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Source: DESIGN FOR MANUFACTURABILITY HANDBOOK

CHAPTER 4.21PARTS PRODUCED BY OTHER ADVANCED MACHINING PROCESSES

ABRASIVE-JET MACHINING

The Process

Abrasive-jet machining (AJM) achieves its cutting effect from the action of fine-pow-dered abrasive impinged on the workpiece by a high-velocity stream containing the abrasive in a gas carrier. The stream is focused through a nozzle opening of 0.13 to 0.81 mm (0.005 to 0.032 in) and travels at 150 to 300 m/s (500 to 1000 ft/s). The car-rier gas is normally air but may be carbon dioxide or nitrogen and is at a pressure of 200 to 830 kPa (30 to 120 lbf/in2). The abrasive is aluminum oxide, silicon carbide, or glass, and the nozzle is tungsten carbide or sapphire.

For precision cutting, the nozzle is mounted on apparatus that provides accurate positioning; for rough cutting, deburring, or stripping, the nozzle is usually handheld. In some cases, rubber, glass, or copper masks are used to confine the abrasive action to a certain portion of the workpiece surface.

Applications and Characteristics

Cutting, drilling, slotting, trimming, etching, cleaning, deburring, carving, and strip-ping can be performed by abrasive-jet machining. Some current applications are trim-ming resistors to precise values; stripping varnish from wires; cutting patterns and shapes in silicon semiconductors; abrading and frosting glass; drilling, cutting, and trimming thin sheets of tungsten or hardened steel; etching trade names and numbers on parts; removing plating or other surface coatings, particularly in a portion only of the workpiece surface; removing broken tools from holes; and making final adjust-ments or minor modifications in hardened-steel molds. Other products machined by AJM are dental devices, jewelry, hard-alloy-steel thrust bearings, and laminated opti-cal filters. (See Fig. 4.21.1.)

Very little heat is generated during abrasive-jet machining; there is no heat damage to the workpiece. Therefore, the process enjoys a major advantage, applicability to the machining of heat-sensitive components. The minimum slot width machinable with AJM is 0.13 mm (0.005 in). Taper in the walls of AJM cuts is inherent; it increases as nozzle-to-work spacing increases.

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PARTS PRODUCED BY OTHER ADVANCED MACHINING PROCESSES

4.238MACHINED COMPONENTS

FIGURE 4.21.1 Abrasive jet linked to a pantograph cuts intricate patterns in silicon semiconductors. (Courtesy S. S. White Industrial Products.)

Costs and Economic Production Quantities

Manual abrasive-machining equipment is inexpensive. Tooling costs also are low, making the process economical for small quantities. In any case, the choice of the AJM process does not depend on production quantity. The process is slow0.016 cm3 (0.001 in3) of material is normally removed per minuteand is used only when more conventional and more rapid metal-removal processes cannot be employed because of the nature of the workpiece material. The process is also used primarily for finishing and light cutting operations when the slow cutting rate is not too serious an adverse factor.

Suitable Materials

Abrasive-jet machining is used most advantageously for hard, fragile, heat-sensitive materials. Porcelain, glass, ceramic, sapphire, quartz, tungsten, chromium-nickel alloys, hardened metals, and semiconductors such as germanium, silicon, and gallium are suitable for the process.



OTHER ADVANCED MACHINING PROCESSES4.239

Design Recommendations

Designers specifying abrasive-jet machined parts should make allowance in their designs for the following:

1. The taper of the sidewalls of holes, slots, and other cuts should be at least 0.05 mm/cm (0.005 in/in) of depth.

2. The part configuration should allow access room for the abrasive-jet nozzle; i.e., cuts should not be specified immediately alongside steps or bosses.

3. Severing cuts should allow for kerf, at least 0.13 mm (0.005 in) but preferably 0.45 mm (0.018 in).

4. Corners cannot be sharp. As a minimum, allow a radius of 0.1 mm (0.004 in).

These design recommendations are illustrated in Fig. 4.21.2.

FIGURE 4.21.2 Abrasive-jet machining design recommendations.

