Important facts about spark erosion

Important facts about
spark erosion
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Important facts about spark erosion
Important facts about spark erosion
Table of contents
1. Technology of spark erosion
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2. The use of Dielectrics in spark erosion
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3. Functions of the Dielectric
17
4. Requirements for Dielectric
18
5. Criteria for assessing Dielectrics
18
6. The flushing process during spark erosion
22
7. Filtering the Dielectric 23
8. The effect of spark erosion on the work piece
26
9. „IonoPlus®“ – a new way to better dielectrics
30
10. Dielectrics IME 63, IME 82, IME 110, IME 126
32
11. Gases produced during spark erosion
35
12. Dielectrics and the human skin
35
13. „7 Golden Rules“ for working with IME Dielectrics
36
14. The effect of spark erosion on metal surfaces
37
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Important facts about spark erosion
Important facts about spark erosion
1. Technology of spark erosion
Introduction
Spark erosion is a modern machining technique
with decisive advantages as a result of which its
use is becoming more and more widespread.
Only one practical example is given here out of its
countless applications in the machining of metal.
lt is a moulding die for glassware. In the bottom is
the ejector opening. To the right it is the ejector.
Both were eroded in a single operation.
Difficult workpieces, machined quickly and accurately. But how does the process work? How can we
visualize the removal of material by spark erosion?
Unfortunately most of the processes are invisible.
We shall try to obtain a picture of them with the
aid of models and diagrams. (Fig. 1)
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Important facts about spark erosion
Principle
The principle of spark erosion is simple. The workpiece and tool are placed in the working position
in such a way that they do not touch each other.
They are separated by a gap which is filled with
an insulating fluid. The cutting process therefore
takes place in a tank. The workpiece and tool are
connected to a D.C. source via a cable. There is a
switch in one lead. When this is closed, an electrical potential is applied between the workpiece and
tool. At first no current flows because the dielectric
between the workpiece and tool is an insulator.
However, if the gap is reduced then a spark jumps
across it when it reaches a certain very small size.
In this process, which is also known as a discharge,
current is converted into heat. The surface of the
material is very strongly heated in the area of the
discharge channel. If the flow of current is interrupted the discharge channel collapses very quickly
. Consequently the molten metal on the surface of
the material evaporates explosively and takes liquid
material with it down to a certain depth. A small
crater is formed. lf one discharge is followed by
another, new craters are for med next to the previous ones and the workpiece surface is constantly
eroded. (Fig. 1)
Figure 1
Spark Gap
The voltage applied between the electrode and
workpiece and the discharge current have a time
sequence which is shown under the illustrations
of the individual phases. Starting from the left, the
voltage builds up an electric field throughout the
space between the electrodes. As a result of the
power of the field and the geometrical characteristics of the surfaces, conductive particles suspended in the fluid concentrate at the point where the
field is strongest. This results in a bridge being for-
med, as can be seen in the centre of the picture.
At the same time negatively charged particles are
emitted from the negatively charged electrode.
They collide with neutral particles in the space between the electrodes and are split. Thus positively
and negatively charged particles are formed. This
process spreads at an explosive rate and is known
as impact ionization. This development is encouraged by bridges of conductive particles. (Fig. 2)
Important facts about spark erosion
Figure 2
Here again we see what in fact is invisible. The
positively charged particles migrate to the negative
electrode, and the negative particles go to positive.
An electric current flows. This current increases to
Figure 3
a maximum, and the temperature and pressure
increase further. The bubble of vapour expands,
as can be seen in Figure 3.
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Important facts about spark erosion
Connection between the path of electric
power and heat
The model shows how the supply of heat is reduced by a drop in the current. The number of
electrically charged particles declines rapidly, and
the pressure collapses together with the discharge
channel. The overheated molten metal evaporates
explosively, taking molten material with it. The
vapour bubble then also collapses, and metal particles and breakdown products from the working
fluid remain as residue. These are mainly graphite
and gas. (Fig. 4)
Figure 4
By means of the model we will now try to demonstrate the relationship between the flow of current
and heat. In a detail enlargement below we see
the negative electrode surface, and above it a
part of the discharge channel. Positively charged
particles strike the surface of the metal. These are
shown in red. They impart strong vibrations to
particles of metal, which correspond to a rise in
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Figure 5
temperature. When a sufficient velocity is reached,
particles of metal, which are shown in grey and
yellow here, can be torn out. A combination of
positively charged particles, which are shown in
red, and negatively charged particles, which are
shown in blue, augments the vibration and thus
raises the temperature of the particles, which are
now uncharged. (Fig. 5)
Important facts about spark erosion
We know that electrical energy is converted
into heat when the discharge takes place. This
maintains the discharge channel, leads to the formation of discharge craters on the electrodes, and
raises the temperature of the dielectric. (Fig. 6)
Figure 6
Polarity
Now let us examine the question of polarity. The
exchange of negatively and positively charged particles, which are respectively shown in blue or red,
results in a flow of current in the discharge channel. The particles thus generate heat which causes
the metal to melt. With a very short pulse duration
more negative than positive particles are in motion.
The more particles of one kind move towards the
target electrode, the more heat is generated on it.
It is also important that as a result of their greater
size the positively charged particles generate more
heat with the same impact velocity. In order to
minimize the material removal or wear on the tool
electrode, the polarity is selected so that as much
heat as possible is liberated on the workpiece by
the time the discharge comes to an end.
With short pulses the tool electrode is therefore
connected to the negative pole. Its polarity is
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Figure 7
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Important facts about spark erosion
With short pulses the tool electrode is therefore
connected to the negative pole. Its polarity is thus
negative. With long pulses, however, it is connected to the positive pole so that its polarity is
positive. The pulse duration at which the polarity is
changed depends upon a number of factors which
are mainly connected with physical characteristics
of the tool and electrode materials. When steel
is cut with copper the marginal pulse duration is
about 5 microseconds. (Fig. 7)
Machining time
As in all machining processes, in spark erosion time
and accuracy are important factors. The erosion
time is determined by the volume of material to be
removed from the workpiece and the rate of removal, which is represented by Vw. This is measured
in cubic millimetres per minute or cubic inches per
Figure 8
hour. The wear on the tool electrode is another
factor influencing the machining accuracy. It is
represented by a small Greek theta ( ) and a v.
This figure is the volume of material lost from the
electrode by wear, expressed as a per centage of
the volume removed from the workpiece. (Fig. 8)
Important facts about spark erosion
Surface finish
In a similar way to conventional machining
methods, spark erosion does not produce a completely smooth surface but a slightly rough, indented one. This surface is typical of spark erosion,
and its quality must be known for the function or
fitting of individual workpieces. For the purpose
of measurement a reference system and surface
dimensions have been created so as to allow the
surface quality to be specified. Frequently used
measurements and characteristics are Rmax and
Ra. Rmax represents the greatest roughness height.
In Germany and France this value is also known as
Rt, and in USA it is known as Hmax. Rmax becomes an important characteristic if, for example, a
part has to be polished or lapped. The arithmetical
mean roughness is represented by CLA in Britain.
