Unconventional or Nontraditional Machining - III

Ultrasonic Machining (USM)

• In ultrasonic machining, a tool which is negative of desired shape, vibrates at low amplitude (0.013 to 0.08 mm) and ultrasonic frequency (about 20 kHz) in an abrasive grit slurry at the workpiece surface.

• The tool is gradually moved down maintaining a constant gap between the tool and workpiece surface.

• As the tool vibrates over the workpiece, the abrasive particles act as the indenters and indent both the work material and the tool. The abrasive particles, as they indent, the work material, would remove the same, particularly if the work material is brittle, due to crack initiation, propagation and brittle fracture of the material. 

At full indentation, the indentation depth 'h' and assuming brittle fracture takes place leading to hemi-spherical fracture of diameter ‘D’ under the contact zone.

Volume removed by the single grain is

\[V = \frac{2 \pi}{3}(\frac{D}{2})^3\]

\[D \approx \sqrt{dh}\]

\[V = \frac{2 \pi}{3}(dh)^{3/2}\]

 If number of particles impacting per cycle is n, frequency of operation f and efficiency is η then Material Removal Rate can be expressed as:

\[MRR = η V Zf = η \frac{2 \pi}{3}(dh)^{3/2} nf\]

Applications: USM is best suited for hard, brittle materials, such as ceramics, carbides, glass, precious stones, and hardened steels.

Water Jet Machining (WJM)

WJM works by forcing a large volume of water through a small orifice in the nozzle, at high pressure and velocity against work surface. 

This jet of water erodes the surface of workpiece. 

Applications: Mostly used to cut lower strength materials such as wood, plastics, rubber, paper, leather, composite, etc. 

>Good for materials that cannot withstand high temperatures.

Abrasive Water-Jet Machining (AWJM)

The water jet also contains abrasive particles such as SiC, hence material removal rate is higher than WJM.

Abrasive Jet Machining (AJM)

A high-velocity jet of gas containing abrasive particles is directed at the workpiece surface under controlled conditions. It removes material through the eroding action of a high velocity stream of abrasive-laden gas. Abrasive particles are generally of Al2O3, SiC with particle size 10 to 50 µm. The gas supply pressure is in order of 7 atm and the jet velocity about 300 m/s. It is used to cut materials which are hard to cut, e.g., composites, ceramics, glass.

General Observations

• ECM has the highest material removal rate (MRR).
• EDM has the lowest specific power requirement.
• USM and AJM have low MRR and combined with high tool wear, are used for non-metal cutting.
• LBM and EBM have high penetration rates with low MRR and, therefore, are commonly used for micro drilling, sheet cutting, and welding. 

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