Classification table
| C1-C7 Physical Principle |
Process/Material Interaction |
|||
| subtractive | additive | mass containing | joining | |
| C1 mechanical force |
C1.1.1
Micromilling C1.1.2 Microturning C1.1.3 Fly Cutting C1.1.4 Microdrilling C1.1.5 Microgrinding C1.1.6 Ultrasonic Machining C1.1.7 ECF |
C1.3.1
Rolling C1.3.2 Forging C1.3.3 Deep Drawing C1.3.4 Bending C1.3.5 Blanking C1.3.6 Embossing C1.3.7 Cold Forging |
C1.4.1 Ultrasonic Welding | |
|
C2
melting
vaporization ablation |
C2.1.1
EDM C2.1.2 ELID C2.1.3 Laser Micromachining |
C2.2.1
CVD C2.2.2 PVD |
C2.3.1 Laser Bending | C.2.4.1
Resistance
Welding/Soldering C.2.4.2 Laser Welding/Soldering C.2.4.3 Bonding |
| C3 dissolution |
C3.1.1
ECM C3.1.2 Lithography & Etching |
|||
| C4 solidification |
C4.3.1 Casting C4.3.2 Injection Molding |
|||
| C5 recomposition |
C5.2.1 Electroforming C5.2.2 Electroplating |
|||
|
C6
sintering
|
||||
| C7 LIGA |
C7 Combination of dissolution (x-ray lithography), recomposition (electroforming) and mass containing (molding) | |||
Overview of Metal Processing Technologies suitable for mass production
Overview of Metal Processing Technologies suitable for small batch production
Serial Manufacturing |
||||
| C1 mechanical force | C2 melting/vaporisation/ablation | C4 solidification | ||
| C1.1 subtractive | C1.3 mass containing | C1.4 joining | C2.4 joining | C4.3 mass containing |
| C1.1.1 Micromilling | C1.3.1 Rolling | C1.4.1 Ultrasonic Welding | C2.4.1 Resistance Welding/Soldering | C4.3.1 Casting |
| C1.1.2 Microturning | C1.3.2 Forging | C2.4.2 Laser Welding/Soldering | C4.3.2 Injection Molding | |
| C1.1.3 Fly Cutting | C1.3.3 Deep Drawing | C2.4.3 Bonding | ||
| C1.1.4 Microdrilling | C1.3.4 Bending | |||
| C1.1.5 Microgrinding | C1.3.5 Blanking | |||
| C1.3.6 Embossing | ||||
| C.1.3.7 Cold Forging | ||||
Small Quantity Production |
|||||||
| C1 mechanical force | C2 melting/vaporisation/ablation | C4 dissolution | C5 recomposition | C7 LIGA | |||
| C1.1 subtractive | C2.1 subtractive | C2.2 additive | C2.3 mass containing | C2.4 joining | C.3.1 subtractive | C5.3 mass containing | |
| C1.1.6 Ultrasonic Machining | C2.1.1 EDM | C2.2.1 CVD | C2.3.1 Laser Bending | C2.4.3 Bonding | C3.1.1 ECM | C5.3.1 Electroforming | |
| C1.1.7 ECF | C2.1.2 ELID | C2.2.2 PVD | C3.1.2 Litography & Etching | C5.3.2 Electroplating | |||
| C2.1.3 Laser Micromachining | |||||||
Technology
suitable
for both serial and small quantity production
Perhaps
the most versatile
direct machining process in conventional manufacturing is milling. This
would
also seem logical at the microscale. Peripheral end-milling and slot
milling
present the most severe machining environment of any of the
micromechanical
processes. The geometry of a diamond cutting tool, even when its width
is
small, is a relatively strong structure. At the extremity of the
cutting edge,
there is primarily compressive stress. The loading condition away from
the
cutting edge becomes bending but the section modulus becomes large
quickly as
the depth of the cutting edge region becomes larger. Micromilling
however,
requires the tool to perform as a cantilever beam which is the weakest
structure
of the three types of cutting tools for micro chip-making processes.
Development of the micromilling process began by mimicking a
conventional
four-fluted peripheral end milling tool. Milling tools are made using
the
focused ion beam process. The focused ion beam process (FIB) is widely
used in
the semiconductor industry for tasks such as mask repair and junction
and
metallization layer sectioning. Both tasks make use of the fact that
the ion
beam process can remove material at the atomic scale. The
micromechanical
milling tools begin as a cylindrical piece of hard material such as
tool steel
or tungsten carbide. The tool blank can be nearly any diameter; however
for the
22-micrometer tools described in this paper, the blank is a broken
microdrill with
a nominal cutting tip diameter of 25 micrometers. The blank is part of
a center
less ground and lapped mandrel. The blank-mandrel combination operates
in a
vee-block (see section on microdrilling) with four convex diamond pads.
So long
as the tool was originally ground in, and is used in vee-blocks
calibrated to
each other, the blank turns with no measurable eccentricity. The
vee-block
arrangement is commonly used for high precision rotation applications.
Micromilling
is characterized
by mechanical interaction of a sharp tool with the workpiece material,
causing
breakage inside the material along defined paths, eventually leading to
a
removal of the useless part of the workpiece in the form of chips. The
tool
edge radius must be in the order of the dimension of the cut thickness
or
smaller. Monocrystalline diamond is the most suitable tool material but
it
implies a limitation with regards to the workpiece materials because of
its
high chemical affinity with steel.
Tool
fabrication is another
important issue for the application of microcutting technology. For
industrial
applications, micro powder (0.3 µm particle sizes) tungsten
carbide two flutes
end mills up to 100 µm diameter are commercially available
with an edge radius
in the order of 1-2 µm, while smaller sizes are still at the
research phase.