This value is always important when a part is being
machined in order to achieve a fit. In the USA it is
represented by AA, and in Switzerland by Ra.
(Fig. 9)
Figure 9
In exactly the same way as with cutting operations,
fine or coarse surfaces can be produced by erosion. The following two examples show how wide a
range of roughness the eroded surface can have.
(Fig. 10)
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Figure 10
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Important facts about spark erosion
Different spark gaps
The spark gap separates the workpiece from the
tool electrode. Even at a small cutting depth a
distinction must be made between the frontal and
the lateral gap. The frontal gap is determined by
the control system, while the lateral gap depends
upon the duration and height of the discharge
pulses, the combination of materials, the no-load
voltage and other predetermined values. (Fig. 11)
Figure 11
Power supply unit
The power supply unit is an important part of any
spark erosion system. It transforms the AC supply
from the mains and provides rectangular voltage
pulses. This can visualized by plotting a graph of
Figure 12
voltage against time. By a number of switching
devices the size of the rectangles and the distance
between them can be adapted to any operational
requirements. (Fig. 12)
Important facts about spark erosion
The sequence of the rectangle is a graphic representation of the opening and closing of the switch,
or in other words the pulse duration and pulse
interval, or of the discharge time and pause, and
also of the voltage and current at the spark gap. In
the AGIEPULS-L power supply units the discharge
current, pulse duration and pulse interval can be
set completely independently of each other. The
discharge current is proportional to the height
of the rectangle, and the width corresponds to
the pulse duration, which is measured in micro
seconds or millionths of a second. The distance
between the individual pulses can also be altered so as to set the length of the intervals during
which the flow of current is interrupted. The pulse
interval is expressed as a percentage of the pulse
duration. For example, if the interval lasts 25 micro
seconds and the pulse 100 micro seconds, Tau is
80 per cent. This means that the pulse lasts for 80
per cent of a switching cycle and the interval for
20 percent of the cycle. (Fig. 13)
Figure 13
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Important facts about spark erosion
Electrode wear
Erosion with a light current gives a low rate of
removal, while conversely a heavy current gives
a high rate of removal. But the wear on the tool
electrode expressed as a percentage of the volume
also increases if steel workpieces are eroded with
copper electrodes. Graphite electrodes behave differently. The wear declines up to a certain current
level and then remains more or less constant.
(Fig. 14)
Figure 14
Eroding with short pulses means increasing electrode wear. Conversely the wear is smaller when the
pulses are long. In practice, when roughing with
copper and graphite electrodes into steel a pulse
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Figure 15
length Iying between maximum removal and minimum wear is selected. (Fig.15)
Important facts about spark erosion
Off time
Not least, the interval between two discharges is
a factor of considerable importance. In general we
can say that rapid removal with little wear can be
achieved with small intervals, or in other words a
high duty factor. The limit must not be exceeded
because a point is then reached beyond which the
process is impaired resulting in reduced erosion
and greater wear. This critical value is also known
as the marginal duty factor. (Fig. 16)
Figure 16
Impulse current
This diagram shows that the surface roughness and
the size of the spark gap are decisively influenced
by the discharge energy, which is represented
by the area of a current pulse in the picture. The
energy contained in a pulse is proportional to the
orange-coloured area. It can clearly be seen that
the roughness is less marked with a small discharge energy than high discharge energy. For exam
ple, in pre-finishing and finishing a certain surface
quality must be attained. This corresponds to a
given discharge energy which must be found by
suitable adjustment of the discharge current or
pulse height and the discharge time or pulse width
A compromise between maximum erosion and
minimum wear is chosen from the range of possible settings. (Fig. 17)
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Figure 17
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Important facts about spark erosion
Surface quality in relation to current
A rougher surface is machined to a finer one by
eroding with reduced discharge energy. The roughness is reduced, while the electrode wear is some
what increased. The picture shows how big a difference there can be in practice between two subsequent machining stages. (Fig. 18)
Figure 18
In workshop practice, in roughing or pre-machining a degree of roughness should be attained
which needs only to be evened out in the next
machining stage. Experience has shown that the
Figure 19
roughness of the subsequent stage is about a third
to a fifth of the initial roughness. This procedure
gives a very economic overall eroding time in relation to the degree of accuracy attained. (Fig. 19)
Important facts about spark erosion
2. The use of Dielectrics in spark erosion
In 1943 the Russian research scientists, Mr. and
Mrs. B. R. and N. J. Lazarenko, discovered that the
erosive effect of capacitor discharges could be utilized in the processing of metals. At first they used
ordinary air as a dielectric. Very soon it became
clear, however, that liquid mineral oil derivatives
had considerable advantages. Disruptive strength
was greater. Smaller spark gaps could be used,
Density at 15° C
Viscosity at 20° C
Flashpoint ° C
Initial boiling point ° C
Final boiling point ° C
Evaporation no. (ether = 1)
Aromatic compounds % in vol.
making higher precision possible. Spark frequency
could be increased and metal particles could be
removed without difficulty. Without these mineral
oil products the industrial utilization of spark erosion would never have become possible. Initially
products containing petroleum and products
derived from white spirit (e. g. Kristallöl 60) were
used.
Crude Petroleum
0.790
1.8 cSt.
54–58
180
220
220–250
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White Spirit (Kristallöl 60)
0.790
2.0 cSt.
60
180
210
295
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From 1960 onwards the mineral oil industry began developing industrial fluids specifically for use in spark
erosion machines.
3. Functions of the Dielectric
Insulation
One important function of the dielectric is to insulate the workpiece from the electrode. The disruptive discharge must take place across a spark gap
which is as narrow as possible. In this way efficiency and accuracy are improved.
lonization
As quickly as possible optimum conditions for the
production of an electrical field must be created
and then a spark path must be provided. After the
impulse the spark path must be deionized quickly
so that the next discharge can be made. The di-
electric ought to constrict the spark path as much
as possible, so that high energy density is achieved,
which increases discharge efficiency at the same
time.
Cooling
The spark has a temperature of 8000–12000° C
when it punctures the workpiece and so the dielectric must cool both the electrode and the workpiece. Overheating of the electrode must be avoidRemoval of waste particles
Metal particles that have been eroded away
must be removed from the area of erosion by
the dielectric to avoid disruptions in the process.
ed, so that excessively high electrode wear cannot occur. It must be possible for the metal gases
which develop during spark erosion to condense
in the liquid.
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Important facts about spark erosion
4. Requirements for Dielectric
Theoretically all insulating liquids can be used as
dielectrics. However, due to the requirements set
out below, only de-ionized water (for polishing)
and hydrocarbons are used for this purpose today.
These hydrocarbons can either be produced by
distillating and refining mineral oil, or synthetically
by processing gases in a synthetizing oven with the
help of a catalyst. Synthetically produced hydrocar-
bons are characterized by otherwise unparalleled
purity. In addition, precisely those chains of hydrocarbon molecules can be synthetized which have
the best possible erosive effect as well as offering
optimum protection against electrode wear. In this
way they are far superior to those mineral oil products which are produced from certain mineral oil
fractions.
