Can You Choose AJM Nozzle with Arbitrary Inner Diameter?

In abrasive jet machining (AJM), the nozzle directs high velocity abrasive jet towards work surface at a particular incidence angle (called impingement angle) in order to gradually erode material. Basic purpose of nozzle is to convert the pressure (hydraulic energy) into kinetic energy and this is done by converting pressurized gas-abrasive mixture into high velocity abrasive jet. Along with pressure difference, the internal diameter, length and profile of the nozzle determine flow characteristics of abrasive jet coming out of the nozzle. Arbitrary value of diameter, length and profile can severely hamper abrasive jet machining performance.

Nozzle inner diameter needs to be about 20 times larger than the average media size (abrasive grain size).

Influences of inner diameter of nozzle on AJM

For a fixed inlet pressure, if nozzle diameter is large, the jet velocity will be less and vice versa. This is attributed to constant volume flow rate of gas-abrasive mixture (since fluid mass is neither accumulated nor injected within the nozzle), which is proportional to both cross-sectional area and velocity. Low velocity abrasive jet quickly lose kinetic energy required for cutting while moving through stand-off distance (SOD) as it experiences much lower surrounding pressure. Consequently jet loses its cutting ability due to reduction in velocity. However, if SOD is small, such jet can cut a wide area in a single pass because of higher cross-sectional area of the jet.

On the other hand, minimum nozzle diameter is limited by the abrasive grit size. There must be sufficient clearance in order to avoid chocking within the nozzle. Provision for wear should also be taken into account while selecting nozzle diameter. Usually nozzle diameter varies between 0.15 – 1.5mm. Therefore, for same gas pressure, flow rate and mixing ratio, larger nozzle diameter results in lower jet velocity thus less kinetic energy for erosion and lower MRR.

Effects of choosing arbitrary inner diameter of nozzle

In abrasive jet machining (AJM), following problems may arise if selection of nozzle diameter is inappropriate.

  • Inaccurate kerf width of machined slot.
  • Tapered or parabolic hole instead of cylindrical hole.
  • Lower material removal rate.
  • Insufficient depth of penetration.
  • Choking within the nozzle.
  • All these factors will basically lead to machining inaccuracy and rejected parts.

References

  • Book: Advanced Machining Processes by V. K. Jain (Allied Publishers Private Limited).
  • Book: Nonconventional Machining by P. K. Mishra (Narosa Publishing House).
  • Paper: M. W. Chastagner and A. J. Shih, Abrasive Jet Machining for Edge Generation, Transactions of NAMRI/SME, Volume 35, 2007.