EDM video

Document Sample
EDM video Powered By Docstoc
					      Electrical discharge machining or EDM is a thermal erosion

process in which the work piece material is removed through a series of

rapidly reoccurring electrical discharge between electrode (cutting tool)

and an electrical conductive work piece in a presence of a dielectric fluid.

There is a voltage gap between the electrode and the work piece that

form the spark vaporize minute particles of the work piece material,

which is then washed out from the gap by the continuously flushing

dielectric fluid. The two main types of the EDM are Ram and Wire

cutting. EDM is a diverse process that produces products ranging from

tiny electric connectors, medical parts, and automatic stamping dies and

aircraft body panels. EDM has replaced much of the machining, grinding

steps needed in die making which represents the largest single use of

EDM. Die component cut with EDM can often be made in a single piece

no matter how complex the internal form. The single piece die are

always stronger than those made of segments.

Die cut are from hardened steel, heat-treated steel, carbide. Other

materials that can be EDMed are polycrystalline diamond, titanium, hot

rolled and cold rolled steel, copper, brass and high temperature alloys.

      The process of EDM has many benefits since the work piece and

electrode never touch, no cutting forces are generated this makes EDM

will suited to produce fragile parts that can not take the stress of

conventional machining, burr free edges, intricate details, superior
surfaces. EDM process eliminates the risk of damaging expensive work

pieces, allows heat treatment before final machining. Final EDM tools

with build in process knowledge helps reduce the training workers need

to produce complex parts.

      The EDM process also has its limitations. The metal removal rates

are low compared to conventional metal cutting processes. Complex

materials require lead-time for fabrication and are consumable while

cutting and the work piece materials must be conductive.

      The basic components of the electrical discharge machinery are

relatively simple. The electrode is attached to the RAM, which is

connected to one pole of the electric power supply. The work piece is

connected to the other power supply. The work piece is then positioned

so that there is a small piece between work piece and electrode. This gap

is flooded with dielectric fluid which acts as insulator until the power is

turned out. Once on, the machine delivers thousands of electric pulses per

second to the gap and erosion begins. The sparks are generated at on time

but it ranges from 100s up to 1000s of time per second. Each spark has a

temperature of 1400 up to 21000 degrees Fahrenheit. As erosion

continues the machine control advances the electrode through the

material and is containing a constant gap distance. To understand how

EDM removes the material lets examine a single spark in the erosion

process.
      As a pulse of DC Electricity reaches the electrode and part an

intense electric field develops in the gap, microscopic contaminates

suspended in the dielectric fluid are attracted by the field and

concentrated in the field’s strongest point. These contaminates builds a

high conductivity along the gap as the field’s voltage increases. These

materials in the conductive bridge heat up. Some pieces ionize to form a

spark between the electrode and the work piece. At this point the both the

temperature and pressure in the channel increase generating a spark. A

small amount of the materaial melts and vaporizes from the material and

the work piece at the point of spark contact. A bubble composed of

gaseous by products vaporize rapidly expands rapidly outward from the

spark generated. Once the pulse ends the spark and the heating action

stop collapsing the spark channel. Dielectric fluid then rushes into the gap

forcing melted material out from the surfaces. This EDM residue consists

of solidified parts of material and gas bubbles.

      The resulting EDM cut can have several observable surface layers.

The top surface is created when expelled molten material and small

amount of electrode material melt forms spheres on part of the surfaces,

this layer is easily removed. The next layer is a recast or white layer as

EDMing has altered the piece metallurgical structure. This layer can be

reduced by using the right settings or by polishing the part. The third

layer is the heat affected zone or annealed layer. It has been only heated
not melted. The part surface is a function of EDM cycles that has on time

and off time expressed in microseconds. All work occurs during the on

time so pulse duration and the number of cycles per second. The sum of

on time and off time are important. Metal removal is proportion to the

amount of energy during on time that energy is controlled by the time

variables. The peak Amperage or intensity of the spark and the length of

the on time. The larger the on time the more the metal erodes. May also

produce a larger recast layer and a deeper heat affected zone. Off time

also affects speed and stability of the EDM cut. Too shorten off time, the

ejected material will not flood easily with the dielectric fluid. The next

spark may then be not stable. The duty cycle is the percentage of on time

relative to the total cycle time. The higher the duty cycle means increased

cutting efficiency. Gap distance between work piece material and the

electrode also impact the material removal rate. Generally the smaller the

gap the better the accuracy and the surface finish are and the metal

removal rate is. The EDMing cavity is always larger than the electrode

with the difference called the Kerf. The average off time and on time

increases also the over cut. It is important to know the expected over cut

during EDMing because the electrode can probably be under sized.

RAM EDMing

      In Ram EDMing machines the work piece must be inside a tank

and is covered with Dielectric fluid. The electrode is lowered to a
distance of few thousands of an inch from the work piece. To begin

EDMing. Ram types EDMing have the ability to produce complex

cavities out of a solid piece of metal. Ram EDMing machines are also

referred to as die sinkers or vertical EDMs and range in sizes and

automation from manual operating table top systems to large CNC ones.

A ram EDM a number of main subsystems:

-Power supply

-Dielectric fluid

-Electrode

-Servo system.

The power supply provides series of DC current Electric discharges

between the electrode and the work piece. It also controls:

-Pulse voltage

-Current

-Pulse frequency

-Electrode polarity

      There is a direct relationship between the voltage and the gap. The

servo system must sense the voltage between the electrode and the work

piece. This signal controls the servo system and maintains a constant gap

distance between the electrode and the work piece through out the EDM

cycles. Most power supplies provide a cut off or fault protection system
to stop power flow to the system if a short circuit between the electrode

and the work piece occurs.

