Learn Advanced Machining Processes Explained – Hybrid Systems, Abrasive/Water Jet, Plasma/Laser/Electron Beam, EDM (Die & Wire), Electrochemical Machining (ECM & ECG), and Chemical/Photochemical Blanking & Milling

Introduction:

The manufacturing industry has long relied on mechanical methods for material removal, including chip formation, abrasion and microchipping. However, situations have arisen where these traditional methods prove unsatisfactory, uneconomical, or impossible. Several challenges have driven alternative approaches. The strength and hardness of certain workpiece materials present obstacles, particularly when materials exceed 400 HB in hardness. These materials resist cutting tools leading to rapid wear and unacceptable production times. Brittle materials like highly heat treated alloys, glass, ceramics and powder metallurgy parts are prone to damage during mechanical operations. Components that are too flexible or slender cannot withstand machining forces. Parts that are difficult to clamp present challenges. Geometric complexity often exceeds conventional capabilities including internal and external profiles or holes with high length to diameter ratios in hard materials. Special surface finish and dimensional tolerance requirements may exist that cannot be obtained otherwise. Temperature rise and residual stresses are often undesirable.

These difficulties led to chemical, electrical, laser and high-energy beams as energy sources for material removal. These advanced methods, called nontraditional or unconventional machining, began in the 1940s. Such processes remove material through chemical dissolution, etching, melting, evaporation and hydrodynamic action, sometimes with abrasive particles. A major advantage is efficiency independent of workpiece hardness. When applied properly, advanced processes offer major technical and economic advantages.

Chemical Machining:

Chemical machining (CM) was developed from the observation that chemicals attack and etch most metals, stones and some ceramics. The process uses reagents or etchants like acids and alkaline solutions. CM is the oldest advanced process and has been used in engraving, deburring and producing printed circuit boards and microelectronic devices.

Chemical Milling:

In chemical milling, shallow cavities are produced on plates, sheets, forgings, and extrusions for weight reduction. Depths of removal can be as large as 12 millimeters. Selective attack is controlled by removable masking layers or partial immersion. The procedure consists of several steps. If the part has residual stresses, they should be relieved to prevent warping. Surfaces are degreased and cleaned for good masking adhesion and uniform removal. Scale from heat treatment should be removed. Masking material is then applied. Masking with tapes or paints (maskants) is common though elastomers like rubber and neoprene, and plastics including polyvinyl chloride, polyethylene, and polystyrene are also used. The maskant should not react with the reagent. If required, maskant covering regions requiring etching is peeled off by the scribe and peel technique.

Exposed surfaces are chemically machined with appropriate etchants. Sodium hydroxide is used for aluminum, hydrochloric and nitric acid solutions for steels, and iron chloride for stainless steels. Temperature control and agitation are important for uniform removal. After machining, parts are washed thoroughly to prevent further reactions. Remaining maskant is removed, and parts are cleaned and inspected. Additional finishing may be performed. This sequence can be repeated for stepped cavities and contours. Chemical milling is used in aerospace for large aircraft components, missile skin panels, and extruded airframe parts. Tank capacities for reagents are as large as 3.7 by 15 meters. The process is also used for microelectronic devices and is referred to as wet etching. Some surface damage may result from preferential etching and intergranular attack. Chemical milling of welded and brazed structures may cause uneven removal. Castings may result in uneven surfaces due to porosity and nonuniformity.

Chemical Blanking:

Chemical blanking is similar to sheet metal blanking except material is removed by chemical dissolution rather than shearing. Applications include burr free etching of printed-circuit boards, decorative panels, thin sheet metal stampings, and complex or small shapes.

Photochemical Blanking:

Photochemical blanking or photoetching modifies chemical milling using photographic techniques on flat thin sheets enabling complex burr free shapes on metals as thin as 0.0025 millimeters. It is sometimes called photochemical machining. The procedure involves several steps. The design is prepared at magnification up to 100 times. A photographic negative is made and reduced to finished size. The reduced negative is called the artwork. The enlarged drawing allows design errors to be reduced by the reduction amount. The sheet blank is coated with photosensitive material (photoresist) by dipping, spraying, spin casting, or roller coating, then dried in an oven. This coating is often called the emulsion. The negative is placed over the coated blank and exposed to ultraviolet light, hardening exposed areas. The blank is developed, dissolving unexposed areas. The blank is immersed in or sprayed with reagent, etching exposed areas. Masking is removed and parts are washed.

