2015
Wire EDM Machining
AN ADVANCED MACHINING PROCESS
BY: CHRISTOPHER HECKLE
1.0 Introduction
Wire Electrical Discharge Machining, or Wire EDM, is an advanced material removal process that uses
electrical sparks from a vertically held wire to essentially erode away material into complex shapes. The
Wire EDM is a precise machine that can achieve dimensional accuracies of .0001”. That translates to
cutting parts with accuracy of about 1/30 the thickness of printer paper. Wire EDM machined are
Computer Numerical Controlled (CNC) which means it runs without an operator as long as the user has
programmed the machine.
2.0 How it Works
EDM machines work by slowly melting away material from the work piece via spark erosion. High
current in the electrode creates a spark when it gets close to the work piece. The sparks duration can be
controlled by the program and thousands of sparks occur every second. Each spark erodes a small chip
of material from the work piece and melts it at such high temperatures that it turns into a plasma that is
quickly carried away from the cut zone by a high velocity coolant. In wire EDM, a wire, typically .012” in
diameter serves as the electrode. A simple analogy is to compare wire EDM to a band saw where instead
of using a saw blade to cut, a wire EDM uses a high current wire. One of the differences however is that
the wire doesn’t actually contact the work piece like a saw blade does. This proves to be beneficial when
cutting very small or thin walled parts.
TOP DOWN VIEW OF A WIRE EDM CUTTING THROUGH A PART.
3.0 Components
The major components of a wire EDM are the wire and feed, the work piece, the table, and the dielectric
fluid. Each component
3.1
Wire Feed
The wire, typically copper or brass, is wound around a spool on the machine. As the wire is fed into the
work piece, a high current passes through that allows the creation of sparks when it gets close enough
to the conductive work piece. Each spark lasts fractions of a second but results in temperatures in excess
of 10,000 F which is high enough to melt away the work piece material. Consequently, the wire also is
partially eroded away with each spark so it is necessary to feed fresh wire at a constant rate through the
work piece. The wire is then re-wound onto receiving spool that will be discarded after machining is
complete; the wire could break if sustains too much damage. The wire has to guides that are above and
below the work piece that hold the wire in place and continuously feed the new wire through the work
piece.
WIRE GUIDES (WHITE) HOLD THE WIRE AS IT CUTS THE WORK PIECE.
3.2
Work Piece
The work piece is the material you want to machine into your final part. Say you want to machine a gear,
your starting work piece would be a block of steel that you would cut pieces from to get your final gear.
Wire EDM requires the work piece to be conductive to allow for a spark to form between the work piece
and wire. For this reason, wire EDM is limited to cutting of electrically conductive materials which is
typically only metals; though some advanced ceramics are possible. The work piece needs to be held in
place either by clamps or a specially designed fixture. It is important the work piece is held tightly so if
doesn’t move during machining. One of the benefits of Wire EDM is the wire never actually touches the
work piece. This eliminated the tool force that’s is seen in most material removal processes. However,
the force of the dielectric fluid flush as well as residual stresses in the work piece still require it to be
held securely in place. Work piece setup can be time consuming but proper setup is critical to the EDM
process.
3.3
Table
The table on a wire EDM is the part of the machine that moves in the X and Y directions. This movement
allows contours to be cut into the part. In most cases, the wire feeds are fixed in place and the table
moves, similar to a milling machine. The table is actuated by two stepper motors capable of steps less
than 1/10,000 of an inch resulting in precise movement.
3.4
ARROWS SHOW THE TABLE TRAVEL DIRECTIONS.
Dielectric Fluid
A dielectric fluid serves two purposes to cool and to
clean. Just like most material removal processes, the
work piece becomes extremely hot. High work piece
temperatures can result in part distortion and residual
stresses once the part cools. For this reason, it is critical
the parts temperature is controlled using coolant.
During EDM machining, the part must be cooled using a
dielectric coolant. Typical dielectric fluids are de-ionized
water or oil. The benefit of deionized water is its ability
to transfer heat away from the part much quicker than
oil. However, the water will evaporate much quicker
than the oil which means it is important to monitor the
machine to guarantee a sufficient supply of water is
available.
In addition to cooling the part, the dielectric fluid is also
flushed into the cutting area to remove chips formed during the material removal process. It is
important the material is moved away from the part and the wire so that the chips aren’t melted again
and reattached to the work piece. Most wire EDMs flush coolant onto the wire above and below the
part to ensure maximum cooling and chip clearing.
As the dielectric fluid is flushed into the cut zone it tends to become charged and thus no longer
dielectric. It is important to ensure the fluid stays dielectric and free of conductive metals chips so that it
doesn’t interfere with the sparks between the wire and work piece. To ensure the fluid stays dielectric, a
pump forces it through a filter to remove any chips from the fluid and then into a deionizer. The fresh
fluid is them pumped back into the cutting zone. Many EDMs actually submerge the entire part into a
bath of dielectric fluid to enhance the cooling rate allowing the machine to cut more efficiently.
EXAMPLE OF A FULLY-SUBMERGED WIRE EDM MACHINE. PART IS COMPLETELY COVERED IN DIELECTRIC FLUID.
4.0 Summary
Wire EDM uses an electrode in the form of a wire to remove material using spark erosion. Each
component of the machine is equally critical to the proper performance. It is a valuable tool for
engineers across all industries. The main advantages are high accuracy, repeatability, and dependability.
Understanding the key fundamentals of Wire EDM can greatly aid an engineer who wishes to take
advantage of this process.