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STEELFLUID on EDM öljyjen erikoisvalmistaja, jonka valikoimista löytyy oikea ratkaisu kaikkiin kipinätyöstösovelluksiin: -uppokipinäkoneisiin eri vaatimuksiin -alkureikäporiin -öljykylvyssä tapahtuva lankasahaus -öljykylvyssä tapahtuva hionta ja sahaus
Valikoimissa myös esimerkiksi lankasahojen veteen sekoitettava ruosteenestoaine. |
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Artikkelilainaus Steelfluid Srl kotisivustosta. Käyttöoikeus vain Steelfluidin luvalla.
Dielectric Fluids for die-sinking EDM
Why so many viscosity grades?
The most frequently asked questions, either directly or indirectly, are the following:
· Why is there a dielectric fluid range for die-sinking electroerosion comprising of six or seven different fluids;
· Why for years only two fluids have been used in electroerosion processing, one of which is a low viscosity fluid (between 1.6 and 1.8 cSt at 20°C) that was the most frequently used, and the other a high viscosity fluid (> of 6 cSt at 20°C) normally used for hot pressing of metals, with the exception of often wrong choices;
· one fluid is much the same as another, they do not affect electroerosive processing; why should I switch over?
· I have always topped up with a new fluid;
· why is it necessary to thermostat the fluid in use?
The above questions and comments are, in my opinion and without prejudice to anyone, the normal outcome of deep misinformation; in fact, it would be better to call it for what it really is, that isignorance in its correct meaning of ignoring facts or missing knowledge of the electrical physical process of electroerosion, which is an innovative and unconventional method for processing conductive metals.
All of the above are justifiable as arising from being unaware of all that EDM processing involves and what really is this manufacturing method. Which is really absurd, considering it is only the metal processing method with the highest technological content.
To answer the first question, namely " Why is there a dielectric fluid range for die-sinking electroerosion comprising of six or seven different fluids?" is, allow me to simplify, like explaining to the user of an endothermic engine why it is supplied with a 4, 5 or 6 gear change and what are the benefits of a 6-gear engine compared to a 4-gear engine of the same mechanization standard; which makes it necessary to examine in detail the electrical physical principle of electroerosion.
We must go back to the electrical physical principle of EDM processing to clarify the role played by the dielectric fluid and the reasons behind the above question. The EDM fluid has the same function in electroerosion processing that the gears have in a vehicle. It is well known that gear changing is necessary to obtain the best performance from an endothermic engine in the most favourable torque curve area, and the same principle holds true for the use of the correct fluid, necessary to best suit the electrical parameter settings (amperage and frequency), the required volumes and final surface roughness.
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Fig. 1 |
If the fluid’s role were to be limited to these three functions, it would also be right to say that it is not necessary to have a whole range of fluids with different viscosity grades starting from 1.7 cSt at 20°C and ending at 5.5 cSt at 20°C to use in EDM processing. If the dictum were true, any fluid from kerosene to lamp oil would have the necessary requirements to answer the processing demands, except for the odour emissions and the evaporation.
As a matter of fact, the main function of an EDM fluid is to determine the diameter of the ionisation channel at parity of amperage employed. After generating the ionisation channel, the surface on which to concentrate the electrical energy, converted to thermal energy owing to local temperature of between 4000°C and 12000°C from each single spark, not to be mistaken with the voltaic arc that is to be avoided as the antithesis of the electroerosive process, will be determined.
Fig. 2 Keeping in mind that the electroerosion principle consists of nine stages (see Fig.1, Fig.2, Fig.3, by kind permission of AGIE) repeating between each spark, the fluid acts as insulator only between the first and the third stage, that is from the moment the generator has closed the electrical circuit between the two electrodes (stage 1) to the end of stage three. |
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This period known as td firing delay is necessary to allow formation of the ionisation and deionisation channel, through which is discharged the electrical spark, conveying the near totality of the thermal energy on the metal to be machined.
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Fig. 3 |
Fig. 4 |
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It should be common knowledge, at least among EDM users, that the higher the amperage, the higher the tendency to increase the diameter of the ionisation channel, given fluids with the same viscosity grade.
