Process Parameters of AJM

The process parameters in AJM can be grouped into the following categories. The Ishikawa cause and effect diagram depicts the effect of various process parameters on the accuracy and quality of the machining operations by the abrasive jet machine.

1. The Abrasive: types, composition, strength, size, mass flow rate

2. The Gas: composition, pressure and velocity

3. The nozzle: geometry, material, stand-off distance (SOD), feed rate, inclination to work

4. The workpiece: Type of material

The selection of abrasive particles to be used in AJM depends upon the type of work material and type of machining operation which needs to be carried out. Different machining operations such as finishing, roughing require different types of abrasive for AJM operations. Commonly used abrasive for cutting include aluminum oxide and silicon carbide. In cleaning, etching and polishing operations glass beads and dolomites are recommended. The size of the abrasive particles also plays an important role in type of machining operations of AJM. Coarse grain particles are recommended for cuttingoperations while fine grains are recommended for finishing or polishing operations


The gas used in the AJM process must be non-toxic. It should be cheap and easily available. Common types of gas used in AJM applications are air, nitrogen and carbon. The recommended velocity of gas abrasive mixture ranges between 100 m/sec to 300 m/ sec depending upon the cutting or finishing operation.

The velocity of gas abrasive mixture is a function of nozzle design, nozzle pressure, and abrasive particle size. Stand-off distance (SOD) is a very important parameter. SOD is defined as the distance between the tip of nozzle and the work surface. The larger the SOD the poorer is the quality and accuracy of the cut. The effect of SOD on the accuracy of the cut is 10 – 30 micron Cutting, grooving

Abrasive Jet Machining (AJM)

In abrasive jet machining (AJM) material removal occurs on account of impact of high velocity air / gas stream of abrasive particles on the workpiece. The abrasives are propelled by a high velocity gas to erode material from the workpiece. As an outcome of impact of the abrasive particles on the workpiece, tiny brittle fractures occur at the surface of the workpiece and the carrier gas carries away the fractured fragments. AJM is also called as abrasive blasting process. It is also known by several other names such as abrasive micro-blasting, pencil blasting and micro-abrasive blasting. AJM is an effective machining method for hard and brittle materials such as glass, silicon, tungsten and ceramics. Typically the process is used for cutting intricate shapes or forms of specific edges. The process is inherently free from chatter, vibration and heat problems because the tool never touches the substrate. The schematic of AJM process set up is shown in Figure

Principle of AJM

The principle of machining / cutting by abrasive jet process is explained through the
following steps:

1. Abrasive particles of size between 10 m to 50 m (depending upon the requirement of either cutting or finishing of the work piece) are accelerated in a gas stream (commonly used gas stream is air at high atmospheric pressures).
2. The smaller abrasive particles are useful for finishing and bigger are used for cutting operations.
3. The abrasive particles are directed through the nozzle, towards the work piece surface where-ever cutting or finishing is to be done. The distance between the tip of the nozzle and the work surface is normally within 1 mm.
4. As the abrasive particles impact the surface of the work piece, it causes a small fracture at the surface of the work piece. The material erosion occurs by the chipping action.
5. The erosion of material by chipping action is convenient in those materials that are hard and brittle.
6. As the particles impact the surface of
7. The abrasive particles once used, cannot be re-used as its shape changes partially and the work piece material is also clogged with the abrasive particles during impingement and subsequent flushing by the carrier gas.

Advantages
 AJM process is a highly flexible process wherein the abrasive media is carried by
a flexible hose, which can reach out to some difficult areas and internal regions.
 AJM process creates localized forces and generates lesser heat than the conventional machining processes.
 There is no damage to the workpiece surface and also the process does not have tool-workpiece contact, hence lesser amount of heat is generated.
 The power consumption in AJM process is low. Disadvantages
 The material removal rate is low
 The process is limited to brittle and hard materials
 The wear rate of nozzle is very high
 The process results in poor machining accuracy
 The process can cause environmental pollution
Applications:
Metal working:
 De-burring of some critical zones in the machined parts.
 Drilling and cutting of the thin and hardened metal sections.
 Removing the machining marks, flaws, chrome and anodizing marks.
Glass:
 Cutting of the optical fibers without altering its wavelength.
 Cutting, drilling and frosting precision optical lenses.
 Cutting extremely thin sections of glass and intricate curved patterns.
 Cutting and etching normally inaccessible areas and internal surfaces.
 Cleaning and dressing the grinding wheels used for glass.
Grinding:
 Cleaning the residues from diamond wheels, dressing wheels of any shape and size.

