EDM has been commercially available and a part of the manufacturing landscape for fifty years. Though considered a "Black Magic" technology for many years, it has come into its own and is now looked upon as an indispensable and highly reliable manufacturing process.
Even today, unfortunately, there are still those who feel uncomfortable with EDM.
Much of our modern day life would be cost prohibitive or unthinkable if it were not for EDM’s ability to produce products or tooling cheaper and more accurately than ever before.
Over the years EDM has achieved the impossible, even when conventional wisdom said it couldn’t be done.
How? By using one’s imagination and asking why EDM works.
"If I adjust this or that... this seems to happen... why?"
By taking advantage of one’s natural curiosity and the power of observation the possibilities are endless.
KEY FACTORS OF EDM
Since EDM uses electrical energy to remove material, it stands to reason that the make-up of the material will have a direct impact on the rate of removal.
This being the case there are three critical things to keep in mind.
Since machining with EDM occurs by using electrical energy the electrical conductivity plays a key role in how fast the Spark Gap canionize allowing a spark to occur.
Materials that are less electrically conductive will machine slower.
Pulse energy (heat) is conducted away from the surface of the part and is dissipated through the piece.
Material removal will be slower and less efficient when compared tosteel.
Electrical Conductivity of the material being machined.
Thermal Conductivity of the material such as copper or bronze alloys used in plastic molds or parts production can adversely affect machining speed.
Melting Temperature is a key factor in material removal rates.
Steel melts at about 2,5000F while Tungsten’s high melting temperature of 3,5000F requires more energy to disintegrate. Aluminum on the other hand melts at about 16000F, and will machine faster.
THE EDM PULSE
When we see a flash of lightning we are witnessing EDM on a grand scale. The lightning bolt is electrical energy flowing between anelectrode (cloud) and a (grounded) workpiece in a natural kind of EDM phenomena. This discharge is the same as what occurs in an EDM machine.
Thankfully EDM is more controllable.
An EDM pulse is highly controllable by the operator who can adjust power settings and parameters to determine the size and intensity of the spark. On-Time is the length of time the spark is turned on and the period of time between each spark is referred to as Off-Time. Off-timeis necessary to allow for efficient chip removal.
The combination of the EDM Pulse On and the Pulse Off is one cycle.
If my machine is burning efficiently the reading on the ammeter should equal the Peak Current multiplied by the Duty Factor as calculated below. It is helpful to know how to calculate duty factor in order to confirm your efficiency.
Duty Factor Calculation
25msecx100=2500=33% DUTY FACTOR
75msec (Total Cycle)
In the example above the Ammeter should read 8.3 Amps Average Current if the EDM generator is set for 25 Amps (peak current). In other words 33% of 25 Amps = 8.3 Amps. This is important when calculating the time an EDM job will take.
The general "rule of thumb" is that you can achieve a metal removal rate of about .55 in3 /hr using 25 amps of current (Graphite +/ Steel-).If the duty factor is 33% then the metal removal rate per hour would be0.18in3 /hr.
.55 in3 MRR
1.20 in3 MRR
Maximum Current vs. Electrode size for Manual EDM machines (Multiply Current Density by two for CNC EDM).
Current is the Power used to drive the Cutting Tool (Electrode) into the workpiece.
Current can be thought of as the diameter of your cutting tool since it will produce a larger crater as power increases.
On-Time can be thought of as the depth of cut. The longer the On-Time the deeper the crater becomes.
When voltage is first applied there is no amperage as shown in Figure 4.As the electrode approaches the workpiece (squeezing the gap) the voltage drops and amperage will increase. The readings on the volt and ammeters are averages achieved over a period of time.
If for some reason you are not achieving the level of amperage or metal removal that you have calculated you may be having problems ionizing the spark gap. In other words you may not be evacuating chips fast enough to maintain a clean enough spark gap in order to ionize.
If the gap remains contaminated with debris the time it takes to prepare the gap for a spark may increase. This slowdown is sometimes referred to as (Ionization) Delay Time.
Delay Time may vary dramatically from pulse to pulse, making your cut inefficient since it is actually adding non-cutting Off-Time to your burn. The cycle between each pulse (e.g., Off-time + Delay Time) may vary producing fewer sparks per length of time.
Electrode wear may occur for a number of reasons. It can be caused by inappropriate power supply settings, or mechanical abrasion of the electrode caused by excessive flushing or mechanical movement, such asorbiting.
