MECHANICAL ENERGY BASED PROCESSES
• Abrasive Jet Machining (AJM)
• Abrasive Water Jet Machining (AWJM)
• Water Jet Machining (WJM)
• Ultrasonic Machining (USM)
1 ABRASIVE JET MACHINING (AJM)
In Abrasive Jet Machining (AJM), abrasive particles are made to impinge on the work material at a high velocity. The high velocity abrasive particles remove the material by micro-cutting action as well as brittle fracture of the work material.
In AJM, generally, the abrasive particles of around 50 μm grit size would impinge on the work material at velocity of 200 m/s from a nozzle of I.D. of 0.5 mm with a standoff distance of around 2 mm. The kinetic energy of the abrasive particles would be sufficient to provide material removal due to brittle fracture of the work piece or even micro cutting by the abrasives.
1. Process Parameters and Machining Characteristics
Abrasive: Material – Al2O3 / SiC / glass beads
Shape – irregular / spherical
Size – 10 ~ 50 μm
Mass flow rate – 2 ~ 20 gm/min
Carrier gas: Composition – Air, CO2, N2
Density – Air ~ 1.3 kg/m3
Velocity – 500 ~ 700 m/s Pressure – 2 ~ 10 bar
Flow rate – 5 ~ 30 lpm
Abrasive Jet : Velocity – 100 ~ 300 m/s
Mixing ratio – mass flow ratio of abrasive to gas
Stand-off distance – 0.5 ~ 5 mm
Impingement Angle – 60 ~ 90
Nozzle: Material – WC
Diameter – (Internal) 0.2 ~ 0.8 mm
Life – 10 ~ 300 hours
.2. Modeling of material removal
Material removal in AJM takes place due to brittle fracture of the work material due to impact of high velocity abrasive particles.
3. Modeling has been done with the following assumptions:
(i) Abrasives are spherical in shape and rigid. The particles are characterized by the mean grit diameter
(ii) The kinetic energy of the abrasives are fully utilized in removing material
(iii) Brittle materials are considered to fail due to brittle fracture and the fracture volume is considered to be hemispherical with diameter equal to choral length of the indentation
(iv) For ductile material, removal volume is assumed to be equal to the indentation volume due to particulate impact
2. WATER JET MACHINING (WJM)
Water jet cutting can reduce the costs and speed up the processes by eliminating or reducing expensive secondary machining process. Since no heat is applied on the materials, cut edges are clean with minimal burr. Problems such as cracked edge defects, crystallization, hardening, reduced wealdability and machinability are reduced in this process.
Water jet technology uses the principle of pressurizing water to extremely high pressures, and allowing the water to escape through a very small opening called “orifice” or “jewel”. Water jet cutting uses the beam of water exiting the orifice to cut soft materials. This method is not suitable for cutting hard materials. The inlet water is typically pressurized between1300 – 4000 bars. This high pressure is forced through a tiny hole in the je el, hich is typically 0.18 to 0.4 mm in diameter. Picture of water jet chining process.
Water jet cutting is mostly used to cut lower strength materials such as wood, plastics and aluminium. When abrasives are added, (abrasive water jet cutting) stronger materials such as steel and tool steel.
2.3. Advantages Of Water Jet Cutting
• There is no heat generated in water jet cutting; which is especially useful for cutting tool steel and other metals where excessive heat may change the properties of the material.
• Unlike machining or grinding, water jet cutting does not produce any dust or particles that are harmful if inhaled.
• Other advantages are similar to abrasive water jet cutting
2.4. Disadvantages of water jet cutting
• One of the main disadvantages of water jet cutting is that a limited number of materials can be cut economically.
• Thick parts cannot be cut by this process economically and accurately
• Taper is also a problem with water jet cutting in very thick materials. Taper is when the jet exits the part at different angle than it enters the part, and cause dimensional inaccuracy.
3. ABRASIVE WATER-JET MACHINING (AWJM)
Abrasive water jet cutting is an extended version of water jet cutting; in which the water jet contains abrasive particles such as silicon carbide or aluminium oxide in order to increase the material removal rate above that of water jet machining. Almost a ny type of material ranging from hard brittle materials such as ceramics, metals and glass to extremely soft materials such as foam and rubbers can be cut by abrasive water jet cutting. The narrow cutting stream and computer controlled movement enables this process to produce parts accurately and efficiently. This machining process is especially ideal for cutting materials that cannot be cut by laser or thermal cut. Metallic, non-metallic and advanced composite materials of various thicknesses can be cut by this process. This process is particularly suitable for heat sensitive materials that cannot be machined by processes that produce heat while machining.
