Abrasive Jet Machining
Abrasive Jet Machining (AJM) is a machining process that makes use of a stream of fine grained abrasive mixed with air or some other carrier gas at high pressure. This stream is directed by means of a suitably designed nozzle on to the work surface to be machined. Metal removal occurs due to erosion caused by the abrasive particles impacting the work surface at high speed. The figure below shows a schematic diagram of the working of this process.
Applications of Abrasive Jet Machining (AJM):
- Removing flash and parting lines from injection molded parts.
- Deburring and polishing plastic, nylon and teflon components.
- Cleaning metallic mold cavities which otherwise may be inaccessible.
- Cutting thin sectioned fragile components made of glass, refractories, ceramics, mica etc.
- Producing high quality surfaces.
- Removing glue and paint from paintings and leather objects.
- Reproducing designs on a glass surface with the help of masks made of rubber, copper etc.
- Frosting interior surfaces of glass tubes.
- Etching markings of glass tubes.
Advantages and disadvantages of Abrasive Jet Machining (AJM):
The advantage of this process is its ability to machine brittle materials with thin sections, especially in areas which are inaccessible by ordinary methods. Absence of tool work contact and metal removal at microscopic level leads to very little or no heat generation, resulting in insignificant surface damage. The process is characterized by low capital investment and low power consumption. However, the application of AJM is restricted to brittle materials due to lower rates of metal removal attainable in cases of ductile materials. Sometimes, the part to be machined needs to undergo an additional operation of cleaning as there is a possibility of the abrasive grains sticking to the surface. The machining accuracy is poor and the nozzle wear rate is high. Also, the process tends to pollute the environment.
Variables in Abrasive Jet Machining (AJM):
These are the parameters which influence the material removal rate and accuracy of machining. The following are such parameters:
- Carrier gas
- Type of abrasive
- Size of abrasive grain
- Velocity of abrasive jet
- Mean number of abrasive particles per unit volume of the carrier gas
- Work material
- Stand off
- Nozzle design
- Shape of cut
Carrier gas which is to be used in AJM must not flare excessively when discharged from the nozzle into the atmosphere. Further, the gas should be non-toxic, cheap, easily available and capable to being dried or cleaned without difficulty. The gases that can be used as:
- Carbon dioxide
Due to low cost and easy availability, air is the preferred gas.
Type of abrasive:
The choice of abrasive depends upon the type of machining operation, e.g. roughing, finishing etc., work material and cost. The abrasive should have a sharp and irregular shape and be fine enough to remain suspended in the carrier gas and should also have excellent flow characteristics. The abrasives used for cutting are aluminum oxide and silicon carbide, whereas sodium bicarbonate, dolomite, glass beads etc., are used for cleaning, etching, deburring and polishing. Re-use of abrasives is not recommended because not only does it reduces the cutting ability, it also contaminates or clogs the orifice of the nozzle.
The material removal rate depends on the size of the abrasive grain. Finer the grains, less irregular they are in shape and hence possess less cutting abilities. Also, finer grains tend to stick together and choke the nozzle orifice. The preferred grain sizes range from 10 to 50µ. Coarse grains are recommended for cutting, whereas finer grains are useful in polishing, deburring etc. The effect of grain size on the MRR is shown the figure below.
The kinetic energy of the abrasive jet is utilized for metal removal by erosion. For erosion to occur, the jet must impinge the surface with a certain minimum velocity. For the erosion of glass by silicon carbide (grain size 25µ), the minimum jet velocity has been found to be around 150 m/s. The jet velocity is a function of the nozzle pressure, nozzle design, abrasive grain size and the mean number of abrasives per unit volume of the carrier gas.
Mean number of abrasive grains per unit volume of the carrier gas:
To get an idea about the mean number of abrasive grains per unit volume, we can look at mixing ratio, M. It is defined as:
M = (volume flow rate of the abrasive per unit time)/(volume flow rate of the carrier gas per unit time)
A larger value of M results in higher rates of metal removal but a large abrasive flow rate has been found to adversely influence jet velocity, and may sometimes even clog the nozzle. Thus for the given conditions, there is an optimum mixing ratio that leads to a maximum metal removal rate.
This process is recommended for the processing of brittle materials like glass, ceramics, refractories etc. Most of the ductile materials have been found to be unmachinable by AJM process. The metal removal rate for brittle materials depend upon the Mohr's hardness of the material.
Stand off distance (SOD):
Stand off is defined as the distance work surface and the face of the nozzle. The SOD has been found to have considerable effect on the metal removal rate and the accuracy of the process. If the value of SOD is large then the jet flares up, thus decreasing the accuracy of the process. Low MRR at low value of SOD is mainly due to decrease in pressure of jet with a decreasing distance between nozzle face and work surface. The drop in MRR at high value of SOD is due to decrease in the jet velocity with an increase in the distance.
Metal Removal Rate in AJM:
The MRR in AJM is obtained by the relation,
Where Q = metal removal rate
N = number of abrasive particles taking cut at a time
d = size of abrasive particle
v = velocity of sound
= density of abrasive material
= yield stress of material