Emerging trends in Electric Discharge Machines
Spark erosion is a modern machining technique with a distinct advantage, as a result of which its use is becoming more & more decisive.
EDM continues to be an indispensible technique. Today EDM has revolutionized the way, majority of highly accurate moulds, dies and complex machined parts are being produced. Most enthusiastic EDM users do find solutions to upcoming challenges in metal cutting industry. The ability to machine both hardened steel & carbide parts to highest level of accuracy and superior finish without much operator intervention would have been a dream, when spark erosion came in to existence.
These days EDM is not as an obscure term like it was some years ago, but there are still all kinds of vague statements available. For years, scholars have done lot of studies on the discharge process of EDM. A variety of hypotheses have been made but still even now, the differences of understanding the discharge process exist.
It is very difficult to describe such a complex process, of which most parts are invisible and are accompanied by some physical and chemical phenomenon such as sound, heat, light, electromagnetic radiations etc. Until today, they are neither completely understood, nor computable. Various theories, mathematical models, about the estimation of gap condition has been put forward, but all are approximations, and whichever manufacturer has approximated the gap condition better, is a leader in EDM field.
EDM is a process where cutting material and material being cut does not touch each other; there is always a physical gap between the two. Probably this gap, which has created perceptional gap in understanding EDM, so it is also said that EDMing is an Art. Over a period of last 20 years, many innovative ideas have made the process lot predictable.
Ater the world war II, Russian Scientist B. R. Lazarenko & N. I. Lazarenko invented the EDM process and since then, there have been continuous researches. In last 20 years, EDMs have become 6-8 times more productive.
It is very essential to know the basic Input / Output of EDM to understand & appreciate the innovation in the field of EDM
v Electrode material
v Work piece material
v Di electric media.
v High temperature
v Electromagnetic forces
v Cutting speed
v Job accuracy
v Surface finish.
Traditionally, EDM has been classified into two distinct categories because of different ways adopted in performing the process.
1. SINK EDM or PLUNGE EDM or RAM EDM
2. WIRE EDM
We all know the fact that finished EDM job can be no better than the electrode used. Output performance parameters like surface finish, accuracy and overall productivity mainly depends upon selection of electrode material. It is a known fact that 70% is labor and 20-30% is the cost in manufacturing of ELECTRODE. In last 15 years, there has been phenomenal development in the quality of electrode material available especially Graphite. Imported graphite is now readily available which is mostly used in overseas countries.
In India, we are seeing a shift from copper as an electrode material to graphite. To choose the correct material, the supplier provides almost 15 types of Graphite material and complete performance characteristic
Deciding which material to use e.g ‘Copper or Graphite’ mainly depends upon efficient method of producing the electrode, Job size, finish requirements, multiple electrode options. In general, if you ask Asian user of EDM, choice is Copper and Graphite if the user is from USA. In short, Graphite is preferred when electrode size is bigger; it is easily and faster machined on machining center. Good quality fine grain structure and sintered imported graphite is now available in India. Major problem in machining graphite is the dust. It is a major headache unless machines are essentially equipped with special vacuum system to handle dust.
Copper is the most popular material in India because it is cheap and can be machined on a simple machine like Lathe. Copper electrode manufacturing on wire EDM is the most popular and faster method whereas Graphite on wire EDM is very slow and troublesome, hence mostly avoided.
From the EDM process point of view, Graphite is thermally very stable material and hence, has very low electrode wear, where as Copper has got comparatively high wear.
Even though, the EDM machine should produce replica of electrode used, electrode wear does not allow it to happen easily and affect the accuracy and finish of the job.
1. Use of multiple electrodes e.g. roughing and finishing electrode.
2. Dressing of electrode during EDM process using auto electric discharge dressing.
3. Change of electrode during EDM process.
4. Use of 2D/3D Orbit cycles to compensate for electrode wear.
5. Use of adaptive control to precisely monitor EDM process to reduce electrode wear. These various features and strategies are used to accurately achieve the dimension and finish on the die or mould.