Applications include fine screens, printed circuit cards, electric motor laminations, flat springs and components for miniaturized systems. Skilled labor is required, but tooling costs are low, the process can be automated, and it is economical for medium to high production volume. Photochemical blanking can make very small parts when traditional dies are difficult to produce and is effective for fragile workpieces. Handling reagents requires precautions against chemical exposure. Disposal of by products is a major drawback though some can be recycled.

Design Considerations for Chemical Machining:

Because the etchant attacks all exposed surfaces avoid sharp corners, deep narrow cavities, severe tapers, folded seams or porous materials. Because etchant attacks vertically and horizontally, undercuts may develop. Tolerances of plus or minus 10% of material thickness can be maintained. To improve production rate, bulk material should be shaped by other processes first. Dimensional variations can occur from size changes in mask pattern due to humidity and temperature, minimized by selecting appropriate media and controlling environment. Product drawings must be translated into a protocol compatible with artwork generation equipment.

Electrochemical Machining:

Electrochemical machining (ECM) is the reverse of electroplating. An electrolyte acts as current carrier and high electrolyte movement in the tool workpiece gap typically 0.1 to 0.6 millimeters washes metal ions from the workpiece (anode) before they can plate onto the tool (cathode). The cavity produced is the female mating image of the tool shape. The shaped tool solid or tubular is made of brass, copper, bronze or stainless steel. The electrolyte is a highly conductive inorganic fluid like aqueous sodium nitrate, pumped through passages at 10 to 16 meters per second. A DC power supply from 10 to 25 volts maintains current densities of 20 to 200 amperes per square centimeter.

Material removal rate (MRR) for 100% efficiency can be estimated from MRR = CI, where MRR is in mm³/min, I is current in amperes, and C is a material constant in mm³/A-min. For pure metals, C depends on valence, higher valence gives lower C. Machines with current capacities as high as 40,000 amperes and as low as 5 amperes are available. Penetration rate is proportional to current density. MRR typically ranges from 1.5 to 4 mm³ per A-min. Because MRR depends only on ion exchange rate, it is unaffected by strength, hardness or toughness.

Process Capabilities:

The concept was patented in 1929 and developed in the 1950s and 1960s. ECM is used for complex cavities in high strength materials, particularly in aerospace for turbine blades, jet-engine parts, and nozzles. Automotive applications include engine castings and gears, and medical industries use it for components. ECM is used for die sinking and producing small holes. Versions are used for turning, facing, milling, slotting, drilling, trepanning, profiling and producing continuous metal strips and webs. Recent applications include micromachining for electronics.

Shaped tube electrolytic machining (STEM) is a modification for drilling small-diameter deep holes in turbine blades. The tool is a titanium tube coated with insulating resin. Holes as small as 0.5 millimeters can be drilled at depth to diameter ratios as high as 300 to 1. ECM leaves a burr free bright surface and can be used for deburring. It causes no thermal damage, and absence of tool forces prevents distortion. There is no tool wear since only hydrogen is generated at the cathode. However mechanical properties should be compared with other methods. ECM systems are available as numerically controlled machining centers with high production rates, flexibility and close tolerances. ECM can be combined with electrical discharge machining (EDM).

Design Considerations:

Because electrolyte erodes sharp profiles, ECM is not suited for sharp square corners or flat bottoms. Controlling electrolyte flow may be difficult, so irregular cavities may not achieve desired accuracy. Designs should provide a small taper for holes and cavities.

Pulsed Electrochemical Machining:

Pulsed ECM (PECM) uses very high current densities on the order of 1 A/mm², but pulsed rather than direct current. Pulsing eliminates the need for high electrolyte flow rates, which limit ECM in die and mold making (die sinking). PECM improves fatigue life and has been proposed for eliminating recast layer left by EDM. Tolerances typically range from 20 to 100 micrometers. Machines can perform both EDM and PECM. However, maintaining precise alignment is difficult; if misaligned, polishing occurs where gap is smallest and passivation where gap is largest. The process leaves metal residues harmful to the environment if discarded untreated. ECM can be effective for micromachining; absence of tool wear allows precision electronic components though stray current erosion must be overcome.