      To successful EDMing flushing is very essential, where the EDM

dielectric system introduces clean dielectric to the EDM cutting zone

flushing away the EDM debris and cools the work piece and electrode.

Popular dielectric fluids for EDM systems are hydrocarbon and silicon

based oil.

      The dielectric fluid is pumped through nozzles through the

electrode or through the work piece or some combination of them to

continuously flush the work area. Flushing requires careful consideration

because of the high forces involved forcing the fluid through small

passageways. Many low-pressure holes are preferred to many high-

pressure holes.

      The shape of the work shape of the electrode or cutting tool is a

negative or reverse of the cavity generated on the work piece. The ram

EDM electrical polarity is usually has a positive charge and the work

piece polarity is negative. Although the metal removal is smaller than if

the polarity is reversed. A positive electrode polarity protects the

electrode from excessive wear and preserves its dimensional accuracy. So

EDMing requires multiple electrodes that perform roughing then

finishing operations. Copper and graphite are the most common electrode

materials. Whatever the electrode material is used it should combine high
strength and high melting point. To help the electrode resist the erosion of

the corners where electric field is concentrated. Making the electrode is

an important step in ram EDM. The electrodes are shaped on all types of

machines. Since the graphite material turns to dust, the machine must be

equipped with a system of dust control and evacuation.

      The servo system controls the in feed of the electrode to precisely

determine the rate of material removal. If the gap voltage system

determine that there is a residue reducing the gap between the electrode

and the work piece. The servo system will reverse the feed until

contamination is flushed away. The greatest benefit of the CNC ram

EDM machine comes from the capability to change tool automatically

boosting unattended time. The tool changer holds multiple electrodes and

the part program automatically change them. This allows multiple

cavities of a variety of work piece to be EDMed. Orbiting is a pattern

movement between electrode and the work piece increasing the EDM

capabilities not achieved by straight ram sinking. Orbiting the electrode

distributes wear eventually so that corners do not wear as fast. Less

cutting is done by the bottom and the bottom corners of the electrode and

more with the sides. Orbiting is commonly accomplished on a non-CNC

machine by using an orbiting device on the machine. Implying a motion

on a standard or electrical electrode can be used without a shape or

machined electrode to be of the shape of the work piece.
Wire EDMing.

CNC wire cut EDM machines use a traveling wire electrode to cut a

complex outline of fine details of stamping and blanking dies of too

harden steel. The wire die system continuously delivers fresh wire under

constant tension to the work area guided by a set of guides or diamond

wire guide. New wire is always exposed to the part so electrode wear is

not a problem like in ram EdMing. Wire EDM machines are also easier to

learn than ram EDM. They can run un attended for long periods including

overnight and weekends. There are four basic wire EDM subsystems, all

of which are CNC controlled. These subsystems include:

   - Power supply

   - Dielectric system

   - Wire feeding system

   - Positioning system.

The wire EDM power supply principles are similar to ram EDM. It

provides the series of DC electric discharges between the wire electrode

and the work piece. As with ram EDM flushing is critical. The wire EDM

dielectric system uses deionized water, which can remove heat more

efficiently than conventional than conventional dielectric systems and

produce extremely thin recast layer. Flushing is done along the wire with

flushing nozzles. In some cases the work piece and wire are completely

submerged under water. The downsize of submerged EDMing is that the
deionized water can cause corrosion to the work piece additionally

machines with tanks are more expensive and cycle time are increased by

fill and drain time.

      The positioning system use a CNC two axis table and provide a

variety of multi axis wire positioning capabilities depending on the cuts

that must be made. A two-axis wire EDM can make only right angle cuts

the worktable. An independent 4-axis machine can create tapered angles

even with a top profile with a different shape than the bottom profile.

This is necessary for producing extrusion dies and flow valves. As in ram

EDM the electrode never touches the work while cutting. The servo

system maintains at least thousands of an inch gap between wire and

work piece. During spark erosion each wire produces a kerf or over cut in

the work surface that is slightly larger than the wire diameter. As a 0.012

diameter wire for example can create 0.015-diameter kerf. The wire cuts

along a programmed path. The wire starts to cut from the side of the work

piece or through drilled holes made by EDM drilling machines made for

that purpose. Wire EDM machines can also process parts that are stacked,

cuts perhaps so thin parts in one path. Under these conditions flushing

can be problematic.

      The third type of EDM involves drilling small deep holes and slots

of round and regular shapes. These machines use long rounded

electrodes. These electrodes rotate with a moderate speed about 100
revolutions per minute. During drilling this rotation aids in flushing the

cut and tends to create uniform electrode wear. These machines can drill

very minute holes. This process can obtain length to diameter rotation of

30:1 or better

      Fire is the worst with ram EDMing. If the dielectric falls bellow the

level of which sparking occurs. When this happens the dielectric can be

heated above its flushing point and sparks can ignite it. Machines should

turn of automatically when the dielectric drops below a safe level. Smoke

from the process can be irritant. Smoke should be properly ventilated or

smoke-consuming units can be attached to the machines. Finally EDM

can produce hand skin irritation which is remedy with hand creams.

				
DOCUMENT INFO
Shared By:
Categories:
Stats:
views:172
posted:12/1/2010
language:English
pages:11