Innovations in CNC technology has also found its way into EDM too, EDM being a very slow process, speed of CNC does not play any significant role in improving productivity.
Enhances EDM process by generation of few-canned cycle, in EDM terms,
which is called as ORBIT CYCLE.
Uses Adaptive EDM process control by continuously monitoring the gap
condition and modulate EDM pulse.
Simplifies the manufacture of electrode by using multiple segmented
electrodes and use of ATC (Automatic Tool Changer).
Automates manual interruptions like change of electrodes by use of tool
magazine and therefore change of EDM parameters by using automatic
selection of predefined EDM codes.
Provides Powerful user interface, which enables user to define only the
Performance &, output and the machine automatically select the steps of
EDMing and its EDM parameters.
6. Equips machine with fuzzy logic based adaptive control
to significantly improve the EDM process, thereby the productivity.
Direct coupled AC brushless Motors are also used by few manufacturers to achieve faster RAM speeds. This enhance the EDM performance during deep cavity machining, where very fast retraction of the electrode for a short duration is necessary to remove EDM debris, in the cavity, hence, avoid damage of electrode / cavity due to electrical arcing. Few manufacturers use Linear motors to achieve higher RAM speeds.
EDM being a slow process, EDMing a bigger dies and moulds is time consuming. Alternatively, high speed milling machines, popularly known as HSM have come-up in the market place which can machine directly hardened die plates.
The theory of hard milling is basically the same as conventional milling. Hard milling is done on a specially designed machine tool & cut is taken at a very high RPM (15000 to 40000). In such cases, majority of material removal of typical die / mould plate is accomplished by high-speed machine and only intricate cavities are machined by EDM process. This scheme doesn’t become economical unless dies / moulds are of big size and repetitive in nature.
Both the processes, EDM & HSM have certain limitations & now a days they compliment each other in Indian job shops & tool rooms.
EDM process has been refined with a flexibility, repeatability and accuracy that have elevated EDMing from specialty technique into a place of prominence as preferred methods for many different types of jobs.
Basic concept of material removal is same as of a sink EDM; however, a Wire EDM machine by its design is quite different. For example, all WireEDM machines are CNC where as Sink-EDM may or may not be a CNC machine.
These machines have been commercially available in India in early 80’s. Development in the field of electronics and CNC has made a significant impact on Wire EDM machines, to say, it has now enhanced the performance 6-8 times, compared to earlier version of machines.
Unlike Sink EDM, electrode in case of Wire EDM is a simple, fixed diameter wire, so the complexity of design and manufacturing of electrode simply goes away.
Efficient and high performance machining of hardened parts mainly depends upon various peripheral factors apart from the machine itself. These are as important to be taken care as the machine itself. They are
Wire electrode material.
Work piece metallurgy and preparation.
Clean water dielectric.
Clean & controlled environment.
1. Wire Electrode Material :
EDM Wire has also evolved with machine developments to meet the changing demands of machine technology and new applications.
First ever wire used by WEDM was copper wire. Soon brass wire came into existence which is essentially is alloy of copper and Zinc. It is an established fact that higher the percentage of Zinc, better the cutting speed and less wire breakage (We will not go into theoretical aspect of this.) It also improves performance in terms of better surface finish and integrity of cut parts especially carbide and PCD – poly crystalline diamond.
Due to the manufacturing process limitations, pure zinc wire cannot be drawn / manufactured. Existing manufacturing process prohibit reliable and economic drawing of wire if zinc content is more than 40%. Coated wire came into existence with coating of 100% pure zinc but due to process limitations coating thickness can be only few microns.
In coated wire Zinc gets consumed quickly due to its lower vaporizing temp., hence its effective use is limited up to maximum 80mm thick Work piece. Diffused annealed wire with a coating thickness of 30-40 micron and heat treated by a process called as diffused annealing. It contains 45% Zinc gives optimum speed and many manufacturers including Electronica’s technology is based on such type of wire.
