Unit Manufacturing Processes: Issues and Opportunities in Research (1995)

Mass-change processes are characterized by the removal of material through the use of mechanical, thermal, chemical, or electrical energy.¹ In most instances, the workpiece density is not altered; however, the material microstructure may be modified, particularly at the work surface. Workpiece chemical composition is, in some cases, affected in a small surface region. Mass-change processes are employed in most manufacturing enterprises in intermediate and final processing operations. Workpiece materials span the spectrum of metals, ceramics, polymers, and their composites. High-performance workpiece materials generally are processed by tooling made from higher-strength materials. For example, diamond is used as a tooling coating to process ceramics and ceramic matrix composites.²

Processing costs associated with mass-change processes are directly related to the material properties of the workpiece and to the tolerance and surface finish requirements of the final part. Considerations of operation setup time and cost of fixtures and tooling must be included in the evaluation of the process economics.

Mass-change processes can be grouped into traditional (chip-making) and nontraditional processes. Chip-making processes remove unwanted workpiece material by exploiting shear deformation and fracture mechanisms. The basic

chip-making processes include shaping, turning, milling, drilling, sawing, punching, broaching, and grinding (abrasive machining).

Nontraditional processes replace the chip-making mechanisms of material removal with alternative mechanical, electrical, thermal, or chemical removal techniques. These processes usually are used for applications that involve complex shapes or materials that are not easily handled with traditional processes. Typical examples are laser processing, electrodischarge machining (EDM), and electrochemical machining (ECM).

Current research trends have the objective of increasing material removal rates with no loss of part quality or precision. Improved process understanding is guiding advancements, such as the identification of advanced cutting tool and grinding materials for conventional and advanced workpiece materials.

Traditional chip-making processes are mature and have been studied extensively for over a century. Future improvements are projected to be incremental in nature and are expected to be in the areas of:

• machine design and utilization for high-speed operations;
• tooling design and materials;
• precision and high-speed machining; and
• processing of new materials such as ceramics.

The prime productivity goal of machining is increased material removal rates (MRRs), along with improved precision and accuracy levels in the final part. Current material removal rates are attained by using relatively low feed rates, low depths of cut, and high cutting speeds. These conditions result in reduced chip loads and lower machining forces on tooling and ensure precision of part shape and geometry, particularly for advanced materials in which the present abrasive processes have limited removal rates.

Achieving the goal of increased material removal rates requires advances in the process, equipment, and machine control. Specifically, improvements related to increased depth of cut and feed rates, as well as high cutting speeds, are needed. The technical challenges to attaining this goal are in the areas of machine tool stiffness, high-level servo drive control, advanced computer numerical control technology, tool materials and coatings, and thermal management of the process.

The grinding of steel is a mature commercial process; most of the process conditions are based on empirical experience. State-of-the-art science and technology is often not applied in equipment design, process design, or process control architecture, and the consequence is that cycle times and productivity are not optimized. For example, grinding wheel sharpness greatly influences part surface finish. It demands careful control during production, since grinding wheels can dull and cause surface integrity problems in the workpiece.³ Advanced grinding materials, such as cubic boron nitride, offer improved performance at an increase in wheel costs. As with machining, the machine stiffness governs the tolerance potential of the process. Also, as is the case for machining fluids, grinding fluids are being categorized as hazardous wastes and may require replacement or elimination in the future.

High-efficiency deep grinding is an example of an advanced grinding process. This technology involves a high-performance surface grinder modified to attain large depths of cut using high traverse speeds. High-efficiency deep grinding achieves material removal rates up to 100 inches³/minute per inch of width, compared with an upper limit of 5 inches³/minute per inch of width for traditional high-performance grinding processes.

The grinding of ceramics is becoming a viable commercial process. The operating principles are partially developed, and the understanding of the process properties and performance for these materials is in the early stages of development. Further research will document the mechanisms active in the grinding contact zone and will establish the effect of material removal rate on part precision and quality. High geometric accuracy depends on precise machine tool motions, which are controlled by both the static and dynamic machine stiffness and the grinding wheel design and wear. Part quality is also influenced by material handling and fixturing, since ceramic parts are often brittle and prone to mechanical damage. Advancements in grinding ceramics require combinations of strategies. For example, a cast iron wheel with bonded pieces of diamond, combined with numerical control and in-process electrodischarge dressing of the wheel, can yield improved material removal rates for the production of complex ceramic parts.