Electrode wear is caused by the ability, or inability as it were, for the machined particles from the workpiece to plate the electrode following a machining pulse.
This plating looks like a white or "Silver" layer (of steel) on the graphite electrode.
Positive (+) electrode polarity must be used in order to achieve No-Wear(.01% or less). As a rule of thumb, the higher the duty factor and amperage the lower the overall electrode wear will be.
In finishing a part or cavity we reduce the current used and operate at shorter On-Times, increasing electrode wear, producing smaller and shallower craters and smaller chips.
Negative (-) electrode polarity is used for high-speed roughing when electrode wear (5 to 15%) is of little concern, or for obtaining finer finishes.
In order to achieve no wear the electrode must be positive (+) and theworkpiece negative (-). The spark is discharged from the workpiece to the electrode as shown (arrows) in Figure 6.
Following the EDM pulse, the molten chip will plate the electrode unless
There is a flushing hole nearby.
Flushing fluid rapidly cools the chip preventing the plating action.Flushing pressure that is too high will cause uneven electrode wear and instability.
Mechanical action of the Ram positioning the electrode in the Gap isunstable, either through retraction or orbital motion, preventing the chip from reaching the electrode while still in the molten state.
Short pulse duration and low current is used. Smaller chips cool faster due to the small mass, reducing the plating action.
Negative Polarity is applied to the electrode.
Electrode position at the spark Gap is essential to obtaining the lowest possible electrode wear.
Though EDM machining does not produce conventional cutting forces, large hydraulic pressure and Servo performance will determine the accuracy of the electrode position in the Gap. Erratic Servo performance or inaccurate position causes electrode wear and will affect part finish.
For this reason machine rigidity is very important.
Low electrode wear is unachievable with certain types of materials. Carbide, copper/bronze are materials that will not plate. High (15to 50%) electrode wear is common when machining these materials.
The first thing we are taught is that flushing is the key to efficient EDMing. We are told that without flushing, flushing and (more)flushing we cannot succeed in EDM.
Efficient flushing takes time to learn. The problem is that flushing can change dramatically from job to job.
There are choices to make about flushing. The best results will be achieved with the least amount of flushing (pressure) required to obtainstability. The exception is in small hole EDM drilling applications where pressure can substantially improve cutting speed. Volume is more important than pressure.
There are several methods commonly used to flush the EDM gap: lateral, pressure, vacuum and mechanical. Each type has its benefits and drawbacks.
Job preparation time and cost are key factors in determining the best method of flushing to use.
There are several questions to ask oneself when trying to develop the best flushing strategy for a particular job. What is most important in the job?
Electrode Manufacturing Time (including drilling of flushing holes)?
Setup & Preparation Time?
Unattended Time (when using CNC)?
Having set job priorities, you can develop the best overall flushing strategy.
Lateral Flushing - using hoses or fixtures directed at the electrode to help wash debris away.
Care must be taken when manually setting hoses. Operator experience plays a key role in the performance of this type of flushing.
Through Pressure or Vacuum - forces chips out of the Gap by directly creating dielectric flow through drilled holes in the electrode,workpiece or fixtures.
Time of manufacturing and number of electrodes may increase.
Pressure Flushing = More Electrode Wear
Wear Vacuum Flushing = Less Electrode Wear
Mechanical/Orbital Flushing - is the least aggressive of all the flushing types and relies on hydraulics to clear chips out of the Gap.
Though burn time may be slower in most cases, total time for a job is most critical.
Depending upon the application there may be several advantages realized with this type of flushing and might include:
Lower electrode wear
More consistent part finishes (less secondary discharging)
Keeping in mind that fluid will always follow the "path of least resistance," placement of hoses, flushing devices or location of drilled flushing holes is critical to achieving consistent performance from yourEDM machine.
Even before CNC EDM provided computer control of orbiting, attachments were available to allow orbiting on manual machines.
Though less efficient than today’s CNC control, orbiting attachments promoted chip removal, reduced electrode wear and helped to improve surface finishes.
Orbital motion of the electrode also reduces the need to produce roughing and finishing electrodes as is done on manual machines.
During orbiting the electrode is driven by Servo control and repositioned in its machining plane (X-Y, X-Z or Y-Z) by the amount of undersizing programmed, expanding outwardly. This movement of as little as several thousandths of an inch will assist flushing of the Gapduring machining.
Three dimensional orbital patterns can be found in most CNC controls.The benefits in reduced electrode manufacturing that these orbits provide can be substantial.
