Effects of Carrier Gas on Abrasive Jet Machining Performance

In abrasive jet machining (AJM), fine abrasive particles, accelerated by dry pressurized gas, are allowed to strike the work surface in the form of a jet with the help of a nozzle. This gas with which abrasive particles are mixed is called carrier gas. Carrier gas must be dehumidified and made dust free. Mass of abrasive particle mixed per unit mass of carrier gas is called mixing ratio. Various parameters relevant to carrier gas that can affect abrasive jet machining performance are enlisted below. Effects of such parameters are also discussed in the successive sections.

  • Carrier gas pressure
  • Carrier gas flow rate
  • Carrier gas type (insignificant effect)
  • Carrier gas temperature (insignificant effect)

Effects of carrier gas pressure on AJM performance

Dried and dust free carrier gas is compressed to high pressure (15 – 20bar) using an air compressor. Abrasives are then mixed with this compressed gas in a mixing chamber (maintained at constant pressure) according to mixing ratio. A nozzle then converts hydraulic energy (pressure) of the gas-abrasive mixture to kinetic energy by substantially increasing velocity. This high velocity jet (100 – 300m/s) is allowed to strike the work surface for material erosion. Therefore, ultimate jet velocity depends on carrier gas pressure; in fact, they are proportional. So higher gas pressure indicates high velocity jet and thus higher erosion rate (or MRR).

However, carrier gas pressure cannot be increased indefinitely to enhance MRR. The machine set-up must be capable of withstanding such high pressure. Every machine set-up has certain rated capacity and if parameters exceed that limit then malfunctioning, leakage, and even burst may occur.

Effects of carrier gas flow rate on AJM performance

Along with carrier gas pressure, its flow rate also influences machining capability in abrasive jet machining. Under same pressure, higher volume flow rate can degrade mixing ratio if mass flow rate of abrasives are not increased proportionally. Decreased mixing ratio indicates lower MRR, which is usually undesirable. Moreover, if abrasive flow rate and gas flow rate both are increased at the same rate maintaining mixing ratio constant, then jet velocity will drop unless gas pressure is also increased. For same compressor delivery pressure, if more abrasives are mixed then final pressure after mixing will reduce as abrasives are fed from atmospheric pressure (this is attributed to conservation of momentum before and after mixing).

Therefore, in conclusion it can be emphasized that carrier gas pressure, mixing ratio, gas flow rate, losses during mixing and flow, SOD, and also nozzle diameter—all are interrelated factors and an optimum value of them is indispensably required for harnessing best result while abrasive jet machining of a particular work material.

References

  • Paper: N. Ramachandran and N. Ramakrishnan (1993); A review of abrasive jet machining; Journal of Materials Processing Technology; Vol. 39; pp. 21-31.
  • Book: Nonconventional Machining by P. K. Mishra (Narosa Publishing House).
  • Book: Unconventional Machining Processes by T. Jagadeesha (I. K. International Publishing House Pvt. Ltd.).