5. Criteria for assessing Dielectrics
The following criteria are generally used today to
assess different dielectric fluids:
a) Degree of metal removal and electrode wear
b)Effects on health:
skin irritation
toxicity
smoke
odours
c) Flash point
d) Density
e) Evaporation number
f) Viscosity
g) Conductivity
In general it can be said that it is easy to develop a
product which achieves excellent results according
to one or another of the above criteria. However, it
is important for the utilized product to achieve an
optimum in them all, if possible. Thus it is possible
for a product of the highest mechanical efficiency,
h) Dielectric constant
i) Disruptive
j) Particle suspension
k) Filterability
l)Compatability with other machine components
(machine parts, varnish, sealing material)
m)Aging stability
n) Constancy of quality
o) Availability
p) Price
combining high metal removal with low electrode
wear, to be unusable in practice, because of physiological reasons, or because it eats into engine
parts.
Effects on health
In the present, and certainly even more so in future, the effects of industrially used hydrocarbon
fluids on health are becoming increasingly impor-
tant. Smoke, odours and skin irritation have a decisive influence on working conditions at spark erosion machines.
Skin irritation
Products, which are so pure that they are unharmful from a dermatological point of view, should
always be given preference over others. As far as
possible these products ought to consist of completely saturated hydrocarbons and should contain
as few aromatic compounds as can be. An aromatic content of less than 1% in vol. is desirable.
Hydrocarbons from the normal paraffin series of
C12 to C14 often cause skin irritation and ought
not to be used. If at all possible only such products
ought to be used which have been proven to be
unharmful to the skin by independent medical
tests.
Important facts about spark erosion
Toxicity
for aro-
There are as yet no legal provisions i.r.o. toxicity
(or rather physiological properties) for the industrial utilization of dielectric fluids. Low aromatic
content in an unused product is not on its own an
indication of good quality. Far more important is
the question, to what extent there is a tendency
matic compounds to develop during erosion (aging
stability). Even after the product has been in use
for some time it must not develop any polycyclic
aromatics (e.g. benzpyrene), which are today considered to be carcinogenic.
Smoke
The amount of smoke given off during erosion is
largely dependent on the varying rates of metal
removal. Thin-bodied dielectrics usually give off
less smoke than more viscous ones. The higher the
flow of the dielectric over the place of erosion, the
less it smokes. (According to German engineering
guidelines – VDI 3402 – the dielectric level must be
at least 40 mm above the place of erosion.) A ventilator should always be provided at a spark erosion
machine, unless it is used exclusively for fine work.
Odours
The unused dielectric should be odourless and
should not begin to smell, even when heated.
After it has been used for some time, it is quite
usual for a faint ozonic smell, caused by the electri-
cal discharges, to develop. A sour, acrid smell,
however, is often an indication that the dielectric
ought to be renewed.
Flash point (German standard - DIN 51755)
The flash point is the lowest temperature at which
a dielectric gives off sufficient vapours to produce
an inflammable mixture of air and gases in a standardized apparatus. The higher the flash point, the
Danger class:
A I
A II
A III
under 21° C
21–550° C
55–100° C
safer is the use of the dielectric. Dielectrics are divided into different danger classes according to their
differing flash points.
e. g. benzine
e. g. crude petroleum, white spirit
e. g. diesel, light fuel oil
According to German engineering guidelines - VDI
3402 - substances with flash points below 21°
C may not be used in spark erosion machines. It
must also be pointed out that crude petroleum and
white spirit are in danger class A II and that special
safety regulations must therefore be complied with
when they are used. Most of the dielectrics in use
today are in danger class A III. Dielectrics whose
flash point is over 100° C are not considered to be
inflammable as defined by German law. No special
safety measures are therefore needed for them.
To determine the flash point of fluids in accordance with the German legal provisions for industrial substances, flash points up to 50° C must be
measured with the Abel-Pensky apparatus, while
flash points of over 50° C must be measured with
the Pensky-Martens apparatus (Flp. PM). It is not
permissible to use an open cup apparatus, such as
the one developed by Cleveland.
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Important facts about spark erosion
Density (German standard - DIN 51757)
Irrespective of viscosity, the influence of density is
greater during the finishing process than in rough
cut operations. „Heavy“ products remove more
metal. The density of a substance is the ratio of its
mass to its volume (usually measured at a temperature of 15° C). Dielectrics normally used today have
a density of 0.750–0.820. The shorter the chain of
hydrocarbon molecules, the lower usually is its specific gravity. Changes in the specific gravity
of a dielectric before and after use indicate that
alien substances, such as hydraulic fluid, have
entered it. Density increases in a dielectric which
was blended from different fractions show to what
extent the more volatile parts have evaporated.
Density can easily be checked with a densimeter
(hydrometer). This is a floating glass instrument
with a density scale (units of 0.001) also containing
a thermometer.
Evaporation number (German standard - DIN 53170)
The evaporation number (VD) is the ratio of evaporating time for the dielectric to that for ether.
Dielectrics for polishing work should have an eva
poration number of 500–1000. For economic reasons, substances that evaporate more quickly (e. g.
Petroleum VD 260) are not suitable as dielectrics.
Viscosity (German standard - DIN 51562)
Viscosity is the property of a fluid whereby it tends
to resist the displacement of two neighbouring
layers. The physical unit of measurement of absolute viscosity is the Pascal second. One mPa.s is equal
to one Centipoise (cP). The ratio of absolute viscosity to density is called kinematic viscosity. The unit
of measurement is the square metre per second
(M2/s). A centistoke (cSt) is equal to 1 mm?/s. The
viscosity of thin-bodied dielectrics is usually measured at a temperature of 20° C.
Dielectrics of 2 to 3.5 cSt at a temperature of 20° C
are suitable for polishing work. 4 to 6.5 cSt at 20° C
is suitable for rough cut operations. The disadvan-
Figure 1
tage of dielectrics which have been produced from
two fractions of differing viscosity is that the more
volatile, less viscous components evaporate more
quickly, leaving behind a dielectric which is so
viscous after prolonged use that it is suitable only
for rough cut operations. The surface roughness
of the processed workpiece is also dependent on
viscosity. Thus a narrow spark gap can be used
with a thin-bodied dielectric, leading to a finer
finish. When more viscous dielectrics are used, a
larger spark gap must be chosen to avoid flushing
difficulties. This leads to greater roughness in the
processed workpiece (see fig. 1).
Important facts about spark erosion
Conductivity
Conductivity is equal to the reciprocal of volume
resistivity. The unit is the Siemens. A conductivity
AC bridge on the Whetstone bridge principle, at
frequencies of 50 or 3000 Hz, is used for measu-
rement. Hydrocarbon dielectrics for industrial use
have a conductivity of about 2x10 -14 ohm x cm-1
when new.
