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Manufacturing Processes > Non-Traditional Machining > Ultrasonic Machining

 

Ultrasonic Machining

 

Process description

The tool, which is negative of the workpiece, is vibrated at around 20 kHz with an amplitude between 0.013mm and 0.1mm in an abrasive grit slurry at the workpiece surface. The workpiece material is removed by essentially three mechanisms: hammering of the grit against the surface by the tool, impact of free abrasive grit particles (erosion) and micro-cavitation. The slurry also removes debris away from the surface. The tool is gradually moved down maintaining a constant gap of approximately between the tool and workpiece surface.

Materials

Any material, however, brittle hard materials are preferred to ductile, for example, ceramics, precious stones, tool steels, titanium and glass.

USM

Process variations

  • Vibrations are either piezo-electric or magnetostrictive-transducer generated.
  • Tool materials vary with application and allowable tool wear during machining. Common tool materials are: mild steel, stainless steel, tool steel, aluminum, brass and carbides (higher wear rates are experienced with aluminum and brass).
  • Abrasive grit is available in many grades and material types. Materials commonly used are: boron carbide, aluminum oxide, diamond and silicon carbide.
  • Liquid medium can be water, benzine or oil. Higher viscosity mediums decrease material removal rates.
  • Rotary USM: a rotating diamond coated tool is used for drilling and threading, but with no abrasive involved.
  • Ultrasonic cleaning: uses high-frequency sound waves in a liquid causing cavitation, which cleans the surface of the component, similar to a scrubbing action. Used to remove scale, rust, etc.

Economic considerations

  • Production rates very low.
  • Material removal rates low, typically 13mm³/s.
  • Linear penetration rates up to 0.4 mm/s.
  • Lead time typically days depending on complexity of tool. Special tooling required for each job.
  • Material utilization poor. Scrap material cannot be recycled.
  • High degree of automation possible.
  • Economical for low production runs. Can be used for one-offs.
  • Tooling costs high.
  • Equipment costs generally moderate.
  • Direct labor costs low to moderate.

Typical applications

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

Design aspects

  • Limited to shape of tool and control in 2-dimensions.
  • Tool and tool holder designed with mass, shape and mechanical property considerations.
  • Sharp profiles, corners and radii should be avoided as abrasive slurry erodes them away.
  • Overcut will be produced which is approximately twice the grit size.
  • Suitable for small diameter holes with length to diameter ratios typcially 3:1. Up to 4:1 using special equipment.
  • Waste removal limits hole depths.
  • Maximum hole size =90 mm.
  • Minimum hole size =0.08mm.

Quality issues

  • Tapering of slots and holes occurs.
  • Through holes in brittle materials should have a backing plate.
  • Amplitude and frequency of vibration, tool material, impact force, abrasive grit grade and slurry viscosity and concentration all impact on accuracy, surface roughness and material removal rate.
  • Finishing cuts made at lower material removal rates.
  • Tool wear problematic. Tool changes can be frequent.
  • Part is burr free with no residual stresses, distortion or thermal effects.
  • Difference in wear rate between the tool and workpiece materials should be as high as possible.
  • Surface detail good.
  • Surface roughness values ranging 0.2–1.6 µm Ra.
  • Finer surface roughness values obtained with finer grit grades.
  • Achievable tolerances ranging ±0.005–±0.05 mm. (Process capability charts have not been included. Capability is not primarily driven by characteristic dimension.)

costing