The most attractive advantage of micromilling is the possibility to machine 3D micro structures characterized by high aspect ratios and high geometric complexity. However, an important issue in microcutting is burrs removal. Since the dimensions of the machined parts make handling after machining difficult, conventional methods for burr removal are impossible to apply. Therefore special techniques for burr removal as well as burr-free machining strategies have to be developed.
|
Micromilling
Data Sheet |
|
|
Mould Materials |
Tool steel up
to HRC 62, Al7075, Cu |
|
Cutting tool |
Tungsten
carbide end mill up to Ø0.1 mm |
|
Machine |
Ultra-precision
milling machine 3-5 axes |
|
Removal rate |
1-3 mm3/h |
|
Machining of channels & ribs |
|
|
Minimum width |
100-110 µm |
|
Aspect ratio |
10-15 |
|
Accuracy |
5 µm |
|
Roughness |
0.3
µm Ra |
|
Machining of holes & pins |
|
|
Minimum diameter
|
110-150 µm |
|
Aspect ratio |
5 |
|
Accuracy |
5 µm |
|
Is a 3D
freeform surface possible? |
Yes |
C1.1.2.1
Hard Milling
The
micro-cutting of steel with tungsten carbide tools can meet many of
the demands of miniaturised components. A prerequisite for a stable
machining
process is a homogeneous, hard workpiece material with no internal
stresses.
The achievable surface roughness is between Rz = 0.5 μm and 1
μm. The most
important process parameter is the cutting velocity. Depending on the
hardness
of the material, higher cutting velocities lead to better surface
qualities. In
order to improve the achievable surface quality a deeper understanding
of the
influence of the material properties on the minimum cutting depth is
needed.
C1.1.2.1
Diamond Milling
In diamond
milling mainly two different types of processes for the
generation of microstructures can be found, i.e. circumferential
milling, so
called fly-cutting, and ball-end milling. Before micro structuring, the
relative position of workpiece and diamond tool have to be known inside
the
machine tool with respect to each other. Besides a geometrical error of
the
cutting edge and the tool included angle a profiled diamond tool
exhibits three
degrees of freedom inside the tool fixture: tilt error κ
(kappa) in the
Y-Z-plane of the tool, tilt errors ξ (ksi) in the X-Y-plane of
the tool, and
tilt error ψ (psi) in the X-Z-plane of the tool. These tilt
errors lead to a
misalignment of the tool which has to be avoided for the manufacturing
of
defined microstructures. Therefore scratch traces and reversal tests as
well as
the machining of witness samples were used for the alignment of the
diamond
tools.
(Technology
suitable for both serial and small quantity production)
Microturning
is a method for
producing micropins, but it is more difficult to realize other
applications.
The reason for this is the deformation of the workpiece. This is
similar to the
case of the microendmilling, however the workpiece in micro turning is
often
more elastic than the tool in microendmilling. Diameters
larger than 100 µm is
the practically applicable range in case of microturning. It is
possible to
fabricate another kind of microparts using conventional ultraprecision
turning.
After cutting microsteps on the surface of a plate, microparts can be
cut out
by other methods such as WEDM or grinding. As a result, microparts with
microsteps can be fabricated [1,2]
Diamond
turning is a cutting
process capable to obtain an absolute accuracy better than 1
µm and 0.002~0.005
µm Ra in some metals. Its application is the production of
mirror surfaces in
optical quality components, moulds or reference parts. Tool geometry
must be
accurate, being the control of the edge radius and the tool tip radius
the key
parameters to obtain mirror finishing. The control must be performed
with an
accuracy of 3~75 nm, natural and synthetic diamond tools are usually
applied.
Diamond at high temperature reacts with those metals that present
affinity for
the carbon of its structure forming carbides that contaminate the tool
that
looses its properties and wears. Favourable metals are: aluminium
alloys,
brass, bronze, copper, gold, silver, zinc, beryllium, plumb, tin,
indium,
plutonium, magnesium (not able for steel, nickel, titanium, molybdenum,
cobalt,
chrome, vanadium, rhodium and tungsten).
For
micromachining the
process is suitable to produce small diameter shafts
(Ø0.2~Ø0.02 mm) and small slots
(using small tailor-made tools). Part cutting becomes an important
issue.
Mirror finishing is right now its main market, some applications are:
laser
driving optics, wavelength filtering surfaces, moulds for components of
optical
quality, etc.
In [A_1],
referring to Masuzawa [A_2], in micro metal cutting, the first
requirement for micro machining, small unit removal, is satisfied when
a high
stress that causes shearing of material is applied to a very small area
or
volume of the workpiece. This means that a highly concentrated force
must be
applied to an appropriate position of the workpiece. Therefore,
assuming that
the desired unit removal is around 100 nm, a tool that has its edge
sharpened
to a radius smaller than 1μm is necessary. The tool material for
these
processes must be stronger than the workpiece material, and for the
case of
very small unit removal diamond and hard ceramics are suitable as tool
or
abrasive material for all processes of this type when the metals are
machined.
This is gives reason for the application of monocrystalline diamond
tools in
diamond cutting processes.
[A_1] Metal
cutting in microstructures, E.
Brinksmeier, O. Riemer, Multi-material Micro Manufacture, W. Menz
& S.
Dimov 2005, pp. 1-7
[A_2] Masuzawa, T., 2000, State of the art of micromachining, Annals of the CIRP, 49/2:473-488.
Technology
suitable for both serial and small quantity production
back to overview
Technology
suitable
for both serial and small quantity production
Microdrilling
is
characterized not just by small drills but also a method for precise
rotation
of the microdrill and a special drilling cycle. In addition, the walls
of a
microdrilled hole are among the smoothest surfaces produced by
conventional
processes. This is largely due to the special drilling cycle called a
peck
cycle. The smallest microdrills are of the spade type. The drills do
not have
helical flutes as do conventional drills and this makes chip removal
from the
hole more difficult. Drills with a diameter of 50 micrometers and
larger can be
made as twist drills. Drills smaller than this are exclusively of the
spade
type because of the difficulty in fabricating a twist drill of this
size.