Traditional mass-change processes remove material by mechanical action. In contrast, nontraditional processes remove material by the individual or combined action of thermal, chemical, and electrical processes. Twenty-one such processes are described in Nontraditional Manufacturing Processes (Benedict, 1987). Discussed below are laser machining processes,⁴ which are probably the most rapidly developing nontraditional techniques, and EDM and ECM, which are other widely used nontraditional processes.

Laser machining is a class of processes in which material removal occurs through either phase change (i.e., melting or vaporization) or oxidation reaction with a gas jet. The laser types used most widely in manufacturing are carbon dioxide (10.6-µm wavelength), neodymium-yttrium aluminum garnet (1.06-µm wavelength), and excimer (0.193- to 0.356-µm wavelength) lasers.

There are several advantages of laser machining over mechanical methods. First, since laser processing is principally thermal based, the effectiveness of laser machining depends on the material's thermal properties and its absorption of laser energy rather than on its mechanical properties. Therefore, brittle and hard materials can be machined easily by a laser if their thermal properties (e.g., conductivity, heat of fusion, etc.) are favorable. Second, energy transfer between the laser and material occurs without mechanical contact; therefore, there is no mechanically induced material damage, no tool wear, and no machine vibration effects, and the need for heavy fixturing is eliminated. Third, lasers do not require special processing environments, such as a vacuum.

Along with these advantages, however, there are several disadvantages that inhibit wider adoption of laser processes. First laser systems are currently expensive to purchase and operate. This effectively restricts the use of lasers to the processing high-value parts, high-speed applications, and special applications for which no alternative process exists (e.g., drilling holes with high aspect ratios at high angles of incidence). Second, the reliability of laser systems has not reached the level of traditional machine tools; therefore, maintenance expenses are significantly higher than those for mechanical processes. Finally, laser processing of polymers, composites, and ceramics must be carefully controlled

or unacceptable thermally induced bulk material damage can result, such as charring and microcracking.

Although most of the industrial applications to date have emphasized the high-speed aspect of laser processing, an emerging development area exploits the flexible nature of laser machine tools. When combined with a multi-axis workpiece positioning system or robot, the laser beam can perform a variety of unit processes on many classes of engineering materials by changing process parameters (e.g., beam diameter, scanning velocity, beam focus, assist gas, etc.) instead of changing machine tools.

There have been numerous experimental and theoretical studies on laser machining (for example, laser drilling, cutting, and scribing). Laser drilling is usually performed by either impingement of successive laser pulses onto the workpiece surface (i.e., percussion drilling) or cutting a workpiece in a circular beam trajectory (i.e., trepanning). The advantages of laser over mechanical drilling are the ability to drill small-diameter holes (on the order of several millimeters), high achievable drilling rates, and the ability to drill holes at high angles of incidence. For some applications, such as creating cooling holes in superalloy turbine blades and combustor liners, laser drilling is the preferred manufacturing process. In laser drilling of ceramic materials, microcracks develop near the cutting front due to thermally induced stresses but may be controlled through proper selection of parameters.

Laser cutting of two-dimensional shapes can be performed with either a continuous-wave beam or pulsed beam. Beam impingement on the workpiece surface results in material removal by either melting, vaporization, or reaction with a gas jet. Laser cutting allows a great deal of flexibility, since the cutting geometry is set by programming the kinematics of the beam and workpiece instead of the cutter geometry. High processing speeds (up to several meters per second) are also achievable. Laser cutting experiments for metals have been limited mostly to sheets less than 15 mm thick for continuous carbondioxide beams with a power range between 100 watts and 850 watts and scanning velocities between 0.5 and 5 m/minute (Babenko and Tychinskii, 1973; Decker et al., 1983).

