In this article we will discuss about:- 1. Meaning of Abrasive Jet Machining (AJM) 2. Effect of Various Parameters on Metal Removal Rate (MRR) 3. Characteristics 4. Applications 5. Advantages 6. Limitations.
Meaning of Abrasive Jet Machining (AJM):
In this process a focused steam of abrasive particles (of size 10 to 40 microns) carried by high pressure gas or air at a velocity of about 200 to 400 m/sec is made to impinge on the work surface through a nozzle, and the work material is removed by erosion by the high velocity abrasive particles.
The inside diameter of the nozzle through which abrasive particles flow is about 0.04 mm and the stand-off distance (i.e. distance between nozzle tip and workpiece) is kept about 0.7 to 1.0 mm. The process can be easily controlled to vary the metal removal rate which depends on flow rate and size of abrasive particles.
This process is best suited for machining super alloys and refractory type of materials, and also machining thin sections of hard materials and making intricate hard holes. The cutting action is cool because the carrier gas serves as a coolant.
When an abrasive particle (like Al2O3 or SiC) having sharp edges hits a brittle and fragile material with a high speed, it makes dent into the material and lodges a small particle from it by a tiny brittle fracture. (Refer Fig. 10.39). The lodged out or wear particle is carried away by the air or gas.
For best cutting results Al2O3 abrasive particles of size 15 µm to 20 µm are used. Since the cutting capacity of particles decreases after one cutting action, these are normally not reused.
Fig. 10.40 shows a schematic layout of AJM. Gas (nitrogen or CO2 or air) is supplied under pressure (2 to 8 kg/ cm2) and after filter and regulator, it is passed to a mixing chamber (containing abrasive particles) vibrating at 50 c/s. From the mixing chamber, the gas along with the entrained abrasive particles of size 10—50 µm passes on to a nozzle having its tip of tungsten carbide and diameter around 0.45 mm, with a velocity of 150 to 300 m/sec. The air consumption is of the order of 0.6 m3/hour.
The nozzle tip distance is of the order of 0.81 mm. The abrasive powder feed rate is controlled by the amplitude of vibration of the mixing chamber. The relative motion between the nozzle and workpiece is obtained by cams, pantographs to control the size and shape of the cut.
Dust removal equipment is incorporated to protect the environment. The material removal rate, geometry of cut, surface roughness, and nozzle wear rate are influenced by the size and distance of nozzle, composition, strength, size and shape of abrasives flow rate; and composition, pressure and velocity of carrier gas.
The abrasive particles should have irregular shape and consist of short edges. The rounded shape will be useless. Abrasives may be Al3O3, SiC, sodium bicarbonate, dolomite, glass beads; their selection and their grain size depending on the machining operation.
The material removal rate is mainly dependent on the flow rate and size of the abrasive particles. High grain size will always produce more metal removal. At a particular pressure the material removal rate increases with the abrasive flow rate but after reaching an optimum value, the material removal rate decreases with the further increase in abrasive flow rate.
This is due to the fact that mass flow rate of the gas decreases with the increase of abrasive flow rate and hence the mixing ratio increases causing a decrease in material removal rate because of the decreasing energy available for erosion. The abrasive particles are generally not used again and again.
Effect of Various Parameters on Metal Removal Rate (MRR):
Figs. 10.41 to 10.45 show how the metal removal rate is affected by various parameters.
Typical material removal rate is 16 mm3/min for cutting glass and minimum width of cut can be 0.1 mm. An accuracy of 0.1 mm and surface roughness of 0.2 µm (with 10 µm size particles) to 1.5 µm (with 50 µm size) particles can be attained.
The carrier gas used is usually air, CO2 or N2. The nozzle pressure is maintained between 2 and 8.0 kgf/cm2. Higher nozzle pressure results in greater material removal rate, but it decreases the nozzle life.
The material removal rate first increases with the increase of tip distance from work upto a certain limit after which it remains unchanged for a certain tip distance and then falls gradually.