There
are several important
geometric characteristics of spade-type microdrills. First, the point
of the
drill is not a point at all. Even on conventional twist drills, the end
is not
truly pointed. Instead, the end of the microdrill consists of a cutting
edge
(called the chisel edge) made by two intersecting planes which also
define the
two primary cutting edges of the drill. The chisel edge removes
material
primarily by extrusion and cutting at high negative rake angle. The
specific
cutting energy along the chisel edge is relatively large compared to
the
drill's primary cutting edges. The chisel edge also adds to the
drilling
complexity because of the lack of a point. As the rotating drill first
contacts
the work piece (remember the drill has a very small structural
rigidity)
anything on the surface, including microroughness and material slope,
will
cause the drill to walk on the surface as it is trying to begin
removing
material. Walking is characterized by an eccentric motion of the drill
as it
turns perhaps coupled with a non-time- varying bending of the drill
about its
longitudinal axis. Depending on the feed per revolution of the drill
during
hole start-up, the drill may begin drilling at a slant with the drill
deflected
like an end-loaded cantilever beam (which it is with superimposed
column
loading). If permitted to continue, the drill will quickly break. If
the drill
is strong enough to survive the large stress imposed in it due to
drilling at a
slant, the resulting hole will be slanted rather than normal to the
work
surface.
A
second consequence of the
chisel edge is its relatively long length compared to the drill
diameter. This
results in a relatively high thrust force along the drill axis. While
the
sloped cutting edges are increasing the diameter of the hole machined
by the
chisel edge, the specific cutting energy along the cutting edge is
normally
lower than at the chisel edge. The result is a large thrust force
compared to
the diameter of the drill. Again, this size effect works against the
microdrilling
process similar to the size effect in micromilling.
Microdrills
are typically
made of either cobalt steel or micrograin tungsten carbide. The steel
drills
are less expensive and easier to grind but are not as hard or strong as
the
tungsten carbide drills. The drill point angle is based on the material
to be
drilled. The normal point angle is 118 degrees and 135 degrees is used
for hard
materials. The larger included point angle provides more strength at
the drill
point.
A
microdrilling spindle uses
a vee-block bearing arrangement. The drill is mounted in the mandrel
and is
fabricated integral with the mandrel. The mandrel rides against four
convex
diamond surfaces which are the only points of contact. So long as the
drill was
ground with the mandrel supported in a similar manner, the drill will
be
concentric about an axis. That axis may not coincide with the mandrel
axis but
that is not significant as long as the offset is not sufficient to
cause
excessive vibration, and it is not normally. A small pulley is fastened
to the
drill mandrel and a drive belt passes around the pulley and drives the
drill
from an external motor. The belt tension is the only force holding the
mandrel
against the diamond pads and a slight upward component of the belt
tension is
used to retract the drill. The upper end of the mandrel rides against a
ceramic
material which provides the drill thrust force. This disk may also rest
against
a force sensor to measure drill thrust force which is often used to
indicate
the extent of drill wear.
Microdrills
must be used in a
peck cycle wherein the drill is repeatedly withdrawn and reinserted
into the
hole being drilled. This is necessary to help clear chips from within
the hole.
A thin cutting fluid is also recommended to aid in chip clearing. The
fluid
should be moving, as in an air-oil mist rather than stagnant. Stagnant
fluid
will allow chips to reenter the hole along with the drill. The effect
of not
using a fluid is clearly shown. The hole has more, large chips, on the
order of
5 micrometers in size, and the drilling thrust force under such a
condition is
typically higher than if the chips are cleared from the hole. With no
fluid to
help clear chips, the hole is packed with chip debris and the axial
force on
the drill is typically several times (2-3) higher than with fluid. In
very soft
materials, complete removal of the drill from the hole each peck cycle
can
cause a slight taper near the hole entrance. This can be avoided by
incomplete
removal of the drill. For softer materials, chip removal is not a
severe
problem since the machining forces for such materials are normally
lower than
for hard materials but chips left in the hole can cause the drill to
wander
from an axial path and can result in a drilled hole with a center which
does
not lie along a line.
The
recommended speeds and
feeds for microdrilling are as varied as the materials which can be
drilled.
Microdrilling is not generally a high speed process since dwelling of
the drill
at the bottom of the hole can cause hardening of the work piece leading
to
increased drilling forces. For most metals, typical spindle speeds are
in the
range of 2000 to 4000 rpm and feeds are in the range of a approximately
micrometer per revolution. Care must be taken when drilling plastics to
avoid melting
of the material which can lead to adhesion of the plastic to the drill.
This
can cause drill breakage or poor sidewall smoothness.
The applicability of
microdrilling
as a complementary process with features produced by lithography and
electroplating has been investigated. The average roughness of the hub
wall of
a copper microgear is 0.4 micrometers. A microdrilled hole in the same
material
gave a roughness of 0.15 micrometers over a much longer bore length.
Microdrilling can also be used to augment lithography for mesoscopic
(millimeter and larger) sized components. Often parallelism of deep
holes is of
concern. To determine typical values for parallelism of microdrilled
holes,
glass fibers were inserted into a number of holes drilled with a very
slow
starting sequence. This is necessary to ensure the drill does not walk
on the
surface of the part and that the hole axis aligns with the undeflected
axis of
rotation of the drill. Holes with a length-to-diameter ratio of 8 were
drilled
at 4000 rpm. The threedimensional misalignment of the inserted
fibers was
measured to be 0.08 degrees (1.5 milliradians), which included skewing
of the
fiber in the hole due to oversize of the hole which was estimated to be
1
micrometers. Microdrilling presents the advantage that the electrical
properties of the workpiece do not influence the process. Therefore,
most metal
and plastics, including their composites, can be machined easily. One
typical
example is the drilling of holes in laminated circuit boards. Another
advantage
is that machining time can be controlled easily because the process is
stable
when an appropriate feed rate per rotation is set. Microdrilling has
one major
disadvantage because of the drill geometry. Because of the drill point,
a
flat-bottomed hole can not be produced. If one is attempting to produce
cylindrical cavities in a micromold, there must be a relatively thick
plating
base under the mold material, or the structural substrate of the mold
could act
as the plating base. To fully develop the diameter of the hole,
projected onto
a plane perpendicular to the drilling direction, requires the drill
point to
extend 30% of the drill diameter beyond the depth of the fully
developed hole.
