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