Ajm unit 2

PRINCIPLE:-PRINCIPLE:- In Abrasive Jet Machining (AJM),

abrasive particles are made to impinge 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 converting the pressure energy of the carrier gas or air to its kinetic energy and hence high velocity jet.

The nozzle directs the abrasive jet in a controlled manner onto the work material, so that the distance between the nozzle and the work piece and the impingement angle can be set desirably. The high velocity abrasive particles remove the material by micro-cutting action as well as brittle fracture of the work material. Shown in fig 1.1.

FIG:-1.1 REPRESENTED AJM

Process:-Process:- In AJM, air is compressed in an air compressor and

compressed air at a pressure of around 5 bar is used as the carrier gas. Gases like CO2, N2 can also use.

The carrier gas is first passed through a pressure regulator to obtain the desired working pressure. The gas is then passed through an air dryer to remove any residual water vapour. To remove any oil vapour or particulate contaminant the same is passed through a series of filters. Then the carrier gas enters a closed chamber known as the mixing chamber. The abrasive particles enter the chamber from a hopper through a metallic sieve. The sieve is constantly vibrated by an electromagnetic shaker.

Abrasives:Al2O3

Silicon carbide (cutting harder materials)

Glass powder (light polishing,deburring)

Dolomite (light cleaning and etching)

Sodium bicarbonate

The abrasive particles are then carried by the carrier gas to the machining chamber via an electro-magnetic on-off valve. The machining is carried out as high velocity (200 m/s) abrasive particles are issued from the nozzle onto a work piece traversing under the jet.

Fig:- AJM set up

Applications • For drilling holes of intricate shapes in hard and brittle materials

• For machining fragile, brittle and heat sensitive materials

• AJ M can be used for drilling, cutting, deburring, cleaning and etching.

• 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 work material is ductile

• Tapering occurs due to flaring of the jet

• Environmental load is rather high.

Characteristics of AJM:Work material → hard and brittle materials like glass,ceramics,mica.

Abrasive → Al2O3,Silicon carbide, Glass powder

Dolomite

Size of abrasive → around 25µm

Flow rate → 2-20g/min

Medium → N2 (or) CO2 (or) air

Velocity → 125-300m/s

Pressure → 2 to 8 kg/cm2

Nozzle material → tungsten carbide or synthetic sapphire

Life of nozzle → tungsten carbide (12 to 20 hours)

sapphire (300hours)

Gap → 0.25-0.75mm

Tolerance → ±0.05mm

Machining operation → drilling,cutting,cleaning etc

Metal removal rate process parameters:

Mass flow rate

Abrasive grain size

Gas pressure

Velocity of abrasive particlesMixing ratio (mass flow rate of abrasive / mass flow rate of

gas)

Nozzle tip clearance

AdvantagesAdvantages• Low initial investment• No direct contact between tool and

workpiece• Good surface finish• Used to cut intricate hole shapes in hard and

brittle materials

DisadvantagesDisadvantages

Material removal rate is slow

Soft material cannot be machined

Machining accuracy is poor

Nozzle wear rate is high

Abrasive powder cannot be reused

ApplicationsApplications

Fine drilling and micro welding

Machining of semiconductors

Machining of hard and brittle materials

Cleaning and polishing of plastics ,nylon components

WATER JET MACHININGWATER JET MACHININGWork material → soft and non metallic materials like paper board, wood,

plastics, rubber

Mass flow rate → 8 lit /min

Pressure of water – 100 to 1000 Mpa

Tool - water or water with additives.

Additives – glycerin, polyethylene oxide.

Nozzle material → tungsten carbide or synthetic sapphire

Power – 45 Kw

Metal removal rate – 0.6 mm3 /s

Feed rate – 1 to 4 mm/s

Stand off distances – 2-50mm

PROCESS PARAMETERS OF WJM

Material removal rate

Mass flow rate

Velocity

Nozzle diameter

Fluid pressure

Geometry and surface finish of work material

Nozzle design

Jet velocity

Cutting speed

Depth of cut

Wear rate of the nozzle

Pressure of jet

Velocity of jet

nozzle design.

Advantages of WJMCheap , non toxic and easy to dispose

Low operating cost

Low maintenance cost

Disadvantages of WJM

• Initial cost is high • Difficult to machine to hard material