Recommended Tolerances

The normal tolerance for the dimensions of machined areas is 0.13 mm (0.005 in), although 0.05 mm (0.002 in) is possible if extra care is taken. A desirable surface-finish tolerance is 1.3 m (50 in), which would allow the use of larger, faster-cutting abrasives. If necessary, surface finish can be held to 0.25 m (10 in) through the use of fine, slower-cutting abrasives.




ABRASIVE-FLOW MACHINING

Abrasive-flow machining (AFM) involves the use of a viscous semisolid medium rather than the gas-abrasive mix of AJM. In AFM, the workpiece is clamped between two cylinders. The putty-like medium is pumped hydraulically from one cylinder to the other and extruded through or over the workpiece. Abrasive grains in the medium rub against the surfaces of the workpiece, particularly where there are restrictions, sharp corners, or burrs, and gently remove material. One to hundreds of reversals of direction of the medium compound may take place. Pressure of the medium is 690 to 20,000 kPa (100 to 3000 lbf/in2). Medium compounds are proprietary and contain thickening agents and lubricants as well as the abrasive particles. Particle size ranges from No. 8 to No. 500. Abrasive materials may be aluminum oxide, silicon carbide, boron carbide, or diamond. Fixtures used to hold the workpiece should be abrasive-resistant in areas contacted by the medium; they are made of hardened steel, ceramic, or urethane.


AFM is primarily a deburring method for burrs in locations not easily accessible with conventional methods. (If burrs can be removed by vibratory or barrel tumbling, that method is usually more economical than AFM.) Other uses of AFM are the radiusing of sharp corners, particularly at the intersection of internal machined surfaces, polish-ing, and minor surface removal. One example of surface removal is the removal of the recast layer from holes produced by EDM or laser-beam machining. Holes as small as 0.4 mm (0.016 in) in diameter can be processed on AFM equipment. Although it may not be practical to remove large burrs by AFM because of its low rate of material removal, results are normally more uniform with AFM than with manual methods. Figure 4.21.3 illustrates examples of abrasive-flow-machined parts.


Equipment of moderately high cost and the need for fixturing and for development of proper operating parameters dictate the need for middle-sized or higher production quantities of most parts to justify the AFM process economically. Given a moderate production quantity, one-time costs can be recovered from the significant reduction in each-piece production times with the process. However, for particularly inaccessible surfaces, AFM may be the only practical finishing method, and small production lots or even prototype quantities may be processed advantageously.

Suitable Materials

All materials that can be machined by abrasive methods are suitable for abrasive-flow machining. Softer materials, of course, are processed most quickly, but AFM is best applied to aerospace and similar components with complex machined surfaces and hard, tough materials of poor machinability.



FIGURE 4.21.3 Examples of parts processed with abrasive-flow machining. (a) Deburring and polishing ratchet teeth. (b) Polishing turbine blades. (Courtesy Extrude Hone Corp.)





Designers should follow these recommendations:

1. Avoid blind holes if AFM is to be used. The process requires a through flow of abrasive compound.

2. Allow the same degree of out-of-roundness in holes after AFM as existed before the operation; AFM will not improve the roundness of holes.

3. Wide holes and slots (over about 12 mm, or 0.5 in) suffer from some loss of AFM efficiency because of boundary effects. For maximum process effectiveness, keep holes and slots narrow, down to about 0.6 mm (0.024 in). (The minimum practical hole diameter, as noted, is 0.4 mm, or 0.016 in.)

4. If AFM efficiency is important, make sure that parts are not too fragile. Cutting is most efficient when high pressures and more viscous media are used.

5. Corners that are to be rounded should have a radius of 0.013 to 2.0 mm (0.0005 to 0.080 in).

Tolerance Recommendations

When product function permits, maximum production economy results when 25 percent is allowed on the radii of rounded corners and on the amount of stock to be removed by AJM. If necessary, however, radii and surface-stock removal can be con-trolled within 10 percent. The latter limit requires extra care and production time and more extensive testing on the establishment of process parameters.

Surface-finish improvement normally results in roughness readings 50 percent finer than those which existed before the AJM operation. In extreme cases, roughness can be reduced by 90 percent.