The quality of laser-cut surfaces is often a critical consideration. For example, dross formation along the bottom edges of the kerf is a significant issue in surface quality of laser cut stainless steel (Arata et al., 1979). Due to the viscosity of the molten metal, the gas jet can only expel a portion of the molten material out of the kerf. The remaining material resolidifies along the bottom edge, forming dross, which must be removed mechanically after laser processing. Surface quality can be improved by using a pile cutting technique to reduce the viscosity of the molten stainless steel through mixing it with molten mild steel (Arata et al., 1979). A second method for improving cut quality uses a rear gas

jet in tandem with the laser beam to expel molten material from the kerf. In addition to dross formation, several other surface quality issues exist. Beam divergence effects influence the taper of the kerf edge. Heat conduction into the workpiece causes the formation of a heat-affected zone. These surface quality concerns of dross formation, kerf taper, and heat-affected zone formation also exist for laser cutting of ceramics, plastics, and composites.

Laser grooving and scribing differ from laser cutting in that the laser beam does not penetrate through the entire thickness of the workpiece. Laser grooving and scribing have been used increasingly in applications ranging from marking or engraving identification labels on parts to creating cooling channels for electronics packaging. Laser scribing and grooving processes are used for ceramics, plastics, and composite materials. The issues of resolidified material accumulation, heat-affected zone and microcrack formation, and uniformity in groove depth and profile are of primary concern.

EDM relies on material removal by erosion of the workpiece resulting from spark discharge with the tool (i.e., electrode). This process is often used for die sinking and machining. Rates of 480 mm³/minute have been reported for a 30-amp current. EDM may also be used to produce thin slots in flat or curved surfaces. Traditional EDM used kerosene as the dielectric fluid, which limited its use in unsupervised situations. The development of low-viscosity, highflashpoint fluids has minimized this problem.

There are several key factors that presently limit the extent of application of ECM: the production of precise shapes requires compensation for tool wear; machining rates are typically low; and surface integrity can be an issue, as exemplified by a potential surface layer of residual tensile stress.

A major innovation in the development of electric discharge wire machining (EDWM) was the use of a disposable, continuously moving wire as the electrode. Commercial equipment operates with wires 0.010-0.002 inches in diameter; machines have been modified to use wires with a diameter as small as 0.001 inches. Since deionized water is used as an electrolyte, EDWM can operate for extended periods under computer control without supervision. Surface integrity problems with EDWM or with EDM can be minimized at the expense of decreased cutting rates. With larger diameter wires, cutting rates of 250 mm²/minute have been reported for EDWM. An advantage of EDM and ECM is that hard materials may be cut as easily as soft ones, but the material being cut must have at least a limited electrical conductivity.

ECM employs electrolytic dissolution as the material removal process and is typified by machining rates of 2-2.5 cm³/minute per 1,000-amp current, surface roughness of 0.1-1.2 µm, and accuracy of 10-300 µm. The limiting factor in the ECM process, as with EDM and EDWM, is that materials with low electrical conductivities cannot be processed. In addition, ECM cannot produce sharp radii of less than 0.02 mm.

Tool design, electrolyte processing, and sludge disposal are major technical barriers to wider acceptance and development of ECM. Designing ECM tools is complex and costly, usually employing a trial-and-error approach. And sludge, which may contain toxic and hazardous materials, is a costly environmental problem.

Specific research opportunities in the development of high-speed machining include:

• new tool designs and materials capable of improved service life at the projected speeds and depth of cut;
• high-speed, high-performance spindle designs, including refined bearings and lubrication techniques;.
• enhanced machine control hardware (e.g., servo drives and open architecture controllers) that incorporates feed forward compensation⁵ to reduce contouring errors during machining that requires high feed and turning speeds and to minimize stiction effects. This enhanced control hardware must also communicate easily with other hardware (e.g., sensors), as well as link to process planning and factory level activities;
• refined part fixturing for restraint during the high loading encountered during machining with a high metal removal rate;
• high-capacity chip removal and flushing systems to handle the increased material removal rate; and

• Improved machine tool control methods and alternative equipment and tooling designs for machining thin-walled workpieces for parts such as gas turbine impellers and ribbed aircraft structures. The technical challenge is overcoming the deflections that are due to the low effective stiffness of the tool⁶ or workpiece.