Classification of Advanced Machining / Material Removal Processes:

These processes are referred to a typical group of advanced machining processes in which the excess material is removed by non-traditional source of energy arising from electrical, mechanical, thermal or chemical source. Most of these processes don’t use a sharp cutting tool, as in the conventional case. Advanced material removal processes are generally classified according to the type of energy used to remove material. The classification of these processes based on the energy is given as below The processes based on use of Electrochemical Energy are:
 Electro-Chemical Machining (ECM),
 Electro-Chemical Grinding (ECG),
The processes based on the use of Thermal Energy are:
 Electric- Discharge Machining (EDM),
 Wire-Cut Electric Discharge Machining (WEDM)
 Laser Beam Machining (LBM),
 Electron Beam Machining (EBM).
The processes based on the use of Mechanical Energy are:
 Abrasive Flow Machining (AFM)
 Abrasive Jet Machining (AJM),
 Water Jet Machining (WJM),
 Abrasive Water Jet Machining
 Ultrasonic Machining (USM),

Why are Advanced Machining / Material Removal Processes Needed?

With the advent of new materials and the requirements of complex features on them, there was a necessity to develop new processes. Some of these features are:
1. Related to material properties:
 High hardness  High strength  High brittleness 2. Related to workpiece structure:
 Complex shapes  Typical thin and delicate geometries  Parts which are difficult in fixturing 3. Related to requirements in high surface finish and tight tolerances.
4. Related to controlling of temperature rise and residual stresses.

Need For Advanced Material Removal Processes

Advanced Material Removal Processes represent one of the technologies, which emerged after the second world war to cope up with the demands of sophisticated, more durable and cost competitive products. With the advent of new materials such as metal-matrix composites, super-alloys, ceramics, aluminates and high performance polymers etc. and the stringent requirements to machine complex geometrical shapes with high precision and accuracy, a strong need existed for the development of advanced material removal processes. The processes in this category differ from conventional processes in either utilization of energy in an innovative way or, in using forms of energy that were unused for the purpose of manufacturing. The conventional machining processes normally involve the use of energy from electric motors, hydraulics, gravity, etc. and rely on the physical contact between tools and work components. On the contrary, advanced material removal processes utilize energy from sources such as electrochemical reactions, high temperature plasma, high velocity jets and loose abrasives mixed in various carriers etc. Although these processes were originally developed to handle unique problems in aerospace industry (machining of very hard and tough alloys), today wide range of industries have adopted this technology in numerous manufacturing operations.

Why dry compressed air?

The air we breathe contains contamination in the form of water vapour and airborne particles. During the compression process an air compressor concentrates these contaminants and depending on the design and age will even add to the contamination in the form of oil carry over.

Modern air compressors generally have built in after coolers that reduce the discharge temperature of the compressed air and with the help of water separators, remove the bulk of liquid water.
In some applications this may be sufficient, but the remaining dirt and moisture content suspended in aerosol form, can, if not removed, damage the compressed air system and cause product spoilage.
Air Contaminants lead to increase down time and reduced productivity. it lead to corrosion , damaged Tools , poor finish to painting Jobs etc .

Compressors & Compressed Air Systems - Post 3

Rotary compressor

Rotary compressors have rotors in place of pistons and give a continuous pulsation free discharge. They operate at high speed and generally provide higher throughput than reciprocating compressors. Their capital costs are low, they are compact in size, have low weight, and are easy to maintain. For this reason they have gained popularity with industry. They are most commonly used in sizes from about 30 to 200 hp or 22 to 150 kW.

Types of rotary compressors include:
Lobe compressor (roots blower)
Screw compressor (rotary screw of helical-lobe,where mail and female screw rotors moving in opposite directions and trap air, which iscompressed as it moves forward,)
Rotary vane / sliding- vane, liquid-ring, and scroll-type
Rotary screw compressors may be air or water-cooled. Since the cooling takes place right inside the compressor, the working parts never experience extreme operating temperatures. The rotary compressor, therefore, is a continuous duty, air cooled or water cooled compressor package.
Because of the simple design and few wearing parts, rotary screw air compressors are easy to maintain, operate and provide great installation flexibility. Rotary air compressors can be installed on any sur face that will support the static weight.

Dynamic Compressors
The centrifugal air compressor is a dynamic compressor, which depends on transfer of energy from a rotating impeller to the air. The rotor accomplishes this by changing the momentum and pressure of the air. This momentum is converted to useful pressure by slowing the air down in a stationary diffuser. The centrifugal air compressor is an oil free compressor by design. The oil lubricated running gear is separated from the air by shaft seals and atmospheric vents.