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  1. 1. Electron Beam Machining (EBM) 19.1 Process Principles 19.2 Equipment 19.2.1 Electron Beam Gun 19.2.2 Power Supply 19.2.3 The Electron Beam Machining Systems 19.3 Process Parameters 19.4 Process Capabilities 19.5 Application Examples 19.6 Process Summary 19.7 References 19.1 PROCESS PRINCIPLES Electron beam machining (EBM) is a thermal material removal process that utilizes a focused beam of high-velocity electrons to perform high-speed drilling and cutting. Just as in electron beam welding (Chap. 18),
  2. 2. material-heating action is achieved when high-velocity electrons strike the work piece. Upon impact, the kinetic energy of the electrons is converted into the heat necessary for the rapid melting and vaporization of any material. Invented in Germany in 1952 by Dr. K. H. Steiger wald, EBM is able to drill materials up to 10-mm (0.394-in.) thick at perforation rates that far exceed all other manufacturing processes. Although EBM is capable of producing almost any programmable hole shape, it is most often applied for high-speed drilling of round holes in metals, ceramics, and plastics of any hardness. The EBM process begins after the work piece is placed in the work chamber and a vacuum is achieved. The creation of a hole by an electron beam occurs in four stages (Fig. 19.1). First, the electron beam is focused onto the work piece to a diameter that is slightly smaller than the final desired hole diameter. Power is adjusted so that the electron beam will generate a power density at the work piece in excess of 108 W/CM2 (1.5 x 107 W/in.2). A power density of that magnitude is more than sufficient to instantly melt any material regardless of thermal conductivity or melting point. Drilling is accomplished through the combination of an electron beam pulse and an organic or synthetic backing (auxiliary) material, which is applied to the exit side of the surface being drilled.
  3. 3. When the focused beam strikes the work piece, local heating, melting, and vaporization take place instantly. Only about 5% of the affected material is actually vaporized. The pressure of the escaping vapor is sufficient to form and maintain a small capillary channel in the material. The beam and capillary rapidly penetrate through the work piece and a finite distance into the backing material. The volume of backing material that is contacted by the beam is almost totally vaporized resulting in the explosive release of backing material vapor. As a result of the comparatively high pressure of the backing material vapor, the molten walls of the capillary are expelled in a shower of sparks leaving a hole in the work piece and a small cavern in the backing material.
  4. 4. Figure 19.1 The four steps that lead to material removal by electron beam drilling (Source: courtesy, Messer Griesheim GmbH, Puchheim, W. Ger.). As mentioned, a single pulse is often used to produce a single hole; however it the material is very thick, multiple pulses may be required. If the desired hole shape is not round, the beam, pulsing at rates up to 1000 pulses/sec, is deflected by computer to cut out the shape along its perimeter. Using this technique, almost any hole shape can be generated. 19.2 EQUIPMENT The appearance of electron beam machining equipment is very similar to electron beam welding equipment. Most system subassemblies such as the vacuum system, work piece-positioning system, and vacuum chamber are essentially identical with those used for EBW. There are however significant differences between the two systems with respect to the electron beam gun and power supply. 19.2.1 Electron Beam Gun The function of the electron beam gun is to generate, shape, and deflect the electron beam to drill or machine the work piece. The EBM guns resemble
  5. 5. those used for welding; however the similarities end there. For example. the EBM gun is designed to be used exclusively for material removal applications an can be operated only in the pulsed mode. A typical triode EBM gun functions in a manner very similar to an EBW gun (Chap. 18). An electron quot;cloudquot; is generated by a superheated tungsten filament, which also acts as the cathode. A combination of repewng forces from the negative cathode and the attracting forces from the positive anode causes the free electrons to be accelerated and directed toward the work piece. Before passing through the anode, the beam travels through a bias electrode, which controls the flow of electrons and acts as a switch for generating pulses. After passing through the anode, the electron beam is diverging rapidly and traveling at approximately one-half the speed of light. A magnetic coil, which functions as a magnetic lens, repels and shapes the electron beam into a converging beam. The beam is then passed through a variable aperture which results in the removal of stray electrons from the beam's fringe areas, thus reducing the fmal focused spot diameter and producing a more favorable beam energy distribution for machining applications. Beneath the aperture are three final magnetic coils that are used as the final magnetic lens, deflection coil, and stigmator. Pinpoint focusing is accomplished with the lens, and a small amount of controllable beam
  6. 6. deflection is achieved with the deflection coil. The stigmator corrects minor beam aberrations and ensures a round beam at the work piece. To protect the electron beam gun from metal spatter and vapor, a series of rotating slotted disks are often mounted directly beneath the gun exit opening. The rotational rate of the disks is synchronized such that the beam pulse will pass through the slots, but the spatter is blocked. 19.2.2 Power Supply The high-voltage power supply used for EBM systems generates voltages of up to 150 kv to accelerate the electrons. The most powerful electron beam machining systems are capable of delivering enough power to operate guns at average power levels of up to 12 kw. Individual pulse energy can reach 120 joules/pulse. To avoid the possibility of arcing and short circuits, the high-voltage sections of the power supply are submerged in an insulating dielectric oil. AB power supply variables, such as the accelerating current, focus current, pulse duration, and others, are controlled by a CNC unit or by a microcomputer. To ensure process repeatability, the process variables are monitored and compared with set-points by the power supply computer. If a discrepancy arises, the operator is alerted.