Electrochemical Grinding:

Electrochemical grinding (ECG) combines ECM with conventional grinding. Equipment is similar to a conventional grinder except the wheel is a rotating cathode embedded with abrasive particles. The wheel is metal bonded with diamond or aluminum oxide abrasives and rotates at surface speed from 1,200 to 2,000 m/min. Abrasives serve as insulators between wheel and workpiece and mechanically remove electrolytic products. A flow of electrolyte solution, usually sodium nitrate, is provided. Current densities range from 1 to 3 A/mm². Most metal removal is by electrolytic action; typically less than 5% by abrasive action. Therefore, wheel wear is very low and workpiece remains cool. Finishing cuts are made by grinding action for good finish and accuracy.

ECG is suitable for applications similar to milling, grinding and sawing, but not for cavity sinking operations. ECG can be applied to carbides and high strength alloys, offering advantage over traditional diamond wheel grinding for very hard materials. ECG machines are available with numerical controls. Electrochemical honing combines honing with electrochemical action. Although costly, it is as much as 5 times faster than conventional honing, and tool lasts as much as 10 times longer. Electrochemical honing is used primarily for finishing internal cylindrical surfaces.

Design Considerations:

In addition to ECM guidelines, ECG requires: avoid sharp inside radii. Flat surfaces should be narrower than wheel width.

Electrical Discharge Machining:

Electrical discharge machining (EDM), also called electrodischarge or spark-erosion machining, is based on erosion of metals by spark discharges. When current carrying wires touch, an arc is produced and metal is eroded leaving a crater. Though known since electricity's discovery, a machining process was not developed until the 1940s. EDM has become one of the most important production technologies.

Principle of Operation:

The basic EDM system consists of a shaped tool (electrode) and workpiece connected to a DC power supply and placed in a dielectric (electrically nonconducting) fluid. When potential difference is high enough, dielectric breaks down and a transient spark discharges, removing a small amount of metal. Capacitor discharge repeats at rates between 200 and 500 kHz with voltages from 50 to 380 V and currents from 0.1 to 500 A. Volume removed per spark is typically from 10⁻⁶ to 10⁻⁴ mm³. EDM can be used on any electrical conductor.

Melting point and latent heat of melting determine volume removed per discharge as these increase, MRR decreases. MRR can be estimated from MRR = 4 × 10⁴ / (I × Tw^1.23), where MRR is in mm³/min, I is current in amperes, and Tw is melting point in °C. Workpiece is fixtured in the dielectric tank, movements controlled by numerical control. The gap (overcut) is critical, downward feed is controlled by a servomechanism maintaining constant gap. Hardness, strength and toughness do not influence removal rate. Removal rate and surface roughness increase with increasing current density and decreasing spark frequency.

Dielectric Fluids:

Dielectric fluid functions as an insulator until potential is sufficient, provides cooling and acts as flushing medium to carry away debris. Common fluids are mineral oils though kerosene and distilled/deionized water are used. Clear, low viscosity fluids are available, more expensive but make cleaning easier. Machines have pump and filtering systems.

Electrodes:

Electrodes are usually graphite, though brass, copper or copper tungsten alloys are used. Tools can be shaped by forming, casting, powder metallurgy, or CNC machining. Tungsten wire electrodes as small as 0.1 mm diameter have produced holes with depth to diameter ratios up to 400 to 1. Sparks also erode the electrode, affecting dimensional accuracy. Wear ratio is volume of workpiece removed to volume of tool wear, ranging from about 3:1 for metallic electrodes to as high as 100:1 for graphite. Lower melting point gives higher wear rate; higher current gives higher wear. Graphite electrodes have highest wear resistance. Tool wear can be minimized by reversing polarity and using copper tools, called no-wear EDM.

Process Capabilities:

EDM applications include dies for forging, extrusion, die casting, injection molding and large sheet metal automotive components. Other applications include deep small diameter holes with tungsten wire electrodes, narrow slots, cooling holes in superalloy turbine blades and intricate shapes. Stepped cavities can be produced by controlling relative movements. High removal rates produce rough surface finish with poor integrity and low fatigue properties due to recast layer. Finishing cuts are made at low removal rates or recast layer is removed by finishing. Surface finish can be improved by oscillating electrode in planetary motion at amplitudes of 10 to 100 micrometers.

Design Considerations:

Parts should be designed so electrodes can be shaped properly. Deep slots and narrow openings should be avoided. Specified surface finish should not be too fine for economic production. Bulk material removal should be by conventional roughing out.