Dielectric constant (German standard – DIN 53483)
The relative dielectric constant (DK) of a particular
dielectric shows to what extent the capacitance of
an empty capacitor is increased by introducing that
dielectric. A (dielectric constant) DK-meter is used
to measure the dielectric constant. The capacity of
a capacitor is measured by connecting it to a high
frequency resonant circuit, both when filled with
dielectric and when empty. The dielectric constant
is the ratio of the two different values obtained.
A dielectric suitable for spark erosion ought to
have a dielectric constant of 2–2.5.
Disruptive voltage (German standard - DIN 53481/
German electrical guidelines - VDE 0303)
The voltage required to disrupt a 2.5 mm layer of
dielectric between two spherical electrodes is called
disruptive voltage. Good dielectrics should have a
disruptive voltage of 50–60 kv when new. It must
be noted that the least amount of moisture added
to the dielectric (e. g. condensation water) will
have a negative influence on this value.
Particle suspension
Waste particles eroded away from the workpiece
and the electrode, as well as carbon particles resulting from electrical discharges, are impurities in
the working substance. The dielectric must remove
these particles from the work area. Adequate particle suspension is necessary for this task. However,
particle suspension must not be too high, otherwise these impurities will not separate from the Di-
electric during filtration. Too many impurities lead
to arcing. On the other hand, a dielectric will only
function in the best possible way if a few microparticles are to be found in the dielectric, as this
is conducive to ionization. These tiny particles can
even be added to the dielectric artificially when it
is new to improve erosion from the start.
Compatability with other machine components
Dielectric fluids in industrial use must remain neutral towards other machine components with which
they come into contact, e.g. sealing material, tubes
and varnish used in containers. The dielectric must
not cause these materials to swell up, shrink or
dissolve.
Aging stability
Aging stability in dielectrics is very important for
economic reasons. The longer a product can be
used, the better is the relationship of price to performance. In ordinary erosion practice it ought to
be possible to use a dielectric with paper filtration
for one or two years. When using precoated filters,
dielectrics have now been known to last for almost
20 years without having been renewed. In these
cases nothing more was done than to replenish
the dielectric tank as the need arose. Age can be
assessed by means of infrared spectrographic analysis, but the alternative method, by which neutralization value is determined (NZ/German standard
DIN 52558), has also proved to be reliable up to
the present. Dielectrics with an acid number of
more tha 1 mg/KOH/g ought to be renewed as
soon as possible.
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Important facts about spark erosion
Quality and availability
The producer of a dielectric must be able to guarantee its quality for an adequate period of time.
In addition, the quality of a dielectric sold under
the same name in different countries must always
be the same. Dielectric fluids for industrial use
ought to be available in those quantities, in those
localities, and within those time periods, in which
they are required.
Prices
When prices are compared, all the above criteria
must be taken into consideration, as the dielectric
which is cheapest at first is often the most
expensive in the long run.
6. The flushing process during spark erosion
Every experienced spark erosion expert knows that
the flushing process is of utmost importance, when
metals are subjected to this procedure. The dielectric must flush away the eroded particles from the
gap between electrode and work piece, otherwise
they may form bridges, which cause short circuits.
Such arcs can burn big holes in the work piece and
in the electrode. Modern spark erosion plants
therefore have a built in power adaptive control
system, which increases pulse spacing as soon as
this happens and reduces or shuts off the power
supply completely. The more thin-bodied a dielectric and the lower its surface tension, the better it
is able to meet flushing requirements.
Open flushing
Open flushing is the most common form of flushing and is used when it is impossible to flush
through the electrode or workpiece.
Figure 1: Open flushing
Pressure flushing
Next to open flushing, pressure flushing is the
most important form. The dielectric is either
pushed through a flushing hole in the electrode
from above, or through a flushing hole in the work
piece from below. The amount of dielectric flowing
through is more important for effectivity than the
pressure of flushing.
Figure 2: Pressure flushing
Important facts about spark erosion
When calculating the smaller than specified dimension of the electrode, it must be remembered that
in this type of flushing particles rising up through
the lateral gap are continuously causing additional
erosion. This leads to minor defects in precision,
sometimes called secondary erosion.
Figure 3: Pressure flushing
Suction flushing
In suction flushing the eroded particles are sucked
out of the gap between electrode and work piece.
This type of flushing is best in those cases, where
a fine finish and paralell walls are required in the
work piece. When using this method with narrow gaps and small amounts of dielectric flowing
through, care must be taken that enough dielectric
gets into the spark gap, so that the spark erosion
process will remain stable.
Figure 4: Suction flushing
Combined flushing
In very complex jobs it may be advisable to combine suction and pressure flushing.
Interval flushing
In interval flushing the erosion process is interrupted after a while and the electrode is retracted.
This improves the flushing out of the eroded particles. The retraction and return of the electrode has
the additional effect of suction and pumping
Figure 5: Combined flushing
respectively, which improves the effectivity of the
flushing process. This method is particularly suitable, when deep depressions or thin electrodes are
involved, and also during finishing work.
7. Filtering the Dielectric
In order for the dielectric to perform its flushing
function in the best possible way, eroded particles
from the workpiece and the electrode, as well as
the cracked parts of the dielectric itself, must be
removed. In addition the dielectric, which heats
up during spark erosion, has to be cooled down
again to a normal working temperature of 20°
a)Storing the dielectric
b)Cleaning the dirty dielectric coming from the
work tank
c)Providing the required amount of clean fluid and
the necessary pressure for rapid filling as well as
C–30° C. If it is too hot, there will be inaccuracies
in the work and much of the dielectric will be lost
through evaporation. For this reason every spark
erosion machine has a filtering plant, which has
the following functions to perform:
for pressure and suction flushing
d)cooling the dielectric (by air, water or cooling
plant)
e)processing backwashed fluid and filtrate
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Important facts about spark erosion
Cartridge filter system (see Fig. 1)
In practice cartridge filter systems have proved
very effective for filtering dielectric in smaller spark
erosion plants, in which up to approx. 450 mm2/
min are eroded. Cartridge filter systems are simple,
and, as far as the cost of acquisition is concerned,
inexpensive apparatuses. In the main they consist
of a storage tank, filter pump, machine pump,
cartridge filter, cooler and the requisite piping. The
plant is operated manually. The filter element itself is housed in a pressure resistant container and
consists of a piece of paper, folded like a star and
arranged around a central pipe. The filter cartridge
is not reusable. Once it has attained its maximum
capacity for retaining dirt, it has to be replaced by a
new one. The fineness of the filtering effect of such
a plant lies between 1 and 5 µm, depending on the
paper used. Under normal conditions the dielectric
IME can be used with a paper filter plant for about
1–2 years.
Figure 1
Precoated filter system (see Fig. 2)
In big spark erosion plants it is advisable to mount
a so called precoated filter system. In these systems
the filter elements are coated with an even layer of
filter aid, before filtering begins. This layer may consist of diatomite, Rixid or cellulose. After precoating
is completed, the filter cycle of the plant is started,
either by hand or by machine. After a maximum differential pressure has been reached, the entire filter
system is flushed back and all the dirt on the filter
elements, plus the filter aid, are expelled via a mud
valve into the after-filter. After the flushing back
process is completed, the filter plant can be precoated anew and the filter cycle restarted. The filter area
should be large enough, so that all the dirt accumulating during one shift can be absorbed, before flushing back becomes necessary. A fineness of up to
1 µm can be achieved with precoated filter systems.
