Ultrasonic machining (USM)

Process Description:

The tool, which is a negative of the workpiece, is vibrated at around 20KHz with and amplitude of between 0.013mm and 0.1mm in abrasive slurry at the workpiece surface.

Material removal is by 3 mechanisms:

  1. Hammering of grit against the surface by the tool.
  2. Impact of free abrasive grit particles (erosion).
  3. Micro-cavitation.
  • Parts are burr-free with no residual stresses, distortion or thermal effects. There are no changes to the metallurgical, chemical or physical properties of the workpiece.
  • The tool is gradually moved down and a constant gap is maintained between the workpiece surface and the tool.
  • Good for machining very brittle materials.
  • Slicing, coining, dicing, lapping, engraving, deburring, broaching, boring and trepanning operations are also practical.
  • Parts are burr-free with no residual stresses or thermal effects.

USMFig.1: Ultrasonic machining.

Typical Uses:

  • Burr free holes and slots in hard, brittle materials.
  • Complex cavities.
  • Coining operations.

Design guidelines:

  • Can be used for any material, however, brittle hard materials (eg. ceramics, precious stones, tool steels, glass, titanium) are preferable to ductile materials.
  • Finer surface roughness available with finer grit grades.
  • Limited to shape of tool and control in 2-dimensions.
  • Sharp corners difficult as they are eroded by abrasive slurry.
  • Good for small diameter holes with 3:1 to 4:1 length to diameter ratios, although tapering occurs.

Process variations:

  • RUM (Rotary Ultrasonic Machining). A rotating diamond coated tool allows drilling, threading and end-milling functionality.
  • It is common to use an ultrasonic assist in other material removal processes (e.g. drilling).
  • Rotary Ultrasonic Machining.
  • Ultrasonic cleaning.

Tradenames/alternative names:

  • Ultrasonic impact grinding.
  • Impact grinding.

The environment:

  • Scrap material cannot be recycled.

The economics:

  • Suitable for low production rates only.
  • Special tooling required for each job. High tooling costs.
  • Multiple cuts required for progressively better finish. Tool wear is a problem requiring frequent changes.

Technical notes:

  • A high frequency electrical signal is converted into a oscillatory mechanical motion. This motion is acoustically transmitted through a metal tool holder and cutting tool assembly. This linear oscillation is typically at a rate of 20,000 times per second, and, when used with an abrasive slurry flowing around the cutting tool, microscopic grinding occurs.
  • The process is very difficult to control due the large number of variables involved. These include the type, size and concentration of the abrasive powder, the fluid, the flux intensity, the vibration amplitude and frequency and the tool geometry and material. The optimisation of the process regarding to these parameters is not well established in literature.

About: Gareth Weeks

Chief clicker of engineeringclicks.com, Gareth holds a BSc (Hons) in Product Design, followed by an MPhil in Engineering. 20+ years in consumer products, bio-science, telecoms industries. Likes electric guitar, piano, sci-fi and horror, designing stuff, listening to and lifting heavy metal (not necessarily at the same time).

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