Nontraditional Machining and Thermal Cutting Processes Nontraditional machining refers to a group a processes which removes excess material by various techniques involving mechanical, thermal, electrical or chemical energy These processes do not use a sharp cutting tool in the conventional sense
Nontraditional processes have been developed in response to new and unusual machining requirements, including The need to machine newly developed materials with special properties (high strength, high hardness, high toughness) The need for unusual and/or complex geometries The need to avoid surface damage
Classification of nontraditional manufacturing processes by principle form of energy Mechanical - mechanical energy in some form different from the action of a conventional cutting tool; erosion of the workpiece material is typical Electrical - electrochemical energy to remove material
Thermal - thermal energy generally applied to a small portion of the work surface, causing removal by fusion and/or vaporization; thermal energy is generated by conversion of electrical energy Chemical - most materials are susceptible to chemical attack by certain acids or other etchants; chemicals selectively remove material from portions of the workpiece, while other portions of the surface are protected
Available nontraditional material removal processes Mechanical AFM - abrasive flow machining AJM - abrasive jet machining HDM - hydrodynamic machining LSG - low stress grinding RUM - rotary ultrasonic machining TAM - thermally assisted machining TFM - total form machining USM - ultrasonic machining WJM - water jet machining
Electrical ECD - electrochemical deburring ECDG - electrochemical discharge grinding ECG - electrochemical grinding ECH - electrochemical honing ECM - electrochemical machining ECP - electrochemical polishing ECS - electrochemical sharpening ECT - electrochemical turning ES - electro-stream STEM - shaped tube electrolytic machining
Thermal EBM - electron beam machining EDG - electrical discharge grinding EDM - electrical discharge machining EDS - electrical discharge sawing EDWC - electrical discharge wire cutting LBM - laser beam machining LBT - laser beam torch PBM - plasma beam machining
Chemical CHM - chemical machining ELP - electropolish PCM - photochemical machining TCM - thermochemical machining TEM - thermal energy machining While many processes are available, only the most commercially important processes are discussed here
Mechanical Energy Processes Ultrasonic machining (USM) Abrasives contained in a slurry are driven at high velocity against the work by a tool vibrating at low amplitude (.003in) and high frequency (20-100khz) The tool oscillates in a direction perpendicular to the workpiece surface and is fed slowly into the workpiece so that the shape of the tool is formed in the part
The action of the abrasives impinging against the work surface performs the cutting Tool materials - soft steel, stainless steel Abrasive materials - boron nitride, boron carbide, aluminum oxide, silicon carbide and diamond The vibration amplitude should be set approximately equal to the grit size, and the gap size should be maintained at about two times the grit size
The ratio of work material to tool material removed during the cutting process ranges from ~100:1 for cutting glass down to ~1:1 for cutting tool steel Workpiece materials: hard and brittle such as ceramics, glass and carbides; successfully used on certain metals such as stainless steel and titanium Shapes obtained by USM include nonround holes, holes along a curved axis and coining operation, in which an image pattern on the tool is imparted to a flat work surface
Water jet cutting (WJC) Nozzle diameter: 0.004-0.016 in Pressure: up to 60,000psi Jet velocity: up to 3000 ft.Sec Nozzle made of sapphire, ruby or diamond Cutting fluids: polymer solutions; preferred because of their tendency to produce a coherent stream
Important process parameters: standoff distance, nozzle operating diameter, water pressure and cutting feed rate Typical feed rates: 12 in/min to well over 1200 in/min The water jet cutting process is usually automated using CNC robots to manipulate the nozzle unit along the desired trajectory Materials cut by water jet: plastic, textile, composites, tiles, carpet, leather and cardboard
Advantages: no crushing or burning of the work surface, minimum material loss because of the narrow cut slit, no environmental pollution, and easy automating the process Limitation: not suitable to cut brittle material because of their tendency to crack during cutting
Abrasive water jet cutting (AWJC) Introduction of abrasive particles into the stream adds to the number of parameters that must be controlled; among these are: abrasive type, grit size and flow rate Type of abrasive materials: aluminum, oxide, silicon dioxide and garnet (a silicate mineral) Grit size: ranges between 60 and 120 Flow rate: approximately 0.5 lb/min Nozzle orifice diameter: 0.010 - 0.025in; somewhat larger that in water jet cutting to permit higher flow rates and more energy to be contained in the stream prior to the infection of abrasives
Abrasive jet machining (AJM) A high velocity stream of gas containing small abrasive particles Pressure: 25 - 200 psi Nozzle orifice diameter: 0.003 - 0.040 in Velocities: 500 - 1000 ft/min Gases: dry air, nitrogen, carbon dioxide and helium The process is usually carried out manually by an operator AJM is normally used as a finishing process Applications: deburring, trimming and deflashing, cleaning and polishing Applied on hard, brittle materials (glass, silicon, mica and ceramics) that are in the form of thin flat stock Typical abrasives: aluminum oxide (for aluminum and brass), silicon carbide (for stainless steel and ceramics), and glass beads (for polishing)