For holes in the 100 micrometer region, requires a thick plating base
to be
deposited. One method for creating flat-bottomed blind holes is to use
an end
milling tool instead of a drill. The disadvantage to this procedure is
that
drills have a typical L/D ratio of 4 to 14 while end mills typically
have a
ratio of only 2 to 3.
An
other disadvantages is
that machined holes are often inclined because the already- machined
part of
the hole influences the orientation of the drill. In order to avoid
inclination, correct positioning is necessary when the drill tip begins
to cut
the workpiece. If the tip position shifts by even a small distance from
the
target center of the hole, the drill bends to follow a certain angle
guided by
the hole which the drill itself produces.
Very
hard or brittle
materials are difficult to machine. A diamond drill with small depth of
cut may
be a solution to this problem; however, it is difficult to suppress
completely
the brittle chipping of the workpiece and the breakage of microdrills.
C1.1.4.1
Diamond Drilling
For
special applications
diamond turning and milling processes applying monocrystalline diamond
tools
may not be useful due to their geometric and kinematic limitations.
Therefore,
diamond contour boring has been developed for the manufactured of micro
structured
optical molds [A_1]
[A_1] Metal cutting in microstructures, E. Brinksmeier, O. Riemer, Multi-material Micro Manufacture, W. Menz & S. Dimov 2005, pp. 1-7
Technology
suitable for both serial and small quantity production
Microgrinding
is a material
removal technique by means of mechanical force used for machining pins
and
grooves with small dimensions and for obtaining flat surfaces with very
fine
finishing. The main reason for this is that the chip size in
microgrinding is
very small because cutting is realized by means of micrograins.
Due
to the very small
obtainable depth of cut, microgrinding is particularly advantageous for
brittle
materials which can be mirror finished. The tool is generally in the
form of a
wheel, constituted of an abrasive and a matrix. In order to accomplish
smooth
surfaces of less than 10 nm peak to valley depth, the grain depth of
cut has to
be kept to less then 100 nm [4]. Two possible approaches to achieve a
very
small grain depth of cut are: maintaining a small wheel depth of cut
using
coarse abrasives or using grinding wheels consisting of very fine
abrasives.
The first method requires high precision machine tools and an accurate
dressing
of the grinding wheel. Ultra-precision 4-axes grinding machines with a
maximum
resolution of 1 nm for 3-axis linear feeding and a prototype
ultrastiff
machine tool, Tetraform C, which produced a repeatable surface finish of less than 10 nm using
76 µm CBN grit on
hardened bearing steel have been developed for this first method [5].
Special
techniques for wheel dressing,
as
electrolytic in-process dressing (ELID), have been developed for
controlling
and maintaining the desired protrusion of the grains on the surface of
the
wheel., enabling mirror finishing of silicon wafers and other materials
as
ceramics, ferrite, glass, steel, etc. [6].
Concerning
the second method,
abrasive pellets composed of ultrafine silica particles of 10 to 20 nm
have
been obtained applying electrophoretic deposition [4]. A particular
technique
is referred to as Nanogrinding, where diamond abrasive grains are
embedded in a
soft tin plate, resulting in a very minimal grain protrusion. The
average
surface roughness obtained with this technique was Ra = 1.14 nm for
Al2O3-TiC
and Ra = 0.79 nm for SiC [7].
The
recent development in the
fabrication technology of grinding tools has led to the application of
grinding
in the fabrication of 2D or 3D microcavities in a system similar to
mechanical
or EDM/milling. In this case a tool with a microsized tip is used.
Because of
the considerable grinding force, the aspect ratio of the tool has to be
low.
Therefore, deep microholes or deep, narrow cavities are not promising
targets
of microgrinding. The machinable shapes are almost the same as those in
milling
by mechanical cutting.
Among the limitations of microgrinding is the minimum obtainable tip radius of the tool which is strongly influenced by the grit size [5]. It determines the rounding radius when machining concave shapes as V grooves. Ultrasonic vibrations have been applied to grinding in order to reduce the grinding force. This has led to the production of pins in cemented carbides of 11 µm in diameter and 160 µm in length as well as micro flat drills of 17 µm in diameter and 100 µm in length. Microgrinding is also applied to surface finish thermally sprayed hard coatings [8]. However, considerably high values of compressive biaxial subsurface stresses are generated, with a large gradient in the thickness direction. One of the technological problems is the fact that the tool must be made up of an abrasive and a matrix. When the tool size is very small, the grain size cannot be ignored; this leads to certain difficulties in forming the precise shape of the grinding tool.
C1.1.5.1
Diamond Grinding
Micro
diamond grinding has some constraints in wheel size and shape that
limits achievable quality and geometry of micro parts and structured
surfaces.
Moreover, difficulty in grinding tool-making and preparation including
truing
and dressing also hampers the widespread application of diamond
grinding into
micro fabrication significantly. In [A_1], recently some innovative
grinding
tool concepts were proposed by Hoffmeister et al. [A_2] in which
grinding
wheels become endurable and slightly easier to make, and enhanced
process
stability was demonstrated with the assistance of ultrasonic vibration.
[A_1] Metal
cutting in microstructures, E.
Brinksmeier, O. Riemer, Multi-material Micro Manufacture, W. Menz
& S.
Dimov 2005, pp. 1-7
back to overview
Technology
suitable for small quantity production
Micro
ultrasonic
machining (MUSM) is a method derived from conventional ultrasonic
machining, in
which a tool and free abrasives are used. The tool that is vibrated at
ultrasonic frequency drives the abrasive to create a brittle breakage
on the
workpiece surface. The shape and the dimensions of the workpiece depend
on
those of the tool. Since the material removal is based on brittle
breakage,
this method is suitable for machining brittle materials such as a
glass,
ceramics, silicon and graphite [9]. The chip size required for
micromachining
can be realized when submicron particles are available for the use as
an
abrasive. Microtools can be supplied in the same way as that in
micro-EDM,
because the same types of tools are used to specify the corresponding
shapes of
products, although the microscopic removal phenomenon is completely
different.
