIC SETUP FOR WJM:


33/39

3/5/13

Intensifier is driven by a hydraulic power pack.

The heart of the hydraulic power pack is a positivedisplacement hydraulic pump.

The power packs in modern commercial systems are oftencontrolled by microcomputers to achieve programmed rise ofpressure etc.

The hydraulic power pack delivers the hydraulic fluid to theintensifier at a pressure of ph.

The ratio of cross-section of the two cylinders in the intensifieris say

Thus, pressure amplification would take place at the smallcylinder as follows.


34/39

3/5/13

if the hydraulic pressure is set as 100 bar and area ratio is 40,pw = 100 x 40 = 4000 bar.

By using direction control valve, the intensifier is driven by thehydraulic unit.

The water may be directly supplied to the small cylinder of theintensifier or it may be supplied through a booster pump, whichtypically raises the water pressure to 11 bar before supplying itto the intensifier.

Sometimes water is softened or long chain polymers are addedin additive unit.

as the intensifier works, it delivers high pressure water. As thelarger piston changes direction within the intensifier, therewould be a drop in the delivery pressure.

To counter such drops, a thick cylinder is added to the deliveryunit to accommodate water at high pressure. This is called an


35/39

3/5/13

High-pressure water is then fed through the flexible stainlesssteel pipes to the cutting head, which carry water around 4000bar (400 MPa).

Water jets do not create hazardous materials or vapours. It istruly a versatile, productive, cold cutting process.

The most important benefit of the water jet cutter is its abilityto cut material without interfering with the materials inherent

structure as there is no "heat affected zone" (HAZ).

This allows metals to be cut without harming their intrinsicproperties.

WJM can be achieved using two approaches as enumeratedbelow:

1. WJM - Pure2. WJM - with stabilizer

. In pure WJM, pure water (tap water) is used for machining


36/39

3/5/13

Effect of Stand of distance (SOD) with MRR:

MRR increases with the increase of SOD up to a certainlimit after which it remains unchanged for a certain tip distancesand then falls gradually.

A large SOD produces divergence of jet which affects theaccuracy and quality.

1. Small MRR at low SOD is due to reduction in nozzlepressure with decreasing distance.

1. Drop in MRR at large SOD is due to a reduction in thejet velocity with increasing distance.


37/39

3/5/13

OPERATING PARAMETERS:

Fluid:

Type : Water or Water with additives Additives : Glycerin, Polyethylene Oxide or long chain

polymers. Pressure : 1 to 10 kbar (100 to 1000 MPa)Jet Velocity : 300 to 1000 m/min.

Flow rate : upto 8 Lit/min.Jet force on work piece : 0.5 to 15 kg (5 to 15 N).

Nozzle:

Material : Hardened Steel, WC, Synthetic Sapphire. Diameter : 0.075 to 0.4 mm. SOD : 2.5 to 50 mm. Angle: normal to 30 positive rake.


38/39

3/5/13

Advantage:

1) Energy transfer media (water) is cheap, non toxic and easyto dispose.

2) Work area remains clean and dust free.3) Process is environmentally safe and friendly manufacturing.4) Low maintenance and operating cost is low as it has no

moving parts.

5) Intricate contour can be cut.6) There is no thermal damage.7) Cut can be started anywhere without the need for predrilled

holes8) Minimum burr produced.

9) Extremely fast set-up and programming10) Very little fixturing for most parts11) Machine virtually any 2D shape on any material12) Very low side forces during the machining

Disadvantage:


39/39

Thank you

Documents

Nontraditional Machining Techniques Unit - 2