Electron Beam Machining (EBM)
19.1 Process Principles
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
19.1 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),
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
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
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.
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.
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.
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
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
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
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.
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.
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
with EBM explains the ability of the process to rapidly drill very deep and
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.
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.).
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
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.
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
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
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
Figure 19.4 An EBM drilling rate homograph (Source: courtesy, Messer
Griesheim GmbH, Puchheim, W. Ger.).
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 .
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).
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
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,
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.
magnification). (Source: courtesy, Messer Griesheim GmbH, Puchheim, W.
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
Very high drilling rates
Drills any material
No mechanical or thermal distortion
High capital equipment cost
High level of operator skill required
Limited to 100-mm material thickness
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
D. (1977). Drilling with electron beams. Ingenieur Dig. 16:38.