The advantage to CNC EDM machining is more than simply the orbital control and positioning routines found in most machines today. It is in the reliability that can be obtained to operate unattended overnights, weekends and holidays.
In order to achieve a reliable level of unattended operation it is criticalthat the EDM process be controlled. Without this control part damage might occur, or worse yet, damage to the machine and/or shop.
CNC EDM machines today offer highly advanced control software and hardware that provides a high level of automation and safety to theprocess.
Automatic control of the EDM machining process, including spark generation and mechanical (e.g. Servo) corrections are needed if efficiency is to be achieved.
Today’s CNC EDM machines offer the performance and intelligence within the control to reliably adapt the EDM parameters during the cut, even under the most difficult burn conditions.
The most important thing in your EDM machine is your electrode since it represents your final EDMed shape. If the electrode is wrong your part will not be right.
Choosing the best electrode for the application can sometimes be difficult.
Though cost must be considered as an important selection criteria, it can be very expensive over the long haul to purchase your electrodes based upon material costs alone.
Every electrode material has specific characteristics that make them desirable for a particular application.
Fine grain high density graphite is an excellent material for an electrode when wear resistance and surface finish is critical to the application. Lower density graphite may be the electrode of choice when surface finish or wear is not as critical to the job.
Metallic electrodes such as copper that do not have a grain to contend with may be desirable when doing small intricate work that requires mirror (polished) finishes.
The chart below gives approximate surface finishes achievable with various electrode materials types.
Material 5inch Ra VDI Fine Grain Gr. 8 0 Medium Grain Gr. 30 12 copper 2 NA .
Dielectric fluids provide several critical functions in the EDM process. They cool the workpiece and electrode between each discharge as well as carry chips out of the Gap providing ever-critical flushing. Dielectric oils also provide electrical insulation between the work and electrode.
Since the very beginning the most common dielectric used in die sinking EDM machines has been mineral seal, petroleum-based fluids.
Even highly flammable kerosene has been used as a dielectric fluid in some shops doing very delicate low current machining.
Over the past five years or so specially formulated dielectric fluids have been developed specifically for the EDM process.
The dielectric used in your EDM can have a dramatic impact on your machine’s performance and therefore your bottom line.
Regardless of the type of work you do your dielectric must be maintained in order to provide the best performance.
Clean oil will provide the best surface finishes and the minimum amount of arcing or pitting.
The temperature of your dielectric should be tightly controlled for the best accuracy and cutting speed.
Since electrical conductivity or resistance is affected by heat, your dielectric temperature can adversely affect your machines’ performance.
A chiller will improve cutting SPEED!
In choosing a dielectric to put into your EDM machine you should ask your supplier -
What is the anticipated life expectancy of the dielectric?
Are there any potential health hazards to be concerned with?
What is the flash point (safety) of the dielectric?
What is the viscosity of the oil which will determine the ability to flush. Light oils are easier to flush than thicker (high viscosity)oils.
Some special applications might require that you consider other dielectric fluids in your EDM machine.
For instance deionized water type fluids can improve machining speed in a conventional EDM machine since it does not have the insulation value of an EDM dielectric oil.
Though water type solutions provide better machining speed there are other problems that may be encountered, including but not limited to health risks caused by additives used in the water.
In general, dielectric oils are still the best coolant for most applications. As stated earlier, the electrical resistance (e.g.,conductivity) of the material to be EDMed will impact machining speed.
Temperature increases the electrical resistance of a material. It is important, therefore, to maintain the temperature of the dielectric fluid that surrounds the part in your machine.
In small part production, even a 10% to 15% increase in dielectric temperature can add substantially (20~30%) to cycle times.
Location of the electrode in the quill and the work on the table of the EDM machine is critical to reducing setup times and the overall time it will take to perform an EDM job.
New tooling systems and pallets have been developed that provide extremely accurate repeatability and therefore the ability to automate and set up EDM jobs off line. (Accurate probing and machine pick-uproutines may also be used to improve overall precision.)
Even an old EDM machine tool can find new life and benefit from the accuracy and rigidity provided by any one of the newer tooling systemsavailable today. These systems feature exceptional rigidity and repeatability (better than 0.000080").
Reduced costs associated with setup times and work quality will provide an impressive return on investment (ROI) for your (EDM) shop.
After all, if an electrode is out of alignment by as little as 10.0005",machining time can be expected to increase as secondary electrodes are used.