Unit – 4 �Unconventional Machining Process

Need for Unconventional Machining

• Traditional machining is mostly based on removal of materials using tools that are harder than the materials themselves.
• Traditional machining methods are often ineffective in machining hard materials like ceramics and composites or machining under very tight tolerances as in micromachined components.
• Very hard fragile materials difficult to clamp for traditional machining
• When the workpiece is too flexible or slender
• When the shape of the part is too complex

Classification of Unconventional Machining

• Unconventional manufacturing processes is defined as a group of processes that remove excess material by various techniques involving mechanical, thermal, electrical or chemical energy or combinations of these energies but do not use a sharp cutting tools as it needs to be used for traditional manufacturing processes.
• Mechanical: Erosion of the work material by a high velocity stream of abrasives or fluids (or both)
• Thermal: The thermal energy is applied to a very small portion of the work surface, causing that portion to be removed by fusion and/or vaporization of the material. The thermal energy is generated by conversion of electrical energy.
• Electrochemical: Mechanism is reverse of electroplating.
• 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 workpart, while other portions of the surface are protected by a mask.

Classification of Non-Traditional Machining

Mechanical Machining

• Ultrasonic Machining (USM) and Water jet Machining (WJM) are typical examples of single action, mechanical non traditional machining processes.
• The machining medium is solid grains suspended in an abrasive slurry in the former, while a fluid is employed in the WJM process.
• The introduction of abrasives to the fluid jet enhances the machining efficiency and is known as abrasive water jet machining. Similar case happens when ice particles are introduced as in Ice Jet Machining

Thermal Machining

• Thermal machining removes materials by melting or vaporizing the work piece material.
• Many secondary phenomena occur during machining such as microcracking, formation of heat affected zones, striations etc.
• The source of heat could be plasma as during EDM and PBM or photons as during LBM, electrons in EBM, ions in IBM etc.

Chemical and Electrochemical Machining

• Chemical milling and photochemical machining or photochemical blanking all use a chemical dissolution action to remove the machining allowance through ions in an etchant.
• Electrochemical machining uses the electrochemical dissolution phase to remove the machining allowance using ion transfer in an electrolytic cell.

Water Jet Cutting (WJM)

• WJM is a form of micro erosion. It works by forcing a large volume of water through a small orifice in the nozzle.
• The extreme pressure of the accelerated water particles contacts a small area of the workpiece and acts like a saw and cuts a narrow groove in the material.

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Process parameters

• Standoff distance: small to avoid dispersion of the fluid stream (3.2 mm)
• Nozzle opening diameter: affects precision
• Water pressure: high for thicker materials
• Cutting feed rate: the velocity at which the WJC nozzle is traversed along the cutting path

• Nozzle diameter: 0.1 to 0.4 mm
• Pressure: up to 400 MPa
• Velocity: up to 900 m/s
• Fluid is pressurized by a hydraulic pump

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

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.

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Applications

• Mostly used to cut lower strength materials such as wood, plastics, rubber, paper, leather, composite, etc.
• Food preparation
• Good for materials that cannot withstand high temperatures of other methods for stress distortion or metallurgical reasons.

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 any 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.

Applications

• Aerospace, automotive and electronics industries, In aerospace industries, parts such as titanium bodies for military aircrafts, engine.
• 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.

Advantages of abrasive water jet cutting

• In most of the cases, no secondary finishing required
• No cutter induced distortion
• Low cutting forces on workpieces
• 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

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 cut

Abrasive Jet Machining

• In AJM, the material removal takes place due to impingement of the fine abrasive particles.
• The abrasive particles are typically of 0.025mm diameter and the air discharges at a pressure of several atmosphere.

Mechanics of AJM

• Abrasive particle impinges on the work surface at a high velocity and this impact causes a tiny brittle fracture and the following air or gas carries away the dislodged small work piece particle.

Fracture of work surface Formation of cavity

Process Parameters

The process characteristics can be evaluated by judging

• the MRR,
• the geometry of the cut,
• the roughness of the surface produced, and
• the rate of nozzle wear

The major parameters which control these quantities are:

• The abrasive (composition, strength, size and mass flow rate).
• The gas (composition, pressure and velocity).
• The nozzle (geometry, material, distance from and inclination to the work surface).

