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INTRODUCTION
As the world is advancing forth technically in the field of space research, missile and nuclear industry, very complicated and precise components having some special requirements are demanded by these industries. This challenge is taken by new development taking place in the manufacturing field.
The most basic requirements of future manufacturing technology are:
1. Sustained productivity in the face of rising strength barrier.
2. Higher accuracy consistent with increasing demand for higher tolerance.
3. Versatility of automation.
In recent years some non-traditional of manufacturing have been invented in order to supplement affectivity the machining problems of hard to machine and brittle materials. Once of these non- traditional techniques is Abrasive Jet machining (AJM). The abrasive jet machining can be suitable employed for machining super alloys and refractory type material. The process is also very much suitable for cutting, grooving, cleaning, finishing and debarring operations of hard and brittle materials like germanium, glass, ceramics and mica.
The process criteria are greatly influenced by various process parameters as enumerated below:
A) Abrasives: composition, Shapes, Size and flow rate of abrasive.
B) Carrier Gas: Pressure, Viscosity, Molecular, Weight, flow rate of carrier gas etc.
C) Nozzle: Geometrical features, material for construction, orientation with
Horizontal and stand-off distance.
Abrasive jet machining (AJM) is the process in which a material is removed from the work piece due to the impingement of fine grained abrasive by high velocity gas stream. The stream of abrasive mixed gas is directed to the work piece by suitably designed nozzles. The process differs from conventional sand blasting. In that abrasive particles used are finer and the process parameters and cutting actions is carefully controlled. Abrasive jet machining is applied to cut hard and brittle material such as mica, germanium, glass, ceramics etc. The process is free from vibrations and chatter problems. As no current passes from the tool and the work piece. There is no restriction to material to be machined. Thus it cuts conductive as well as non-conductive materials. The process however is no conductive to machine soft due to abrasive particles getting embedded in the work material.
PRINCIPLE OF OPERATION OF AJM
The operating principle of the process is very simple. High pressure air from the compressor passes through filters and control valves into a mixing chamber. The abrasive particles and carrier gas are thoroughly mixed in the mixing chamber and a stream of abrasive mixed gas passes through a nozzle on the work piece. It causes indentation on the work piece.
The indentation ultimately results in rupture of particles from the work surface.
The nozzle geometry and its inclination, size of grit, the abrasive used fro cutting and the carrier gas pressure and the velocity are used as criteria for evaluating AJM process.
A high velocity jet containing abrasive particles is directed on to the work surface through the nozzle. Due to this the nozzle has to sustain maximum wear due to abrasion. Secondly, the accuracy of working and the metal removal rate depends upon the nozzle wear. The material used for nozzle should are therefore have high wear resistance. In practice the nozzle are made of tungsten carbide or sapphire having regular round or square hole. Nozzle made from tungsten carbide last for 12 to 30 hrs. When used with 27 micron abrasive in the present study the tungsten carbide nozzle 1mm and 2 mm diameters are used.
AJM AS UNCONVENTIONAL MACHINE
The word unconventional is used in the sense that the metals are such that they can not be machined by conventional methods, and require some special techniques.
AJM is included in these methods carried by high pressure as at high velocity, which is made to impinge on the work interface.
This eliminates tool to metal contact, which are the main criteria of unconventional machining method used in AJM. Following are the parameters, which affect the material removal are of the work piece.
1) Standoff distance.
2) Abrasive particle size and type.
3) Abrasive Jet Velocity.
4) Carrier Gas Pressure.
5) Mass flow rate of Abrasive.
6) Nozzle diameter.
PROCES PARAMETERS OF AJM
NOZZLE: -
The abrasive particles are directed into work surface at high velocity through nozzle. Therefore, the material of nozzle is subjected to a great degree of abrasion wear and hence these are made of hard such as tungsten carbide or synthetic sapphire. Tungsten carbide nozzle is used for circular c/s in the range of 0.12 to 0.8 mm dia. For rectangular sections of size 0.0805 to 0.18, 3.8mm and for square section of size up to 0.7mm2, Sapphire nozzle from 0.2-0.7mm dia. Nozzles are made with an external taper to minimize secondary effects due to nocoheffing of abrasive particles.
Nozzles made of tungsten carbide have an average life of 8-12 hours while nozzles made of sapphire last for 300 hours of operation, when used with 0.27 abrasive powders.
CARRIER GAS:-
Carrier gas to be used in AJM must not flare excessively when discharged from the nozzle into the atmosphere. Further the gas should be Nontoxic, cheap, cagily available and capable of being dried and cleaned without difficulty. Air, Nitrogen and Carbon Dioxide is generally used as carrier gas. Commercially filled cylinder gases can also be used satisfactorily. Air widely owes to easy availability and little cost.
AJM units are operated usually at pressures of 2-8 Kg/cm2. Higher pressure results in high nozzle wear and lower pressure results in reduced material removal rate (m).
