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GENERAL OVERVIEW OF PROCESS
It is a process of material removal through the action of a focussed stream of fluid
with abrasive particles.It is especially used for machining super alloys,ceramics,glass
and refractory material.It is one of the non-polluting methods available for machining
hard materials to precision details . In abrasive water jet machining the abrasive
particles utilise water jets to accelerate or deaccelerate their speeds depending upon the
type of material to be machined. This machining process carries an additional advantage
for it cannnot be reactive with any of the job material and for it's versatility that any
material, how much harder it is can be machined. The best of all no tool changes are
required ,fixtures to hold the workpiece are minimal in AJM.It can be used to virtually
machine any parts with any intricacies of sharp corners or tight radius at corners etc.
The material which are being machined by this process do not experience any hardening due
to process because heat generated is very less. Also, since major cutting forces are
directed in downward direction it can be used to machine materials with very small wall
thickness.The depth of cut or thickness of part to be machined is a function of speed and
best material machining is obtained for thickness less than 1inches.
PROCESS CAPABILITIES AND LIMITATIONS
Abasive Jet Machining utilises the presure of fluid stream to remove material
from the surface of the job.When using air as a medium the mixture of air and abrasives
are allowed to impinge on the work surface at about 200 to 400m/s through the nozzle and
work material is eroded by the high velocity abrasive particles.The inside diameters of
the nozzle is about 0.04mm and standoff distance is kept about 0.7 to1.0mm. The process
can be easily controllled to vary the metal removal rate which depends on flow rate and
size of abrasive particles .The cutting action is cooled because the carrier gas serves as
a coolant.
Gas used are nitrogen or carbondioxide or even air which are supplied underpressure(2-8
kg/cm^2) filtered through regulator is passed to a mixing chamber(containing abrasive
particles) vibrating at 50c/s.From the mixing chamber, the gas alongwith the entrained
abrasives particles of size 10-50 micro meter passes on to nozzle having its tip tungsten
carbide and diameter of around 0.45mm,with a velocity of 150 to 300 m/s.The air
consumption is of order of 0.6m^3/hr.The nozzle tip distance is of order of 0.81mm.The
abrasive powder feed rate is controlled by the amplitude of vibration of mixing
chamber.The relative motion between the nozzle and the workpiece is obtained by the
programable torch or by cams and pantographs to control the size and shape of the cut.Dust
removal equipment is incorporated to protect the enviroment. The material removal rate,
geometry of cut, surface roughness, and nozzle wear rate are influenced by the size and
distance of nozzle, composition,strength,size and shape of abrasive flow rate;and
composition,pressure and velocity of carrier gas.
The abrasive particles should have irregular shape and consist of short edges rather than
having rounded shapes.Abrasives generally used are Aluminium Oxide,Silicon Carbide, Sodium
bicarbonate,dolomite,glass beads;their selection and their grain size depending on the
machining operation.
The material removal rate is mainly dependent on the flow rate and size of the abrasive
particles .High grain size will always produce more metal removal.At a paticular pressure
metal removal rate increases with the abrasive flow rate but after reachining a optimum
value ,the material removal rate decreases with increase in abrasive flow rate .This is
because mass flow rate of the gas decreases with increase of abrasive flow rate and mixing
ratio increases causing a decrease in material removal rate because of less energy
available for erosion.The abrasive particles are generally not used again and again.
The material removal rate first increases with the increases of tip distance from work
upto a certain limit after which it remains unchanged for a certain tip distance and then
falls gradually.In this process the limitations are that the material removal rate is
low,stray cutting can't be avoided,tapering effect may be found because of unavoidable
flaring of the abrasive jets ,abrasives may get embedded i the work surface,and suitable
dust collecting system has to be provided.
The advantages of this process are that it can be used to cut intricate hole shapes in
hard and hard and brittle materials;even fragile and sensitive materials can be cut
without damage ,and the initial cost is low.It's disadvantages are that it removes
material at very low rate,stray cutting can occur resulting in poor accuracy, and soft
materials can't be machined by this process.
PROCESS PERFORMANCE-