ULTRASONIC MACHINING

The Process

Ultrasonic machining (USM) actually involves two different processes: ultrasonic impact grinding and rotary ultrasonic machining. Ultrasonic impact grinding (USIG) involves the rapid oscillation of a shaped tool immersed in a slurry of abrasive that is also in contact with the workpiece. This oscillation drives abrasive particles against the workpiece and cuts in it a cavity that has the same shape as the tool. The oscillat-ing frequency of the tool is from 19,000 to 25,000 Hz, and its amplitude is only 0.013 to 0.063 mm (0.0005 to 0.0025 in). The gap between the tool and the workpiece is small (0.025 to 0.1 mm, or 0.001 to 0.004 in), and the abrasive slurry is pumped through this gap. The tool is normally of low-carbon or stainless steel and is fastened to an ultrasonic generator through a horn of Monel metal. The abrasive particles may be aluminum oxide, silicon carbide, or boron carbide.

Rotary ultrasonic machining is similar to conventional drilling or milling of glass or other nonmetallics except that the rotating diamond-coated tool is also vibrated at an ultrasonic frequency (20 kHz). There is no abrasive slurry as in ultrasonic impact grinding, but there is a coolant (usually water) that flushes away the removed material.




The ultrasonic vibration reduces the pressure on the cutting tool and the friction at the point of cutting. It provides better coolant flow and better flushing of removed materi-al. These factors result in faster cutting action.

Typical Applications and Characteristics

Ultrasonic impact grinding is most advantageous when applied to the machining of irregular holes in thin sections or shallow, irregular cavities. Materials not suitable for other processes can be machined by USM (see subsection Suitable Materials). Fragile parts and materials like honeycomb are processible without undue difficulty. Drilling, cutting, deburring, etching, polishing, cleaning, and machining to produce a coined-like or embossed-like surface are normal applications. Holes as small as 0.08 mm (0.003 in) across, round or nonround, can be drilled. The maximum hole size with currently available 2.4-kW machines is about 90 mm (3.5 in) in diameter, although larger holes can be made by trepanning or by feeding the cutter in a transverse direc-tion. The normal maximum hole depth is 25 to 50 mm (1 to 2 in). Multiple holes machined in one pass, cavities with curved axes, and screw threads can be produced by the process.

Rotary ultrasonic machining is used primarily to drill glass and other hard non-metallic materials. Milling and grinding of these materials are also accomplished. The rotary ultrasonic method provides faster material removal and less danger of cracking or chipping these brittle materials than can be achieved with regular diamond-tool machining. The ultrasonic assist enables holes to be drilled straighter, deeper, and closer to edges than otherwise would be feasible. Holes up to about 50 mm (2 in) in diameter can be core drilled.

Surfaces cut by both processes have low surface stresses and no heat effects and are free from burrs. With ultrasonic impact grinding, there is an overcut on the diame-ter or width of the machined cavity of twice the average particle size. Most holes, especially deep ones, have a sidewall taper due to cutting on the sides of the tool. Figure 4.21.4 illustrates typical parts machined ultrasonically.


USIG is a relatively slow cutting method. Costs of tooling and equipment are moder-ate. However, choice of the process is not based particularly on production quantity. USIG is chosen when it is the most suitable method for the workpiece and material involved. When other conventional processes are usable, they are generally more eco-nomical. Cutting rates with USIG vary greatly with different materials, ranging from 0.03 to 4 cm3/min (0.002 to 0.25 in3/min). Tool wear, which ranges in ratio from 1:1 to 1:200 with workpiece-material removal, is another adverse cost factor. Rotary ultra-sonic machining of hard nonmetallic materials, however, is faster than conventional machining of these materials.

Suitable Materials

USIG is most advantageous for hard, brittle, nonconductive materials. Actually, all materials can be cut by USIG, but processing materials softer than Rc 45 is not recom-mended; materials harder than Rc 64 are best suited to the process. EDM is a compet-




FIGURE 4.21.4 Alumina, glass, and ferrite parts machined ultrasonically. (Courtesy Branson Sonic Power Co.)

ing process and provides more rapid cutting of metals and other conductive materials; USIG is used primarily for nonmetallic materials not suitable to EDM. Materials com-monly machined ultrasonically, listed in order of decreasing ultrasonic machinability, are glass, mother-of-pearl, ferrite, glass-bonded mica, germanium, carbon and graphite, quartz, ceramic, synthetic ruby, boron carbide, tungsten carbide, and tool steel. Table 4.21.1 lists metal-removal rates for some of these materials. Materials for rotary ultrasonic machining are similar: glass, quartz, ferrite, ruby, sapphire, and vari-ous ceramics. Softer, ductile materials are not so suitable; they tend to clog the dia-mond tool.

