Gas used is carbon-di-oxide or nitrogen or compressed air. Most commonly, aluminium oxide or silicon carbide particles are used. Mixing chamber: It is used to mix the gas and abrasive particles. Filter: It filters the gas before entering the compressor and mixing chamber.
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In AJM, why abrasive particles cannot be reused? Carrier gas for abrasive jet machining Basic purpose of carrier gas in abrasive jet machining is to accelerate fine abrasive particles by momentum transfer. A compressor is used to elevate pressure of the carrier gas as high as 20bar ; abrasive grits are mixed with it in a mixing chamber as per mixing ratio , and a nozzle is used to convert pressure energy into kinetic energy in the form of high velocity jet.
Carrier gas pressure along with nozzle diameter determines final jet velocity and thus machining performance. Among various gases, air is commonly used in AJM as it is abundantly available at free of cost. Sometime commercially pure carbon di-oxide and nitrogen are also used to harness better performance for a particular purpose.
However, pure oxygen is not used as it can quickly oxidize the work surface. Before compressing, carrier gas is dehumidified properly as presence of steam can block pipelines. When gas is compressed to high pressure, steam may condense and tiny water particles can create a larger globule after agglomerating with abrasives. Carrier gas is also made dust free before compressing to high pressure.
Can pure oxygen be used as a carrier gas in AJM? Nozzle used in abrasive jet machining Primary function of nozzle in abrasive jet machining is to convert pressure energy of the pressurized gas-abrasive mixture into kinetic energy in the form of high velocity jet. Nozzle also directs high velocity jet towards work surface from a specific distance called SOD and at a particular predefined angle, called impingement angle.
Inner diameter of the nozzle is paramount parameter as it determines final velocity and cross-sectional area of the jet for certain gas pressure. As flow rate and compressor delivery pressure are constant, jet velocity will be inversely proportional to the jet cross-sectional area.
Choosing nozzle material is another decisive factor from economic point of view. Usually tungsten carbide WC or sapphire nozzles are used in industrial applications. WC nozzles are cheaper but have limited life 20—30hr ; while sapphire nozzles have extended life —hr but are costlier.
Frequent changing of nozzle is associated with idle time during machining. Can you choose AJM nozzle with arbitrary inner diameter? Equipment for abrasive jet machining Air compressor: It compresses the carrier gas to a pressure of 15 — 20bar. Compressor unit also consists of drier and filter. So it removes water vapor and dust particles to avoid condensation or jamming during compression.
Pressure gauges: A number of such gauges are employed for measuring pressure of carrier gas as well as gas-abrasive mixture. Flow regulating valves: These valves controls volume flow rate of carrier gas in order to maintain constant mixing ratio. Hopper: In AJM, usually circular hopper with gradual compression is employed for continuously supplying fresh abrasive to the mixing chamber.
Hopper is sometime vibrated to avoid bridging jamming at outlet. Mixing chamber: Its purpose is to mix abrasives with pressurized carrier gas. Here momentum transfer takes place and abrasives start flowing with carrier gas. Chamber is vibrated to obtain homogeneous mixing. Nozzle: As an isentropic steady flow device, nozzle converts hydraulic energy pressure of the gas-abrasive mixture to the kinetic energy and thus high velocity jet is obtained.
Working chamber: A close working chamber, inbuilt with proper exhaust system, is usually maintained in order to avoid environmental pollution. It also helps protecting workers from lung dieses caused by exposing into atmosphere containing excessive tiny abrasive dust. Servo controller: Sometime movement of work table is controlled by servo mechanism. This gives easy, accurate and precise control and is suitable for cutting intricate profiles and contours.
Various components of AJM set-up and their functions. Process parameters and their influence on AJM There are many factors that can influence abrasive jet machining performance. Important process parameters include i abrasive particles—its shape, size, strength, material and flow rate; ii carrier gas—its nature, composition, flow rate, pressure and temperature; iii abrasive jet—mixing ratio, striking velocity, impingement angle and stand-off distance; iv nozzle—its profile and inner diameter; and v work material—its mechanical properties and stress concentration.
AJM performance is usually assessed by analyzing three output responses, namely i material removal rate MRR , ii surface roughness and accuracy of machined feature, and iii nozzle life or nozzle wear rate. Effects of process parameters on AJM performance are discussed below. List of factors that affect abrasive jet machining performance.
