Non Traditional Manufacturing Processes

Introduction
Non-traditional 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.
Extremely hard and brittle materials are difficult to machine by traditional machining processes such as turning, drilling, shaping and milling. Non traditional machining processes, also called advanced manufacturing processes, are employed where traditional machining processes are not feasible, satisfactory or economical due to special reasons as outlined below.

• 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

Several types of non-traditional machining processes have been developed to meet extra required machining conditions. When these processes are employed properly, they offer many advantages over non-traditional machining processes. The common non-traditional machining processes are described in this section.
Electrical Discharge Machining (EDM)
Electrical discharge machining (EDM) is one of the most widely used non-traditional machining processes. The main attraction of EDM over traditional machining processes such as metal cutting using different tools and grinding is that this technique utilises thermoelectric process to erode undesired materials from the workpiece by a series of discrete electrical sparks between the workpiece and the electrode. A picture of EDM machine in operation is shown in Figure 1.

Figure 1: Electrical discharge machine The traditional machining processes rely on harder tool or abrasive material to remove the softer material whereas non-traditional machining processes such as EDM uses electrical spark or thermal energy to erode unwanted material in order to create desired shape. So, the hardness of the material is no longer a dominating factor for EDM process. A schematic of an EDM process is shown in Figure 2, where the tool and the workpiece are immersed in a dielectric fluid.
Figure 2: Schematic of EDM process

EDM removes material by discharging an electrical current, normally stored in a capacitor bank, across a small gap between the tool (cathode) and the workpiece (anode) typically in the order of 50 volts/10amps.

Application of EDM
The EDM process has the ability to machine hard, difficult-to-machine materials. Parts with complex, precise and irregular shapes for forging, press tools, extrusion dies, difficult internal shapes for aerospace and medical applications can be made by EDM process. Some of the shapes made by EDM process are shown in Figure 3.

Figure 3: Difficult internal parts made by EDM process

Working principle of EDM
As shown in Figure 1, at the beginning of EDM operation, a high voltage is applied across the narrow gap between the electrode and the workpiece. This high voltage induces an electric field in the insulating dielectric that is present in narrow gap between electrode and workpiece. This cause conducting particles suspended in the dielectric to concentrate at the points of strongest electrical field. When the potential difference between the electrode and the workpiece is sufficiently high, the dielectric breaks down and a transient spark discharges through the dielectric fluid, removing small amount of material from the workpiece surface. The volume of the material removed per spark discharge is typically in the range of 10-6 to 10-6 mm3.
The material removal rate, MRR, in EDM is calculated by the following foumula:
MRR = 40 I / Tm 1.23      (cm3/min)
Where, I is the current amp,
Tm is the melting temperature of workpiece in 0C
Advantages of EDM
The main advantages of DM are:

• 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
The main limitations of this process are:

• 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.

Dielectric fluids
Dielectric fluids used in EDM process are hydrocarbon oils, kerosene and deionised water. The functions of the dielectric fluid are to:

• Act as an insulator between the tool and the workpiece.
• Act as coolant.
• Act as a flushing medium for the removal of the chips.

 
The electrodes for EDM process usually are made of graphite, brass, copper and copper-tungsten alloys.
Design considerations for EDM process are as follows:

• Deep slots and narrow openings should be avoided.
• The surface smoothness value should not be specified too fine.
• Rough cut should be done by other machining process. Only finishing operation should be done in this process as MRR for this process is low.

Wire EDM
EDM, primarily, exists commercially in the form of die-sinking machines and wire-cutting machines (Wire EDM).  The concept of wire EDM is shown in Figure 4. In this process, a slowly moving wire travels along a prescribed path and removes material from the workpiece. Wire EDM uses electro-thermal mechanisms to cut electrically conductive materials. 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 area where discharge takes place is heated to extremely high temperature, so that the surface is melted and removed. The removed particles are flushed away by the flowing dielectric fluids.
The wire EDM process can cut intricate components for the electric and aerospace industries. This non-traditional machining process is widely used to pattern tool steel for die manufacturing.

Figure 4: Wire erosion of an extrusion die

 

The wires for wire EDM is made of brass, copper, tungsten, molybdenum. Zinc or brass coated wires are also used extensively in this process. The wire used in this process should posses high tensile strength and good electrical conductivity. Wire EDM can also employ to cut cylindrical objects with high precision. The sparked eroded extrusion dies are presented in Figure 5.

Figure 5: Sparked eroded extrusion dies

This process is usually used in conjunction with CNC and will only work when a part is to be cut completely through. The melting temperature of the parts to be machined is an important parameter for this process rather than strength or hardness. The surface quality and MRR of the machined surface by wire EDM will depend on different machining parameters such as applied peak current, and wire materials.