  7. 7. Figure 19.2 Computer-controlled multi axis EBM system (Source: courtesy, Messer Griesheim GmbH, Puchheim, W. Ger.). 19.2.3 The Electron Beam Machining Systems An EBM system, as shown in Fig. 19.2, has an appearance very similar to an electron beam welding system. A vacuum chamber is required for EBM and should have a volume of at least 1 m3 to minimize the chance of spatter sticking to the chamber walls.
  8. 8. The time necessary to pump the chamber to an operating level of 10-2 mbar is approximately 3 min for each cubic meter of volume. Inside the chamber a positioning system is used for the controlled manipulation of the work piece. The positioning system may be as simple as a single, motor-driven rotary axis or as complex as a fully CNC, closed- loop, five-axis system. 19.3 PROCESS PARAMETERS Beam current, pulse duration, lens current, and the beam deflection signal are the four most important parameters associated with electron beam machining. Determining the initial parameter settings for new applications usually involves some amount of trial-and-error testing. However once established, each parameter is computer controlled during processing to ensure repeatability on a day-toA day basis. Beam current is continuously adjustable from approximately 100 gamp to 1 amp. As the beam current setting is increased, the energy per pulse delivered to the work piece is also increased. Electron beam machining systems are available that can generate pulse energies in excess of 120 joules/pulse, a value that is 200400% higher than that available from industrial laser-drwing systems. The extremely high pulse energy available
  9. 9. with EBM explains the ability of the process to rapidly drill very deep and large-diameter holes. Pulse duration affects both the depth and the diameter of the hole. The longer the pulse duration, the wider the diameter and the deeper the drilling depth capability will be. To a degree, the amount of recast and the depth of the heat-affected zone will be governed by the pulse duration. Shorter pulse durations will allow less interaction time for thermal affects to materialize. Typically, electron beam systems can generate pulses as short as 50 tisec or as long as 10 msec. The lens current parameter determines the distance between the focal point and the electron beam gun (the working distance) and also determines the size of the focused spot on the work piece. The diameter of the focused electron beam spot on the work piece will, in turn, determine the diameter of the hole produced. As mentioned earlier though, to achieve a desired hole diameter, the beam power must be sufficient to generate more than 108 W/CM2; otherwise the power density will be insufficient to promote vaporization and drilling. The depth to which the focal point is positioned beneath the work piece surface determines the axial shape of the drilled hole. By selecting different focal positions, the hole produced can be tapered, straight, inversely tapered, bell shaped, or center-bowed. A cross-sectional view of tapered electron-bearn drilled holes is shown in Fig. 19.3.
  10. 10. When hole shapes are required to be other than round, the beam deflection coil is programmed to sweep the beam in the pattern necessary to cut out the shape at the hole's periphery. Beam deflection is usually applicable only to shapes smaller than 6 mm (0.236 in.).
  11. 11. Figure 19.3 Cross-sectional view of electron-beam-drilled holes through 5- mm thick copper sheet. Holes were drilled at the rate of six holes per second. Hole diameter is 0.2 mm (Source: courtesy, Messer Griesheim GmbH, Puchheim, W. Ger.). The deflection coil can also be used to track the work piece when drilling quot;onthe fly.quot; For the duration of the pulse, the inertia-less electron beam tracks the proper location on the work piece thus avoiding elongation of the hole. After the hole has been completed and before drilling the next one, the beam is directed back to its initial position, ready for the next hole. Using this technique holes can be drilled at the rate of several thousand each second without the necessity of stopping the work piece at each hole location. An electron beam deflection coil is able to scan the electron beam a distance of 0.1 mm (0.004 in.) in 1 msec, meaning that effective tracking can be accomplished with work piece motion as fast as 100 mm/sec. (236 in./min). 19.4 PROCESS CAPABILITIES A wide range of materials, such as stainless steel, nickel and cobalt alloys, copper, aluminum, titanium, ceramics, leather, and plastics, can be successfully processed by EBM. Some materials are easier to process than
  12. 12. others. For example, aluminum and titanium require less beam power to remove a given volume of material than either steel or tungsten. Because EBM is a thermal machining process, some thermal effects remain on the machined edge after processing. However because of the extremly high beam power density and the short duration of the beam/work piece interaction time, thermal effects are usually limited to a recast layer and the heat-affected zone, which seldom exceeds 0.025 mm (0.001 in). Typically, no burr is generated on the exit side of the hole, although, a small lip of solidified material may remain around the rim of the hole on the entrance side. Another capability of the electron beam process made possible by the high power density is the ability to drill deep, high aspect ratio holes. Aspect ratios as large as 1 5: 1 can be achieved in most materials. The hole diameters that can be drilled range from 0.1 to 1.4 mm (0.004 to 0.055 in.) in thicknesses up to 10 mm (0.390 in.). The tolerance on the hole diameter is typically ± 5% of the diameter or 0.03 mm (0.001 in.), whichever is greater. The rate at which holes can be drilled is a function of the hole geometry and material thickness. Figure 19.4 is a homograph that can be used to predict drilling rates for a given hole diameter and depth or volume. To use this chart, first find the diagonal lines that correspond to the desired hole diameter and depth. After locating the intersection point of the two lines, follow the vertical line up to the center of the white band in the top chart. From that point, move left to read
  13. 13. the corresponding drilling rate. This drilling rate is applicable when drilling materials such as steels, nickel, and cobalt alloys. An electron beam does not apply any force to the work piece, thereby allowing brittle or fragile materials to be processed without danger of fracturing.