TABLE 4.21.1 Typical Material-Removal Rates for Materials Machined by

Ultrasonic Impact Grinding

Volume of material removedTool feed rates per

per minute, cm3 (in3)minute, mm (in)

Glass3.87 (0.236)3.8 (0.150)

Ferrite3.21 (0.196)3.2 (0.125)

Mica, glass-bonded3.21 (0.196)3.2 (0.125)

Germanium2.18 (0.133)2.2 (0.085)

Graphite2.05 (0.125)2.0 (0.080)

Quartz1.67 (0.102)1.7 (0.065)

Ceramic1.54 (0.094)1.5 (0.060)

Boron carbide0.39 (0.024)0.38 (0.015)

Tungsten carbide0.36 (0.022)0.36 (0.014)

Tool steel0.26 (0.016)0.25 (0.010)






1. Shallow holes and cavities are more suitable for USIG than deep ones. Holes should not be deeper than 212 times diameter.

2. Through holes or holes with through passages for abrasive slurry are preferred to blind holes. (See Fig. 4.21.5.)

3. If the workpiece material is brittle and a through hole is machined, the part should be designed so that a backup plate can be cemented or clamped to the exit surface. This will prevent chipping of the workpiece at the exit surface. (See Fig. 4.21.6.)

4. Allow for taper in holes machined by USIG, especially deep holes. Taper averages 0.05 mm/cm (0.005 in/in), as illustrated in Fig. 4.21.7. If necessary, taper can be reduced by using two successive machining operations.

5. Allow large radii at the bottom of blind holes, especially with USIG, because tool wear is concentrated at the corners of tools. For the same reason, do not specify sharp detail at the bottom of blind holes. (See Fig. 4.21.8.)

FIGURE 4.21.5 Through holes or holes with through passages for theUSIG abrasive slurry are preferable.

FIGURE 4.21.6 Parts that are ultrasonically machined from brittle materials should have a backup plate attached to the exit surface to prevent chipping at the edge.

Dimensional Factors and Recommended Tolerances

Overcut and tool wear are the two primary factors affecting the accuracy of ultrasoni-cally impact-ground surfaces. Other factors are the rigidity of the setup, the size of the




FIGURE 4.21.7 Normal taper of sidewalls of cavities made by USIG.

abrasive, the temperature of the slurry, and the design of the cutting tool . Nevertheless, very good accuracies are attainable. The recommended dimension-al tolerance for USIG surfaces is 0.025 mm ( 0.001 in). However, 0.013 mm ( 0.0005 in) can be held if necessary. A surface-finish tolerance of 1 mm (40 in) should be allowed, but slower-cutting, finer abrasive can produce surfaces as fine as 0.25 m (10 in).

FIGURE 4.21.8 Do not specify sharp corners at the bottom of cavities machined ultrasonically.

HYDRODYNAMIC MACHINING

Hydrodynamic machining, sometimes called water-jet machining, uses a high-velocity narrow jet of liquid as a cutting agent. The jet of water, sometimes with polyethylene oxide or another long-chain-polymer additive, travels at about 600 m/s (2000 ft/s), or twice the speed of sound. Material is removed from the workpiece by the impingement of this jet. Pressures of 69 to 415 MPa (10,000 to 60,000 lbf/in2) drive the liquid through a fine sapphire nozzle orifice. The resulting jet is 0.05 to 1.0 mm (0.002 to 0.040 in) wide.

The process is presently applicable commercially to soft nonmetallic materials, though very thin, soft metals also can be cut. Gypsum board, urethane, and poly-styrene foam, 18-in plywood, rubber, various thermoplastics, and fiberglass-reinforced plastics are suited to hydrodynamic cutting. Shoe soles, asbestos brake-shoe linings, and furniture parts made from laminated paperboard are examples of production parts cut from other materials.