The higher the viscosity grade, the higher the quantity of metal eroded by each single spark – but this holds true up to values of between 5.1 and 5.6 cSt at 20°C, with amperage values over 90A, above which there are no benefits in terms of more velocity of erosion but only greater difficulty in flushing the gap area. This is due to more efficiency in resisting the expansion tendency of the ionisation channel of a higher viscosity fluid, compared to a lower viscosity fluid, at high amperage values; in other words, it generates a channel with a smaller diameter, and the electricity converted to thermal energy is concentrated on a smaller area, but which goes much deeper. As a matter of fact, the higher velocity during the electroerosive process is reached going deeper into the surface (roughing stage) and not by processing a wider surface area with a larger diameter of the ionisation channel. Die-sinking EDM operators know that the higher the amperage values during the roughing stage, the longer will be the duration of the discharge, resulting in a superior surface roughness. Conversely, during finish and super finish processing the amperage values are lower, the discharges more frequent and very low final surface roughness, but with much higher wear and tear of the electrode when compared with roughing processing (see Fig. 4, by permission of AGIE).
The above accounts for the old practice we referred to in the introduction, that manufacturers of moulds for hot pressing of metals use fluids whose viscosity is around 6-7 cSt at 20°C when amperage values are above 80-90 Amperes. This choice is motivated by the fact that the shapes required to form are not of a complex geometry, and the final roughness required is not very low, but the main point is to contain the running costs and to have a higher speed of erosion. But, by taking into account the electrical physical phenomenon known as EDM processing, which is subject to the kinematic viscosity of the fluid in use, it would certainly be illogical to have the widely-spread doubt of whether it is advisable, or even required, having an EDM range of fluids with six different viscosity grades between 1.75 and 5.5 cSt at 20°C.
We must stress the fact that it is not enough to have a fluid with a viscosity grade suited to the processing conditions, that is, to the type of machining amperage during roughing and finishing, and to the kind of volumes and final surface finish required, but it is also extremely important to ensure that this chemical-physical property of the fluid remains stable in time.
This can only be achieved if the fluid is formulated with hydrocarbons (for precision and clarity, it must be said that also the so-called synthetic fluids are hydrocarbons), that it has a short range between initial and end distillation, and that it is thermostated. These are necessary conditions to avoid instability of the fluid’s viscosity, followed by changes in the diameter of the ionisation channel, during variations of the processing temperature at par of amperage values.
The distillation range is particularly important for fluids with a kinematic viscosity between 1.7 and 3 cSt at 20°C. Within this extent, the distillation range should be between 5 and 17°C tops; still largely available are some old formulation fluids with nominal viscosity of 1.7-1.8 cSt at 20°C, and with a distillation range above 40°C. A product such formulated, aside from having a high evaporation level, cannot have a stable viscosity grade, but will reach viscosity values above 2 cSt at 20°C, thus altering the processing conditions and the end results.
Following the above statements, and to answer the third question, "one fluid is much the same as another, they do not affect electroerosive processing; why should I switch over?” there is first-hand experience at the tool room of a very important spectacle frame manufacturer.
The tool room was equipped with three identical die-sinking EDM machines for the production of industrial moulds for spectacle frame forming, which must be very precise and have a high level of final roughness and polish. When I arrived as technical support to the assistance service of the EDM machine manufacturers which were installed there, the client was using a conventional fluid, widely available on the national market, of rather good quality but with an obsolete formulation. The fluid used at the time has a nominal viscosity grade of around 1.7 cSt at 20°C, a flash point (PM) ca. 74-75 °C and a content of ca. 0.1-0.2% (when new), and with a presumed distillation range of about 45°C (quite long for the viscosity level and the application requirements, which could be described as extreme).
The main problem experienced by the tool room workers, aside from excessive evaporation and fume and odour emissions, was irregular opacity found on the polished surfaces (that could only be seen through a magnifying glass), which had then to be removed by hand, involving extra labour costs and with results of erratic quality. After examining the problem in depth, and following a frank discussion with the client and the EDM machine technicians to rule out anomalies of the machine generator, we came up with the possibility of an excessive variation of the fluid’s viscosity in the ionisation channel area. Supposedly, this phenomenon was a consequence of changes in the gap area temperature that could promote the evaporation of the lighter hydrocarbons, due to the long distillation range of the fluid in use.
All things considered, we agreed to test a new generation EDM fluid, with a similar viscosity grade, on one of the machines without changing the operation parameters. The new fluid had a distillation range of only six °C and a content of aromatic hydrocarbons <0.001%. This product was the .
The first mould produced the satisfaction of discovering that the opacity found before was completely gone, and also that the running times were sensibly reduced.
