DI-SPARK LTD: PART FIVE OF OUR GUIDE TO EDM WIRE EDM AND Spark EDM Basic Theory
Spark & Wire EDM Services, at Di-Spark Ltd
5.2 MACHINING PARAMETERS – Spark & Wire EDM
On and off time are much more than just a switching cycle. They combine with the other basic parameters of the Spark & Wire EDM process and effect metal removal rates, surface finish and electrode wear.
5.2.1 ON-TIME
Speed: Material is removed during on-time. The spark gap is bridged, current is generated, and work is being accomplished. The longer the spark is sustained (the higher the duty cycle), the more work-piece material will be vaporised.
Finish: Consequently, with a longer period of spark duration, the resulting craters will be broader and deeper; therefore, the surface finish will be rougher. Conversely, with shorter spark duration, the finishes will be finer.
Wear: Sometimes during certain roughing operations using elevated on-times, a phenomenon occurs where every spark leaving the electrode can take a microscopic particle with it. More sparks produced within a unit of time will produce proportionately more wear, therefore paradoxically; roughing electrodes can last much longer than finisher electrodes.
5.2.2 OFF-TIME
Speed: While metal removal is only accomplished by the spark during the on-time, the duration of rest required for re-ionisation of the dielectric can drastically affect the metal removal rate. The longer the period of rest (or off-time), the longer the job will take. Unfortunately, off-time is a necessity and an integral part of the Spark & Wire EDM process, generally the smaller this period of time is, the greater the metal removal rate.
Stability: This is the key to maintaining an efficient metal removal rate. Although increasing the off-time will slow down the process, it can provide the stability required to successfully complete a given Spark & Wire EDM application.
Wear: It is sometimes assumed that electrode wear is also a factor influenced by off-time. The logic being that by increasing the off-time, the job would take longer and as a consequence the total wear will increase. However, this is not the case because when the current is switched off, it is exactly that – off! Nothing other than the recovery of the dielectric is occurring. No material is being removed, nor any wear occurring. If only the off-time is changed, then the time required to complete the job will also change.
Minimal off-time is a key to machining speed, but unfortunately a sufficient amount of off-time is required to maintain machining stability. Stability is more important than inconsistent speed.
5.2.3 FREQUENCY
This is the number of times that the current is switched on and off during a given period of time, it is usually expressed as a number of cycles per second. For roughing operations, the on-time is usually extended to achieve high metal removal rates and since this will mean fewer cycles per second, it would be described as a low-frequency setting. Finishing operations, with much shorter on and off times, will have many cycles per second and would be considered high frequency. The duty cycle, which is a measure of efficiency, should not be confused with frequency
The finish left by long on-times is quite rough. This is because the long duration of the spark is sufficient enough to allow a great deal of heat to sink into the work-piece, melting a large crater, rather than vaporising a small one. In addition, the recast layer will be considerably thicker with a potentially deep heat affected zone. This can present problems with the surface integrity of the part although these can be minimised with secondary operations.
Sparks of shorter duration remove little material and much smaller craters are produced. This is the method often used to finish work-pieces and by lowering the power and the on-time an improved surface finish can be obtained. There is also much less potential of thermal damage to the workplace using high frequencies.
5.2.4 CURRENT
This is the amount of power used in Spark & Wire EDM, measured in units of amperage. In both spark erosion and wire erosion applications, the maximum amount of amperage is governed by the surface area of the electrode – the greater the amount of surface area, the more amperage that can be applied. High amperage is used in roughing operations and electrodes with large surface areas.
Although electrodes can withstand high amperages, they are seldom used except in instances of large surface areas. While the electrode itself might withstand intense current, the work-piece, depending upon the material, can suffer from the associated problems of excessive heat. In high amperage/low frequency roughing operations, the intense heat that is generated can sink deep into the material surrounding the area being sparked. Also, when using maximum amperages, the spark gap can be quite large, and smaller details may have to be omitted and produced with subsequent electrodes using less power.
The actual spark gap will be determined by the amount of current and the on-time. Usually when elevated currents are applied, an undersized electrode will be used which will leave sufficient material that will be removed later by subsequent finishing operations either using less power and orbiting, or by using finishing electrodes that will have an adjusted spark gap.
5.2.5 POLARITY
In Spark & Wire EDM, polarity describes which side of the spark gap is positive or negative. Polarity can effect speed, finish, wear, and machining stability.
Spark erosion machines can use both positive and negative polarity, depending upon the particular application, but most operations are performed using a positive electrode. Positive polarity will remove material more slowly than negative polarity, but is used most of the time to protect the electrode from excessive wear.
Negative polarity is used for high-speed metal removal when using graphite electrodes, and should be used when machining carbides, titanium, and refractory metals using copper electrodes. Negative electrode polarity is sometimes used with copper electrodes when no other method is as successful. With graphite electrodes, negative polarity is much faster than positive polarity by as much as 50% or more, but with as much as 30% to 40% electrode wear
Wire EDM machines almost always run with negative polarity – that is, the wire is negative and the work-piece is positive. As in spark EDM applications, metal removal rates are higher using negative polarity, but since the wire electrode is constantly renewed, electrode wear is not a consideration. However, if the wire is burned deep enough, usually about 20% of its diameter, it can no longer withstand the tension and will break. Increasing the speed of the wire will reduce the severity of the wire erosion and help eliminate wire breakage, at the small expense of increased wire consumption.