  14. 14. Figure 19.4 An EBM drilling rate homograph (Source: courtesy, Messer Griesheim GmbH, Puchheim, W. Ger.).
  15. 15. Figure 19.5 Minimum distance between hole centerlines for EBM (Source:courtesy, Messer Griesheim GmbH, Puchheim, W. Ger.). further benefit from the non contact nature of the process is the ability to drill holes at angles as shallow as 200 off the surface. Figure 19.5 illustrates the minimum required spacing between holes for successful electron beam drilling. The relationship is a function of hole diameter, with the minimum center line-to-center line distance being twice the hole diameter. Even with this limitation, work pieces can be perforated with small holes 2 at up to 1000 holes/cm .
  16. 16. 19.5 APPLICATION EXAMPLES Applications best suited to EBM are those that require either thousands of simple holes to be drilled in each work piece, or those that require more than 200 holes that are difficult to drill conventionally because of material hardness or hole geometry. Most current applications of EBM are for the aerospace, insulation, food and chemical, and clothing industries. The drilling of a turbine engine combustor dome made of a CrNiCoMoW steel has been performed for several years using EBM. The part has a wall thickness of 1.1 mm (0.043 in.) and is perforated with 3748 holes that are 0.9 ± 0.05 mm (0.035 ± 0.002 in.) in diameter. Each part is drilled in 60 min for a drilling rate of approximately one hole every second (Messer Griesheim GmbH, 1982). The insulation industry relies on EBM to drill thousands of small holes in cobalt alloy fiber spinning heads that are used in the production of glass fiber and rock wool materials. A example spinning head, shown in Fig. 19.6, requires that 11,766 holes be drilled through a material thickness that varies from 4.3 to 6.3 mm (0.17 to 0.25 in.). The hole diameter requirement is 0.81 ± 0.03 mm (0.032 ± 0.001 in.). The EBM drilling rate of five holes per second is 100 times faster than the alternate method of EDM drilling and results in production rates of one part in 40 min (Closs, 1977).
  17. 17. Filters and screens used in the food-processing industry require thousands of holes to be drilled through relatively thin, formed sheet metal. Electron beam machining is a cost-effective method of producing these holes for approximately 40 cents/ 1000 holes (von Dobeneck, 1977). Figure 19.7 shows holes of various diameter that are used for filtering applications. Figure 19.6 A glass fiber spinning head by EBM drilled with 11,766 smalldiameter holes at a rate of five holes per second (Source: courtesy,
  18. 18. Messer. A rather surprising use of electron beam perforation involves the clothing industry. A percentage of the shoes made today are fabricated from an artificial leather consisting of a plastic-coated textile substrate. This artificial leather is not permeable to moisture and air, thus making its level of comfort poor. A more comfortable breathing material can be produced by electron beam perforation of the plastic surface. Figure9.7 ExamplesofEBM-drWedholesinl-mmthicksheetmetal(IOX
  19. 19. magnification). (Source: courtesy, Messer Griesheim GmbH, Puchheim, W. Ger.). Thus treated, the material is acceptable for use in shoes, clothing, and upholstery. Electron beam machining drills these materials with 0.12-mm (0.0047-in.) diameter holes at a rate of 5000 holes/sec (Steigerwald, 1978). 19.6 PROCESS SUMMARY Advantages Very high drilling rates Drills any material No mechanical or thermal distortion Computer-controlled parameters High accuracy Disadvantages High capital equipment cost High level of operator skill required Limited to 100-mm material thickness
  20. 20. 19.7 REFERENCES Closs, W. W. and Drew, J. (1977). Electron beam drilling. Farrel Company Divvision, USM Corporation, Ansonia, Conn.Messer Griesheim GmbH (1982). EBOPULS CNC electron beam drilling machines. Publication No. 47.20 1 Oe, Puchheim, W. Ger.Steigerwald, K. H. ( 1978). Electron beam machining - the process and its applications. Farrel Company Division, USM Corporation, Ansonia, Conn. von Dobeneck, D. (1977). Drilling with electron beams. Ingenieur Dig. 16:38.