Ultrasonic Machining

Working Principle :
 
In ultrasonic machining, a tool vibrating longitudinally at 20 to 30 kHz with amplitude between 0.01 to 0.06 mm is pressed on the work surface with a light force.
As tool vibrates with a specific frequency, an abrasive slurry, usually a mixture of abrasive grains and water of definite proportion is made to flow under pressure through the tool-workpiece interface. This causes micro-indentation fracture on the material. 

Small abraded particles are removed along the surface which is perpendicular to the direction of the tool vibration. When the material has removed a cavity of the same profile of the tool face is formed. The abrasive particles gradually erode as the machining process continues. As a result, fresh abrasive particles are needed to be supplied in the machining zone. Abrasive particles associated with the liquid is fed to the m/c zone and it ensures the removal of the worn-out grains and material.

The commonly used abrasives are :

aluminium oxide ( alumina ), boron carbide, silicon carabid and diamond dust. 

The abrasive slurry is circulated to the work-tool interface by pumping. A refrigerated cooling system is used to cool the abrasive slurry to a temperature of 5 to 6 degree C. A good method is to keep the slurry in a bath in the cutting zone. The liquid to produce abrasive slurry should have the following characteristics :

• Good wetting characteristics
• Low viscosity
• High thermal conductivity
• Anti-corrosive property
• Low cost

The size of abrasive varies in between 200 and 2000 grit. Course grades are good fro roughing whereas finer grades (1000 grit) are used for finishing.

Cutting rate of USM depending upon the following factors :

• The grain size of abrasive
• Abrasive materials
• Concentration of slurry
• Amplitude of vibration
• Frequency

Accuracy :

• The maximum speed of penetration in soft and brittle materials such as soft ceramics are of the order of 20 mm/min but fro hard and brittle materials penetration rate is lower. 
• Dimensional accuracy upto (+0.005 to -0.005 mm).
• Minimum corner radius of 0.10 mm is possible in finish machining. 

Applications :

• In performing machining operations like drilling grinding, profiling and milling operation on all materials both conduction and non-conducting also.
• In machining of glass, ceramics, tungsten and other hard carbides.
• In making tungsten carbide and diamond wire drawing dies and dies for forging and extrusion processes.
• For USM is used in drill a hole in teeth of any shape without creating any pain.
• Circular, as well as non-circular holes, can be done with straight or curved axes.
• Fabrication of silicon nitride turbine blades.

 Limitations of USM :

• The machining rate is extremely low when compared to the conventional machining process.
• The power consumption is also very high when compared to other similar processes.
• This method is limited for the machining of small workpieces only.
• Machining of deep holes is difficult, as the slurry movement is restricted.
• The tool wear is relatively high.

Recent Development in USM :

Recently a new development in USM has taken place in which a tool impregnated with diamond dust is used and no slurry is used. The tool oscillates at ultrasonic frequencies as well as rotated. If it is not possible to rotate the tool the workpiece may be rotated.

This innovation has removed some of the drawbacks of the conventional process of drilling deep holes. for instance, the hole dimensions can be kept within +0.125 to -0.125 mm. Holes up to 75 mm, in depth have been drilled in ceramics without any fall in the rate of machining as is experienced in conventional processes.

You can also know more :
Advantages and disadvantages of ultrasonic machining

Abrasive Jet Machining

The principle of Abrasive jet machining involves the use of a high-speed stream of abrasive particles carried by a high-pressure gas or air on the work surface through a nozzle. The metal removal occurs due to erosion caused by the abrasive particles impacting the work surface at high speed.

Working Principle :

The filtered gas supplied under a pressure of 2 or 8 kgf/cm2 to the mixing chamber containing the abrasive powder and vibrating at 50 Hz and is then passed into a connecting hose.  

This abrasive and gas mixture emerges from a small nozzle mounted on a fixture at a high velocity ranging from 150 to 300 m/min. The abrasive powder feed rate is controlled by the amplitude of vibration of the mixing chamber. A pressure regulator controls the gas flow and pressure.

The carrier gas should be cheap, nontoxic and easily available. Air and nitrogen are two of the most widely used gas in AJM.

The abrasives generally employed are aluminium, oxide, silicon carbide, glass powder or specially prepared sodium bicarbonate.

The average particle sizes vary from 10 to 50 microns. Larger sizes are used for rapid removal rate while smaller sizes are used for good surface finish and precision work.

Since nozzles are subjected to a great degree of abrasion wear, they are made of hard materials such as tungsten carbide or synthetic sapphire to reduce the wear rate. 

Water jet machining is another variation where a high-pressure jet of water is directed on a surface of removal material.

The metal removal rate depends upon :

• Diameter of nozzle
• Composition of the abrasive-gas mixture 
• Jet pressure 
• The hardness of abrasive particles
• Particle size
• Velocity of Jet 
• The distance of workpiece from jet

Accuracy :
With close control of the various parameters a dimensional tolerance of 
(+0.05 mm to -0.05 mm) can be obtained. on normal production work accuracy of
(+0.01 mm to -0.01 mm) is easily held.

Applications :

• Cutting slots
• Thin sections
• Contouring
• Drilling 
• For producing shallow crevices
• Deburring 
• Producing intricate shapes in hard and brittle materials
• Cleaning and polishing of plastics, nylon and Teflon components
• Frosting of the interior surface of the glass tubes 

Advantages :

• AJM is suitable for materials of any hardness and brittleness like ceramics, germanium, glass.
• Ability to cut fragile and heat-sensitive materials without damage as no heat is generated due to the passing of gas or air.
• Low capital cost
• Holes of intricate shapes could be produced efficiently.
• It can be utilized for cutting, drilling, polishing, deburring, cleaning of the materials.

Disadvantages :

• The material removal rate is slow 
• The machining accuracy is poor and the nozzle wear rate is high.
• Additional cleaning of the work surface may occur because the abrasive particles may remain embedded in the work surface
• Abrasive particles cannot be reused.
• A dust collecting chamber is a basic requirement to prevent atmospheric pollution to cause health hazards.

Non traditional machining process selection

Process Selection :

The points which should be looked before the selection of these process are as below : 

1. Physical parameters.
2. Properties of the work material and shape to be machined.
3. Process capability or machining characteristics.
4. Economic survey. 

Materials application of the various methods are given in the table below :
  

Process	Aluminium	Steel	Super alloy	Titanium	Refractories	Ceramics	Plastic	Glass
USM	G	F	P	F	G	G	F	G
AJM	F	F	G	F	G	G	F	G
ECM	F	G	G	F	F	N	N	N
CHM	G	G	F	F	P	P	P	F
EDM	F	G	G	G	G	N	N	N
EBM	F	F	F	F	G	G	F	F
LBM	F	F	F	F	P	G	F	F
PAM	G	G	G	F	P	N	P	N

Where

G = Good

F = Fair

P = Poor

N = Not applicable
The processing capability or machining characteristics can be analyzed with respect to :

1. Metal removal rate obtained.
2. Tolerance maintained.
3. The surface finishes obtained.
4. Depth of surface damage.
5. Power requires machining.

The economics of the various process is analyzed by considering :

1. Capital cost.
2. Tooling cost.
3. Consumed power cost.
4. Metal removal rate efficiency.
5. Wear of tooling.