Grit sizes are small, 15-40m in diameter and must be very uniform in size for a given application No recycling of abrasives; abrasive grains are fractured, worn and contaminated
Electrochemical Machining Processes Electrochemical machining (ECM) It removes metal from an electrically conductive workpiece by anodic dissolution, in which the shape of the workpiece is obtained by a formed electrode tool in close proximity to, but separated from the work by a rapidly flowing electrolyte Underlying principle: material is deplated from the anode and deposited onto the cathode in the presence of an electrolyte bath
The difference in ECM is that the electrolyte bath flows rapidly between the two poles to carry off the deplated material The electrode tool, usually made of copper, brass or stainless steel, is designed to posses approximately the inverse of the desired final shape of the part Gap distance: usually from 0.003 - 0.030 in A water solution of sodium chloride is commonly used as the electrolyte
Electrolyte serves for: Carrying off the material that has been removed from the workpiece Removing hear and hydrogen bubbles created in the chemical reactions of the process Removed material in the form of microscopic particles must be separated from the electrolyte through centrifuge, sedimentation or other means Large amount of electrical power is required to perform ECM
The material is very hard or difficult to machine or Voltage is kept relatively low to minimize arcing across the gap Use when: The material is very hard or difficult to machine or Where the workpiece geometry is difficult or impossible to accomplish by conventional machining methods Typical ECM applications Die sinking Multiple hole drilling Holes that are not round Deburring
Little surface damage to the work part Advantages: Little surface damage to the work part No burrs as in conventional machining Low tool wear Relatively high metal removal rates for hard and difficult to machine metals Disadvantages Significant cost of electrical power to drive the operation Problems of disposing of the electrolyte sludge
Electrochemical deburring (ECD) An adaptation of ECM designed to remove burrs or round sharp corners The same ECM principles of operation apply to ECD Much less material is removed in ECD, thus cycle times are much shorter
Electrochemical grinding (ECG) Special form of ECM A rotating grinding wheel with a conductive bond material is used to augment the anodic dissolution of the metal workpart surface Bond material: metallic (diamond abrasives) or resin bond impregnated with metal particles (aluminum oxide) Most of the machining is accomplished by electrochemical action, therefore the grinding wheel lasts much longer
Applications: Sharpening of cemented carbide tools Grinding of surgical needles, other thin wall tubes and fragile parts
Thermal Energy Processes Electric discharge machining (EDM) One of the most widely used nontraditional processes Shape of the finished work surface is produced by a formed electrode tool EDM process must take place in the presence of a dielectric fluid
Discharge region heated to extremely high temperature so that a small portion of the work surface is melted and removed Individual discharges occur hundreds or thousands of times per second to give a gradual erosion of the entire surface Process variables: Discharge current Frequency of discharges
The high spark temperature causes the tool to melt, resulting in a small cavity opposite the cavity produced in the work Wear ratio: Work material removed/tool material removed Ranges from 1.0 - 100 depending on the combination of work and electrode materials Electrode materials: graphite, copper, brass, copper tungsten, silver tungsten, etc. Metal removal rate: MRR = KI/Tm1.23
Dielectric fluids used: hydrocarbon oils, kerosene and distilled or deionized water Applications: tool fabrication and parts production delicate parts hole drilling with hole axis at an acute angle to the surface production machining of hard and exotic metals
Electric Discharge Wire Cutting (EDWC or wire EDM) special form of EDM using a wire as the electrode cutting action achieved by thermal energy from electric discharges between the electrode wire and the workpiece
Workpiece fed continuously and slowly past the wire to achieve cutting path NC used to control workpart motions Wire EDM must be carried out in the presence of a dielectric Wire diameters: 0.003 - 0.012 in. Wire materials: brass, copper, tungsten and molybdenum Dielectric fluids: deionized water or oil
Overcut ranges from 0. 0008 - 0. 002in Overcut ranges from 0.0008 - 0.002in. And remains fairly constant and predictable once cutting conditions are established
Electron Beam Machining (EBM) A high velocity stream of electrons is focused on the workpiece surface to remove material by melting and vaporization Electron beam gun accelerates a stream of electrons to ~3/4 c and focused through an electromagnetic lens Kinetic energy of beam converted to thermal energy of extremely high density, melting or vaporizing material in a very localized area EBM must be carried out in a vacuum
