23-06-2012, 06:06 PM
ELECTRICAL DISCHARGE MACHINING
EDM PPT2003.ppt (Size: 496.5 KB / Downloads: 73)
POUR TEMPERATURE
Pour point is an indicator of the ability of the fluid to flow freely at cold temperatures, and the temperature given is the lowest at which the fluid will flow. Many dielectric fluids typically become cloudy and more viscous as the temperature approaches their pour point.
DIELECTRIC STRENGTH
Dielectric strength is a measure of the insulating capacity of a given fluid in an EDM environment. Higher dielectric strength helps minimize DC arcing and is frequently touted as an indicator of overall EDM performance
OXIDATIONAL STABILITY
Oxidation occurs when oxygen attacks and degrades EDM fluids. The process is accelerated by heat, light, and metal catalysts; and the presence of water, acids and solid contaminants. The higher oxidation stability your EDM fluid has, the longer it will last in your system.
ELECTRODE FEED CONTROL
An electrical discharge machining device or EDM machine having an electrode movable toward and away from a workpiece by a hydraulically operated double acting cylinder. A control valve, combined with a servo solenoid and manually operated lever, is operable to control the flow of hydraulic fluid to the cylinder. A light responsive electrical control operates the servo solenoid in response to the size of the electrical discharge gap between the electrode and the workpiece.
The manually controlled lever is operable to engage switches to override the lock valve control during the electrical machining and elevation of the electrode operations of the device. Hydraulic pressure is continuously supplied to the control valve from a pair of liquid storing chambers connected to the control valve with a switching valve. Fluid under pressure in either one of the chambers operates the switching valve to provide a continuous source of hydraulic fluid under pressure to the control valve
EDM tool design
The EDM system consists of a shaped tool or wire electrode, and the part. The part is connected to a power supply. Sometimes to create a potential difference between the work piece and tool, the work piece is immersed in a dielectric (electrically nonconducting) fluid which is circulated to flush away debris.
The cutting pattern is usually CNC controlled. Many EDM machine electrodes can rotate about two-three axis allowing for cutting of internal cavities. This makes EDM a highly capable manufacturing process.
EDM comes in two basic types: wire and probe (die sinker). Wire EDM is used primarily for shapes cut shapes through a selected part or assembly. With a wire EDM machine, if a cutout needs to be created, an initial hole must first be drilled in the material, then the wire can be fed through the hole to complete the machining. Sinker (diesinking) EDMs are generally used for complex geometries where the EDM machine uses a machined graphite or copper electrode to erode the desired shape into the part or assembly. Sinker EDM can cut a hole into the part without having a hole pre-drilled for the electrode.
Design Considerations
Relax the surface-finish for the part, if feasible. This allows the manufacturer to produce the part with fewer passes, at a higher current level and a higher metal-removal rate.
Design or prepare the part such that the amount of stock removed by EDM is relatively small. Use traditional machining techniques to remove the bulk of the stock with the finishing operations performed by EDM. This significantly reduces the amount of time and cost for each part.
The EDM manufacturer should consider fixtures such that several parts can be stacked and machined simultaneously or a single part can have several EDM operations performed simultaneously.
When existing holes are to be enlarged or reshaped by EDM, through holes are preferred to blind holes as they permit easier flow of dielectric fluid past the area being machined
Dimensional Accuracy (+/- 0.0005 inches per inch)Feature Profile accuracy of .0003 is obtainable with cutting pathFeatures to feature true position of .002 is reasonable and down to .001 is possible when geometry requires removal and reattachment of wire.Surface Finish (microinches) Features created by EDM have an "orange peal" appearance.16 Ra is achievable, 64 or higher Ra is typical and less expensive.
Wall ThicknessMin Wall Thickness (inches): 0.01 (over a 5" inch span)
As always, consult with EDM vendor for specific capabilities.
EDM electrode tube 1. Single hole tube(brass/copper) 2. Multi-hole tube(brass/copper) 3. Precision EDM rod 4. Ceramic guide
EDM Electrode tubes
1. Single hole tube (brass / copper)
- Outer diameter range : 0.10 ~ 6.00 mm
- Tube length : 300 / 400 mm (12/16) standard (Other length is available up to request)
- Thin or thick all tube is avaible
- All inch outer diameter size is available up to request
2. Multi-hole tube (brass/copper)
1) Specification
- Outer diameter range : 0.5 ~ 6.00 mm
- Tube length : 300/400 mm (12/16) standard (Other length is available up to request)
- Multi-hole tubes have various section shape, leave no core inside tube hole and
accordingly reduce drilling time at deep hole
- Inch outer diameter size is available up to request
2) Types of multi-hole tube (coreless)
- 2 channel normal type (2 wires inserted)
- 2 channel normal type (2 tubes inserted)
- 1 web type (1 bar inserted)
- 3 channel normal type (3 tubes inserted)
- 3 channel Y type (3 tubes inserted)
- 4 channel + type (4 tubes inserted)
3. Precision EDM rod (brass/copper)
- Outer diameter range : 0.25 ~ 3.00 mm
- Tube length : 200/300/400 mm (8/12/16) standard (Any other length is available up to request).
FLUSHING
Flushing is used in EDM to remove the eroded particle from the gap for efficient cutting. There are two level of flushing setting, for low level flushing is not used and at high level flushing is used. Electrode dimension for low level is 10 mm and high level is 30 mm. the factor for depth of cut is 10 mm at low and 20 mm at high level. The dielectric fluid is used kerosene. The effects of jet flushing was analyzed and discussed
TYPES OF FLUSHING
Pressure flushing
Pressure flushing is also called injection flushing , it is most common method of flushing. One great advantage of pressure flushing is that the operator can visually see the amount of oil that is being used for 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.
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.
There are two types of pressure flushing
they are:-
Pressure flushing through electrode.
2) Pressure flushing through workpiece.
Suction flushingIn 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.
PROCESS CHARACTERISTICS
material removal rate (MRR).
Recent researches indicated that adding powder in EDM
process will enhance MRR, therefore improving the surface
roughness, corrosion resistance and wear resistance. Additives can improve the surface quality of workpiece quite effectively by increasing the material
removal rate (MRR) and decreasing the tool wear rate
especially in mid-finish machining and finish machining
Other than that, we can also use oil as the
dielectric fluids which will affect the tool electrode wear
where it depends on the significance of the pulse duration
Recently green manufacturing has become very
important to all manufacturing industries, thus the use of pure
water has been proved to have more good effect on the
workpiece since water has a high thermal conductivity, a low
viscosity coefficient, and a high flowing rate and pure water
will not be affected by a long working time. Thus a constant
high material removal rate will be obtained .
Tool removal rate
The ratio of amount of electrode to the amount of
workpiece removal is defined as the wear ratio
There are four methods that are
known to evaluate the electrode wear ratio by means of
measuring weight, shape, length, and total volume
respectively. A common one is by calculating the volumetric
wear ratio (υ). Usually we will measure the weight differences
and transfer them into the volumes by the density of materials.
However this method is unsuitable for micro-EDM because
the weight change is so small making it difficult to measure it
accurately.
(d) Effects of the process parameters on the surface finish
There are also a number of published works that solely study the effects of the machining parameters on the WEDMed surface.That the increasing pulse duration, open circuit voltage and wire speed increases with the
surface roughness, whereas the increasing dielectric fluid pressure decreases the surface roughness.