Researchers are working on technologies to improve upon Zinc content. Five years ago, Dr. Dan Tomalin discovered a breakthrough process called Gamma phased wire, which allows 65% Zinc to be retained cutting speed increases by 10-50% in many cases.
Now we have options of using almost twenty different types of wire to achieve greater productivity. Brass, coated brass, diffused brass, steel cored brass, hard / soft brass molybdenum wires are available to name a few in the world.
As the wire diameter is only 0.1 to 0.3 mm, higher current densities are not possible. So current pulses with very small duration, down up to 1.5 to 2 mSec. with high peak currents up to 500A or more are required to be discharged uniformly. Due to the innovations in the field of electronics, fast switching power devices like MOSFET have made it possible to switch short duration high peak currents. Higher the number of pulse discharge per unit time, higher the material removal, which is called as cutting rate, normally specified as mm2/min.
In last 20 years, cutting speed has gone up from 30 mm2/min. to 300 mm2/min., almost 10 times.
WORK PIECE METALLURGY
Normally, all machine manufacturers show cutting speed and finish on a particular grade of hardened steel e.g. Electronica Machine indicate cutting speed and surface finish on HcHc steel in the brochure.
Cutting speed and accuracies differ for various materials due to variation in the behavioral pattern of metal during EDMing.
This is not very commonly used in Indian Tool Room but it is an important metal in Aerospace and racing car industries worldwide. Because of its natural properties, it finds its use in the human body for replacement of various joints and tooth implants.
WEDM cuts titanium approx. at 90 % to 100% of the tool steel speed, depending upon the wire used. Due to water as a dielectric, oxidation process makes the cut surface blue while cutting Titanium.
Special generators with anti electrolysis circuits minimize such effects.
Use of copper is very large in many industries mainly in jewellery, electrical wires, coins, tubes and electrodes for Pulse EDM etc. Wire cut EDM cuts it approx. at 80-85% of tool steel speed.
This metal is commercially available in various alloys. Al. has lower melting temperature; hence it cuts approx 250-300% faster than steel. Faster speeds and rougher finish are the results while EDMing Aluminum.
It is extremely hard material, mixed with cobalt as a binder material and then sintered. Cobalt is highly conductive, and gives strength to carbide. While WEDming, cobalt melts first, leaving loosely bound carbide. This exposes carbide particle and due to water as a dielectric, electrolysis and oxidations will further leach away exposed carbide. This is a common phenomenon on all WEDM machines and is known as cobalt loss which essentially degrades the tool life.
Technology guidelines are given by manufacturer to minimize this effect.
CLEAN WATER DIELECTRIC
Water as a dielectric plays a vital role in WEDM in achieving optimum performance.
Essential characteristics of water dielectric are :-
1. It should be ‘Mechanically CLEAN’ i.e. dissolved particle size should be filtered to micron level. Paper cartridge filter or Mineral bed filtration systems are mostly offered.
2. It should be ‘Thermally CLEAN’. Water heats up due to EDM process and water pumps, circulating water to run the EDM process. Water Chillers comes as a standard accessory to maintain dielectric temperature during metal cutting.
3. It should be ‘Chemically CLEAN’. Deionization of water occurs during cutting and conductivity of water increases. Deionizer mixed bed resins are offered along with automatic conductivity controllers to maintain the conductivity to a recommended level.
CLEAN AND CONTROLLED ENVIRONMENT
Environment in which machine is kept should be maintained at a uniform temperature. Normally, it should be like a tool room environment maintained between 20 to 22 ? C.
Temperature changes during cutting can cause –
– Parts to bend or twist
– Oil deform
– Elongations due to temperature coefficient, this ultimately deteriorates the part quality.
In India, due to extremities of summer heat, many problems related to part accuracy surfaces.