 

Chemical Machining (CM)
Introduction
Chemical machining (CM) is the controlled dissolution of workpiece material (etching) by means of a strong chemical reagent (etchant). In CM material is removed from selected areas of workpiece by immersing it in a chemical reagents or etchants; such as acids and alkaline solutions. Material is removed by microscopic electrochemical cell action, as occurs in corrosion or chemical dissolution of a metal. This controlled chemical dissolution will simultaneously etch all exposed surfaces even though the penetration rates of the material removal may be only 0.0025–0.1 mm/min. The basic process takes many forms: chemical milling of pockets, contours, overall metal removal, chemical blanking for etching through thin sheets; photochemical machining (pcm) for etching by using of photosensitive resists in microelectronics; chemical or electrochemical polishing where weak chemical reagents are used (sometimes with remote electric assist) for polishing or deburring and chemical jet machining where a single chemically active jet is used. A schematic of chemical machining process is shown in Figure 6.

Figure 6: (a) Schematic of chemical machining process (b) Stages in producing a profiled cavity by chemical machining (Kalpakjain & Schmid)

Chemical milling
In chemical milling, shallow cavities are produced on plates, sheets, forgings and extrusions. The two key materials used in chemical milling process are etchant and maskant. Etchants are acid or alkaline solutions maintained within controlled ranges of chemical composition and temperature. Maskants are specially designed elastomeric products that are hand strippable and chemically resistant to the harsh etchants.

Steps in chemical milling Residual stress relieving: If the part to be machined has residual stresses from the previous processing, these stresses first should be relieved in order to prevent warping after chemical milling. Preparing: The surfaces are degreased and cleaned thoroughly to ensure both good adhesion of the masking material and the uniform material removal. Masking: Masking material is applied (coating or protecting areas not to be etched). Etching: The exposed surfaces are machined chemically with etchants. Demasking: After machining, the parts should be washed thoroughly to prevent further reactions with or exposure to any etchant residues. Then the rest of the masking material is removed and the part is cleaned and inspected. Applications: Chemical milling is used in the aerospace industry to remove shallow layers of material from large aircraft components missile skin panels (Figure 7), extruded parts for airframes. Figure 7: Missile skin-panel section contoured by chemical milling to improve the stiffness- to- weight ratio of the part (Kalpakjain & Schmid)

Electrochemical Machining (ECM)
Introduction
Electrochemical machining (ECM) is a metal-removal process based on the principle of reverse electroplating.  In this process, particles travel from the anodic material (workpiece) toward the cathodic material (machining tool). A current of electrolyte fluid carries away the deplated material before it has a chance to reach the machining tool.  The cavity produced is the female mating image of the tool shape.

Figure 8: ECM process

Similar to EDM, the workpiece hardness is not a factor, making ECM suitable for machining difficult-to –machine materials. Difficult shapes can be made by this process on materials regardless of their hardness. A schematic representation of ECM process is shown in Figure 8. The ECM tool is positioned very close to the workpiece and a low voltage, high amperage DC current is passed between the workpiece and electrode. Some of the shapes made by ECM process is shown in Figure 9.

Figure 9: Parts made by ECM

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.
• Complex geometrical shapes in high-strength materials particularly in the aerospace industry for the mass production of turbine blades, jet-engine parts and nozzles can be machined repeatedly and accurately.
• Deep holes can be made by this 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 specialised applications.

Material removal rate, MRR, in electrochemical machining:
MRR = C .I. h    (cm3/min)
C: specific (material) removal rate (e.g., 0.2052 cm3/amp-min for nickel);
I: current (amp);
h: current efficiency (90–100%).
The rates at which metal can electrochemically remove are in proportion to the current passed through the electrolyte and the elapsed time for that operation. Many factors other than current influence the rate of machining. These involve electrolyte type, rate of electrolyte flow, and some other process conditions.

 

Ultrasonic Machining (USM)
Introduction
USM is mechanical material removal process or an abrasive process used to erode holes or cavities on hard or brittle workpiece 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 workpiece 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 workpiece 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. A schematic representation of USM is shown in Figure 10.

Figure 10: Schematic of ultrasonic machine tool
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 mm in dia.) in the (water-based, up to 40% solid volume) slurry includes:  Boron carbide, silicon carbide and aluminum oxide.

Applications
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 11: A non-round hole made by USM

Advantage of USM
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

Some 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.