Research opportunities for improved efficiency and precision in machining and drilling processes include:

• New cutting tool materials and wear-resistant coatings for machining and drilling. Advancements in traditional materials, as well as engineered materials, such as composites, require better cutting tools, including drills, to improve productivity and quality.
• Tool designs and materials for dry machining and drilling. Future environmental regulations are restricting and may eliminate the use of traditional metalworking fluids. This requires the development of new tooling capable of dry operation at established productivity rates and quality levels.
• Intelligent control of drilling and machining. Application of advanced control methodology (e.g., fuzzy logic, neural networks, and expert systems) with innovative in situ sensors and process models will lead to improved part quality and process productivity by minimizing tool breakage and dulling. Problem-free tooling designs are needed to minimize the need for operator intervention during processing.
• Understanding the effects of machining process conditions on product integrity. Part integrity and performance can be compromised by machining operations that produce microstructural damage or residual stress distributions in the finished part. Enhanced understanding of the process conditions related to the occurrence of such situations will lead to better process control methods to minimize integrity problems and enhance product performance.
• Innovative component fixturing. Flexible fixturing techniques will provide for rapid changeover from lot to lot, resulting in lower setup times and improved productivity.

• Improved machine tool accuracy. Advanced process design, machine design, and process control methods can lead to enhanced process accuracy, resulting in finished parts with tighter tolerances and better performance. For example, eliminating secondary finishing operations for drilled holes offers substantial savings in process costs. On-machine inspection also offers significant opportunities for productivity and quality improvements.

Specific research opportunities to improve the economics and precision of grinding operations include:

• Process understanding. Simulation of grinding processes, especially newer processes such as ductile regime grinding of ''brittle'' materials, incorporating appropriate constitutive models of both the grinding media and the workpiece, can be used to develop optimum process conditions. Constitutive models of the grinding media will include the influence of media characteristics (i.e., shape, composition, and size distribution) on finish, geometric tolerance, and surface integrity for a wide range of workpiece materials. In particular, identifying the chip-formation mechanisms in advanced materials, such as structural ceramics, metal matrix composites, and fiber-reinforced composites, is necessary to identify additional influences of the grinding process on material integrity and part performance. This understanding will aid researchers in the development of optimal grinding media.
• Control of part surface integrity and material damage. Cracking and other surface defects occur during the grinding of high-strength materials (e.g., M50 steel) and structural ceramics. These problems are exacerbated by efforts to maximize material removal rates. Investigation of the process variables that control such material integrity problems will establish the optimal process conditions for producing defect-free components. This understanding can be incorporated into process control strategies to improve part quality.
• Real-time control of surface integrity. Degradation of the grinding wheel during extended, continuous wheel use causes variation in wheel sharpness, which leads to thermal damage and taper and size variations in the workpiece. Process modification based on continuous monitoring of grinding wheel sharpness would reduce such damage and improve productivity.
• Technology development for superfinishing and honing of advanced materials. The growing use of structural ceramics will require a commercial capability to finish component surfaces to less than one microinch for applications ranging from fuel injectors to read-write heads in disk storage

• devices. Achieving such precision requires improvements in both grinding media and process understanding. Because residual stresses generated during microfinishing of ceramics and metals can lead to part distortion, the development of nondestructive techniques to quantify and determine the distribution of such stresses deserves special attention. Better modeling of the microfinishing process is also needed to establish the processing conditions for minimizing residual stresses.
• Grinding fluid alternatives. Grinding fluids are destined to become unavailable due to their hazardous waste classification. Alternative formulations or the development of dry grinding media and techniques will be necessary. Separate techniques may be required to perform the combined lubrication and heat transfer functions of current fluids. Research is required to document the behavior of alternative grinding fluids in terms of tribochemistry, damping effects, lubrication, and heat transfer.
• Grinding-wheel dressing optimization. Dressing and truing techniques of used grinding wheels can introduce variation to the grinding process. Aggressive truing produces a sharp wheel, which requires low grinding forces but produces poor surface finishes. Conversely, gentle wheel truing produces a dull wheel, which yields a good surface finish but requires high forces during operation. Development of optimal truing techniques, based on wheel specifications, would lead to refurbished wheels that produce parts with good surface finishes with minimal grinding forces.
• Real-time process control of grinding. Control algorithms are needed to achieve reliable, safe, closed-loop control of the grinding process. These algorithms must incorporate reliable process models and define process parameter limits for the production of defect-free parts.
• Tolerance control for grinding of thin-wall and slender parts. Deflection during grinding of thin-wall and slender parts increases the difficulty of maintaining high precision and roundness. Accurate simulation models of the grinding process are needed to improve process understanding and to define optimal operating conditions.
• Finishing technology for superconductors. Surface finishing is a possible source of defects and detrimental residual stress patterns in high-temperature ceramic superconductors. Development of the process sequence to finish these materials without creating such defects will enhance material performance.