Can be used on any known material Applications: drilling of extremely small diameter holes - down to 0.002 in drilling holes with high depth/diameter ratios, greater than 100:1 Limitations: need of a vacuum high energy required expensive equipment
Laser Beam Machining (LBM) Uses light energy from a laser to remove materials by vaporization and ablation Types of lasers: CO2 solid-state Energy is concentrated optically and in terms of time Light beam pulsed so that the released energy results in an impulse against the work surface, producing evaporation and melting
Used for: drilling - down to 0.001 in slitting slotting scribing marking Not considered a mass production process; generally used on thin stock Range of work materials virtually unlimited
Plasma Arc Cutting (PAC) Plasma - a superheated, electrically ionized gas PAC uses a plasma stream operating at temperatures in the range from 18,000o - 25,000o F to cut metal The high-velocity plasma stream is directed at the workpiece, melting it and blowing the molten metal through the kerf
Plasma arc generated between an electrode inside the torch Plasma flows through a water-cooled nozzle, which constricts and directs the stream Hot enough to cut through metal 6 in thick Gases used: nitrogen, argon-hydrogen or a mixture (primary gases) secondary gases or water directed to surround the plasma jet to confine the arc and clean the kerf
Most applications consist of cutting flat metal sheets and plates Can be used to cut nearly any electrically conductive metal Feed rates: as high as 430 in/min for 1/4 in. aluminum 200 in/min for 1/4 in. steel 20 in/min for 4 in. aluminum
Advantage: high productivity Disadvantages: rough cut surface, metallurgical damage
Air Carbon Arc Cutting arc generated between a carbon electrode and the metallic work High-velocity air jet used to blow away the melted portion of the metal Used to form a kerf for severing the piece or to gouge a cavity in the pat Used on a variety of metals, including cast iron, carbon steel, low alloy and stainless steels Sputtering of molten metal is a hazard
Other Arc Cutting Processes Gas metal arc cutting Shielded metal arc cutting Gas tungsten arc cutting Carbon arc cutting
Chemical Machining Mechanics and Chemistry of Chemical Machining Differences in applications and the ways in which the steps are implemented account for the different forms of CHM; the steps are Cleaning - to ensure that material will be removed uniformly from the surfaces to be etched
Masking - maskant, chemically resistant to the etchant, applied to portions of the work surface not to be etched Etching - the material removal step; part immersed in an etchant that chemically attacks unmasked portions; part removed and washed when desired amount of material has been removed Demasking - maskant removed from the part Masking and etching involve significant variations in methods, materials and process parameters
Maskant materials: neoprene, polyvinyl chloride, polyethylene and other polymers Masking methods Cut and peel - performed by hand, used for large workparts, low production quantities and where accuracy is not a critical factor Photographic resist - normally applied where small parts are produced in high quantities and close tolerances are required Screen resist - used in applications that are between the other two masking methods in terms of accuracy, part size and production quantity
Etchant selection - depends on work material, desired depth and rate of etch, and surface finish requirements
Chemical Milling First CHM process to be commercialized Used largely in the aircraft industry Applicable to large parts where substantial amounts of metal are removed Cut and peel maskant method employed As depth increases, surface finish becomes worse Metallurgical damage very small
Chemical Blanking Uses chemical erosion to cut very thin sheet-metal parts, down to 0.001 in. and/or for intricate cutting patterns Produces burr free parts Photoresist or screen resist method applied Maximum stock thickness ~0.030 in. Hardened or brittle materials can be processed
Chemical Engraving A chemical machining process used for making flat panels that have lettering and/or artwork on one side Can be used to make raised or recessed lettering by reversing the portions of the panel to be etched Masking done by either photoresist or screen resist methods Filling operation to apply paint or other coating follows etching
Photochemical Machining (PCM) Chemical machining in which the photoresist masking method is used Employed in metalworking when close tolerances and/or intricate patterns are required Used extensively in electronics industry (makes VLSI possible) Photoresist materials in current use are sensitive to UV light, but not other wavelengths
No need to carry out process in a darkroom Anisotropy: depth of cut d divided by undercut u; reciprocal of the etch factor A=1/Fe = d/u A: degree of anisotropy Fe: etch factor
Application Considerations Workpart Geometry Features Very small holes - (below 0.005 in. in diameter) use LBM Holes with large depth/diameter ratios - (d/D > 20) use ECM and EDM Nonround holes - use EDM and ECM Narrow slots that are not straight - use EBM, LBM, wire EDM, WJC and AWJC
Micromachining - use PCM, LBM and EBM Shallow pockets and surface details in flat parts - use CHM and its variations Creation of special contoured shapes for mold and die applications - (die sinking) use EDM and ECM
Work Materials
Performance of Nontraditional Processes Nontraditional processes are generally used when conventional methods are not practical or economical