The process is used most often for cutout operations on material in sheet form. It has a number of advantages for such work: The kerf is narrow, only 0.025 mm (0.001 in) wider than the nozzle orifice; the dwell of the jet does not widen the kerf; there is no heat effect to the cut edge; and little or no dust is created. One drawback is the high noise level that can accompany the process.

Cutting rates can be rapid with some materials but vary considerably from one material to another. For cutout operations, tooling is minimal. Equipment is normally designed and fabricated for a specific application.Tolerances for cutout pieces depend primarily on the accuracy of the mechanism




that provides movement of the nozzle with respect to the material to be cut. Tolerances of 0.25 mm (0.010 in) are normally achievable.

When abrasive particles are added to the water jet, the process is referred to as abrasive water-jet machining, and metals and other hard materials can be cut with the process. Abrasive particles are fed into the high-speed steam of water, travel with the water through an orifice, and impinge on the workpiece. The abrasive particles cut the workpiece material as they strike it. The jet diameter is typically 0.5 to 2.5 mm (0.020 to 0.100 in). The abrasive slurry is captured after it leaves the workpiece.

The process is used to machine metals, ceramics, concrete, glass, and composites. Almost all materials can be machined by this method. Reinforced plastics, honey-combed materials, and others difficult to machine by other methods are particularly suitable. It has the advantage over laser and electron-beam machining in not producing any significant heat-affected zone. It is also superior to laser-beam machining for metal sheets thicker than 13 mm (0.5 in). Blind-hole and small-hole drilling generally are not feasible. Kerfs are wider than with laser-beam machining or electron-beam machining, typically 0.75 to 2.3 mm (0.030 to 0.090 in). Except in soft, ductile materials, burrs are not produced. Metal plates up to about 200 mm (8 in) can be cut by the process.

Equipment for abrasive water-jet machining is lower in cost than that required for laser-beam or electron-beam machining, but capital costs are still a major item. Another is the cost of the abrasive used, most commonly garnet. Cutting speeds for materials thin enough for laser-beam machining are somewhat slower than with laser-beam machining. The process is also considerably slower than flame cutting but pro-duces a cleaner edge. In general, it is most economical for those materials for which a heat-affected zone is a problem. As with laser-beam, electron-beam, and flame cutting, no cutting tools are required.

The surface finish of the cut edge typically ranges from 1.6 to 6.3 m (63 to 250 in). Dimensions for cut parts should have a tolerance of 0.25 mm ( 0.010 in), though closer values are possible, if necessary, with precision equipment.

ELECTRON-BEAM MACHINING

The Process

Electron-beam machining (EBM) is, with minor differences, the same process as used for electron-beam welding and described in Chap. 7.1. A high-velocity beam of elec-trons, focused on a small point of the workpiece, intensely heats the workpiece materi-al at that point so that it melts and vaporizes. Whereas in electron-beam welding the objective is to melt the workpiece material so that it flows together and fuses, in EBM the objective is to cut completely through the workpiece. Higher power levels and higher beam velocities are utilized in EBM compared with electron-beam welding. Another difference is the greater need for a full vacuum with EBM. In EBM, the beam impinges on an area of 0.32 to 0.64 mm2 (0.0005 to 0.001 in2) and has a power density of 15 million W/mm2 (10 billion W/in2).


EBM is most suitable for fine cuts in relatively thin workpieces. Drilling fine holes in metals is the major application, but any material can be machined by the process.




Holes and slots only a few thousandths of an inch wide and very precise contoured cuts are quite feasible. The process is particularly suitable for cuts that are too fine for EDM or ECM.

EBM is used for drilling metering holes such as are used for diesel fuel injection, gas orifices for pressure-differential devices, wire-drawing dies, spinnerets, sleeve-valve holes, scribing thin films, and removing broken taps of small diameter. Holes as small as 0.013 mm (0.0005 in) in diameter are practicable in 0.025-mm- (0.001-in-) thick material, as are slots as narrow as 0.025 mm (0.001 in). Length-to-diameter ratios of holes up to 10 or even 15 in in some cases can be achieved. However, 6.4 mm (0.25 in) is a practical maximum depth of cut. If the workpiece material is over 0.13 mm (0.005 in) thick, a 1 to 2 taper in the sidewalls of the through cut should be expected.