The Abrasive

• Mainly two types of abrasives are used (1) Aluminum oxide and (2) Silicon carbide. (Grains with a diameter 10-50 microns are readily available)
• For good wear action on the surfaces, the abrasive grains should have sharp edges.
• A reuse of the abrasive powder is normally not recommended because of a decrease of cutting capacity and clogging of the nozzle orifices due to contamination.
• The mass flow rate of the abrasive particles depends on the pressure and the flow rate of the gas.

The Gas

• The AJM unit normally operates at a pressure of 0.2-1.0 N/mm2 .
• The composition of gas and a high velocity has a significant impact on the MRR even if the mixing ratio is not changed.
• The gas propulsion system supplies clean and dry gas (air, nitrogen, or CO2) to propel the abrasive particles.

Nozzle to Tip Distance

• The nozzle tip distance (NTD) or the stand off distance is a critical parameter in AJM.
• The NTD not only affects the MRR from the work surface but also the shape and size of the cavity produced.

Ultrasonic Machining (USM) Process

• USM process involves a tool (made of a ductile and tough material) vibrating with a low amplitude and very high frequency and a continuous flow of an abrasive slurry in the small gap between the tool and the work piece.
• The tool is gradually fed with a uniform force.
• The impact of the hard abrasive grains fractures the hard and brittle work surface, resulting in the removal of the work material in the form of small wear particles.
• The tool material being tough and ductile wears out at a much slower rate.

Mechanics of USM

• The hammering of the abrasive particles on the work surface by the tool.
• The impact of free abrasive particles on the work surface.
• The erosion due to cavitation.
• The chemical action associated with the fluid used.

Process Parameters

• Frequency
• Amplitude
• Static loading (feed force)
• Hardness ratio of the tool and the workpiece
• Grain size
• Concentration of the abrasive in the slurry

Abrasive Slurry

• The most common abrasives are Boron Carbide (B4C), Silicon Carbide (SiC), Corrundum (Al2O3), Diamond and Boron silicarbide.
• B4C is the best and most efficient among the rest but it is expensive.
• SiC is used on glass, germanium and most ceramics.
• Cutting time with SiC is about 20-40% more than that with B4C.
• Diamond dust is used only for cutting diamond and rubies.
• Water is the most commonly used fluid although other liquids such as benzene, glycerol and oils are also used.

Advantages

USM process is a non-thermal, non-chemical, creates no changes in the microstructures, chemical or physical properties of the workpiece 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

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.

Electrochemical Machining

• In ECM, a dc voltage (10-25 v) is applied across the gap between a pre-shaped cathode tool and an anode work piece. The work piece is dissolved by an electrochemical reaction to the shape of the tool.
• In the ECM process the work-piece is connected to a positive electrode and the tool to the negative terminal for metal removal.
• If the tool is given a downward motion, the work surface tends to take the same shape as that of the tool, and at a steady state the gap is uniform.
• The electrolyte flows at high speed (10-60 m/s) through the gap (0.1-0.6 mm) to dissipate heat and wash away the dissolved metal.

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Electrochemistry of ECM process

• The amount of chemical change produced by an electric current, that is, the amount of any material dissolved or deposited, is proportional to the quantity of electricity passed.

Advantages of ECM

• The components are not subject to either thermal or mechanical stress.
• No tool wear during ECM process.
• Fragile parts can be machined easily as there is no stress involved.
• ECM deburring can debur difficult to access areas of parts.
• High surface finish (up to 25 μm in) can be achieved by ECM process

Limitations of ECM

• ECM is not suitable to produce sharp square corners or flat bottoms because of the tendency for the electrolyte to erode away sharp profiles.
• ECM can be applied to most metals but, due to the high equipment costs, is usually used primarily for highly specialized applications.

