ajm unit 2
TRANSCRIPT
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.