There are a large number of research opportunities in laser processes:⁷

• Development of new lasers. New types of lasers designed specifically for manufacturing applications are needed. The characteristics of these lasers would include shorter pulse duration, smaller focal diameters, compactness, and shorter wavelengths. Promising new laser types include chemical oxygen-iodine, diode-pumped Nd-YAG, crystal-generated harmonics, direct diode, and copper vapor lasers. The benefits of these new lasers include precise control of machining, heat treatment, or welding depth through short pulses; minimization of thermal damage on the material through high energy densities and short pulse duration; smaller wavelengths and focus diameters to create smaller features; and high compactness of the laser. To make these lasers commercially viable, the reliability, size, and cost must all be improved significantly. Precise control of the laser output is also necessary.
• Rapid prototyping. A variety of laser-based rapid prototyping techniques have been developed recently, including laminated object manufacturing, stereolithography, and selective laser sintering. In these processes, a part design resident in a computerized database can be translated directly into a three-dimensional part, either through selectively photocuring a polymer, sintering a powder, or laser cutting two-dimensional profiles on thin sheets and then bonding them together. Although these processes show promise as flexible unit manufacturing processes, they are currently limited to selected materials used for geometric prototyping; these processes are only beginning to be extended to the production of structural parts. Also, since the part is created layer-by-layer, processing times up to 24 hours may be needed to fabricate a part. Future directions for research include the use of new laser types for improved dimensional accuracy and tolerances, the ability to process engineering materials such as metals and ceramics, and the design of new machine tools to increase processing speed significantly.
• Development of laser-assisted processes and machine tools. A potentially large area of investigation is the application of lasers for novel processing of materials beyond heat treatment, surface modification, welding, and machining. Recently, lasers have been used to perform three-dimensional shaping of workpieces through single-beam vaporization and two-beam grooving and

• subsequent chip removal. The depth of the pocket was determined by the beam power and workpiece velocity. A relationship between the effectiveness of vaporization and the direction of workpiece motion relative to the beam polarization was also studied. A concept for three-dimensional laser machining that uses two laser beams has been demonstrated for applications such as gear making, threading, turning, and die making (Chryssolouris et al., 1986). Commercial equipment is already available to machine cavities for dies and molds.
• Lasers also can be used to augment a mechanical cutting process. A technique using a laser to preheat and soften the workpiece material prior to tool contact has been developed in which the laser beam is projected onto the workpiece underneath the flank of the cutting tool. The beam's energy elevates the temperature field and softens the material at the cutting zone, resulting in lower required cutting forces and tool wear (Copley et al., 1983).
• Other possible applications for lasers include secondary surface finishing, fabrication of composite parts, direct fabrication of engineered materials and surface layers, and machining micron-size structures in semiconductor materials.
• Laser process modeling. Effective development of laser processes requires a comprehensive understanding of the physics of beam-material interaction. Current models of laser processing do not account for the products of these complex interactions, including plasma formation, molten layer flow, chemical reaction with a gas jet, and transient thermal effects due to a pulsed beam. A possible area of investigation is detailed numerical modeling of laser processes with the aid of high-speed computing. Another aspect of modeling is the integration of laser processes with traditional processes in a job shop or factory environment. Process planning tools are needed that can generate sequences of manufacturing processes and operating parameters to exploit the advantages of laser processing and to reduce the trial-and-error calibrations required to find acceptable operating conditions for a given part and material.
• Sensors and control. Almost all laser processes currently operate on an open-loop basis. Operating parameters are set through trial and error until satisfactory material removal rates, surface quality, and dimensional accuracy are achieved. Sensing has been limited to beam power and mode monitoring, ultrasonic sensing, and adjustment of the focus head for machining on curved surfaces. However, a controlled beam source does not necessarily result in a controlled heat-affected zone, weld, hole, kerf, or groove. External factors such as gas-jet fluctuations, impurities in the workpiece, and velocity variations contribute to variations in beam-material interaction. One area of investigation may be in developing sensors that monitor the laser process, instead of the beam. Possible approaches include real-time sensing of temperature field fluctuations,