The major problems are the accuracy of the setup and the dynamics of
the
equipment. Ultrasonic vibration of the machining head makes accurate
tool
holding difficult. The implementation of a transducer and rotation
mechanism is
too difficult to maintain high equipment precision. However, recent
development
has overcome most of these difficulties. In the earliest works, the
vibrations
were applied to the tool, resulting in problems in tool holding and in
machining accuracy. In order to overcome tool holding problems, the
on-the machine
tool preparation was introduced. In this approach the tool was soldered
to the
machine head prior to its preparation then machined by wire electro
discharge
grinding (WEDG) to the desired dimensions and the subsequent machining
of
workpiece took place on the same machine tool. However this method
prevents
measurement of the size and the shape of the fabricated tool. By means
of the
on-the-machine fabrication of the tool, holes in silicon with 20
µm in diameter
have been produced [10].
The
main problem in tool
holding accuracy arises from the soldering process that has been
applied to
solve the problem of loosening caused by ultrasonic vibration. If the
tool
material is soldered before it is fabricated into a microtool, the
completed
tool setting is free from the low accuracy of soldering.
A
further development is the
introduction of vibrations applied to the workpiece instead of to the
tool
[10]. This enables a better tool holding and the use of a
high-precision
spindle mechanism. A high- precision rotation/feed mechanism is
essential for
most machine tools. The introduction of a vibration mechanism such as
that in
USM is, therefore, an idea contrary to the concept of the
micromachining. One
of the solutions to this problem is to attach the vibration mechanism
to the
workpiece side [10]. This will cause little problem in terms of
accuracy
because a transducer can simply be inserted between the worktable and
the workpiece.
Such a setup enables the use of a universal, highprecision
machine head and
its influence on workpiece holding accuracy is negligible.
With
MUSM, micro holes of 5
µm in diameter were machined in quartz glass and silicon,
using a tool with
diameter of 4 µm and an abrasive with average grain size of
0.2 µm. Furthermore
a microhole with diameter 9 µm and an aspect ratio of 4 was
realized in quartz
glass. A major problem in MUSM is the high tool wear ratio. Tungsten
carbide is
used for tool fabrication because tools of approximately Ø 5
µm can be machined
and it is tough enough to withstand machining load. However a wear
ratio always
higher than 0.5 leads to limitations of the machining efficiency with
the
impossibility of machining deep holes or multiple holes with the same
tool. The
introduction of sintered diamond tools has enabled to overcome the
problem,
giving a wear ratio of 0.01 when machining soda glass. However, tools
in
sintered diamond are limited to a minimum diameter of 15 µm
since they are
fragile and tend to break during machining.
Rotary
ultrasonic machining
(RUM) is a hybrid machining process that combines the material removal
mechanisms of diamond grinding with ultrasonic machining (USM),
resulting in
higher removal rates than those obtained by either diamond machining or
USM
alone. As main tool is made of diamond, steel is not machinable, and
its main
advantage is obtained machining fragile materials (ceramics, glass), so
its
application to metals is quite marginal. Expected accuracy is in the
order of
few microns and roughness of 0.2 microns Ra.
In
pure ultrasonic machining
(USM), the tool, shaped conversely to the desired hole or cavity,
oscillates at
high frequency, typically 20 kHz, and is fed into the workpiece by a
constant
force. Abrasive slurry composed of water and small abrasive particles
is
supplied between the tool tip and the workpiece. Material removal
occurs when
the abrasive particles impact the workpiece due to the downstroke of
the
vibrating tool.
In
rotary ultrasonic
machining (RUM), a rotating core drill with metal bonded diamond
abrasives is
ultrasonically vibrated in the axial direction while the spindle is fed
toward
the workpiece at a constant pressure. Coolant pumped through the core
of the
drill washes away the swarf, prevents jamming of the drill and keeps it
cool.
By using abrasives bonded directly on the tools and combining
simultaneous
rotation and vibration, RUM provides a fast, high-quality machining
method for
a variety of materials.
In
[A_4], micro USM is widely described. Material removal in micro USM
is by the mechanical action of abrasives as well as by the cavitation
erosion
due to rapid pressure changes caused by the ultrasonic vibration of
fluid in
working zone [A_5] [A_6]. This non-thermal, nonchemical and
non-electrical
process is especially suitable for the micro machining of hard brittle
and
inert insulators such as glass, ceramics, composites, quartz, precious
stones
and for the machining of fragile and porous materials such as
graphite. Irregular shaped hard abrasive particles are dispersed in a
liquid
medium (called abrasive slurry) and fed into the gap between tool and
workpiece. The tool is vibrating with an ultrasonic frequency (usually
20~40
kHz) with an amplitude of several to tens micrometers. When static load
is
applied between tool and workpiece, abrasive particles impact and chip
away
material from both workpiece and to a lesser extent from the tool [A_7].
Condition
of the abrasive and its grain size affect the machining rate [A_8].
A continuous flow of abrasive slurry flushes away the debris from the
working
zone. Since actual machining is carried out by abrasive particles, the
tool can
be softer than the workpiece.
A
vibration of the workpiece improves the machining accuracy [A_9]. It
not only simplifies the structural design of the tool system, but also
stirs
the abrasive slurry during the machining to improve renewing particles
in the
working zone and facilitates debris removal [A_10]. Precise measurement
of the
vibration amplitude at micron level is a challenging task. An online
measurement method proposed in [A_10] drives the tool tip to touch the
workpiece surface and captures two vertical positions of the surface
with
respect to turning on and turning off of the vibration. The difference
between
two positions is treated as the vibration amplitude. The accuracy of
this
measurement method is highly affected by the precision and responding
time of
driving components and force sensor. A force sensor with short
responding time
and high resolution is required for monitoring and controlling the
static load
to avoid tool breakage during machining. In some studies, the static
loads are
recorded under constant tool feed rate [A_11]. The static load under
constant
tool feed rate usually shows cyclic fluctuate pattern. A closed loop
control
can be
employed
for the better evaluation of the effect of static load as a
parameter on machining characteristics. In this case, the tool feed
rate is
adjusted to obtain a stable static load [A_10].
The
mechanism and modeling in macro USM are not yet fully understood.
