non conventional machining

NON-TRADITIONAL MACHINING- The term nontraditional machining refers to this group that removes excess material by various techniques involving mechanical, thermal, electrical, or chemical energy (or combinations of these energies). They do not use a sharp cutting tool in the conventional sense.

The nontraditional processes are often classified according to principal form of energy used to effect material removal. By this classification, there are four types:

1. Mechanical: Mechanical energy in some form other than the action of a conventional cutting tool is used in these nontraditional processes. Erosion of the work material by a high velocity stream of abrasives or fluid (or both) is a typical form of mechanical action in these processes.
2. Electrical: These nontraditional processes use electrochemical energy to remove material; the mechanism is the reverse of electroplating.
3. Thermal: These processes use thermal energy to cut or shape the work part. The thermal energy is generally applied to a very small portion of the work surface, causing that portion to be removed by fusion and/or vaporization. The thermal energy is generated by the conversion of electrical energy.
4. Chemical: Most materials (metals particularly) are susceptible to chemical attack by certain acids or other etchants. In chemical machining, chemicals selectively remove material from portions of the work part, while other portions of the surface are protected by a mask.

Mechanical Energy Processes: Ultrasonic Machining- Ultrasonic machining (USM) is a nontraditional machining process in which abrasives contained in a slurry are driven at high velocity against the work by a tool vibrating at low amplitude and high frequency. The amplitudes are around 0.075 mm, and the frequencies are approximately 20,000 Hz.

• The tool oscillates in a direction perpendicular to the work surface, and is fed slowly into the work, so that the shape of the tool is formed in the part. However, it is the action of the abrasives, impinging against the work surface, that performs the cutting.
• Common tool materials used in USM include soft steel and stainless steel. When the tool is made hollow, the internal contour should be parallel to the external one to ensure uniform wear.
• Abrasive materials in USM include boron nitride, boron carbide, aluminum oxide, silicon carbide, and diamond. Grit size ranges between 100 and 2000.
• The slurry in USM consists of a mixture of water and abrasive particles. Concentration of abrasives in water ranges from 20% to 60%.
• In addition to surface finish, material removal rate is an important performance variable in ultrasonic machining.

Jet Machining- In jet machining, high-velocity stream of water (Water Jet Cutting) or water mixed with abrasive materials (Abrasive Water Jet Cutting) is directed to the workpiece to cut the material. If a mixture of gas and abrasive particles is used, process is referred to as Abrasive Jet Machining and is used not to cut the work but for finishing operations like deburring, cleaning, polishing.

Water Jet Cutting (WJC) uses a fine, high-pressure, high velocity stream of water directed at the work surface to cause cutting of the work, as illustrated in Fig.

• Water is the most common fluid used, but additives such as alcohols, oil products and glycerol are added when they can be dissolved in water to improve the fluid characteristics.
• The fluid is pressurized at 150-1000 MPa to produce jet velocities of 540-1400 m/s. The fluid flow rate is typically from 0.5 to 2.5 l/min. The jet have a well behaved central region surrounded by a fine mist.
• Typical work materials involve soft metals, paper, cloth, wood, leather, rubber, plastics, and frozen food.

In Abrasive Water Jet Cutting (AWJC) a narrow, focused, water jet is mixed with abrasive particles. This jet is sprayed with very high pressures resulting in high velocities that cut through all materials.

• The presence of abrasive particles in the water jet reduces cutting forces and enables cutting of thick and hard materials (steel plates over 80-mm thick can be cut).
• The velocity of the stream is up to 90 m/s, about 2.5 times the speed of sound.
• Abrasive Water Jet Cutting process was developed to cut materials that cannot stand high temperatures for stress distortion or metallurgical reasons such as wood and composites, and traditionally difficult-to-cut materials, e.g. ceramics, glass, stones, titanium alloys.
• The common types of abrasive materials used are quartz sand, silicon carbide, and corundum (Al₂O₃), at grit sizes ranging between 60 and 120.

In Abrasive Jet Machining (AJM) fine abrasive particles (typically ~0.025mm) are accelerated in a gas stream (commonly air) towards the work surface.

• As the particles impact the work surface, they cause small fractures, and the gas stream carries both the abrasive particles and the fractured (wear) particles away.
• The jet velocity is in the range of 150-300 m/s and pressure is from two to ten times atmospheric pressure.
• The preferred abrasive materials involve aluminum oxide (corundum) and silicon carbide at small grit sizes. The grains should have sharp edges and should not be reused as the sharp edges are worn down and smaller particles can clog the nozzle.
• Abrasive Jet Machining is used for deburring, etching, and cleaning of hard and brittle metals, alloys, and nonmetallic materials (e.g., germanium, silicon, glass, ceramics, and mica).