The schematic of abrasive water jet cutting is shown in Figure 15 which is similar to water jet cutting apart from some more features underneath the jewel; namely abrasive, guard and mixing tube. In this process, high velocity water exiting the jewel creates a vacuum which sucks abrasive from the abrasive line, which mixes with the water in the mixing tube to form a high velocity beam of abrasives.
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, and heat resistant alloys), aluminium body parts and interior cabin parts are made using abrasive water jet cutting.
In automotive industries, parts like interior trim (head liners, trunk liners, and door panels) and fiber 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.
.3.4.Advantages of abrasive water jet cutting
• In most of the cases, no secondary finishing required
• No cutter induced distortion
• Low cutting forces on work pieces
• Limited tooling requirements
• Little to no cutting burr
• Typical finish 125-250 microns
• Smaller kerf size reduces material wastages
• No heat affected zone
• Localises structural changes
• No cutter induced metal contamination
• Eliminates thermal distortion
• No slag or cutting dross
• Precise, multi plane cutting of contours, shapes, and bevels of any angle.
3.5 Limitations of abrasive water jet cutting
• Cannot drill flat bottom
• Cannot cut materials that degrades quickly with moisture
• Surface finish degrades at higher cut speeds which are frequently used for rough cuts
The major disadvantage s of abrasive water jet cutting are high capital cost and high noise levels during operation.
A component cut by abrasive water jet cutting is shown in Figure .As it can be seen, large parts can but cut with very narrow kerfs which reduces ma terial wastages. The complex shape part made by abrasive water jet cutting
3.6. Abrasive water jet cutting
• WJM - Pure
• WJM - with stabilizer
• AWJM – entrained – three phase –abrasive, water and air
• AWJM – suspended – two phase –abrasive and water o direct pumping
i. Indirect Pumping
ii. Bypass pumping
4. ULTRASONIC MACHINING (USM)
USM is mechanical material removal process or an abrasive process used to erode holes or cavities on hard or brittle work piece by using shaped tools, high frequency mechanical motion and an abrasive slurry. USM offers a solution to the expanding need for machining brittle materials such as single crystals, glasses and polycrystalline ceramics, and increasing complex operations to provide intricate shapes and work piece profiles. It is therefore used extensively in machining hard and brittle materials that are difficult to machine by traditional manufacturing processes. The hard particles in slurry are accelerated toward the surface of the work piece by a tool oscillating at a frequency up to 100 KHz - through repeated abrasions, the tool machines a cavity of a cross section identical to its own.
USM is primarily targeted for the machining of hard and brittle materials (dielectric or conductive) such as boron carbide, ceramics, titanium carbides, rubies, quartz etc. USM is a versatile machining process as far as properties of materials are concerned. This process is able to effectively machine all materials whether they are electrically conductive or insulator.
For an effective cutting operation, the following parameters need to be carefully considered:
• The machining tool must be selected to be highly wear resistant, such as high-carbon steels.
• The abrasives (25-60 µm in dia.) in the (water-based, up to 40% solid volume) slurry
Includes: Boron carbide, silicon carbide and aluminum oxide.
The beauty of USM is that it can make non round shapes in hard and brittle materials. Ultrasonically machined non round-hole part is shown in Figure 11.
Figure : A non-round hole made by USM
4.2Advantage of USM
USM process is a non-thermal, non-chemical, creates no changes in the microstructures, chemical or physical properties of the work piece and offers virtually stress free machined surfaces.
• Any materials can be machined regardless of their electrical conductivity
• Especially suitable for machining of brittle materials
• Machined parts by USM possess better surface finish and higher structural integrity.
• USM does not produce thermal, electrical and chemical abnormal surface
4.3. Disadvantages of USM
• USM has higher power consumption and lower material-removal rates than traditional Fabrication processes.
• Tool wears fast in USM.
• Machining area and depth is restraint in USM.