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Abrasive Jet Machining (AJM)

Case Studies

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Some Studies on Abrasive Jet Machining

Authors

P.K. Ray and Dr. A.K. PaulDepartment of Mechanical Engineering

Regional Engineering CollegeRourkela

Journal of the Institution of Engineers (India)Volume 68, 1987

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An AJM setup may be of two types:

(i) Using a vortex type mixing chamber – abrasive particles are

carried by the vortex motion of the carrier gas.

(ii) Using a vibratory mixing chamber – abrasive particles are

forced into the path of carrier gas by vibratory motion.

The erosion phenomenon may be considered as two phases.

(i) Transportation problem - flow rate, direction and velocity of the

impinging particles.

(ii) Problem of determining material removal rate or erosion rate.

Introduction

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Erosion is a function of

(i) Speed and angle of impact

(ii) Ductility and/or brittleness of material as well the particles

(iii) Elasticity of the material

(iv) Shape and geometry of impinging particles

(v) Impinging particle diameter to material thickness ratio

(vi) Average flow stress

(vii) Materials density

(viii)Stand-off distance.

Introduction

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According to Finnie (1960), MRR

Where, M & V - Mass flow rate and velocity of abrasives

- Angle of impingement

C, n - Constants

- Minimum flow stress of

the materials

Sheldon & Finnie (1966) optimized ‘’ for max MRR. (For brittle - 90

and for ductile – 20 - 30)

Pandey (1977) and Bhatacharya (1976) studied the effect of

abrasive flow rate (AFR) and stand-off distance (SOD) on MRR.

Authors - studied the effect of gas pressure on MRR, AFR and

MRF (Material Removal Factor) during experimentation on an

indigenous AJM setup developed in their lab.

Literature

nMVCf )(

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AJM Nomenclature

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Experimental Setup

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Nozzle & Mixing Chamber

SS Nozzle life – 2 hrNozzle dia - 1.83 & 1.63 mm

Abrasives – SiC (60 & 120 µm)Workpiece – porcelainProcess – hole drilling

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Effect of Pressure on MRR

MRR increases with pressure.

There is a threshold pressure

below MRR is negligible.

MRR increases with increase in

nozzle diameter, grain size and

SOD.

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Effect of SOD on MRR

They also stated as found by other

researchers that MRR increases

initially and remains constant for a

small range.

Then it falls with further increase in

SOD.

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Effect of Pressure on AFR

It is found that AFR is proportional to the pressure.

As pressure increases, AFR increases.

Also, higher the grain size of abrasive particles, AFR is

higher for the given pressure.

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Effect of Pressure on MRF

MRF = MRR / AFR

MRF decreases with increase in pressure.

Quantity of material removed per gram of

abrasive at a higher pressure is less than that

at a lower pressure.

At higher pressure, AFR is higher – meaning

more number of abrasive particles.

This reduces gas flow rate and affects the

removal of debris (tiny particles from work).

Higher AFR – inter particle collision – loss of

energy.

Cushioning effect of trapped abrasive

particles.

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Conclusion

AJM with SiC – suitable for hard and brittle materials like porcelain.

Use of SS nozzle is justified by its low cost, though it has a very low

life.

Nozzle changeover time – less than half a minute.

MRF – maximum for the pressure range of 2 – 3 kgf/cm2

Increase in MRF beyond 4 kgf/cm2 pressure is marginal.

MRF & MRR – higher for higher SOD.

Thus, higher SOD is preferable where MRR is of prime importance.

In precision work, a higher pressure and a lower SOD may be

adopted for better accuracy and penetration rate.