It is also found that cutting speed of the WEDM drops by more than 15% for temperature rise of 10%.
Limitations of WEDM & their remedies.
Bow & Taper Effect & its remedy.
WEDM machines are designed to generate a taper of required degrees by having an auxiliary axis parallel to main XY axis. But the WireEDM process itself generates certain Bow / Taper effect which affect accuracies on job. Each manufacturer provides certain guidelines to overcome such deficiencies by applying trim cuts, or skim cut or multiple cut after first cut. Accuracy of low energy skim cuts depend mainly upon three parameters like precise pulse energy control, vibration free wire electrode travelling mechanism & perfect water jet control.
Many manufacturers have gone up to 5 Skim cuts technology with their innovative control of above three parameters.
CORNER CONTROL STRATEGY:
Due to high mechanical & electromagnetic forces acting on wire in the spark gap, during wire cutting, wire tends to deviate from its programmed path, thus creating damages to the sharp corners, or corners with very small corner radius on the job. Each manufacturer provides in-built innovative fuzzy logic control strategy to retain the corner shape during the wire cutting.
In these types of machines, Wire cutting operation is done in a water bath. Under difficult machining conditions of poor flushing like cutting a high degree taper, Extrusion die, stacked work pieces, submerged machining is always preferred. Most of the manufacturer now offers submerged machines.
Heat effect on metal surface- surface integrity:
During EDM process the surface of the material is affected by many phenomenons which directly modify the structure of the parent material. It results in to the affected surface that does not remain integral to the parent material.
Material defects are mainly due to various processes that are triggered from the start of the initial strike of EDM spark.
Due to thermal process.
Due to chemical process.
Due to electrolysis & corrosion process.
Damage due to thermal process-
With the initial strike of the spark on the metal surface, material is melted & crater is formed. Molten metal is drawn back in to the spark crater due to surface tension.
This material refreezes on the cut surface producing a layer which is quite different in properties than that of parent material. This layer is called as ‘white layer’ or ‘recast layer’.
Immediately below the white layer, is an area called as “HAZ” ‘Heat affected Zone’.
This re hardened layer is formed due to temperature that has risen above the hardened temperature, but not melted the surface. This layer is hard & brittle.
Damage due to Chemical effect-
EDM surface undergoes a chemical contamination from the electrode & dielectric. From a metallurgical view point carbon is the most important contaminant. Small amount of carbon has a profound effect on behavior of steel. In WEDM machines, chemistry of contaminated ion particles is more or less neutralized by the Deion process. But in RAM EDM no such mechanism exists , hence it is always desirable to change oil dielectric periodically.
Damage due to Electrolysis effect-
Process of WEDM uses water as a dielectric, work piece as anode & wire as cathode. Operating gap voltage gives rise to a reaction. Where water molecules are reduced at cathode & oxidized at anode. Hence more or less anode i.e. work piece is consumed depending upon the type of metal used. This causes surface deterioration of work piece.
Due to EDM process, various layers like white layer, re-hardened layer, Tempered layer is formed over the parent material.
Today’s high speed switching, short duration, high peak current pulse technology do provide a finer control to minimize such effects, but thermal nature of EDM process itself makes it impossible to eliminate such defects entirely. Affected zone thickness may vary up to few microns, depending upon Roughing / Finishing parameter settings.
Skim cuts at lower current can help improving the surface.
Reduced mechanical properties of metal located in affected layer contributed to shortened tool life.
Effect on surface integrity for a die / punch made in hard metal will render surface profile, hence; affect its resistance to wear.
New Trends in EDM Application
EDM IN MOLDING INDUSTRY
Vertical EDM has been a mainstay in mold making operation for decades. In last few years, enhanced WEDM performance saw its usage in Mold making industry too.
Mold making is an age old craft that, in theory, has not changed since ages.
A part is replicated by powering or forcing a molten material in a pre-shaped impression or cavity. Better cavity accuracy and finish of mold will result in better replication of part dimensions and longer life of mould. This is mostly needed for plastic parts which are normally molded as a finished part.
