• 10/06/2019

1. What is EDM ?

Have you ever seen sparks to weld steel frames in construction sites of buildings?
And sparks sometimes come out when you insert wall sockets.
Sparks emitted towards metal in the air melt it in the high temperature. Large sparks make large melt, and small sparks make small melt.

What happens if we make a spark toward a metal in a liquid? The spark melts the metal. When the spark stops, the melted part in the liquid is cooled rapidly and scatters intensely. The remaining portion is depressed like a hole, and it looks like a crater of the moon.

EDM is a processing method using this phenomenon.
It is a machining method to carve metals by hitting sparks intermittently and repeating metal dissolution, cooling, scattering.

2. What kind of shape can it process ?

EDM is called "a machining method by copying. It needs an original shape called "electrode". "Electrodes" are made with materials such as copper by different type of machine tools.

By hitting sparks intermittently to 1000 - 100,000 times per second between the electrode and the metal, it carves a shape of the electrode to the metal. Several hours later (depending on the size of the electrode), a shape of the electrode is copied on the metal.

3. Fields for which EDM is utilized

EDM is utilized when no other machining method is available because the machining speed is slow.

The main features are as follows.

• It can engrave a complex shapes on metals.
• It can engrave shapes on hard metals

etc.

Though it is used for many special fields, the most common field is mold manufacturing.

Mold is models to make plastic parts etc. for cell-phones, TVs, or automobiles in large quantities. From 1,000 to tens of thousands of plastic parts are made with one mold.

Die & mold manufacturing is a strong field of ​​Japan in the world. EDM is the main machining methods of it.

4. History of EDM

EDM came to be known generally since the study by Mr. & Mrs. Lazarenko of the Soviet Union in 1940's.
They found the new machining method when they were studying the phenomenon that switches were worn by the outbreak of sparks.

Commercialized EDM in Japan was put to practical use by Ikegai Ironworks and JAPAX circa 1953. However, different from current machines, EDM at that time was only available in "through-hole machining" because of large electrode wear.

"Bottomed machining" like current EDM became available since the power supply circuit with a transistor was put to practical use around 1968.

Later, "Sinker EDM" for "bottoming machining" by using shaped electrodes and "Wire EDM" for "contour machining" by using wire electrodes were distinguished.

5. Manufacturers

Major Japanese companies that manufacture electrical discharge machines

• Makino Milling Machine Co., Ltd.
• Mitsubishi Electric Corporation
• Sodick Co., Ltd.

6.Electrode wear

Sinker EDM is a kind of machining of transcribing the shape of the electrode to the workpiece, and the shape of the electrode is damaged by sparks. This damage is called "electrode wear".

The ratio of the amount of machining of the workpiece and the amount of electrode wear is called "electrode wear ratio", and it is important on transcribing the shape of the electrode to the workpiece. Electrode wear ratio changes due to "the combination of electrode and workpiece material", "the polarity of the voltage to apply", "the duration of the spark", and so on.

7.Low electrode wear ratio

It is said that the temperature of the spark generated by EDM is about 6000 degrees centigrade. When sparks occur between the electrode and the workpiece, the sparking point is dissolved hot. The spark duration is adjusted between 1/1,000,000 second and 1/1,000 second, and longer duration makes dissolving larger.

Both the electrode and the workpiece are supposed to be dissolved hotly similarly by the outbreak of the spark. Reducing the wear by dissolution of the electrode is necessary for transcribing the shape of the electrode efficiently. Therefore, the materials which are hard to dissolve are suitable for the electrode, and the workpiece of more easily dissolved material can be processed more efficiently.

Currently, copper is used most as a material of the electrodes, and copper is a material which is good for heat conduction and tough for partial dissolution. It is mostly used to process iron workpiece. Iron is a material comparatively easy to partial dissolution. In the material combination of "copper (electrode) and iron (workpiece)" can make electrode wear ratio less than 1 percent.

Electrode wear ratio [%] = Amount of electrode wear [g] / Amount of work machining [g] × 100

8.Discharge gap

A proper "gap" is necessary to generate sparks between an electrode and a workpiece. This gap is called "discharge gap".
The lightning spark occurs at very long distances from the cloud to the ground, and the voltage reaches millions volts. In EDM, the voltage supplied is approximately 50 to 300 volts because higher voltage is not suitable for high precision machining. Therefore, EDM discharge gap is about 0.005 - 1.0mm.

Sparks do not occur in the condition that an electrode and work touch each other (i.e. discharge gap = 0), and the machining does not proceed. EDM moves an electrode up and down minutely depending on progress of the machining, and adjusts the electrode position to make the discharge gap suitable for sparking. This control which keeps the discharge gap proper is called "discharge gap servo".

9.Electrode materials　

Copper, graphite, copper and tungsten are used for electrode materials.

The selection guide is as follows.

• Electrode wear ratio
• Fitness for the electrode production
• Material costs

Etc.

1) Copper

Copper (Standard JIS: more than 99.96% purity) is used. 
As for the temper, H which is the hardest is suitable. H material has limit of the size of the material, and for large electrodes, material around 1/2H is used.

2) Graphite

Graphite fine particles are combined by combination materials called the binder. There are a number of products with the characteristics such as graphite particle size and binder materials. On the choice of materials, not only the performance of discharge machining, cutting efficiency at the time of electrode production is also important.

3) Copper tungsten

On making fine shape electrodes, it is characterized by good machinability.

In addition, the appropriateness of electrode materials should be judged with "the material of the workpiece," "contents of discharge machining" and "method of making the electrode" and so on.

10. Dielectrics　

EDM is done in a machining liquid.
Without a machining liquid, machining does not make progress because molten part of the workpiece cannot be scattered by sparks.

Machining liquid used for sinker EDM is an oil which is a kind of petroleum and is near to kerosene. Kerosene also can be used, but because of the risk of fire, its flash point is raised to about 100 degrees C from about 50 degrees C which is the level of kerosene. Recently, oil-based synthetic machining liquid without harmful aromatic constituents also is used generally.

Dielectrics are sold by oil companies.

11.Orbits and electrode reduction　

In EDM by spark, the machined part becomes about 0.005 - 1.0mm larger than the shape dimensions of the electrode. This amount differs by the power of the spark, and a stronger spark makes it larger. In EDM, strong sparks and weak sparks should be used properly. The stronger the spark is, the rougher the surface becomes, and machining becomes faster.

In general, in the early stages of rough machining, "strong spark" is used, and at the finishing stage, 'weak spark" which can make fine and clean surface is used. However, "weak spark" cannot be generated after using "strong spark" because the gap between the metal and the electrode is already too big. You can make a slightly larger electrode newly, but it will cause cost problems etc.

The machining control method which was contrived for that is "orbit". 
"Orbit" means "swing", and it is a machining control function of swinging electrodes.
By using "orbit", you can use "weak spark" even after using "strong spark", because it can reduce the gap between the electrode and metal.

And, the amount of swinging the electrode is called "amount of orbit," and electrodes are manufactured smaller by estimating this amount beforehand. This amount of making electrode size smaller than objective size is called "electrode reduction amount".