In this process the limitations are that the material removal rate is low, stray cutting can’t be avoided, tapering effect may be found because of the unavoidable flaring of the abrasive jet, abrasives may get embedded in the work surface, and suitable dust collecting system has to be provided.
The distance between the nozzle tip and the work surface has great influence on the diameter of cut, its shape and size, and also rate of material removal.
Following table gives an idea of these parameters (with nozzle orifice diameter 0.45 mm, 25 pm Al2O3 abrasives, flowing at 10 g/min and air at 5 kg/cm2 and work material- glass).
It would be noted that velocity of jet from nozzle (and hence material removal rate) keeps on increasing upto a nozzle tip separation of 10 mm but beyond that it decreases due to drag of atmosphere.
The AJM process is best suited for machining brittle and heat-sensitive materials like glass, quartz, sapphire, ceramics, etc. It is used for drilling holes, cutting slots, cleaning hard surfaces, deburring, polishing, radiusing.
The advantages of this process are that it can be used to cut intricate hole shapes in hard and brittle materials; even fragile and heat sensitive materials can be cut without damage, and the initial cost is low.
Its disadvantages are that it removes material at very low rate, stray cutting can occur resulting in poor accuracy, and soft materials can’t be machined by this process.
Characteristics of Abrasive Jet Cutting (AJM):
i. Abrasive — Al2O3 or SiC to be used once
ii. Size of abrasive — around 25 µm
iii. Flow rate — 2—20 g/min
iv. Medium — Air or CO2
v. Velocity — 150—300 m/sec
vi. Pressure — 2 to 8 kg/cm2
vii. Nozzle — WC or sapphire with orifice area—0.05 to 0.2 mm2
viii. Life of nozzle — WC (12—30 hrs), sapphire (around 300 hrs)
ix. Nozzle tip distance — 0.25 to 15 mm
x. Tolerance — ± 0.05 mm
xi. Surface Roughness — 0.15—0.2 µm with particles of 10 µm size, 0.4—0.8 µm and 1.0 to 1.5 µm, with particle size of 25 and 50 µm
xii. Work material – Hard and brittle materials like glass, ceramic, mica, etc.
xiii. Machining operations – Drilling, cutting, deburring, cleaning
xiv. Advantages – Can cut intricate hole shapes in hard and brittle materials, fragile and heat sensitive materials can be cut without damage because there is no heating of working surface
xv. Limitations – Low material removal rate, low accuracy (0.1 mm) due to stray cutting (taper effect), and abrasives get embedded in surface if material is soft.
Applications of Abrasive Water Jet Cutting (AJM):
Abrasive water jet cutting is highly used in aerospace, automotive and electronics industries. In aerospace industries, parts such as titanium bodies for military aircrafts, engine components (aluminium, titanium, heat resistant alloys), aluminium body parts and interior cabin parts are made using abrasive water jet cutting. Complex shapes can be produced by this process.
In automotive industries, parts like interior trim (head liners, trunk liners, door panels) and fibre glass body components and bumpers are made by this process. Similarly, in electronics industries, circuit boards and cable stripping are made by abrasive water jet cutting. Large parts can be cut with very narrow kerf which reduces wastage of material.
Advantages of Abrasive Water Jet Cutting (AJM):
i. In most of the cases, no secondary finishing required
ii. Typical finish is of the order 125-250 microns
iii. Little to no cutting burr
iv. No cutter induced distortion
v. Low cutting forces on workpieces
vi. No heat affected zone
vii. Eliminates thermal distortion
viii. Limited tooling requirements
ix. Smaller kerf size reduces material wastages
x. Localises structural changes
xi. No cutter induced metal contamination
xii. No slag or cutting dross
xiii. Precise, multi plane cutting of contours, shapes, and bevels of any angle.
Limitations of Abrasive Jet Cutting (AJM):
i. Cannot drill flat bottom
ii. Cannot cut materials that degrade quickly with moisture
iii. Surface finish degrades at higher cut speeds which are frequently used for rough cutting
iv. The major disadvantages of abrasive water jet cutting are high capital cost and high noise levels during operation.