On the average 1 kg of diatomite or 0.5 kg Rixid are
required for 1 m2 filtering area. The residual moisture of a dry sludge cake discharged from a precoated filter system lies between about 20 % and 30 %
of the weight, depending on the type of dielectric
used. The service life of the dielectric in precoated
filter systems is very long, since diatomite and Rixid
not only have a mechanical cleaning effect, but also
filter out acid components from the dielectric to a
certain extent. In precoated filter systems bleaching
earth may also be used as a filter aid, in order to
clean the dielectric even more thoroughly. There is
data available from precoated filter systems, which
were filled twenty years ago with a quantity of the
dielectric IME, which is still fully operative today.
Merely the amounts lost through drag-out and evaporation had to be replaced.
Important facts about spark erosion
between about 20% and 30% of the weight,
depending on the type of dielectric used. The service life of the dielectric in precoated filter systems
is very long, since diatomite and Rixid not only
have a mechanical cleaning effect, but also filter
out acid components from the dielectric to a certain extent. In precoated filter systems bleached
earth may also be used as a filter aid, in order to
clean the dielectric even more thoroughly. There is
data available from precoated filter systems, which
were filled twenty years ago with a quantity of the
dielectric IME, which is still fully operative today.
Merely the amounts lost through drag-out and
evaporation had to be replaced.
The Transor filter system (see Fig. 3)
The Transor filter system is able to produce a filtering effect of 1 µm without the use of filter aids by
employing the edge filter principle. Filtering rods,
on which thousands of extremely fine special paper
discs are mounted, are installed in a pressure tank.
The dirty dielectric is pumped into the pressure
tank and pressed through the filtering rods from
the outside to the inside. As this system works
without filter aids, no precoating is necessary. The
gaps between the paper discs are so narrow, that
all particles that are larger than 1 µm are deposited
on the surface of the filter rods. When the rods are
dirty, backflushing occurs, and the dielectric, which
has already been filtered, is pressed back through
the filter rods in the opposite direction. The dirt
layer on the filter rods is blasted off and can be
taken out of a sludge tank. There is little sludge in
comparison to the precoated filter system, because
no filter aids are used. The service life of the filter
rods is on the average about 8,000 working hours.
In a Transor filter system one must make sure that
the viscosity of the dielectric does not excede 4.0
cSt at 20° C.
Diagram of a filter system for dielectrics working according to the edge filter principle
a)filter container,
b)filter rods,
c)filter pump,
d)sludge tank,
f) clean oil tank,
g)machine pump,
h)oil air cooler,
i)water trap and reducing valve
for compressed air,
j)central valve with single-lever
operation
Figure 3
25
26
Important facts about spark erosion
8.The effect of spark erosion on the work piece
Spark erosion has a completely different effect
on working material than customary methods of
processing. The electrical spark hitting the work
piece heats up the outer layer of the steel so much
(about 10,000° C) that the material evaporates.
The metal gases formed then condense in the
dielectric, usually in the form of hollow balls, open
on one side and having a sharp edge. In the work
piece itself depressions, shaped like craters, are
formed. How great is the danger for the working
material to be so unfavourably affected on the
surface, that the serviceability of the tool suffers?
And what about tool life, resistance to wear and
buffability? Figures 1, 2 and 3 show surface roughness, electrode wear and metal removal in relation
to the firing period.
Figure 1. Surface roughness in relation to firing period
Electrode wear relative value
Figure 2. Electrode wear in relation to firing period
Figure 3. Metal removal in relation to firing period
Important facts about spark erosion
Apart from metal removal, surface roughness and
electrode wear, the effect on the surface quality of
the working material is of utmost importance. In
most cases it was shown that there was no effect
on the functioning of the tool. In some cases, e.g.
in a cutting tool, it even became more resistant to
wear, in others, however, tools broke prematurely.
All changes that could be detected were due to
the high temperatures that were produced on the
rim. In this rim the structure, hardness, state of
stresses and carbon content of the steel are influenced. Fig. 4 shows a section of a surface that has
been roughened down by spark erosion, showing
the various structural changes, which are typical of
such a rim.
Figure 4. Section of a surface that has been subjected to spark erosion with structural changes Material: UHB Rigor, hardened to
57 HRC
27
28
Important facts about spark erosion
The melted zone (Fig. 5) shows clearly that it has
solidified very quickly. Columnated crystals have
grown vertically up out of the metal surface during
solidification. A crack that has formed in this layer
runs inward along the line of crystals.
The melted layer is usually about 15–30 µm thick
after normal rough work. In the hardened zone
the temperature rose above that needed for hardening. A hard and brittle martensite has formed.
In the annealed zone the temperature was not
so high as to harden the steel. It has only been
tempered. Underneath is the unaffected core. The
thickness of the various layers appears to be unrelated to the type of steel used and the electrode
material. However, there is a very clear difference
between hardened and softened materials. In softened steel the layers are thinner and there are
fewer cracks. The brittle, hardened layer is almost
non-existent. During rough work the thickness of
the layers varies much more than during finishing.
The longer the firing period, the thicker the melted
and hardened layers become. Further research has
shown that the strength of current has basically
the same effect as the length of the firing period.
Steel with a high carbon content gets the most
cracks. Steel with a low carbon content only develops few cracks in the melted layer. About 20% of
the cracks extend into the hardened zone and only
a few reach the core. In the core there are seldom
cracks longer than 10 µm. These cracks in the core
are usually found in high alloy tool steel and in
high alloy high-speed steel.
Figure 5
Important facts about spark erosion
Figure 6a. Thicknesses of layers and amount of cracks in the rim
after spark erosion on hardened (52 HRC) UHB Orvar 2
microdized at different lengths of firing period
Figure 6b. The same after spark erosion on UHB Orvar 2 microdized which has been annealed
Number of cracks/cm 1) (in the melted zone)
2) (in the hardened zone)
3) (in the core)
The cracks are caused by stresses, which result
from the repeated, rapid chilling of the work material by the dielectric, as well as from the differences
in volume between the various structural parts in
the different layers. If erosion is properly done and
includes the final finishing process, the surface
errors that result from rough work can largely be
corrected. Where finishing is not possible, the following procedures may be used:
a)stressfree annealing at about 15° C less than
before. This decreases the hardness of the surface without influencing the core.
b)softening and renewed hardening and annealing leads to an almost complete restoration of
the structure (cracks however remain)
c)grinding or scouring removes the surface
structure together with the cracks. The rate
of cut is important here, and should be about
5–10 µm.
In summary it may be said that the structural faults
caused by rough work can be corrected during the
normal process of spark erosion, which includes
rough work and finishing. A certain amount of
structural changes will, of course, always remain.
However, in most cases they are of little importance. There are even instances in which the great
hardness of the hardened layer improves the tool‘s
resistance to wear. In others, the craters on the
surface of the work piece provide a better hold for
lubricants, which also increases the service life of
the tool.