Electrical Discharge Machining (EDM)

• EDM is a thermal erosion process whereby material is melted and vaporized from an electrically conducive workpiece immersed in a liquid dielectric with a series of spark discharges between the tool electrode and the workpiece created by a power supply.
• The EDM system consists of a shaped tool or wire electrode, and the part. The part is connected to a power supply to create a potential difference between the workpiece and the tool.
• When the potential difference is sufficiently high, a transient spark discharges through the fluid, removing a very small amount of metal from the workpiece.

The dielectric fluid

• acts as an insulator until the potential is sufficiently high,
• acts as a flushing medium, and
• Provides a cooling medium

Characteristics of EDM

• Gap control
• Tool or electrode-brass or copper although electrodes (tool) of tungsten, graphite, steel and silver alloys
• Work material
• Spark generators
• Metal removal rate (MRR)

EDM Applications

• Widely used in aerospace, mold making, and die casting to produce die cavities, small deep holes, narrow slots, turbine blades, and intricate shapes.

Cavities produced by EDM

Stepped cavities

Advantages of EDM

• By this process, materials of any hardness can be machined;
• No burrs are left in machined surface.
• One of the main advantages of this process is that thin and fragile/brittle components can be machined without distortion
• Complex internal shapes can be machined

Limitations of EDM

• This process can only be employed in electrically conductive materials;
• Material removal rate is low and the process overall is slow compared to conventional machining processes;
• Unwanted erosion and over cutting of material can occur.
• Rough surface finish when at high rates of material removal.

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Wire Cut Electrical Discharge Machining(WCEDM)

• In this process, a slowly moving wire travels along a prescribed path and removes material from the work piece
• The material is removed by a series of discrete discharges between the wire electrode and the workpiece in the presence of dieelectirc fluid, which creates a path for each discharge as the fluid becomes ionized in the gap.
• The wires for wire EDM is made of brass, copper, tungsten, molybdenum. Zinc or brass coated wires are also used extensively in this process.

Laser Beam Machining

• Add energy to make electrons “jump” to higher energy orbit – Electron “relaxes” and moves to equilibrium at ground-state energy level
• – Emits a photon in this process (key laser component)
• – Two mirrors reflect the photons back and forth and “excite” more electrons
• – One mirror is partially reflective to allow some light to pass through: creates narrow laser beam

Advantage of laser cutting

• No limit to cutting path as the laser point can move any path.
• The process is stress less allowing very fragile materials to be laser cut without any support.
• Very hard and abrasive material can be cut.
• It is a cost effective and flexible process.
• High accuracy parts can be machined.
• No tool wear
• Narrow heat effected zone

Limitations of laser cutting

• Uneconomic on high volumes compared to stamping
• Limitations on thickness due to taper
• High capital cost
• High maintenance cost

Plasma Arc Machining

• When a gas is heated to a temperature above 5500°C, it becomes partially ionized and exists in the form of a mixture of free electrons, positively charged ions and neutral atoms and this mixture is called plasma.
• The temperature of the central part of the plasma may go as high as 11,000 to 28000°C.
• Plasma arc machining is a material removal process wherein the material is removed by directing a high velocity jet of high temperature ionized gas on the work.

• The metal removal in PAM is basically due to the high temperature produced.
• The heating of the work piece is, as a result of anode heating, due to direct electron bombardment plus convection heating from the high temperature plasma that accompanies the arc.
• The heat produced is sufficient to raise the work piece temperature above its melting point and the high velocity gas stream effectively blows the molten metal away.

CHEMICAL MACHINING (CHM)

• The working principle of chemical machining is based on chemical etching .the part of the work piece metal where material is to be removed is brought into contact with a strong corrosive chemical called etchant.
• The etchant react with the workpiece in the material to be cut and causes the solid material to be removed. Thus the metal is removed by the chemical attack of the etchant.
• The portion of the work material where material is not to be removed is protected from chemical attack by means special coating called as maskants.

• The working principle of chemical machining is based on chemical etching .the part of the work piece metal where material is to be removed is brought into contact with a strong corrosive chemical called etchant.
• The etchant react with the workpiece in the material to be cut and causes the solid material to be removed. Thus the metal is removed by the chemical attack of the etchant.
• The portion of the work material where material is not to be removed is protected from chemical attack by means special coating called as maskants.