Cratering usually occurs on the workpiece surface adjacent to the hole entrance. There also may be some spatter on the same surface, but this is easily removed. The edges of holes and slots tend to show nonuniform surfaces. There is a heat-affected zone about 0.25 mm (0.010 in) deep adjacent to the cut. Otherwise, the workpiece is distortion-free, since there is no pressure or contact between the workpiece and any cutter. Figure 4.21.9 illustrates characteristics of a hole machined by an electron beam.

FIGURE 4.21.9 Typical characteristic of an electron-beam-machined hole.


Cutting rates are rapid for thin materials. For example, producing 0.1-mm- (0.004-in-) diameter holes in 0.5-mm (0.020-in) stock requires less than 110 s. Slots 0.05 mm (0.002 in) wide in 0.25-mm- (0.010-in-) thick material can be machined at 65 to 150 mm/min (2.5 to 6 in/min). Nevertheless, the volume of metal removal with EBM is actually low compared with conventional methods, averaging about 0.8 to 2 mm3/min (0.00005 to 0.00012 in3/min). The time required to evacuate the vacuum chamber for each machine load is a factor that increases production time. The high cost of equip-ment and the need for skilled operators are further adverse cost factors.




Suitable Materials

As indicated above, any material can be machined by EBM. Metals, ceramics, plastics, and composites all are easily machined, although the cutting rate is slower for materi-als with high melting and vaporization temperatures. Hardened steel, stainless steel, molybdenum, nickel, cobalt, titanium, tungsten, and their alloys; quartz, ceramics, and synthetic sapphire have all been successfully machined by EBM.



1. Workpieces to be machined by the electron-beam process should be kept as small as possible so that a larger number of pieces will fit into the vacuum chamber. It may even be advisable to create an assembly of several parts rather than having one bulkier part if only a portion of the workpiece requires EBM.

2. The normal minimum radius for internal corners is 0.25 mm (0.010 in). Sharper corners should not be specified.

3. For best results with through cuts, the workpiece should be as thin as possible. A practical maximum is 6.3 mm (0.25 in), but workpieces considerably thinner than this machine more rapidly with less sidewall taper.

4. Designers should allow for the machined surface effects of EBM, which for some applications may be undesirable. In these cases, stock should be allowed for sec-ondary operations. Figure 4.21.9 illustrates these surface effects.


A tolerance of 10 percent should be allowed on hole diameters and slot widths. The normal surface-finish specification should be 2.5 m (100 in), although surfaces as fine as 0.5 m (20 in) can be produced under optimal conditions.

LASER-BEAM MACHINING

Like EBM, laser-beam machining (LBM) utilizes a process that is applicable to weld-ing as well as to machining. The basic process is described in Chap. 7.1 and illustrated in Fig. 7.1.4. When used for machining, the process operates at somewhat higher ener-gy levels than for welding. The narrow, highly focused beam of coherent light of extremely high intensity melts and vaporizes material at the point where it strikes the workpiece. The beam may consist of intermittent pulses or of a continuous beam. A typical focused spot on the workpiece is 25 to 50 m (0.001 to 0.002 in) wide. The density of energy at this spot is extremely high, amounting to millions of watts per square centimeter. When this level of energy strikes the workpiece, material at the




point of impingement is vaporized or melts and is swept away from the beam. In some cases, a jet of gas coaxial with the laser beam is directed against the workpiece. Oxygen is the most common gas for rapid cutting, while inert gases are best to improve the edge-surface finish.

Applications and Typical Characteristics

Laser-beam machining is most commonly used for high-precision machining or micromachining of thin parts that are difficult to machine by conventional methods. Holes smaller than 1.3 mm (0.050 in) and stock thinner than 5 mm (0.200 in) give the best results. Larger holes can be trepanned. The drilling of very small holes with a large depth-to-diameter ratio is a particularly advantageous application. Increasingly, the process is being used to cut out blank parts from sheet metals and other materials up to about 13 mm (0.5 in) in thickness. Slitting, trimming, and perforating are other applications.

FIGURE 4.21.10 Examples of LBM: (a) Cooling holes in turbine-engine blades. (b) Slots in high-strength steel for sprag-type clutch. (Courtesy Apollo Lasers, Inc.)




