27-09-2012, 01:42 PM
WIRE EDM FUNDAMENTALS
[attachment=34606]
HISTORY
The beginning of EDM came during the Second World War, when two Russian
physicists B.R. and N.I. Lazarenko published their study on The Inversion of the Electric
Discharge Wear Effect. which related to the application to manufacturing technology of the
capacity of electrical discharges, under controlled distribution, to remove metal.
EDM was being used at that time to remove broken taps and drills. The early “Tap-
Busters” disintegrated taps with hand fed electrodes, burning a hole in the center of the tap or
drill, leaving the remaining fragments that could be picked out. This saved workpieces and
very expensive parts from being scrapped and having to be made over again.
This process opened the birth of Vertical EDM, also called: Sinker, Conventional,
Ram, Plunge or Diesinker EDM’s. These machines were, and still are primarily used to
make precision cavities in metal primarily for the mold industry.
WIRE EDM
Wire EDM (Vertical EDM's kid brother), is not the new kid on the block. It was
introduced in the late 1960s', and has revolutionized the tool and die, mold, and
metalworking industries. It is probably the most exciting and diversified machine tool
developed for this industry in the last fifty years, and has numerous advantages to offer.
It can machine anything that is electrically conductive regardless of the hardness,
from relatively common materials such as tool steel, aluminum, copper, and graphite, to
exotic space-age alloys including hastaloy, waspaloy, inconel, titanium, carbide,
polycrystalline diamond compacts and conductive ceramics. The wire does not touch the
workpiece, so there is no physical pressure imparted on the workpiece compared to grinding
wheels and milling cutters. The amount of clamping pressure required to hold small, thin and
fragile parts is minimal, preventing damage or distortion to the workpiece.
The accuracy, surface finish and time required to complete a job is extremely
predictable, making it much easier to quote, EDM leaves a totally random pattern on the
surface as compared to tooling marks left by milling cutters and grinding wheels. The EDM
process leaves no residual burrs on the workpiece, which reduces or eliminates the need for
subsequent finishing operations.
MACHINE
Wire EDM’s are manufactured in various sizes and styles of flush or submerged type
machines to fit the needs of the consumer. Large scale EDM’s can handle workpieces
weighing over ten thousand pounds and can cut over twenty inches thick. Automatic Wire
Threaders (AWT) are usually standard equipment on most models. In addition to the X-Y
table travels, wire EDM’s have U / V travels for providing the movement to cut tapers. Most
machines can cut tapers of 20-30 degrees depending on workpiece thickness
PRINCIPLE OF WIRE ELECTRICAL DISCHARGE MACHINING
The Spark Theory on a wire EDM is basically the same as that of the vertical EDM
process. In wire EDM, the conductive materials are machined with a series of electrical
discharges (sparks) that are produced between an accurately positioned moving wire (the
electrode) and the workpiece. High frequency pulses of alternating or direct current is
discharged from the wire to the workpiece with a very small spark gap through an insulated
dielectric fluid (water).
Many sparks can be observed at one time. This is because actual discharges can occur
more than one hundred thousand times per second, with discharge sparks lasting in the range
of 1/1,000,000 of a second or less. The volume of metal removed during this short period of
spark discharge depends on the desired cutting speed and the surface finish required.
The heat of each electrical spark, estimated at around 15,000° to 21,000° Fahrenheit,
erodes away a tiny bit of material that is vaporized and melted from the workpiece.
(Some of the wire material is also eroded away) These particles (chips) are flushed away
from the cut with a stream of de-ionized water through the top and bottom flushing nozzles.
The water also prevents heat build-up in the workpiece. Without this cooling, thermal
expansion of the part would affect size and positional accuracy. Keep in mind that it is the
ON and OFF time of the spark that is repeated over and over that removes material, not just
the flow of electric current.
COMPUTER NUMERICAL CONTROL (CNC)
Today’s numerical control is produced with the needs of the operator in mind.
Programs, machine coordinates, cutting speeds, graphics and relevant information is
displayed on a color monitor, with easy to use menu’s.
The control unit displays menu’s that are designed to give top priority to operability.
Characters and commands are input using the keyboard. The system is very easy to use,
allowing the operator to quickly become familiar with it, resulting in his/her learning curve
being drastically reduced.
Besides executing NC data for positioning movement of the axes, the control amends
these movements when using offsets, tapering, scaling, rotation, mirror images, or axis
exchange. The control also compensates for any pitch error compensation or backlash error
in the axes drives, to ensure high accuracy positioning. The machine has multiple coordinate
systems, and jobs can be programmed in absolute or incremental modes saving valuable
programming time. For example, multiple jobs can be set-up on the worktable, while storing
the separate reference points or locations of these jobs in specific coordinate registers.
The numerical control offers the capabilities of scaling, mirror imaging, rotation, axis
exchange and assist programs. This enables an operator to produce an entire family of parts
from a single program without the need to edit the program. Mirror imaging is great for left
and right handed parts. Scaling is useful when working with "shrink factors" for plastic
cavities or extrusion dies. Assist programs find the edge of parts, vertically align the wire,
and perform centering routines that are very useful to the operator when setting up jobs.
Power Supply
When wire EDM machines were first introduced in the United States, they were
equipped with power supplies that could achieve less than one square inch per hour.
Today, most machines are rated to cut over twenty square inches per hour and faster.
Faster or slower speeds are obtained depending on the workpiece material, part thickness,
wire diameter, type of wire, nozzle position, flushing condition and required part accuracy. .
Adaptive Control is yet another improvement where high speed circuitry has
improved the spark gap sensitivity, reaction time of the servo motors, and changes to the
power. With these improved capabilities, wire breakage is reduced to a minimum, making
today’s machines far more "forgiving" than in the past.