Several models have been proposed to predict the material removal rate,
and
most of them are with rough accuracy in prediction. Material removal
mechanism
in micro USM is believed to be similar to conventional USM. However
theoretical
work in micro USM has rarely been reported and there is a lack of
knowledge
about the process behaviours of micro USM under various conditions.
Theoretical
models of macro USM may not
exactly
be applicable to micro USM due to effects such as difficulty of
refreshing of abrasive particle and debris removal caused by the
downscaling of
tool and abrasive particle [A_10]. Existing knowledge is far from
sufficient to
provide a complete understanding and instructive rules for industrial
users [A_12].
A tentative mechanistic modeling of material removal in micro USM was
proposed
in [A_13]. The basic assumptions in this modeling are similar to those
in
Rotary Ultrasonic Machining (RUSM).
All
materials tending to a brittle fracture behavior can be machined by
micro USM. Examples include highperformance ceramics, glass, graphite
and a
part of the fiber-reinforced plastics [47] [A_14]. Geometrical
capabilities of
micro USM have been testified by drilling, slot machining and 3D
machining.
Micro holes with a diameter less than 10 μm were successfully
drilled on
silicon, quartz glass and alumina [A_9] [A_15] [A_16]. Slotting on low
melting
glass has been reported in [A_11]. The tool wear compensation strategy
“Uniform
Wear Method” originally developed for micro EDM has been
applied in micro USM
to successfully generate 3D micro cavities as shown in Figure 17 [A_17].
Micro
USM has not yet been commercialized with a functional machine tool
similar to micro EDM. However, it is believed that this process could
provide
solutions to easily and quickly achieve the larger MEMS structures as
well as
packaging for both prototype and production in silicon, glass and
ceramic [A_7].
Proper
selection of micro USM process parameters at present is not well
understood due to lack of related experimental results [A_17]. Abrasive
particle size, vibration amplitude, static load and tool rotation are
the main
parameters influencing the micro USM machining speed for the given
workpiece
material [A_10] [A_13] [A_16]. It was found that a slight tool rotation
drastically improves the drilling speed. However, there was no
significant
improvement for speed higher than 50 rpm [159]. The debris accumulation
affects
the machining speed in micro USM due to the poor fluidic circulation
around the
machining zone [A_13]. The dependence of dimensional accuracy on tool
diameter,
vibration amplitude, and abrasive size needs further research [A_12].
By means
of calculating the maximum impact force, combined with microcrack
models
obtained from the research on indentation, it is possible to correlate
the
depth of microcracks with process parameters. A predictive model for
the
microcrack depth can be employed to optimally select the process
parameters [A_12].
High
tool wear is an intrinsic drawback of micro USM. It is difficult to
get a constant depth of cut due to longitudinal tool wear. Tool wear is
affected by parameters such as vibration amplitude, static load and
tends to
increase when harder and coarser abrasives are used. As a consequence,
harder
abrasives, like diamond, cause higher tool wear than softer abrasives
such as
silicon carbide [A_16]. Therefore, it is necessary to account for and
to
compensate the tool wear during machining. The feasibility of applying
the “Uniform
Wear Method” for generating accurate 3D microcavities by
micro USM has been
tested and found that the tool shape remains unchanged and the tool
wear has
been compensated [A_17]. Due to inadequate research done on tool wear
in micro
USM, the selection of tool materials is not well supported by the
experimental
data and related analysis. Also, the tool wear mechanisms, the wear rate
dependence
upon tool hardness, toughness, abrasive type and size,
abrasive hardness and material toughness need to be studied to reduce
and
control the tool wear in micro USM [A_12].
The
micro tools used in micro USM can be prepared by a WEDG unit [A_9].
A micro tool with multi tips was made using batch mode micro EDM. Tool
material
with enough abrasive wear resistance and small deflection under
mechanical load
is preferable in micro USM. A PCD tool is helpful in reducing tool wear
[A_9].
Some
of the important micro USM issues requiring systematical research
include the study of material removal mechanism, innovative tooling,
tool wear
mechanism and reduction, on-line sensing, subsurface
damage
control, and surface roughness improvement. In addition,
in-process monitoring and model-based selftuning strategies are needed
for
improving the process stability and performance.
[A_4] Micro
and Nano Machining by Electro-Psysical and Chemical Processes, K.P.
Rajurkar,
G. Levy , A. Malshe, M.M. Sundaram, J. McGeough, X. Hu, R. Resnick, A.
De
Silva, Annals of the CIRP Vol. 55/2/2006, 643-666.
[A_10] Hu,
X., Yu, Z., Rajurkar , K. P., 2005,
Experimental Study of Micro Ultrasonic Vibration Machining. Fourteenth
International Symposium on Processing and Fabrication of Advanced
Materials,
Pittsburgh, Pennsylvania, USA, 197-210.
[47]
IWF, IPT, 2002, Investigation of the
International State of the Art of Micro Production Technology MickroPRO.
[A_5] Kremer,
D., Saleh, S. M., Ghabrial, S. R.,
Moisan, A., 1981, State of the Art of Ultrasonic Machining., Annals of
the
CIRP, 30 /1: 107-110.
[A_6] McGeough,
J. A., 2002, Micromachining of
Engineering Materials, Marcel Dekker Inc., New York.
[A_12] Zhang,
C., Rentsch, R., Brinksmeier, E.,
2005, Advances in Micro Ultrasonic Assisted Lapping of Microstructures
in
Hard-Brittle Materials: A Brief Review and Outlook, International
Journal of
Machine Tools and Manufacture, 45 /7-8: 881-890.
[A_7] Medis,
P. S., Henderson, H. T., 2005,
Micromachining Using Ultrasonic Impact Grinding, Journal of
Micromechanics and
Microengineering, 15/8: 1556-1559.
[A_8] Adithan,
M., Venkatesh, V. C., 1978,
Appraisal of Wear Mechanisms in Ultrasonic Drilling, Annals of the
CIRP, 27 /1:
119-121.
[A_9] Egashira,
K., Masuzawa, T., 1999, Microultrasonic Machining by the Application of
Workpiece Vibration, Annals of the CIRP, 48 /1: 131-134.
