MECHANICAL ENERGY BASED PROCESSES UNIT II

Syllabus

Abrasive Jet Machining

Water Jet Machining

Abrasive Water Jet Machining &

Ultrasonic Machining

Working Principles

Equipment used

Process parameters

MRR

Variation in techniques used

Applications

Abrasive Jet Machining Working Principles – equipment used – Process parameters – MRR-Variation in techniques used – Applications

Working Principle

A stream of abrasive grains (Al2O3 or SiC) is carried by high pressure gas or air (compressed).

Impinges on the work surface at very high velocity through a nozzle of 0.3 to 0.5 mm diameter

Sand Blasting (SB) - a similar process The major differences between are SB and AJM

smaller diameter abrasives a more finely controlled delivery system

Material removal – by mechanical abrasion action of the high velocity abrasive particles

Working Principle

Equipment Used

Abrasive delivery system

Control system

Pump

Nozzle

Motion system

Working

A gas (Nitrogen, CO2 or air) is supplied at 2 – 8 kg/cm2

Oxygen should never be used. (because, it causes violent chemical action with the work piece chips or abrasive particles).

Gas passes through a mixing chamber after filtration and regulation.

In the mixing chamber, abrasive particles (10 – 40 m) are present and vibrated at 50 Hz.

Amplitude of vibration – to control the feed rate of abrasives.

(Gas + abrasives) - passed through a 0.45 mm diameter tungsten carbide nozzle at a speed of 150 – 300 m/s.

The nozzle is directed over the area to be machined

Working

Aluminum oxide (Al2O3) and silicon carbide (SiC) powders are used for heavy

cleaning, cutting and deburring.

Magnesium carbonate is recommended for use in light cleaning and etching.

Sodium bicarbonate – fine cleaning and cutting of soft materials.

Commercial grade powders are not suitable – b’cos their sizes are not well

classified. Also, they may contain silica which can cause a health hazard.

Abrasive powders are not reused. B’cos, contaminations and worn grits will

reduce the machining rate (MRR).

The nozzle stand off distance is 0.81 mm

Process parameters VS Metal Removal Rate

Mass flow rate

Abrasive grain size

Gas pressure

Velocity of abrasive particles

Mixing Ratio

Nozzle tip clearance

Process Characteristics

Application

Drilling holes Cutting slots Cleaning hard surfaces Deburring and polishing Machining intricate shapes or holes in sensitive, brittle, thin, or difficult-

to-machine materials Frosting glass and trimming of circuit boards, hybrid circuit resistors,

capacitors, silicon, and gallium

Advantage

The process is used for machining super alloys and refractory materials. It is not reactive with any work piece material. No tool changes are required. Intricate parts of sharp corners can be machined No initial hole is required for starting the operation as required by wire

EDM. Material utilization is high. It can machine thin materials

Disadvantage

The removal rate is slow. The tapering effect may occur especially when drilling in metals. The abrasive may get impeded in the work surface. Suitable dust-collecting systems should be provided. Soft materials can’t be machined by the process. Silica dust may be a health hazard

Water Jet Machining Working Principles – equipment used – Process parameters – MRR-Variation in techniques used – Applications

Working Principle

Water is pumped at a sufficiently high pressure, 200-400 MPa (2000-4000 bar) using

intensifier technology.

An intensifier works on the simple principle of pressure amplification using hydraulic

cylinders of different cross-sections as used in “Jute Bell Presses”.

When water at such pressure is issued through a suitable orifice (generally of 0.2- 0.4

mm dia), the potential energy of water is converted into kinetic energy, yielding a high

velocity jet (1000 m/s).

Working

Water jet Equipments

It is consists of three main units

(i) A pump along with intensifier.

(ii)Cutting head comprising of nozzle and work table movement.

(iii) filter unit for debris & impurities

Process parameters

Material Removal Rate

Geometry and surface Finish of work

Wear rate of the nozzle

Advantage

There are no bits or tools touching the material surface, thus there is no tool replacement costs.

Ultrahigh-pressure Water-jets cut to accuracy's of +/-0.010".

Low level of mechanical stress (less than a pound) placed on the work piece preventing damage and deformations.

Application

Used for cutting Composites Plastics Fabrics Rubber wood products etc.

Also used in food processing industry

Abrasive Water Jet MachiningWorking Principles – equipment used – Process parameters – MRR-Variation in techniques used – Applications

WJM - suitable for cutting plastics, foods, rubber insulation, automotive carpeting and headliners, and

most textiles.

Harder materials such as glass, ceramics, concrete, and tough composites can be cut by adding

abrasives to the water jet.

Abrasive water jet machining (AWJM) – Developed in 1974 to clean metal prior to surface treatment

of the metal.

The addition of abrasives to the water jet enhanced MRR and produced cutting speeds between 51

and 460 mm/min.

Generally, AWJM cuts 10 times faster than the conventional machining methods of composite

materials.

Zheng et al. (2002) claimed that the abrasive water jet is hundreds of times more powerful than the

pure water jet.

Introduction

Different approaches and methodologies in WJM & AWJM

WJM – pure

WJM – with stabilizer

AWJM – entrained – three phase (water + air + abrasives)

AWJM – suspended – two phase (water + abrasives)

Direct pumping

Indirect pumping

Bypass pumping

In all the above variants, the basic methodology remains the same.

WJM & AWJM

Water is pumped at a sufficiently high pressure, 200-400 MPa (2000 – 4000 bar).

“Intensifier” works on the principle of pressure amplification using hydraulic cylinders of two different

cross-sections.

When water at such a pressure is passed through a suitable orifice (nozzle having = 0.2 – 0.4 mm),

the potential energy of water is converted into kinetic energy.