29
30
Important facts about spark erosion
9.IonoPlus®
a new way to better dielectrics
german patent no. 41 32 879 and american patent no 5,773,782
Every experienced operator of spark erosion
machines is acquainted with the phenomen that
better results are obtained with a used dielectric
than when it has just been renewed. The reason
for
this is that finely dispersed waste particles make it
possible for ionisation channels to build up more
rapidly. In tests a fresh dielectric is always put to
use for at least half an hour before the actual
test phase is begun. Many years ago our firm also
conducted experiments using dielectrics to which
metal pigments or organometals had been added.
It was intended to induce a „controlled“ effect of
increased metal removal. Unfortunately most of
these additives settled on the bottom of the work
tanks even when their specific gravitiy was very
low (e. g. with powdered aluminium) - or were
taken up the filters. Only after these microparticles
had been reduced in size even further, was there a
real improvement in metal removal.
The starting point for the.development of dielectric
IonoPlus IME-MH® was the idea of formulating a
dielectric that could be used for rough cut as well
as finishing and polishing processes. In addition it
was intended that it should increase metal removal
and decrease electrode wear. From a physiological
point of view the new dielectric was to be absolutely unharmful, so that it would no longer fall
under danger class A III for inflammable liquids.
Of course it also had to be devised for use with all
conventional filter systems and had to be simple to
dispose of.
This goal has been reached by using substances
floating in the dielectric in finest distribution, substances that turn into stronger dipoles than the
surrounding hydrocarbons when they come under
the influence of an electrical field. On application
of an electrical current, these chemical satellite
electrodes align themselves along the lines of
electric flux in the electrical field, an channels of
increased electrical conducting capacity develop
in the dielectric liquid. In this way the discharge
channels required for spark disruption can build
up more rapidly than usual. This in turn leads to
a steeper increase in ignition voltage and in this
way to faster spark disruption. Thus the amount
of metal removal per unit of time is significantly
increased.
In contrast to conventional dielectric liquids the
dielectric IonoPlus IME-MH® does not induce a
direct flow of electrons from cathode to anode.
On their way most of the electrons are attracted
by the finely distributed satellite electrodes and
conducted along a widely ramified network of
channels. Since they lost part of their kinetic energy in the process, they hit the anode with relatively
little energy. A decrease of ignition time delay is
achieved at the same time, because of the steep
increase in ignition voltage. Both of these effects
lead to a decrease in anode wear. In comparison
to conventional dielectric liquids electrode wear is
therefore reduced by up to 30%.
In spark erosion for finishing purposes (with
reversed polarity) the work piece serves as anode.
Again the satellite electrodes dampen the impact
of the electrons, that now hit the work piece with
less kinetic energy and more widely distributed
than when a conventional dielectric has been used.
The satellite electrodes lead to a faster build-up of
the ionisation channel and thus make it possible
for less average space current to be applied in
pocessing the work piece.
By means of this new technique very well polished
workpiece surfaces with a surface roughness of
less than 0.1 pm can be produced. This polishing
performance i. r. o. surface quality and speed cannot be achieved with conventional dielectric fluids.
The use of highly polarized substances in the dielectric IonoPlus IME-MH® also has a very positive
effect on its dispersing qualities. The waste particles produced by the spark erosion process are hurled explosively out of the work area in the finest
distribution. This reduces the tendency for short
circuiting and leads to an undisturbed process in
spark erosion. The reason for these good dispersing qualities are the electrical dipoles aligned in
the satellite electrodes, leading to a quicker distribution of the waste particles due to their electrical
repulsion forces.
Technical data of IonoPlus®:
Danger class VbF:
Transportation class:
Tank truck marking:
Danger number:
Substance number:
GGVSee IMDG-Code:
IATA-RAR
article no.
class
none
none
none
none
none
none
none
none
Important facts about spark erosion
rregular ignition by using Dielectric IME82 for finishing
Continuous ignition by using dielectric IonoPlus IME-MH for finishing
The first dielectric with the
plus of satellite electrodes
IonoPlus IME MH dielectric has been thoroughly
tested by the Institute for Research and Control of
Work Materials in Baden-Württemberg/Germany in
®
Colour flourescent green
0,79 DIN 51757
Viscosity at
+40 o C (mm2/s)
2,50 DIN 51562
Pourpoint C
-15 DIN ISO 3016
Flashpoint C
107 DIN EN 22719
o
o
time (t)
respect to operational safety and industrial hygiene.
Toxic or allergic symptoms cannot occur during use.
A tolerance limit in the air surrounding the place of
work (MAK value) is not reached.
IonoPlus® IME MH dielectric can be used in all conventional filter plants. The regulations for flammable
liquids (VbF) do not apply to IonoPlus® IME MH.
•Greater efficiency
in metal removal
The time needed to build an ionization bridge is
substantially reduced.
Technical Data:
Density at 15 o C (g/cm3)
voltage (v)
After many years of research oelheld introduces
an entirely new, powerful concept into dielectrics:
IonoPlus® IME MH. Unlike conventional mineral oil
products, this combination of highly refined synthetic
products is enriched with satellite electrodes in a
special blending process. As a truly universal dielectric,
IonoPlus® IME MH is suited for all operations from
the finest finishing processes to the most effective
rough cut. Besides having the best possible effectiveness in flushing and the greatest possible disruptive
strength, it offers a whole series of unique advantages.
conventional dielectrics
Aromatic content (weight%)0,01 DIN 51378
•Greater resistance
to electrode wear
Macromolecules surround the electrode
like a protective grid.
•Improved surface quality
Satellite electrodes bring about an optimal
distribution of discharges.
•Shining results in the
polishing process
Within a minimum amount of time a surface
roughness of less than 0.1μ can be achieved.
•Best possible dispersing
capacity
wift dispersion of waste particles helps actively
to prevent burn spots from forming.
32
Important facts about spark erosion
10.Dielectrics IME 63, IME 82, IME 110, IME 126
IME dielectrics are synthetic products manufactured
in a catalytic process and posessing greatest disruptive strength. They are clear fluids and are almost
odourless. They do not change colour during erosion. They have the same purity as pharmaceutical
white oil and contain only a few traces of aromatics. There is no toxic or allergic reaction to contact
with human skin or eyes, when IME products
are used. The Institute for Research and Material
Testing of the State of Baden-Württemberg has
tested this brand of dielectrics i.r.o. operational
safety and industrial hygiene. A tolerance limit for
workroom air (according to German regulations
for the maximum concentration of chemical substances at places of work) is not reached.
IME dielectrics have been subjected to extensive
tests and have proven themselves in practice for
decades. They are explicitly recommended by the
leading manufacturers of spark erosion machines.
DIELECTRIC IME 63
Dielectric IME 63 is an extremely thin-bodied dielectric with the least possible surface tension. It is particularly suitable for very fine work, when a very low overcut is required, e.g. the microboring of spinnerets
and the manufacture of microelectronic parts.