Wire EDM:

Wire EDM, or electrical discharge wire cutting, uses a slowly moving wire traveling along a prescribed path. Used to cut plates as thick as 300 mm and make punches, tools, and dies from hard metals. Also cuts intricate electronics components. Wire is brass, copper, tungsten or molybdenum, zinc or brass coated and multicoated wires are used. Wire diameter is about 0.30 mm for roughing and 0.20 mm for finishing. Wire should have high conductivity and tensile strength, tension is typically 60% of tensile strength. Wire is usually used only once. It travels at constant velocity from 0.15 to 9 m/min, and constant gap (kerf) is maintained. Trend is toward clear, low-viscosity fluids. Cutting speed is cross-sectional area per unit time. Examples: 18,000 mm²/hr for 50-mm thick D2 tool steel and 45,000 mm²/hr for 150-mm thick aluminum, indicating linear speeds of 6 mm/min and 5 mm/min respectively. Multiaxis wire EDM machining centers produce three dimensional shapes with computer controls for cutting path and angle, multiheads for two parts simultaneously, wire breakage prevention, automatic self-threading, and programmed strategies. Two axis machines produce cylindrical shapes. Many modern machines allow independent control for tapered parts.

Electrical Discharge Grinding:

The grinding wheel in electrical discharge grinding (EDG) is graphite or brass with no abrasives. Material is removed by spark discharges between rotating wheel and workpiece. Used for carbide tools and dies, and for fragile parts like surgical needles, thin walled tubes, and honeycomb structures. Electrochemical discharge grinding (ECDG) is faster but consumes more power. EDG can combine with electrochemical grinding to form ECDG, where chemical action with electrical discharges breaking up oxide film is washed away by electrolyte. MRR in EDG can be estimated from MRR = KI, where MRR is in mm³/min, I is current in amperes, and K is a material factor in mm³/A-min. For example, K = 4 for tungsten carbide and K = 16 for steel. In sawing with EDM, a band or circular saw without teeth is used with the same circuit. Narrow cuts can be made at high rates. Because cutting forces are negligible, it can be used on thin and slender components.

Laser Beam Machining:

In laser-beam machining (LBM), a laser focuses optical energy on the workpiece surface. The high density energy melts and evaporates portions. This process, not requiring vacuum, machines metallic and nonmetallic materials. Types include CO₂ (pulsed or continuous wave), Nd:YAG, Nd:glass, ruby, diode and excimer lasers. Important parameters are reflectivity, thermal conductivity, specific heat and latent heats of melting and evaporation. Lower values make the process more efficient. Cutting depth can be expressed as t = C × P / (V × d), where t is depth, C is a constant, P is power, V is cutting speed, and d is spot diameter. Peak energy densities range from 5 to 200 kW/mm². Surface is usually rough with a heat-affected zone, which may need removal or heat treatment. Kerf width is important. Laser beams may be used with a gas stream like oxygen to increase energy absorption. High-pressure inert-gas assisted cutting (nitrogen or argon) is used for stainless steel and aluminum, leaving oxide-free edges that improve weldability. Gas streams blow away molten and vaporized material.

Process Capabilities:

LBM is widely used for drilling, trepanning, and cutting metals, nonmetallics, ceramics, and composites. Holes as small as 0.005 mm with depth-to-diameter ratios of 50:1 have been produced, though 0.025 mm is more practical. Steel plates as thick as 32 mm can be cut. LBM is used in electronics and automotive industries. Bleeder holes for fuel-pump covers and lubrication holes in transmission hubs are drilled with lasers. Cooling holes in Boeing 747 jet engine vanes are laser produced. Significant cost savings have been achieved. Laser beams are also used for welding, localized heat treating, and marking parts with letters, numbers, codes, etc. Though equipment is more expensive, laser marking is common due to accuracy, reproducibility, flexibility, automation and online application. Laser cutting can compete with traditional punching. Extreme caution is required even low power lasers can damage the retina.

Design Considerations:

Avoid sharp corners. Deep cuts produce tapered walls. Reflectivity is important; dull and unpolished surfaces are preferable. Investigate adverse effects from high temperatures. Laser cutting advantages include smaller batches, flexibility, wide thickness range, prototyping, difficult composites and programmable geometries. Punching advantages include requiring large lot sizes, relatively simple parts, small thickness range, fixed geometries, rapid production and integration with subsequent processing. Machines utilize both processes jointly. Turret punch presses have integrated laser heads, they can punch or laser cut, but not simultaneously. Factors in combining include size, thickness, and shape ranges; processing and setup times, programming and process capabilities including vibrations and shock that may disturb laser components.