[attachment=34606]
HISTORY
The beginning of EDM came during the Second World War, when two Russian
physicists B.R. and N.I. Lazarenko published their study on The Inversion of the Electric
Discharge Wear Effect. which related to the application to manufacturing technology of the
capacity of electrical discharges, under controlled distribution, to remove metal.
EDM was being used at that time to remove broken taps and drills. The early “Tap-
Busters” disintegrated taps with hand fed electrodes, burning a hole in the center of the tap or
drill, leaving the remaining fragments that could be picked out. This saved workpieces and
very expensive parts from being scrapped and having to be made over again.
This process opened the birth of Vertical EDM, also called: Sinker, Conventional,
Ram, Plunge or Diesinker EDM’s. These machines were, and still are primarily used to
make precision cavities in metal primarily for the mold industry.
WIRE EDM
Wire EDM (Vertical EDM's kid brother), is not the new kid on the block. It was
introduced in the late 1960s', and has revolutionized the tool and die, mold, and
metalworking industries. It is probably the most exciting and diversified machine tool
developed for this industry in the last fifty years, and has numerous advantages to offer.
It can machine anything that is electrically conductive regardless of the hardness,
from relatively common materials such as tool steel, aluminum, copper, and graphite, to
exotic space-age alloys including hastaloy, waspaloy, inconel, titanium, carbide,
polycrystalline diamond compacts and conductive ceramics. The wire does not touch the
workpiece, so there is no physical pressure imparted on the workpiece compared to grinding
wheels and milling cutters. The amount of clamping pressure required to hold small, thin and
fragile parts is minimal, preventing damage or distortion to the workpiece.
The accuracy, surface finish and time required to complete a job is extremely
predictable, making it much easier to quote, EDM leaves a totally random pattern on the
surface as compared to tooling marks left by milling cutters and grinding wheels. The EDM
process leaves no residual burrs on the workpiece, which reduces or eliminates the need for
subsequent finishing operations.
MACHINE
Wire EDM’s are manufactured in various sizes and styles of flush or submerged type
machines to fit the needs of the consumer. Large scale EDM’s can handle workpieces
weighing over ten thousand pounds and can cut over twenty inches thick. Automatic Wire
Threaders (AWT) are usually standard equipment on most models. In addition to the X-Y
table travels, wire EDM’s have U / V travels for providing the movement to cut tapers. Most
machines can cut tapers of 20-30 degrees depending on workpiece thickness
PRINCIPLE OF WIRE ELECTRICAL DISCHARGE MACHINING
The Spark Theory on a wire EDM is basically the same as that of the vertical EDM
process. In wire EDM, the conductive materials are machined with a series of electrical
discharges (sparks) that are produced between an accurately positioned moving wire (the
electrode) and the workpiece. High frequency pulses of alternating or direct current is
discharged from the wire to the workpiece with a very small spark gap through an insulated
dielectric fluid (water).
Many sparks can be observed at one time. This is because actual discharges can occur
more than one hundred thousand times per second, with discharge sparks lasting in the range
of 1/1,000,000 of a second or less. The volume of metal removed during this short period of
spark discharge depends on the desired cutting speed and the surface finish required.
The heat of each electrical spark, estimated at around 15,000° to 21,000° Fahrenheit,
erodes away a tiny bit of material that is vaporized and melted from the workpiece.
(Some of the wire material is also eroded away) These particles (chips) are flushed away
from the cut with a stream of de-ionized water through the top and bottom flushing nozzles.
The water also prevents heat build-up in the workpiece. Without this cooling, thermal
expansion of the part would affect size and positional accuracy. Keep in mind that it is the
ON and OFF time of the spark that is repeated over and over that removes material, not just
the flow of electric current.
COMPUTER NUMERICAL CONTROL (CNC)
Today’s numerical control is produced with the needs of the operator in mind.
Programs, machine coordinates, cutting speeds, graphics and relevant information is
displayed on a color monitor, with easy to use menu’s.
The control unit displays menu’s that are designed to give top priority to operability.
Characters and commands are input using the keyboard. The system is very easy to use,
allowing the operator to quickly become familiar with it, resulting in his/her learning curve
being drastically reduced.
Besides executing NC data for positioning movement of the axes, the control amends
these movements when using offsets, tapering, scaling, rotation, mirror images, or axis
exchange. The control also compensates for any pitch error compensation or backlash error
in the axes drives, to ensure high accuracy positioning. The machine has multiple coordinate
systems, and jobs can be programmed in absolute or incremental modes saving valuable
programming time. For example, multiple jobs can be set-up on the worktable, while storing
the separate reference points or locations of these jobs in specific coordinate registers.
The numerical control offers the capabilities of scaling, mirror imaging, rotation, axis
exchange and assist programs. This enables an operator to produce an entire family of parts
from a single program without the need to edit the program. Mirror imaging is great for left
and right handed parts. Scaling is useful when working with "shrink factors" for plastic
cavities or extrusion dies. Assist programs find the edge of parts, vertically align the wire,
and perform centering routines that are very useful to the operator when setting up jobs.
Power Supply
When wire EDM machines were first introduced in the United States, they were
equipped with power supplies that could achieve less than one square inch per hour.
Today, most machines are rated to cut over twenty square inches per hour and faster.
Faster or slower speeds are obtained depending on the workpiece material, part thickness,
wire diameter, type of wire, nozzle position, flushing condition and required part accuracy. .
Adaptive Control is yet another improvement where high speed circuitry has
improved the spark gap sensitivity, reaction time of the servo motors, and changes to the
power. With these improved capabilities, wire breakage is reduced to a minimum, making
today’s machines far more "forgiving" than in the past.