A brief description on Abrasive Jet Machining

3.  Abrasives such as Al2O3 (Aluminium Oxide) or SiC (Silicon Carbide) are mixed
with water or air and forced at great velocity against the work piece.
 The work piece is eroded away at the point where the grit impinges on the surface
of the workpiece.
 Production units are available using air or liquid as the propellant.
Features of AJM
 Air is generally used as the carrier medium.
 If water is used as the vehicle for carrying the abrasives, then the process is known
as Water abrasive jet machining.
 The abrasives come out of the nozzle at considerably high pressure (several times
higher than the atmospheric pressure) and velocity.

4.  By controlling the quantity of abrasive flow with the help of valve arrangements,
fine machining and coarse machining can be achieved.
 Generally suitably machined tungsten carbide and diamond nozzles are used.
 Nozzle diameters vary from 0.1mm to 1.0 mm depending on the size of the
abrasive particles used and the quantity of abrasive flow.
 During the machining operation the gap kept between the workpiece and nozzle tip
is about 1mm.
 The depth of the surface layer affected due to the AJM process is about 3µm.
 The amount of abrasives present in the air is an important factor which affects the
performance of the process (machining rate).

5.  Lesser the quantity of abrasives in the air lesser is the machining rate.
 If abrasives present are more than the optimum value, the abrasives start colliding
amongst themselves and the velocity of abrasives striking the work piece gets
reduced.
 Hence, for optimum performance the weight of abrasives present in the air should
be about 1/3 the weight of air flowing through the nozzle. (This ratio is known as
mixing ratio i.e. wt .of abrasives/wt of air at any instance in the flow).
Surface Finish
 The particle size of the abrasives used generally varies from 10µm-50µm.
 Finer the abrasives, better the finish produced on the work piece.
 Finishes down to 0.5µm.

6. Machining rates
 Very slow.
 AJM should not be considered as a mass material removal process.
 Normal material removed rates are about 0.5cm3
/hr; exact value will depend on
several factors such as particle size, work material machined, mixing ratio, etc.
Practical Applications of AJM Process
 AJM is used to drill small holes and to impart desired radii to small holes, to scribe
instrument grooves, and to cut extremely brittle materials such as glass and
ceramics.
 This makes it valuable for the electronics industry to make fine cuts on small non-
metallic components.

7.  Air-abrasive process in the fine trimming of thin film and thick film (resistors) of
hybrid circuits.
 AJM can also be used to machine non-conductive materials where electrical
methods would not work.
 It is usually used for surface finishing rather than for metal removal.
 This process can be used to machine hard and brittle materials and for making
intricate and decorative patterns.
 Another important application is in the preparation of integrated circuits (in
removing different layers of resistors, capacitors, insulators and semiconductors
from printed circuit boards according to a present pattern).

8.  Silicon and Tungsten carbide can also be machined.
 AJM can be used to remove burrs from difficult-to-reach surfaces of components.
Some other applications of this process are –
 Polishing plastic, nylon and Teflon components.
 Cleaning metallic mould cavities which otherwise may be inaccessible.
 Cutting thin sectioned fragile components made of glass, refractories, ceramics,
mica, etc.
 Producing high quality surface.
 Removing glue and paint from paintings and leather objects.

9.  It provides cool cutting action; no heat damage occurs to the delicate work
materials.
 Cutting action is shockless, fragile materials can be handled without breakage.
 The process is characterized by low capital investment and low power
consumption.
 Air-abrasive units are compact and easily transportable.
 AJM units are easy to operate and maintain.
 Process is versatile; can be manually operated; with fixtures the process can be
used in automatic production lines.
 Cutting action is accurate; no variation due to surface irregularities and tool wear
as in conventional machining.

10.  Application of AJM is restricted to brittle materials because of lower rates of metal
removal attainable in case of ductile materials.
 Sometimes, parts machined by this process have to undergo an additional operation
of cleaning as there is possibility of the abrasive grains sticking to the surface.
 The machining accuracy is poor and the nozzle wear rate is high.
 If air is used as propellant a suitable dust collection system is required to prevent
pollution.
 Replacement of rubber hoses which carry abrasive grits may be necessary to keep
it flowing easily.

11. The variables that influence the rate of metal removal and accuracy of machining in
this process are:
Carrier gas –
◦ It must not flare excessively when discharged from the nozzle into the
atmosphere.
◦ The gas should be nontoxic, cheap, easily available and capable of being dried
and cleaned without difficulty.
◦ The gases that can be used are air, carbon dioxide or nitrogen.
◦ Air is most widely used owing to easy availability and little cost.
◦ All abrasive powders supplied by the manufacturers can be run with clean shop
air, provided air filters have been installed in the air lines.

12.  The choice of abrasive depends on the type of machining operation, for example,
roughing, finishing, etc., work material and cost.
 The abrasive should have a sharp and irregular shape and fine enough to remain
suspended in the carrier gas and should have excellent flow characteristics.
 The abrasives used for cutting are aluminium oxide and silicon carbide whereas
sodium bicarbonate, dolomite, are used for cleaning, etching, deburring etc.
 Reuse of abrasives is not recommended because not only does its cutting ability
decrease, but contamination also clogs the orifice of the nozzle.
Grain size
 The rate of metal removal depends on the size of the abrasive grain.
 Finer the grains are less irregular in shape, and hence, posses lesser cutting ability.

13.  Finer grains tend to stick to each other and choke the nozzle.
 The most favorable grain sizes range from 10 to 50µ.
 Coarse grains are recommended for cutting, whereas finer grains are useful in
polishing, deburring, etc.
Jet Velocity
 The kinetic energy of the abrasive jet is utilized for metal removal by erosion.
 For erosion to occur, the jet must impinge the work surface with certain minimum
velocity.
 For the erosion of glass by silicon carbide (grain size 25µ), the minimum jet
velocity has been found to be around 150m/s.

14.  The jet velocity is a function of the nozzle pressure, nozzle design, abrasive grain
size and the mean number of abrasives per unit volume of the carrier gas.
 Mean Number of Abrasive grains per Unit Volume of the Carrier Gas –
An idea about the mean number of abrasive grains per unit volume of the carrier gas
can be obtained from the mixing ratio M. It is defined as –
Volume flow rate of the abrasive per unit time
M =
Volume flow rate of the carrier gas per unit time
 A large value of M should result in higher rates of metal removal but a large
abrasive flow rate has been found to adversely influence jet velocity, and may
sometimes even clog the nozzle.
 Thus, for given conditions, there is an optimum mixing ratio that leads to a
maximum metal removal rate.

15. Work Material
AJM is recommended for the processessing of brittle materials, such as glass,
ceramics, refractories, etc.
Most of the ductile metals are practically unmachinable by AJM.
Stand off Distance(SOD)
It is defined as the distance between the face of the nozzle and the working surface of
the work.
SOD has been found to have considerable effect on the rate of metal removal as well
as accuracy.
A large SOD results in the flaring up of jet which leads to poor accuracy.