Electrochemical machining: ECM uses principle of electrolysis to remove metal from the work piece. Electrolysis is based on faradays laws of electrolysis which is stated as "weight of substance produced during electrolysis is proportional to current passing, length of time the process used and the equivalent weight of material which is deposited".

• ECM is just reverse of electroplating (anode loses metal to cathode). So, in ECM work is made anode and tool is made cathode. So, work loses metal, but before depositing it on to tool, it is carried away be electrolyte.
• In ECM the tool is provided with a constant feed motion and electrolyte is pumped at a high. Pressure through the tool and the small gap between tool and work piece.
• The current used is few thousand amperes and voltage used is 8-20 volts and gap is of the order of to 0.2mm. Practically no tool wears in ECM.

Electrochemical Grinding (ECG)- Electrochemical grinding (ECG) is a special form of ECM in which a rotating grinding wheel with a conductive bond material is used to augment the anodic dissolution of the metal work part surface, as illustrated in Fig.

• Abrasives used in ECG include aluminum oxide and diamond. The bond material is either metallic or resin bond impregnated with metal particles to make it electrically conductive.
• Applications of ECG include sharpening of cemented carbide tools and grinding of surgical needles, other thin wall tubes, and fragile parts.

Electric Discharge Machining- In electric discharge processes, the work material is removed by a series of sparks that cause localized melting and evaporation of the material on the work surface. These processes can be used only on electrically conducting work materials.

• A formed electrode tool produces the shape of the finished work surface. The sparks occur across a small gap between tool and work surface.
• The EDM process must take place in the presence of a dielectric fluid, which creates a path for each discharge as the fluid becomes ionized in the gap.
• The fluid, quite often the kerosene-based oil is also used to carry away debris. The discharges are generated by a pulsating direct-current power supply connected to the work and the tool.
• Electrode materials are high temperature, but easy to machine, thus allowing easy manufacture of complex shapes. Typical electrode materials include copper, tungsten, and graphite.
• The process is based on melting temperature, not hardness, so some very hard materials can be machined this way

Electric Discharge Wire Machining- Wire Electric Discharge Machining (Wire EDM) is a special form of EDM that uses a small diameter wire as the electrode to cut a narrow kerf in the work. Wire EDM is illustrated in the figure.

• The workpiece is fed continuously and slowly past the wire in order to achieve the desired cutting path. Numerical control is used to control the work-part motions during cutting.
• As it cuts, the wire is continuously advanced between a supply spool and a take-up spool to present a fresh electrode of constant diameter to the work. This helps to maintain a constant kerf width during cutting.
• As in EDM, wire EDM must be carried out in the presence of a dielectric. This is applied by nozzles directed at the tool-work interface as in the figure, or the work part is submerged in a dielectric bath.
• Wire diameters range from 0.08 to 0.30 mm, depending on required kerf width. Materials used for the wire include brass, copper, tungsten, and molybdenum.

Laser Beam Machining (LBM)- Laser beam machining (LBM) uses the light energy from a laser to remove material by vaporization and ablation. The setup for LBM is illustrated in the figure:

• The types of lasers used in LBM are basically the carbon dioxide (CO₂) gas lasers. Lasers produce collimated monochromatic light with constant wavelength.
• The light produced by the laser has significantly less power than a normal white light, but it can be highly focused, thus delivering a significantly higher light intensity and respectively temperature in a very localized area.
• Lasers are being used for a variety of industrial applications, including heat treatment, welding, and measurement, as well as a number of cutting operations such as drilling, slitting, slotting, and marking operations.
• Drilling small-diameter holes is possible, down to 0.025 mm. For larger holes, the laser beam is controlled to cut the outline of the hole.

Electron Beam Machining (EBM)- Electron beam machining (EBM) is one of several industrial processes that use electron beams. Electron beam machining uses a high-velocity stream of electrons focused on the workpiece surface to remove material by melting and vaporization. A schematic of the EBM process is illustrated in the figure.

• An electron beam gun generates a continuous stream of electrons that are focused through an electromagnetic lens on the work surface. The electrons are accelerated with voltages of approx. 150,000 V to create velocities over 200,000 km/s.
• The lens is capable of reducing the area of the beam to a diameter as small as 0.025 mm. On impinging the surface, the kinetic energy of the electrons is converted into thermal energy of extremely high density, which vaporizes the material in a very localized area.
• EBM must be carried out in a vacuum chamber to eliminate collision of the electrons with gas molecules.
• The process is generally limited to thin parts in the range from 0.2 to 6 mm thick. Other limitations of EBM are the need to perform the process in a vacuum, the high energy required, and the expensive equipment.
• Applications include drilling of extremely small diameter holes, down to 0.05 mm diameter, drilling of holes with very high depth-to-diameter ratios, more than 100:1, and cutting of slots that are only about 0.025 mm.