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Short notes on Ultra Sonic Machining (USM)

Continuing the short note series for GATE on Unconventional Machining Methods, in this article we will look at Ultra Sonic Machining (USM).

In this machining method, a slurry of small abrasive particles is forced against die work by means of a small vibrating tool removing the workpiece material in the form of extremly small chips. The grains used are silicon carbide, aluminium oxide, boron carbide or diamond dust. This process is suitable only for hard and brittle materials like carbides, glass, ceramics, silicon, preciups stones, germanium, titanium, tungsten, tool steels, die steels, ferrite quartz. The vibrating frequency used for die tool is of the order of over 20,000 oscillations per second. 

The tool is made of relatively soft material and is applied to the workpiece surface and the slurry is applied manually or through a pump. The tool can be attached to the arbor either by brazing, hard soldering or screwing. Sometimes hollow tools are used which facilitate feeding of the slurry through them. The metal removal in this process is facilitated by abrasive action of each grain which is hammered by the high frequency oscillating tool in the work material. 

A high frequency current is sent by ultrasonic oscillator to the ultrasonic transducer. The function of the transducer is to convert this electrical energy into mechanical vibrations. The vibrations so generated are of the order of 20 kHz to 30 kHz, although the amplitude varies from 0.01 mm to 0.1 mm. The transducer is made up of a magneto strictive material, which is excited by the flow of high frequency electric current and this results in the generation of mechanical vibrations. These vibrations are then transmitted to the cutting tool via the intermediate connecting parts, such as transducer cone or horn, connecting body and tool holder. This makes the tool vibrate in a longitudinal direction. 

Advantages of USM:

  • Extremely hard and brittle materials can be easily machined. 
  • Highly accurate profiles and good surface finish can be easily obtained.
  • The machined surfaces are free from stresses.
  • Material removal cost is low.
  • The physical properties of the workpiece material remain unchanged as there is practically no heat generation during the process.
  • The operation is noiseless.

Disadvantages of USM:

  • The MRR is low.
  • The initial equipment cost is higher than the conventional machine tools.
  • This process does not suit heavy metal removal.
  • The tooling cost is high.
  • Softer materials are difficult to machine.
  • Power consumption is high.
  • High wear rate of tool may lead to dimensional inaccuracies.To maintain the dimensional accuracy, frequent tool change needs to be performed.
  • To maintain the cutting efficiency, slurry may have to be replaced periodically.
  • The cavity size that can be machined is limited.

Applications of USM:

  • Tool and die making, especially wire drawing and extrusion dies.
  • Several machining operations like turning, threading, grinding, trepanning, milling etc.
  • Machining of hard to machine or brittle materials.
  • Producing holes of round or such other shapes which can be provided to the cutting tool.
  • Dentistry work to drill holes of desired shape in the teeth.

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