• acoustic sensing, and indirect measurement of spark showers. The sensor methods developed must have fast response time (order of one microsecond), high accuracy, and high reliability, and they must be cost-effective for industrial implementation. Using sensor measurements for feedback, closed-loop control systems can be developed that can actuate changes in beam power, workpiece velocity, or gas-jet parameters.

Specific research opportunities for EDM, EDWM, and ECM include:

• EDM and EDWM for electrically conducting ceramics and composites that offer a potential benefit in producing complex shapes.
• The use of water-based dielectrics for EDM to increase the MRR and decrease fire hazard to facilitate unmanned operation.
• Investigations, monitoring, and control of debris density at the electrode/workpiece gap. For instance, advanced flushing and suction techniques should be improved to reduce debris and increase process efficiency.
• Understanding the fundamentals of parameters governing the surface properties of EDM and EDWM machined parts. Most operations rely on manufacturers' literature for setting controls for rough or fine cutting, but the physics of the removal process, the prediction or measurement of residual stress, and the nature of the recast layer are still incompletely understood.
• Incorporating EDM and particularly EDWM into computer-aided design and manufacturing programs. Such automation offers the benefit of off-line wire path programming and debugging without the cost of machine utilization. Process monitoring and control will maximize part quality, minimize wire breakage, and improve equipment productivity.
• Developing new electrode materials and coating combinations for optimizing EDM performance. Better electrodes can potentially improve the process economics and thus facilitate the use of EDM for a wider range materials. Component surface integrity, finish, and tolerances should be monitored to identify successful candidate electrode materials.
• Expanding current ECM process capability by the incorporation of pulsed current and the implementation of rotating tool electrodes . An additional improvement is the adoption of three-dimensional numerical control of the ECM electrode to produce complex surfaces.
• Numerical simulation and modeling of the ECM process. This approach offers the advantage of process design without the costs associated with ECM equipment. Models of the process dynamics and workpiece materials

• behavior would be integrated to provide a simulation of the process. Investigation of process parameter effects will lead to an optimized process definition for specific workpiece configuration and materials.
• Reclamation techniques for ECM process electrolytes and sludge. The ECM process currently uses potentially toxic electrolytic solutions that often produce sludge containing metallic ions that may be considered toxic waste. Environmentally benign ECM production processes are required.

Other nontraditional processes offer the potential for improved processing performance through the development and understanding of process mechanisms and material behavior. Process simulation based on this understanding and process control, using advanced in-process sensors, will be key technologies. Such processes include electrolyte jet machining, electrolyte abrasive jet machining, three-dimensionalcomputer-numeral-controlelectrochemical grinding, electrochemical discharge machining, electrochemical arc machining with rotating tools, and electrochemical spark machining.

Arata, Y., H. Maruo, L. Myamoto, and S. Takeuchi. 1979. Improvement of cut quality in laser-gas-cutting stainless steel. Journal of the Society for High Temperature Research 5:101-112.

Babenko, V.P., and V.P. Tychinskii. 1973. Gas-jet laser cutting (review). Soviet Journal of Quantum Electronics 2(5):399-410.

Benedict, G.F. 1987. Nontraditional Manufacturing Processes. New York: Marcel Dekker Inc.

Chryssolouris, G., J. Bredt, and S. Kordas. 1986. A New Machine Tool Concept Based on Lasers . Pp. 244-250 in Volume 14 of the Proceedings of the North American Manufacturing Research Conference held May 1986 at the University of Minnesota in Minneapolis. Urbana, Illinois: University of Illinois.

Decker, I., J. Ruge, and V. Atzert. 1983. Physical models and technological aspects of laser gas cutting. Proceedings of SPIE—The International Society for Optical Engineering 455(Sep):81-87.