EDM is a recent machining technique used to cut metals which can’t be cut with traditional methods. EDM works with materials of electrical conductivity.
Two primary EDM methods are
- Ram EDM
- Wire EDM
Between the two methods the main difference is based on the electrodes. In ram EDM graphite electrodes are used, which are machined in traditional tools and with a special shape it is connected to the power source. The electrodes are connected to a ram as well. When the arrangements are done the electrodes are fed into the work piece. The entire process is carried out under a submerged fluid bath. In Wire EDM the electrode is a thin wire. Here processed brass wires are made use of for putting into the material. Wire EDM is done while submerged in a bath of water.
Ram Electro-Discharge Machining (EDM) Method:
Rapid spark produced between the two electrodes control the metal erosion. An appropriate gap is necessary known as spark gap.
The rate of metal removal is related to the spark gap. If both the electrodes are of same materials then the highest erosion takes place on the positive electrode (anode). The two electrodes are kept apart and distanced by a dielectric fluid. A transient electric discharge is created across the gap in the form of spark. With a sufficient potential difference between the electrodes, the dielectric fluid is ionized and broken down to produce a conductive spark channel. The current gets discharged across the channel as spark. If potential difference is lesser than or equal to 12 volts the dielectric fluid is deionised. The process repeats by its own as the condensers begins to recharge.
The spark release energy in the form of local heat and a local temperature found is in the order of 12000°C. Such high temperature and pressure melts away some metals and little of which is vaporized and the rest fine material particles get carried away by the fluid circulating around the electrodes which create a crater on the w/p.
How to make Tool Material choice?
Many factors are considered while selecting the tool material.
- Lower erosion rate and decent work to tool wear ratio
- electrical conductivity
- good machine ability
- low electrical resistance
- high melting point
- high rate of electron emission
EDM has one drawback and it is wear ratio of the tool.
While applying EDM the tool or cathode erodes as well and this is not good. It is unavoidable but limited as the wear of the cathode is lesser than the anode. This occurs due to the following:
- Cations from the dielectric fluid strike the cathode but electrons hit the anode. Though electrons are lighter than the cations it has more energy and moves with greater velocity. So anode gets eroded.
- During spark a compressive force is created at the cathode which lowers the erosion at cathode.
- Fluid medium is made of hydrocarbon. Pyrolysis yield gases which produce carbon particle and they create a thin layer of protection on the cathode. Hence the cathode is safer than anode.
Purpose of Dielectric Fluid
- Effective Coolant for the work piece and the tool.
- It works like an insulating material.
- The eroded materials are carried by the medium.
- It is a coolant to quench the spark and prevent an arc.
What you need for dielectric fluid?
- Optimum Viscosity is necessary.
- It must not react with the work piece, container or tool material.
- Non-expensive, easy availability and inflammable.
- It must not produce toxic gases or vapours during operation.
- It must be a hydrocarbon.
The various dielectric fluids are: transformer oil, oil, spirit (white), and kerosene etc
Advantages of EDM
- Any hard or brittle metal which is tough as well can be machined.
- Harder materials like steel alloys or tungsten carbides and other non-conventional machining like forging and press tools can be made.
- Complicated shapes could be yielded.
- Finer holes can be accurately.
- Wear resistance surface could be made because work pieces produced with EDM have micro-craters which has lubricants.
- Harder metals can be machined faster compared to their conventional process.
- The power needed is quite higher compared to the traditional machining. There are chances of surface cracking when the materials become brittle at room temperature.
- A thin layer ranging from 0.01 mm to 0,10 mm that has 4 % carbon might get deposited on the work pieces made of steel
- A lower Material Removal Rate in comparison.
- Can’t yield sharp corners.
Applications of EDM
- Used for machining to give intricate shapes and narrow slots and blind cavities.
- To generate negative tool geometry
- EDM is quite useful for making small holes.
- It is used in air craft engines and brake valves and etc.
Wire Electrical Discharge Machining
It cuts the work piece with electrical discharge. Here either the work piece or the wire moves. The spark discharge phenomenon which is used to erode the metal same as the traditional EDM. Here, the wire acts as an electrode hence complicated shapes could be cut easily without electrode formation. Another important part is the distilled water supplier with a constant specific resistance.
Features of Wire EDM:
- Electrode formation is not necessary.
- Electrode wears is negligible.
- Smooth machine surface.
- Tight dimensional and geometrical tolerances.
- Straight holes can be produced.
- Machine could be operated without regular supervision for a long time at higher operating rates.
- You don’t need any special skill to run the machine.
- Due to the absence of the split lines, savings of the stages in the tools occurs. It allows more punch opening in every stage.
- No flashes on the moulded parts.
- Necessary for tool manufacturing and storing.
- Work pieces are hardened before cutting.
- Whole work can be done in a machine.
- Very economical.
- Number of rejected work pieces is quite small.
With this I conclude. To know more, keep visiting this space.
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