Electron Beam Machining:

In electron beam machining (EBM), high velocity electrons strike the workpiece surface generating heat. Machines use voltages from 50 to 200 kV to accelerate electrons to 50 to 80% of light speed. Applications are similar to LBM, except EBM requires a vacuum. EBM accurately cuts a wide variety of metals. Surface finish is better and kerf narrower. Interaction produces hazardous X-rays, so equipment should be used only by trained personnel.

Design Considerations:

Because vacuum chambers have limited capacity, parts should match chamber size. If a part requires EBM on only a small portion, consider manufacturing as smaller components and assembling after EBM.

Plasma Arc Cutting:

In plasma arc cutting (PAC), plasma beams (ionized gas) rapidly cut ferrous and nonferrous sheets and plates. Temperatures reach 9400°C in the torch for oxygen plasma gas. The process is fast, kerf width is small, and surface finish is good. Parts as thick as 150 mm can be cut. MRRs are much higher than EDM and LBM. PAC is highly automated with programmable controllers.

Water Jet Machining:

In water jet machining (WJM), the force from a high velocity water stream is used in cutting and deburring. Pressure of about 400 MPa is used though pressures up to 1400 MPa can be generated. Jet nozzle diameters range from 0.05 to 1 mm. A variety of materials can be cut including plastics, fabrics, rubber, wood products, paper, leather, insulating materials, brick and composites. Thicknesses can reach 25 mm and higher. Vinyl and foam coverings for automobile dashboards and body panels are cut by multi axis robot guided WJM. Also used in food processing. Advantages include: cuts can start anywhere without predrilled holes, no heat, no deflection, little wetting, minimal burr and environmentally safe.

Abrasive Water Jet Machining:

In abrasive WJM (AWJM), the water jet contains abrasive particles like silicon carbide or aluminum oxide, increasing MRR. Metallic, nonmetallic, and advanced composites can be cut in single or multilayers. AWJM is suitable for heat-sensitive materials. Cutting speeds can be as high as 7.5 m/min for reinforced plastics, but much lower for metals. Minimum hole size is about 3 mm; maximum hole depth about 25 mm. With multi axis machines, complex 3D parts can be machined. Optimum abrasive level is controlled automatically. Nozzle life improved using rubies, sapphires and carbide-based composites.

Abrasive Jet Machining:

In abrasive jet machining (AJM), a high velocity jet of dry air, nitrogen or carbon dioxide with abrasive particles is aimed at the workpiece. Impact performs operations including cutting small holes, slots or intricate patterns in hard or brittle materials, deburring, trimming, removing oxides, and cleaning irregular surfaces. Gas pressure is about 850 kPa, abrasive velocity up to 300 m/s. Nozzles are tungsten carbide or sapphire. Abrasive size is 10 to 50 micrometers. Because flow rounds corners, avoid sharp corners. Holes tend to be tapered. Hazard from airborne particulates, avoidable by using AWJM.

Hybrid Machining Systems:

Hybrid machining systems combine two or more processes to take advantage of each. Systems handle metals, ceramics, polymers, and composites. Examples include abrasive with ECM, abrasive with EDM, abrasive with electrochemical finishing, water jet with wire EDM, high speed milling, laser ablation and blasting, machining with blasting, electrochemical with EDM (ECSM) and machining with forming like laser cutting and punching. Implementation presents challenges. Considerations include workpiece material and characteristics, compatibility of processing parameters (speeds, sizes, forces, energies, temperature), cycle times, adverse effects of abrasives, chemicals, wear particles, chips, contaminants and consequences of failure in one stage since operations are sequential.

Economics of Advanced Machining Processes:

Economic production run depends on tooling and equipment costs, operating costs, MRR required, operator skill and secondary operations. In chemical machining, cost of reagents, maskants, disposal and cleaning are important. In EDM, cost of electrodes and periodic replacement can be significant. MRR and production rate vary significantly. Cost of tooling and equipment varies considerably. High capital investment for electrical and high energy beam machines must be justified by production runs and feasibility of manufacturing by other means.