The exit velocity of gas stream has not been measured, but it is near the sonic velocity of the air abrasive mixture.
SIZE OF ABRASIVE GRAIN:-
The rate of metal removal depends upon the size of the abrasive grain. Fine grains are less irregular in shape and hence posses lesser cutting ability. Moreover finer grains tend to stick together and check the nozzle. The most favorable grain size from 10 to 50 microns, coarse grains recommended for cutting whereas their grain are useful in polishing , debarring etc.
The material removal rate is mainly dependent upon flow rate and size of the abrasive particles. At a particular pressure the material removal rate increase.
VELOCITY OF THE ABRASIVE JET:-
The kinetic energy of the abrasive jet is utilized for metal removal by erosion FANNIE & SHELDON ( ref. ASIME code book) have shown that for erosion to occurs, the jet must impinge the work surface with a certain minimum velocity for the erosion of glass by silicon carbide ( grain size 25 micron) the minimum jet velocity has been found to be 150 m/ sec.
The jet velocity is the function of nozzle pressure, nozzle design, abrasive grain size and the mean no. of abrasive per unit volume of the carrier gas.
Increase in mass flow of the abrasive will result in a decrease in velocity of found and will thus cause a in available energy for erosion and ultimately the metal removal rate.
The maximum removal rate obtainable from fixed nozzle dimensions and nozzle keep distance usually lies between 2 to 20 mg/ min.
WORK MATERIAL: -
AJM is recommended for the progressing of the brittle material such as glass, ceramics, refractories, semiconductors, cemented carbide etc. most of the ductile material is practically unmachinable by AJM. The rate of metal removal has been found to depend upon the Mohr’s hardness of material to be machined.
EFFECT OF PARAMETERS
In this chapter the effect of different parameters on material removal rate is discussed. Following are the parameters which affects material removal rate.
1) Stand of distance.
2) Abrasive flow rate.
3) Abrasive grit size.
4) Nozzle distance.
DISCUSSION
1) EFFECT OF S.O.D. ON M.R.R.:
The MRR first increases with increase in S.O.D. and after reaching an optimum value at certain S.O.D. the MRR starts decreasing.
The carrier gas attains a maximum velocity at the exit of the nozzle, but abrasive particles being heavier than air tend to lag behind and attain maximum velocity at some distance away from the nozzle exit. At this particular distance the MMR will be maximum. As the S. O. D. increase beyond this optimum distance the jet starts flaring and hence the velocity of abrasive particles decreases and eventually reach a value when erosion ceases completely.
2) EFFECT OF FLOW RATE ON M.R.R.
It is seen that MRR increases with an abrasive flow rate. It is found that fine grain size shall give higher MRR as no. of particles/ unit mass increases with fineness of abrasive. Also fine abrasive particles shall attain readily the air velocity of the jet.
3) EFFECT OF SIZE OF PARTICLES ON S.O.D.
It is clear that as the grain size reduces the S.O.D. at which M.R.R. occurs increases slightly. This fact is due to increase in density of the finer abrasive particles than courses particles.
Based on the discussion in the previous topics the following conclusions are drawn:
I) MRR increases with increase in S.O.D. MRR reaches the max. Value at a particular SOD at max MRR is known as optimum SOD.
II) The MRR increases with increase in abrasive flow rate. Also specific MRR decreases with increase in abrasive flow rate.
III) It is observed that SiC abrasives give more MRR as compared at aluminum abrasive.
IV) It is possible to improve the straightness of the hole by increasing the impacting time but it is not possible to get exact straight hole.
WORKING OF THE MACHINE
The compressor builds up the required pressure of about 2 to 8 kg/cm2. By opening the compressor was the air allowed to flow into main mixing chamber and abrasive chamber through Tee. When the abrasive chamber cock is open, the abrasive powder enters the main mixing chambers. The abrasive powder is carried away by the high pressure air. Thus the mixture of air and abrasive comes out through the nozzle at a very high velocity. In the mixing chamber and the nozzle, pressure energy is converted in to the kinetic energy of the particles of abrasive. The abrasive particles come out in the form of a-line jet. This jet strikes on the work piece placed in front of nozzle at a particular stand of distance. Due to the impact of the high velocity abrasive on the work piece is cut.
STEPS TO BE FOLLOWED IN STARTING THE MACHINE
In order to start the machine following steps are to be taken.
1) Load abrasive feeding chamber with required girt size of abrasive (about 250 gms.)
2) Ensure that all the pipe fitting are air tight and leak proof.
3) Start the compressor to build up the necessary pressure of about 2 of 8Kg/cm2.
4) Fix the nozzle of known diameter to the tapered rod and place the cap properly.
5) Place the glass piece to be machined in the fixture. Adjust the stand of distance.
6) Close the dust collection box and cover properly.