Areas of application of WEDM in Mold making:
1. Electrode making:
This is the first ever use of WEDM in mould making. Complex copper electrode, multiple Accurate and identical electrodes with no burr, this walled electrodes which were very difficult uneconomical and time consuming.
2. Mold Base Work:
Accurately insert pocket in the holder plate of the mold base larger and thicker the mold base larger is the saving. Square inserts can be cut easily without corner radius.
3. Mold Inserts:
Because cut without sectioning and also very fast without accumulated in accuracies due to stacking.
Complicated geometry of the involute gear or spline is virtually impossible to generate on conventional tool room machines. Wirecut has no limitations on pressure angle tooth width, and other involute gear characteristics. Any involute gear cavity can be cut at any shrinkage factor easily.
4. Extrusion die plate:
Extrusion die plate needs essentially a large draft angle which can be cut easily by Wirecut.
5. Watch Gear Molds:
Plastic watch gear is impossible to make due to small size of teeth. With use of 50-70 micron wire, cutting of this part is very easy.
Connector mold inserts require multiple close tolerance laminations, it is quite labor intensive and very expensive to grind and assemble. All this is avoided by using WEDM.
7. Core Pin and Ejector Pin hole
Wire EDM can effectively used to cut all through holes in cavity inserts after hardening. Holes will be straight to the size and will have dead sharp edges at the cavity, resulting in no possibility of flush.
8. Reduce or eliminate polishing
Mold polishing in India is labor intensive hard work, and can be avoided by wire EDM which are now capable of providing down up to 1 µ Rmax finish.
Micro EDM Machining
Micromachining is a basic technology used for the production of miniaturized parts and components.
Until recently, focus has been on production of microelectronic components using etching and other photo fabrication technique. Currently, with the tremendous need arising for the development of micro machines, micromachining using machine tool technology is getting very popular.
Micro-Machining is the generic term for very small machines from millimeter to sub millimeter size like micro-robots, capsules inserted into human body for medical treatment, micro-motor, micro sensors etc. Micromachining using machine tools has the advantage of machining three dimensional shapes with ease.
Over the past few years, there has been an increased demand is Micro machining technology from various industries like
Micro machining techniques generate parts even less than 100 microns just thicker than human hair – human hair is 75 micron.
Such micro machining techniques are used to manufacture parts like
Tooling’s for IC’s
Sub miniature – actuators and motors
Productivity Aspects & Automation
Today’s Job shops strive for achieving higher level of manufacturing efficiency & productivity.
Automation can be a solution, but automation is not viable for the sake of automating, there are many time wasters, which if minimized can give opportunity for competitive edge.
Typical EDM job shop mainly work in four well defined stages.
Workpiece preparation and part programming
Job setting / Alignment / loading / Unloading
While WEDM machine is being run, many factors of human intentions come into play like –
Wire threading / spool changing.
In all above aspects of manufacturing only job cutting yields revenue.
Even though all other aspects are important to run the shop, they are time wasters and one can think of minimizing them.
Let us view this from the point of view of “Lean Job Shop”, in which automation can be means to improve productivity. Lean manufacturing, a shorthand commitment to eliminating waste, simplifying procedures and speeding up production, can be one such concept.
Modern WEDM machine manufacturer and accessories manufacturer have worked towards this to support the user by bringing in many features and accessory to minimize human intervention and thereby reduce time wastages and improve available cutting time. Let us review them.
Job Preparation and programming: It includes planning of cutting strategies like start hole, job clamping margins, rough / skim cuts etc. Machine manufacturers provide guidelines and hands on technology recipes, which can get executed without intervention, thereby saving time.
Tooling: Sufficient amount of time can be saved by using Tooling available now a days of
Single minute exchange of die (SMED)
Single turn clamping of die
And use of single command align function of machine.