DIELECTRIC IME 82
Dielectric IME 82 combines high metal removal with low electrode wear, which makes it suitable for
general use in manufacturing tools and moulds. Even rough cut operations using an electric current of
600 amps can be carried out with IME 82.
DIELECTRIC IME 110
Dielectric IME 110 is always used when a flash point of over 100° C is required for safety reasons, while
much finishing work also has to be done. Dielectric IME 110 lies outside danger class A III.
DIELECTRIC IME 126
Dielectric IME 126 is a dielectric for very high metal removal in rough cut operations, such as in the
manufacture of forging dies. It can only be used for finishing if the best possible flushing conditions
are ensured.
TECHNICAL DATA ON THE DIELECTRICS
Colour
Density at 15° C g/ml
Viscosity cSt at 20° C
Flash point ° C (PM)
Pourpoint ° C
Aromatic content % weight
Disruptive voltage kv at 2,5 mm
Danger class VbF
Transportation class
road ADR/GGVS and
rail RID/GGVE
Tank truck marking
danger number
substance number
GGVSee IMDG-code IATA-RAR
article no.
class
IME 63
IME 82
IME 110
IME 126
clear
0,765
1,8
63
- 40
0,003
58
A III
clear
0,789
3,0
82
- 40
0,02
59
A III
clear
0,775
3,4
106
- 6
0,01
57
none
clear
0,824
5,8
114
-5
0,1
52
none
none
none
none
none
none
none
none
none
none
none
none
none
none
none none
none
none
none
none
none
none
none
none
none
none
none
none
none
Important facts about spark erosion
Testing the various IME dielectrics
IME dielectrics have been tested in practice both
for metal removal and for electrode wear. The fol-
lowing materials and operational steps were
selected for these tests:
a) Materials
Electrode
1) electrolyte copper
2) graphite (Ellor 9)
Workpiece
tool steel X 210 Cr 12
tool steel X 210 Cr 12
b) Operational steps
roughness H max approx. (m)
working time (min.)
electrode shape round 0 (mm)
no-load running voltage (v)
average voltage (v)
average current (amp)
pulse duration (µsec)
pulse spacing (µsec)
flushing hole 0. (mm)
Vw (mm3/min) Rough cut
60
15
35
100
28
36
200
12
7
Finish
10
60
25
100
28-30
6
10
2.6
5
= metal removal
%
=electrode wear expressed as a ratio (in percent) of the volume of electrode material lost, to metal
removed from the workpiece.
The control settings given represent easy to
manage operational steps involving no special
difficulties. Metal removal and electrode wear
were determined by measuring weight differences,
which were then converted into units of volume.
33
34
Important facts about spark erosion
Rough cut
When working with copper and steel, metal removal was lowest for IME 63 during rough cut operations, and highest for IME 126. Electrode wear was
least for IME 63 and most for IME 126 (see fig . 1).
When working with graphite/steel similar results
were obtained. Metal removal was highest for
IME 126 and least for IME 63. It was astonishing
that no measurable electrode wear took place
when IME 110 was used.
Roughening n 12 Tr./200 µ sek
Figure 1
Finish
When working with copper and steel in the
finishing process, IME 126 achieved the highest
metal removal. Least electrode wear took place
when IME 63 was used (see fig 2).
When working with graphite and steel, IME 126
also achieved highest metal removal. The results of
IME 82 were only slightly lower.
Finishung nnn 2 Tr./10 µ sek
Figure 2
All these test results are valid only for the given
control settings and materials. They are intended
to show the varying influence of the dielectric used
on the work process. The excellent results of
IME 126 during finishing can undoubtedly not be
achieved, unless flushing conditions are optimal.
Important facts about spark erosion
11. Gases produced during spark erosion
The gases produced during erosion consist of dielectric vapours and metallic fumes. The vapours of
the dielectrics IME 63, IME 82, IME 110 and IME
126 contain no benzene compounds, such as the
polycyclic aromatics of the Benzpyrene type, even
after they have been in use for some time. There
is no ill effect on health brought about by IME
products. However this does not hold true for the
metallic fumes that may develop during erosion
(e. g. tungsten carbide, titanium carbide, chrome,
nickel and molybdenum). It is therefore important
for the dielectric level to be as high as possible
over the place of erosion, so that most of the
metallic fumes can condense in the dielectric.
German engineering guidelines (VDI 3402) prescribe a depth of 40 mm over the place of erosion.
However a depth of 80 mm is to be recommended
for health reasons. The metallic fumes rising up out
of the dielectric cause the same problems as those
that develop during the welding of metals. It is
therefore advisable to suck off the gases that develop when extensive rough cut work has to be done.
12. Dielectrics and the human skin
Decades of practical experience with the dielectrics
IME 63, IME 82, IME 110 and IME 126, as well as
the knowledge of their composition, permit us to
state that they have no damaging effect on human
skin. Practically only one‘s hands come into direct
contact with the dielectric during work. Remnants
that are left sticking to the skin can be removed
without the use of cleaning agents that have
mechanically or chemically aggressive properties.
In this way secondary damage is also avoided. It is
difficult to make general predictions on the effect
of dielectrics on persons with particularly sensitive
skin or with a tendency to allergies, but practical
experience has shown that a negative reaction only
occurs in very rare cases. (Test reports have been
issued by the Institute for Research and Material
Testing in Baden-Württemberg.) However, metal
particles suspended in the impure dielectric do
have a negative effect on skin. These particles are
microscopically small, hollow, steel globules, open
on one side and with very sharp edges. These globules can easily hurt the epidermis and lead to skin
damage. Certain medicines, such as Penicillin, can
sensitize the epidermis even further. In all these
cases it is advisable for a skin protecting cream
that is not oil soluble to be rubbed into the hands.
Pieces of clothing soaked with dielectric ought to
be changed at once.
35
36
Important facts about spark erosion
13. „7 Golden Rules“ for working with IME Dielectrics
Correct handling of dielectrics begins with the proper storage of packing drums:
are stored out of doors, they should always lie down and never stand upright, so that
•Inof theraindrums
water can seep in.
hen the dielectric is filled into the machine, suitable, clean pumps or containers must be used.
•WPumps
that have been used for acid or caustic solutions destroy the best dielectric at once. PVC
tubes are not oil resistant and will become rigid after they have been used for some time.
nticorrosive agents, used to protect the machine during transport, must be removed before the
•Adielectric
is filled in.
hlorinated hydrocarbons (e. g. trichloride, tetrachloroethylene, trichloroethane or Freon 12) are
•Cdeadly
for the dielectric. The electrical spark causes the hydrocarbons of the dielectric to combine
with the chlorine atoms to form hydrochloric acid. A spark erosion machine must therefore never
be cleaned with trichloride or a similar substance. It is better to use a few liters of dielectric for this
purpose. Moulds that have been cleaned in trichloride must be absolutely dry before being mounted
in the machine.
•Acids, used to pickle the electrode, must not be allowed to get into the dielectric.