Effects of abrasives on AJM performance As discussed earlier, shape, size, strength, material and flow rate of abrasive can influence machining performance. Irregular shape abrasives having sharp edges tend to produce higher MRR as compared to spherical grits. Smaller size grits produce highly finished surface but reduce material removal rate MRR and thus productivity descends.
Larger grits can again create trouble while mixing and flowing through the pipeline. However, variation in size in the entire volume should be low otherwise estimation or assessment will not be accurate. Abrasive materials have varying strength or hardness.
The harder is the abrasive with respect to work surface hardness, the larger will be the volume removal rate. It is basically the relative hardness between abrasives and workpiece that determines machining capability and productivity. Mass flow rate of abrasive is usually controlled by Mixing Ratio, whose effects are also discussed later in this section.
Why different abrasives produce different MRR? Effects of carrier gas on AJM performance Carrier gas pressure and its flow rate are two paramount factors that determine performance and machining capability. Higher gas pressure reduces jet spreading and thus helps in cutting deeper slots accurately. However, various accessories including pipeline must be capable enough to handle such high pressure without failure.
Moreover, increased gas flow rate gives the provision for utilizing higher abrasive flow rate, which can improve productivity. Effects of carrier gas on abrasive jet machining performance. Effects of mixing ratio on AJM performance Mixing ratio M is the ratio between mass flow rate of abrasive particles and mass flow rate of carrier gas.
It basically determines concentration of abrasives in the jet. Mixing ratio can be increased by increasing abrasive percentage and in such case an increasing trend in MRR can be noticed because larger number of abrasives participates in micro-cutting action per unit time. However, excessive concentration of abrasive in the jet can significantly reduce MRR because of lower jet velocity as gas pressure is constant and unavoidable collision thus loss of kinetic energy.
MRR can be enhanced by proportionally increasing both the abrasive flow rate and gas flow rate at same rate so that mixing ratio remains constant. In such case, higher pressure of the carrier gas has to be utilized.
This necessitates thicker and stronger pipelines and other accessories to smoothly handle such high pressure without leakage and rupture. Indefinite increase in MRR is not practically feasible because of limited capability of equipment and accessories. Effects of mixing ratio on abrasive jet machining performance. Higher SOD causes spreading of jet and thus its cross-sectional area increases with the sacrifice of jet velocity.
As a consequent, machining deeper slots or hole becomes difficult; instead a wider area is cut. Alternatively smaller SOD can cut a deeper but narrow slot or hole. It also enhances MRR. Thus an optimum value of stand-off distance is required to set for obtaining satisfactory performance in abrasive jet machining.
Larger angle tends to create deeper penetration, while smaller angle tends to increase machining area. Effects of impingement angle on AJM performance. Material removal rate and its estimation Knowledge of material removal rate MRR is beneficial for selecting process parameters and choosing feed rate of the nozzle.
It also facilitates accurate estimation of productivity, delivery time as well as production cost. Since only kinetic energy of abrasive grits is utilized for erosion, the analytical formula for MRR can be established by equating available kinetic energy with the work done required for creating an indentation of certain cord length on a specific work material.
However, ductile and brittle materials behave differently in indent formation, and thus size of indentation created by the impact of single abrasive grit is different for ductile and brittle materials. Under few assumptions, MRR for abrasive jet machining for different materials can be modeled analytically and can be expressed as provided below.
Abrasive jet machining
In AJM, why abrasive particles cannot be reused? Carrier gas for abrasive jet machining Basic purpose of carrier gas in abrasive jet machining is to accelerate fine abrasive particles by momentum transfer. A compressor is used to elevate pressure of the carrier gas as high as 20bar ; abrasive grits are mixed with it in a mixing chamber as per mixing ratio , and a nozzle is used to convert pressure energy into kinetic energy in the form of high velocity jet. Carrier gas pressure along with nozzle diameter determines final jet velocity and thus machining performance. Among various gases, air is commonly used in AJM as it is abundantly available at free of cost. Sometime commercially pure carbon di-oxide and nitrogen are also used to harness better performance for a particular purpose. However, pure oxygen is not used as it can quickly oxidize the work surface.
Abrasive Jet Machining – Process, Parameters, Equipment, MRR