This yields high velocity (~ 1000 m/s) jet of water.

Such a high velocity water jet can machine thin sheets/foils of aluminium, leather, textile, frozen

foods, etc.

WJM – commercially pure water (tap water) is used for machining.

Basic Methodology

Problem in WJM – as the high velocity water jet is discharged from the orifice, the jet tends to entrain

atmospheric air and flares out – decreasing the machining ability.

Hence, stabilizers (long chain polymers) are added to water (WJM with stabilizer).

Stabilizers hinders the fragmentation of water jet.

In AWJM, abrasive particles are added to the water jet to enhance its cutting ability by many folds.

In entrained type AWJM, the abrasive particles are allowed to entrain in water jet to form abrasive

water jet wit sufficient velocity of as high as 800 m/s.

Such high velocity abrasive jet can machine almost any material.

Variants in WJM & AWJM

Commercial CNC WJM

Paint removal

Cleaning

Cutting soft materials

Cutting frozen meat

Textile, Leather industry

Mass Immunization

Surgery

Peening

Pocket Milling

Drilling & Turning

Nuclear Plant Dismantling

WJM & AWJM - Applications

Steels & Non-ferrous alloys

Ti alloys, Ni- alloys

Polymers

Honeycombs

Metal Matrix Composite & Ceramic Matrix Composite

Concrete

Stone – Granite

Wood

Reinforced plastics

Metal Polymer Laminates

Glass Fibre Metal Laminates

WJM & AWJM - Materials

The cutting ability of WJM can be improved drastically by adding hard and sharp abrasive particles

into the water jet.

Thus, WJM is typically used to cut so called “softer” and “easy-to-machine” materials like thin sheets

and foils, non-ferrous metallic alloys, wood, textiles, honeycomb, polymers, frozen meat, leather etc.

But, the domain of “harder” and “difficult-to-machine” materials like thick plates of steels, aluminium

and other commercial materials, metal matrix and ceramic matrix composites, reinforced plastics,

layered composites, etc. are reserved for AWJM.

Other than cutting (machining) high pressure water jet also finds application in paint removal,

cleaning, surgery, peening to remove residual stress etc.

AWJM can as well be used besides cutting for pocket milling, turning, drilling, etc.

One of the strategic areas where “robotic AWJM” is finding critical application is dismantling of nuclear

plants.

Applications of WJM & AWJM

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Extremely fast set-up and programming

Very little fixturing for most parts

Machine virtually any 2D shape on any material

Very low side forces during the machining

Almost no heat generated on the part

Can machine thick plates

WJM & AWJM - Advantages

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AWJM accelerates a jet of water (70 percent) and abrasive (30 percent) from 4.2 bar up to a velocity

of 30 m/s.

Silicon carbides, sand (SiO2), corundum, and glass beads of grain size 10 to 150 μm are often used

as abrasive materials.

Using such a method, burrs of 0.35 mm height and 0.02 mm width left in steel component after

grinding are removed by the erosive effect of the abrasives while water acts as an abrasive carrier.

The introduction of compressed air to the water jet enhances the deburring action.

AWJM - Elements

Some Machining Components

Ultrasonic MachiningWorking Principles – equipment used – Process parameters – MRR-Variation in techniques used – Applications

In UM the tip of the tool vibrates at low amplitude and at high frequency. This

vibration transmits a high velocity to fine abrasive grains between tool and the

surface of the work piece.

Material removed by erosion with abrasive particles.

The abrasive grains are usually boron carbides.

This technique is used to cut hard and brittle materials like ceramics, carbides,

glass, precious stones and hardened steel

ULTRASONIC MACHINING (UM)

Metal cutting as in this process abrasives contained in a slurry are driven at high

velocity against the work piece by a tool vibrating at low amplitude and high frequency

Amplitude is kept of the order of 0.07 mm and frequency is maintained at

approximately 20,000 Hz.

The work piece material is removed in the form of extremely small chips

Abrasive slurry acts as a multipoint cutting tool and does the similar action as done by a

cutting edge

Process details

Abrasive Slurry

Work piece

Ultrasonic Oscillator or Transducer

Feed Mechanism

Process Parameters

MRR

Tool Material

Tool Wear Rate

Abrasive Material and Abrasive Slurry

Surface Finish

Work Material

Metal Removal Rate

Grain Size of Abrasive

Abrasive Material

Concentration of slurry

Amplitude of Vibration

Frequency of Ultrasonic Waves

Ultrasonic Oscillator or Transducer

Magnetostriction Transducer

Piezoelectric Transducer

Feed Mechanism

Gravity feed Mechanism

Spring loaded Feed Mechanism

Pneumatic or Hydraulic Feed Mechanism

Application

The machining of hard and brittle materials like carbides glass, ceramics,

precious stones, titanium

It is used for tool making and punch and die making

It is widely used for several machining operations like turning, grinding,

trepanning and milling

Advantage

Its main advantage is the work piece after machining is free from any residual stress as to concentrated force or heat is subject to it during the machining process.

Extremely hard and brittle materials can be machined, their machining is very difficult by conventional methods.

Very good dimensional accuracy and surface finish can be obtained.

Operational cost is low.

The process is environmental friendly as it is noiseless and no chemical and heating is used

Limitation

Its metal removal rate (MRR) is very low and it can not be used for large machining cavities.

Its initial setup cost and cost of tool is very high, frequency tool replacement is required as tool wear takes place in this operation.

Not recommended for soft and ductile material due to their ductility.

Power consumption is quite high.

Slurry may have to be replaced frequently

Compiled byD.Vasanth kumar,Assistant Professor, Jansons Institute of technology