[A_11] Moronuki,
M., Saito, Y., Kaneko, A., Miura,
A., Aikawa, C., 2004, Vibration Micromachining of
Low-Melting-Temperature
Glass. 7th International Symposium on Advances in Abrasive Technology,
Bursa,
Turkey, 489-494.
[A_13] Yu,
Z., Hu, X., Rajurkar , K. P., 2005,
Study of Micro Ultrasonic Machining of Silicon. Proceedings of 2005
ASME
International Mechanical Engineering Congress and Exposition, Orlando,
Florida
USA, 1-8.
[A_14] Ghahramani,
B., Wang, Z. Y., 2001,
Precision Ultrasonic Machining Process: A Case Study of Stress Analysis
of Ceramic
(Al2O3), International Journal of Machine Tools and Manufacture, 41
/8:1189-1208.
[A_15] Egashira,
K., Masuzawa, T., Fujino, M., Sun,
X. Q.,1997, Application of USM to Micromachining by On-the-Machine Tool
Fabrication, International Journal of Electrical Machining, 2: 31-36.
[A_16] Choi,
H.-Z., Lee, S.-W., Lee, B.-G., 2003, Micro-Hole Machining Using
Ultrasonic
Vibration, Key Engineering Materials, 238-239: 29-34.
back to overview
Technology
suitable for
small quantity production
ECF
is a new technique to
machine hard materials and other electrochemically active materials.
The tool
movements are similar to conventional milling machines. Nevertheless
ECF is an
electrochemical process, hence both workpiece and tool are submerged in
an
electrolyte and held at a constant potential to prevent corrosion.
Short
voltage pulses in the range of 10ns to 100ns are applied between tool
and
workpiece. With these pulses the electrochemical doublelayer
capacitance on the
surface of the workpiece is charged over the resistance of the
electrolyte.
Because the resistance increases with respect to the distance between
tool and
workpiece the time constant of the charging also changes. Since
charging takes
place only during the time of the pulse, the double-layer is
sufficiently
charged to generate an anodic dissolution of the workpiece only in a
defined
working distance around the tool. Surface areas further away from the
tool are
not affected by the ECF process. This leads to a confined milling with
a high
spatial resolution. Furthermore the working distance is proportional to
the
applied pulse width. Thus the working distance and hence the spatial
resolution
of the process can be varied between 10µm to 5µm
and below just by adjusting
the pulse width.
ECF is a pure electrochemical process, i.e. ion transport from the workpiece into the electrolyte takes place, and due to this, no remelting of the surface of the workpiece or other conversion occurs. Furthermore virtually no forces act on the tool. Thus tools with diameter down to 5µm and below and with arbitrary shapes - like grooving cutters, rounded cutters or ball cutters - can be used. The milling strategies are similar to that of conventional milling, although the tool in the ECF process does not rotate. This allows production of 3D structures and freeform surfaces. Cutting speed is achieved of about 1µm/s in stainless steel 1.4301.
|
ECF Data Sheet
|
|
|
Mould
Materials |
Stainless
Steel |
|
Cutting
tool |
5
– 50 µm home made from tungsten |
|
Machine
|
3
axis, own development |
|
Removal
rate |
about
1000 µm3/s |
|
Machining
of channels & ribs |
|
|
Minimum
width |
10
µm (Literature: <200 nm) |
|
Aspect
ratio |
about
10 with a 20 µm tool |
|
Accuracy
|
2
µm, no burr |
|
Minimum
distance between channels |
<2
µm |
|
Minimum
distance between ribs |
10
µm |
|
Machining
of holes |
|
|
Minimum
diameter |
10
µm |
|
Aspect
ratio |
10
|
|
Accuracy
|
2
µm, no burr |
|
Minimum
distance between holes |
<2
µm |
|
Is
possible 3D freeform surface? |
Yes
|
Technology
suitable for
both serial and small quantity production
Rolling
is a fabrication
process in which the metal is passed through a pair of rolls. There are
two
types of rolling process, flat and profile rolling. In flat rolling the
final
shapes of the product are foil (thickness less than 1 mm), sheet
(thickness
less than 3 mm) or plate (thickness more than 3 mm). In profile
rolling, the
final product may be a round rod or other shaped bar. This process is
gaining
an increasing interest if the foil or sheet metal used for subsequent
production processes like bending, punching, etc. is having a defined
shape of
the surface (tailored tape) in order to enable more complex products to
be
manufactured.
Rolling
is also classified
according to the temperature of the metal rolled. If the temperature of
the
metal is above its recrystallisation temperature then the process is
termed as
hot rolling, If the temperature of metal is below its recrystallisation
temperature the process is termed as cold rolling. If metallic foils
have to be
produced, the first process is not suitable due to the large surface to
volume
ratio yielding high oxidation.
Another approach is the cold forming of sections by rolling technology where continuously metal sheet or foil is formed by several rolls. As a result of the used bending process, at microscale problems like springback depending on the foil thickness have to be considered.
C1.3.2 ForgingTechnology suitable for both serial and small quantity production
As it is defined at conventional length scale, forging processes are usually applied at a specimen temperature of above the recrystallisation temperature. A reduction of the specimen size down to micro scale is always subjected to a significant change in the ratio between surface and volume yielding high oxidation and thus material loss and surface damage. To avoid the oxidation problem, a reduction of the forming temperature down to warm forging is necessary. Warm forging covers the temperature area above room temperature (cold forging) but below recrystallisation temperature (hot forging) and thereby combines the advantages of both forging processes. The benefits are in detail for cold forging an excellent surface quality, necessary for net shape manufacturing, and strain hardening for optimized mechanical properties of the specimen, while the benefits of hot forging are the reduction of the flow stress yielding lower process forces and an enlarged formability because of the thermally activated additional sliding systems. Also the elevated temperature improves the drawbacks of microforming, caused by so called size effects, by homogenizing the material behavior and reducing the occurring scatter of process characteristics. Summarizing the above mentioned advantages of warm forging the oxidation problem can be avoided and the forging limits especially of micro parts can be enlarged while hardening still takes place.