Additionally, by using two intersecting lasers, machining operations equivalent to milling and turning can be performed. However, these are normally limited to materi-als not feasible to machine by conventional methods.

Common examples of LBM are drilling 0.1-mm (0.004-in) holes in glass contact lenses, drilling 0.13-mm- (0.005-in-) diameter holes in plastic aerosol-spray nozzles, trimming thick and thin film resistors to precise values, scribing silicon and ceramic substrates for electronic microcircuits, and making contoured cuts in cloth for garment components. Figure 4.21.10 illustrates additional typical examples.

The LBM process has the advantage of being usable for machining inaccessible places. It can operate through transparent materials and various atmospheres. There is no tool contact with the work. This and the fact that the heating effect is very much localized permit the machining of brittle, heat-shock-sensitive, and fragile workpieces.

The minimum hole diameter is about 0.005 mm (0.0002 in), but 0.13 mm (0.005 in) is more common. Length-to-diameter ratios of up to 50:1 are feasible with 0.13-mm- (0.005-in-) diameter holes. Holes with an angle to the surface as shallow as 15 are possible. Kerf widths for slits and profile cuts as narrow as 0.1 mm (0.004 in) are used, but 0.4 mm (0.015 in) is a better normal value for most applications.

LBM holes deeper than 0.5 mm (0.020 in) exhibit noticeable taper. There is also considerable nonuniformity of hole diameter. Both taper and other irregularities become more pronounced as the depth of the cut increases. Holes normally also are not perfectly round. There is a tendency toward cratering at the entrance surface of the cut, and a narrow heat-affected zone of about 0.13 mm (0.005 in) borders the machined surface.


Actual drilling time for small holes in thin stock is only hundredths of a second. Linear cutting of sheet materials also may be rapid for most materials, particularly with oxygen assist. For example:

Titanium alloys from 0.5 to 10 mm (0.020 to 0.400 in) thick: cut at 2.5 to 12 m/min (100 to 500 in/min)

Mild steel 5 mm (0.200 in) thick: cut at 2.5 m/min (100 in/min)

Plastics 0.8 mm to 3 mm (0.030 to 0.125 in) thick: cut at 2.5 to 7.5 m/min (100 to 300 in/min)

However, equipment costs are high for LBM (though lower than those for EBM, which requires a vacuum chamber). Operational costs may be high due to the com-plexity of the operation. Higher production quantities and the use of hard or difficult-to-machine materials favor the process. Laser-equipped turret machines have found a niche in the job-shop production of sheet-metal parts.

Suitable Materials

All materials are laser-beam machinable, but not necessarily with the same technique or process. The most practical materials for LBM are those which are difficult or impossible to cut by conventional methods. Ceramics, glass, carbides, and some aero-space alloys fall into this category. Copper, aluminum, gold, and silver are not so suit-




able because of their high thermal conductivity. Aluminum also tends to form a dross at the cut edge that requires a secondary operation for removal. Mild steel and titani-um are commonly processed by laser beam. Other materials that have been machined successfully are plastics, rubber, beryllium, zirconium, stainless steel, tungsten, cast iron, brass, molybdenum, cloth, cardboard, wood, boron- and graphite-reinforced epoxy composites, and various laminated materials. Laminates and composites may require special steps because the constituent materials behave differently when exposed to the laser energy.


Since LBM is a process that uses energy in the form of light, best results are achieved when the workpiece surface absorbs rather than reflects the beam energy. Surfaces should be dull and unpolished until after LBM has been completed.

Other design recommendations closely parallel those for EBM. Workpieces should be thin in areas through-cut by LBM because machining time is faster and taper and irregularities are minimized with thinner materials. Corner radii allowances should be 0.25 mm (0.010 in) or greater. Allowances should be made for taper averaging 3 per side and for a heat-affected zone of about 0.13 mm (0.005 in) deep, for cratering on the entrance surface and for spatter and other residue on machined surfaces.


Hole-diameter and slot-width tolerances should be 0.025 mm ( 0.001 in). When a reactive gas assist is used, allowances should be increased to 0.1 mm ( 0.004 in). Dimensions of parts cut by laser from sheet stock should have a tolerance of 0.13 mm ( 0.005 in).


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