Laser–Beam Machining (LBM)
Introduction
Laser-beam machining is a thermal material-removal process that utilizes a high-energy, coherent light beam to melt and vaporize particles on the surface of metallic and non-metallic workpieces. Lasers can be used to cut, drill, weld and mark. LBM is particularly suitable for making accurately placed holes. A schematic of laser beam machining is shown in Figure 12.
Different types of lasers are available for manufacturing operations which are as follows:

• CO2 (pulsed or continuous wave): It is a gas laser that emits light in the infrared region.  It can provide up to 25 kW in continuous-wave mode.
• Nd:YAG:  Neodymium-doped Yttrium-Aluminum-Garnet (Y3Al5O12) laser is a solid-state laser which can deliver light through a fibre-optic cable. It can provide up to 50 kW power in pulsed mode and 1 kW in continuous-wave mode.

Figure 12: Laser beam machining schematic

Applications
LBM can make very accurate holes as small as 0.005 mm in refractory metals ceramics, and composite material without warping the workpieces. This process is used widely for drilling and cutting of metallic and non-metallic materials. Laser beam machining is being used extensively in the electronic and automotive industries.

Laser beam cutting (drilling)

• In drilling, energy transferred (e.g., via a Nd:YAG laser) into the workpiece melts the material at the point of contact, which subsequently changes into a plasma and leaves the region.
• A gas jet (typically, oxygen) can further facilitate this phase transformation and departure of material removed.
• Laser drilling should be targeted for hard materials and hole geometries that are difficult to achieve with other methods.

A typical SEM micrograph hole drilled by laser beam machining process employed in making a hole is shown in Figure 13.

Figure 13: SEM micrograph hole drilled in 250 micro meter thick Silicon Nitride with 3rd harmonic Nd: YAG laser

Laser beam cutting (milling)

• A laser spot reflected onto the surface of a workpiece travels along a prescribed trajectory and cuts into the material.
• Continuous-wave mode (CO2) gas lasers are very suitable for laser cutting providing high-average power, yielding          high material-removal rates, and smooth cutting surfaces.

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.
• Sticky materials are also can be cut by this process.
• It is a cost effective and flexible process.
• High accuracy parts can be machined.
• No cutting lubricants required
• 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
• Assist or cover gas required

 

Water Jet Cutting
Introduction
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, crystalisation, hardening, reduced wealdability and machinability are reduced in this process.
Water jet technology uses the principle of pressurising 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 pressurised between 1300 – 4000 bars. This high pressure is forced through a tiny hole in the jewel, which is typically o.18 to 0.4 mm in diameter. A picture of water jet machining process is shown in Figure 14.

Figure 14: Water jet cutting (http://www.waterjets.org/about_abrasivejets.html)

 
Applications
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 can be cut.

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.

Abrasive Water-Jet Cutting
Introduction
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. 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.

Figure 15: Abrasive water jet machining (http://www.waterjets.org/about_abrasivejets.html)

 

Applications
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.
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
• Eliminates thermal distortion
• No slag or cutting dross
• Precise, multi plane cutting of contours, shapes, and bevels of any angle.

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 cutting.
• The major disadvantages 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 16. As it can be seen, large parts can but cut with very narrow kerf which reduces material wastages. The complex shape part made by abrasive water jet cutting is shown in Figure 17.

Figure 16: Abrasive water jet cutting

 

Figure 17: Steel gear and rack cut with an abrasive water jet

 
References

• Manufacturing Engineering and Technology Fifth Edition - Serope Kalpakjian and Steven R Sschmid. (Prentice Hall), 2006.
• Manufacturing Technology, Volume 2.  R L Timings & S P Wilkinson (Longman) 2000.
• Manufacturing Processes and Equipment – George Tlusty (Prentice Hall).
• E. Paul DeGarmo, J.T. Black, R.A. Kohsern, Materials and Processes in Manufacturing, 8th ed., Prentice-Hall, Englewood Cliffs, NJ, 1997, pp. 941–947.
• Chemical Machining, Metals Handbook, vol. 3, Machining, 8th ed., American Society for Metals, Metals Park, OH, pp. 240–248.
• http://www.dimeg.unipd.it/didattica/31/8_Non_conventional.pdf
•  www.eng.morgan.edu
• http://www.answers.com/topic/electrical-discharge-machining

10. http://www.exitech.co.uk/pdfFiles/Microhole%20Drilling%20using%20Reshaped%20Pulsed%20Gaussian%20Laser%20Beam.pdf
11. http://www.google.co.uk/search?q=ultrasonic+machining+picture&hl=en&start=10&sa=N
12. http://www.ebtecwelding.com/waterjet-cutting/wjetlimitations.html
13. http://www.waterjets.org/about_abrasivejets.html
14. http://www.waterjets.org/index.html

Lecture notes on non-traditional machining processes by:
Dr. H. N. Dhakal
Lecturer in Mechanical and Marine Engineering,
School of Engineering,
University of Plymouth