C1.3.3 Deep drawing
Technology
suitable for both serial and small quantity production
Deep drawing is a process frequently used at conventional length scale. In order to produce e.g. cups with a diameter of a few millimeters or below, some specifics have to be regarded. As it has been shown, there is a relation between the deep drawing clearance and the punch force. If the ratio between clearance and sheet thickness is increased from 1.05 to 1.75, the increase of the force is about 10%. Furthermore, the shape building is also affected by the clearance, yielding a rough cup wall surface in case of a small clearance. Besides the influence of the geometrical design of the tools, the specimen size also affects the forming limits. When scaling down the cup diameter down to 1 mm, experimental investigations have shown that the maximum drawing ratio is reduced. As it is similar to the conventional scale, friction has an influence on the deep drawing process. To quantify the influence, experiments using solid lubricant, extrusion oil and without lubricant have been performed. As the solid lubricant has only minor influence on the deep drawing results, liquid lubricant is leading to lower deep drawing forces and an increase in the maximum drawing ratio.
Technology
suitable for
both serial and small quantity production
In industrial application, micro bending processes are frequently used in case of spring, clamp or lead frame production. Considering the characteristics of this process, namely part dimension being close to sheet thickness, conventional FE-simulation programs, which assume plain strain condition in the deformed area, are not applicable. In order to overcome this, a first model has been developed that enables the calculation under plane stress conditions [11] or in the improved version considering the anisotropy of the material [12].Basic investigations on the relationship between miniaturization and bending process have firstly been done in [13]. The analyses of the experimental results have shown that the process forces relative to the size decrease with miniaturization in case of small grains. In case of large grains (only few grains over the sheet thickness), the force is increasing again. This effect has been confirmed by investigations in [14] performing scaled bending experiments on metallic foils. It has been shown, that depending from the thickness of the sheet (scaled from 200 microns down to 25 microns) and from the material structure, bending moment and thus the spring back angle increases when scaling down. This confirms the previously described theory of strain gradient plasticity [15, 16, 17, 18]. When scaling down the foil thickness, two contrary effects appear: the effect of a reduction of the flow stress due to an increasing fraction of surface grains on the overall volume and the effect of an increasing flow stress caused by the increasing density of geometrically necessary dislocations. When the foil thickness is getting smaller, the latter effect gains superior influence resulting in both, higher normalized bending moment and higher spring back angle.
Technology
suitable for
both serial and small quantity production
Basic investigations on the effects of miniaturization on the blanking process have been first performed by Kals [13]. It has been shown that the normalized forming force is constant while scaling down sheet thickness due to a lack of free surface. When sheet thickness is below a certain value, the forming force and the ultimate shear strength is increasing. Other investigations have shown that there is a dependency between tool and process parameters and the accuracy of the produced leadframes. The deflection of the leads in the plane of the sheet increases with decreasing width of the lead. The deflection is also influenced by different clearances between tool and die in a progressive tool. They also showed that increasing the strip holder pressure has a positive effect on the accuracy in most cases. Also the dynamic behaviour of the tool is affecting the accuracy [19], e.g. increasing blanking speed is resulting in a decreasing accuracy. Performed experiments have additionally shown that increasing the strip holder force is clearly improving the product quality. Other investigations on the deflection of a punch during the blanking process [20] have shown an increase of deflection when the punch is eccentric relative to the die. A particular blanking process, so-called dam-bar cutting has been investigated by [21, 22]. This is a mechanical trimming process removing the dam-bar between the leads after the IC package is encapsulated. Due to the special shape of the specimen rectangular around the IC, investigations have to be performed considering the anisotropic behaviour of the material in the shearing line. Further investigations on different materials have shown that an increase of the angle α reduces the maximum cutting forces but is leading to an increasing burr height. An important aspect for industrial production is tool life. Therefore, investigations in [22] have been performed to show the dependency of the punch forces and thus the tool stresses from the clearance between punch and die and the used sheet material hardness. While tools made of tungsten carbide (WC) have the longest life time, tools made of bare HSS show the highest wear. An improvement can be reached by using coatings like PVD-TiN or plasma-nitriding. In this case, tools made of steel and WC have been compared according to their tool life in an industrial production process of blades for shavers. While the first only reaches a quantity of 50.000 parts, the latter exceeds 1.15 billion parts. Thus, it is obvious that the usage of the more expensive WC as tool material (factor 1.8 compared to steel) is more economical than the usage of steel. In general it can be said that blanking even at microscale using thin foils is an industrially well established technology.
Technology
suitable for
both serial and small quantity production
Studies
concerning coining
aluminium sheet metal have been performed by Ike and Plancak [23].
Using dies
with hole diameters from 0.05 to 1.6 mm, half hard commercially-pure
aluminium
of 2 mm in thickness and 30 mm in diameter, coining results were
evaluated in
terms of aspect ratio, the radial position of the hole and the die and
the
emerging forming load. As it has been shown, the beginning of plastic
flow is
independent from the radial position of the hole, but the height of the
pins is
clearly bound to the position of the holes and its diameter.
At
LWP Saarbrücken, Germany
[24], also coining of smallest cavities with a thickness of about 400
microns
has been investigated. The results show, that in case of micro-coining
the
influence of the tool geometry and the tool deflection on the forming
results
must be considered. This is due to rather high nominal stresses
(minimum three
times the flow stress of the coined material) leading to a large impact
on the
tool system: tool deformation, tool deflection and tool damage.
The
aim of the investigations
on coining technologies at IWU Chemnitz was to establish this
technology for
the production of geometrically defined micro structures for
applications in
the fields of micro-fluidics, micro-optics and information technology
[25, 26].
In a first set of experiments, tools were made out of single crystal
silicon
with an almost smooth surface. Thus, it was possible to create
structures of
100 microns with radii of some 100 nm in aluminium, copper, brass and
steel.
The analysis of the coined material shows that the shape of the rim strongly depends on the material. While aluminium shows a bulging of max. 30 % of the coining depth, steel shows only max. 5 %. Using fine grained ZnAl alloy in a superplastic state at 250 °C, high precision and surface quality are achieved at low compressive stresses of 25 MPa.