We agreed then with the client to carry on the trial for six months at least, keeping a constant check on the results and comparing them with the output of the other two machines, without, of course, changing the operation parameters.
When the trial run ended after six months, the client gave us the results (found below in Table 1), to which must be added that the environmental situation also improved no end because of the great reduction in fumes and odour emissions.
Table 1
Parameters |
Values obtained with conventional fluids |
Values obtained with the |
Initial tank capacity |
1.000 litres |
1.000 litres |
Electrode |
Electrolytic copper 60x11.2 |
Electrolytic copper 60x11.2 |
Erosion depth |
1,48 mm |
1,48 mm |
Material eroded |
Steel 27.67 |
Steel 27.67 |
Erosion time |
2 hrs and 35 mins |
2 ore e 15 min |
Electrode |
Electrolytic copper 9.5x9.5 |
Electrolytic copper 9.5x9.5 |
Erosion depth |
10 mm |
10 mm |
Material machined |
Steel 27.67 |
Steel 27.67 |
Erosion time |
1 hr and 20 mins |
56 mins |
Electrodes (3 pcs: balance tool, finisher, polisher) |
Electrolytic copper 2.5x37.5 |
Electrolytic copper 2.5x37.5 |
Material machined |
K 110 |
K 110 |
Erosion time |
4 hrs and 38 minuti |
4 hrs |
Fluid consumption over 6 months per machine |
ca 550 litres |
ca 155 litres |
Electrodes wear and tear |
- |
Reduced by about 15% |
Polish level |
insufficient, integrated by hand |
perfect |
The results of this trial run prove just how important and decisive is to use a suitable dielectric fluid in a specific EDM application. However, it is not enough to choose the appropriate viscosity grade, but it is necessary that the fluid have the shortest distillation range possible, and even better to be thermostated. This would ensure a stable viscosity and a sensible reduction of fluid consumption through evaporation.
As can be seen from the table above, the shorter erosion times due to the faster polishing translated in the lower wear and tear of the electrode. Electrode wear is of course, higher during finish operations (carried out at high frequency) than during roughing operations.
Fig. 5: Cenospheres The above experience proved untrue the former conviction that a dielectric fluid is much the same as another of a similar viscosity grade. Another frequent anomaly pertaining to EDM processing, particularly when using graphite electrodes, is the premature wear of the electrode, which manifests itself as small holes on the surface when generating complex and deep volume geometries. The phenomenon may be caused by: |
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· inadequate and not constant filtration of the fluid;
· unsuitable viscosity grade of the fluid, usually well above what needed to flush the gap area even when using a powerful machine, like those installed for thermoplastic mould pieces (like for electronic components), moulds for pressure casting of light alloys with complex geometries, or tyre mould manufacturing where old generation fluids are normally employed, with nominal viscosity grades above 6-7 cSt at 20°C and distillation range above 100°C;
· the fluid reaching irreversible chemical and physical decay, specifically when the aromatic hydrocarbon content has exceeded the 1.5% mark, to such an extent that even frequent top-ups are not enough to ensure significant benefits except for very short periods of time.
Given the above operative conditions, and excepted the inadequate filtration that should never be overlooked, the cenospheres (see Fig. 5) which make up the metal and which consolidate again as hollow spheres when the vapour bubble implodes (see Fig. 3 stage n.8) tend to stick to the electrode, causing a reduction in the gap and the beginning of a voltaic arc (which with a manual machine could result in the formation of a crater on the surface, but with a CNC machine slows down the EDM process) and would also cause a formation of holes within the surface of the graphite electrode.
The clinging behaviour of the cenospheres on the tool electrode can be seen mainly during finishing, which is when the gap is reduced and the flushing more difficult, or when using EDM fluids with a viscosity which is too high for the volumes of the electrode shape or which have an aromatic hydrocarbon content above 1.5% due to chemical-physical decay. Aromatic hydrocarbons do have a tendency to deposit resinous adhesive on the tool electrode that cause the cenospheres to cement, thereby reducing the gap and initiating a voltaic arc. Furthermore, aromatic hydrocarbons decrease dielectric rigidity according to the percentage contained in the fluid.
On a last note, the elements to consider when choosing the right fluid according to operative conditions, and to manufacts obtained by die-sinking EDM processing, are the following:
1. the geometries to be formed;
2. the final roughness and polish required;
3. the amperage used during roughing operations.
This is especially true if starting from solid metal workpieces that have not been previously hollowed by milling and/or other conventional metal machining methods with chip removal.