The hydraulic system of a spark erosion machine should be absolutely leakproof. Not more than 1–2%
•additions
of hydraulic fluid should ever get into the dielectric, as the large amounts of additives in
these oils will otherwise lead to malfunctioning. Machines with electric servo motors do not have this
problem.
and again, leaks in the water cooling systems of dielectric units lead to a „miraculous“ increase
•Aofgain
the dielectric and to rusty tables. IME dielectrics separate quickly and completely from water, and
so the water can be drawn off from the bottom of the tank, or the dielectric can be ladled out after
about one day. The dielectric can then be used again.
If you adhere to these rules when working with dielectrics, they will last for about one or two years in
paper filter units and for about 10–20 years in units with precoated filters.
Important facts about spark erosion
14. The effect of spark erosion on metal surfaces
The formation of craters
In the process of spark erosion an electrical potential between workpiece and tool is discharged and
a spark jumps across the gap. At the point where
the work piece is hit, the metal is heated up so
much that it melts and evaporates. A crater is formed. One after another innumerable such sparks
are sprayed on the workpiece, and one crater is
formed next to the other. The diameter of the crater on the photograph is about 200 pm.
The photographs were taken through an electronic
microscope. To make the individual crater more
easily visible a polished metal surface was used.
The spark erosion machine was only switched on
for a fraction of a second, so that the edges of the
craters would not overlap.
Each of these craters has a typical edge with a
thermically influenced zone.
Here a crater on a titanium work piece is shown.
A profile structure, formed by the rapid solidification of the heated titanium, can be clearly recognized. Part of the liquid titanium was flung into
the dielectric.
37
38
Important facts about spark erosion
Eroded surfaces
Characteristic of an eroded surface are the
numerous craters with overlapping edges. In addition microscopic eroded particles can be seen sticking to the surface.
By enlarging the photograph one can clearly see
the crater edges on the work piece and the microparticles sticking to them, as well as a hole in the
surface of the metal.
Enlarged once more one can see microscopic
cracks emanating from these holes. These cracks
are signs of an overheated surface.
Important facts about spark erosion
If the electronic microscope is used to make an
even greater enlargement, these microscopic cracks
become very evident. The eroded particles even
begin to look „human“. „Eyes, ears and a mouth“
can be recognized.
Surface polished by means of spark erosion
If spark erosion is used to polish a surface, the
edges of the craters are largely removed. A crosssection through a work piece polished by spark
erosion shows clearly that there is only a very small
white layer and that the influenced zone is only
about 2 pm thick.
39
40
Important facts about spark erosion
Eroded particles
Seen with the naked eye erosion sludge looks
black. If the sludge of an erosion machine that
works with different materials is washed out with
acetone and then put under an electronic microscope one sees a great many larger and smaller
balls.
After enlargement differences between the eroded
particles become evident. Thus three particles have
melted together to form „triplets“, a big particle
has fused with a small one (mother and child), and
many small particles have gathered together on a
big particle because of electrostatic or magnetic
force.
Some particles look like golf balls, or like our neighbouring planet Mars with its famous Martian
canals.
Important facts about spark erosion
Other eroded particles have a textile structure ...
... or have velvety surfaces like peaches.
Many particles have a cavity on the one side
because of the sudden shrinkage of the metal.
If tool steel is used these microscopic balls are
often hollow and have sharp edges that can
injure the human skin.
41
42
Important facts about spark erosion
For their friendly support during the writing of this brochure
we would like to thank:
AGIE, AG für Industrielle Elektronik, Losone/Schweiz
AGIE CHARMILLES GmbH Schorndorf
CHARMILLES Technologies S.A., Meyrin/Schweiz
exeron GmbH, Fluorn-Winzeln/Deutschland
FAUDI, Stadtallendorf/Deutschland
OPS INGERSOLL GmbH, Burbach/Deutschland
Dr. W. LINDEMANN, Universität Tübingen/Deutschland
MANN & HUMMEL, Ludwigsburg/Deutschland
ONA S.A., Durango/Spanien
TRANSOR Filter GmbH, Usingen/Deutschland
UDDEHOLM, Hagfors/Schweden
ZIMMER & KREIM GmbH & Co. KG, Brensbach/Deutschland
Juli 2007
Dr. Manfred Storr
The information presented herein has been compiled from
sources considered to be dependable and is accurate to the
best of oelheld‘s knowledge; however oelheld makes no
warranty whatsever expressed or implied, of merchantability
or fitness for the particular purpose, regarding the accuracy
of such data or the results to be obtained from the use thereof oelheld assumes no responsibility for injury to recipient
or to third persons or for any damage to any property and
recipient assumes all such risks.
Imprint:
oelheld GmbH
Ulmer Straße 135-139
D-70188 Stuttgart
Tel
+49 (711) 16 86 3-0
Fax
+49 (711) 16 86 3-40
E-mail hutec@oelheld.de
Internet www.oelheld.de
Important facts about spark erosion
43
High-tech products for machines – worldwide!
oelheld GmbH is not only represented with its own sales offices and production plants in France, Great
Britain and the USA, but also has various representatives in most countries.
oelheld U.S., Inc.
1760 Britannia Drive, Unit 1
Elgin Illinois 60124
phone: +1–(847)–531-8501
fax: +1–(847)–531-8511
E-Mail: hutec-us@oelheld.com
web: www.oelheld-us.com
oelheld technologies SAS
Technopôle de Forbach-Sud
140, Avenue Jean-Eric Bousch, 57600 Oeting
Téléphone : +33 (0)3.87.90.42.14
Télécopie : +33 (0)3.87.84.66.91
E-Mail : hutec@oelheld.fr
Internet : www.oelheld.fr
oelheld UK Ltd.
Unit 16, Colomendy Business Park, Erw Las
Denbigh, LL16 5TA.
Tel: +44 (0)1745-814-777
Fax: +44 (0)1745-813-222
E-Mail: sales@oelheldgroup.co.uk
Internet: www.oelheld.com
Storr oelheld (Shanghai) Trading Co., Ltd.
Room 1908, Hitime International Building
No.888, Sichuan Road(N)
Hongkou District, Shanghai, 200080
Telephone: +86-13801724397
E-mail: john.w@oelheld.com.cn
Internet: www.oelheld.com.cn
Argentina
Austria
Australia
Belgium
Bosnia
Bulgaria
Canada
China
Croatia
Czech Republic
Denmark
Estonia
Finland
Indonesia
Israel
Italy
Japan
Kazakhstan
Korea
Latvia
Malaysia
Netherlands
New Zealand
Mexico
Norway
Poland
Portugal
Russia
Serbia
Singapore
Slovakia
Slovenia
South Africa
Spain
Sweden
Switzerland
Taiwan
Thailand
Ukraine
White Russia
oelheld GmbH • Ulmer Strasse 135 -139 • D-70188 Stuttgart
Telefon: +49 (0) 7 11 / 1 68 63 - 0 • Fax: +49 (0) 7 11 / 1 68 63 - 40
E-Mail: hutec@oelheld.de • Internet: www.oelheld.de
07/11
Human Technology
for man, environment
and machines
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