EDM PRIMER

Brief History of EDM:
EDM was originally developed in Russia in the 1950's by researchers who were studying how to reduce the pitting and wear of contacts on electrical relays. EDM soon developed into a viable and original new metal cutting process with many unique properties. R.S. Precision bought their first EDM machine in 1960.

The first EDM machines developed were the Sinker type, then CNC Sinker and CNC Wire EDM Machines were created. There have been tremendous improvements in performance and capabilities for all types of EDM machines over the years, so that while EDM was once used exclusively for Tool and Die work, it has now become an indispensable part of modern manufacturing.

How EDM Works:
EDM is a non-traditional metal removal process in which small amounts of conductive material are eroded away in a very controlled manner. A power supply, or generator, creates hundreds of thousands of tiny electrical sparks per second that jump across a gap between a specifically shaped electrode and the workpiece. As the workpiece is eroded, the electrode wears out as well.

The interface between the electrode and workpiece is submerged under a dielectric fluid, which can be hydrocarbon or water based. The dielectric fluid washes the removed material away. This material, or residue, consists of minute globules of molten and refrozen material. The residue is filtered out before the fluid is used for flushing the cutting gap again. The EDM process is a thermal process that removes material by melting it away. It leaves behind a slightly altered surface which consists of a hard crust or recast layer, and a heat-affected-zone (HAZ) underneath which is usually softer than the base metal. In extreme cases, the recast layer can have micro-cracks and high levels of stress, which can lead to part failure under high levels of cyclic stress loading. The HAZ can also show signs of intergranular attack, as well as microcracking.

For these reasons, the application of EDM in aerospace parts is very carefully controlled and regulated. However, the newer CNC Sinker, and Wire EDM machines, can leave behind recast layers and HAZ's that are as much as ten times thinner than in the past.

The main factors that drive cost in Sinker EDM are the setup and fixturing cost, volume of material to be removed, the cost of the electrode, how easily the cutting gap can be flushed of residue during the cut, the tolerances on the finished features, and the surface finish required. The main factors driving cost in Wire EDM are the length of the cut, the material thickness, the tolerances, and the surface finish.

Some of the unique attributes of EDM are that it is burr-free, can reach hidden and inaccessible areas, can produce any conceivable shape, and works in any electrically conductive material.

Types of EDM Machines:

Sinker EDM (a.k.a Vertical, Plunge, Conventional, or Ram):
This was the original EDM process, in which a "male" electrode "sinks" into a workpiece producing a "female" cavity which is an exact, inverse replica of the electrode. Initially used to produce die openings for stamping dies, it was later used to produce intricate punches, and production parts. These are single-axis machines, usually vertical, which cut in one direction. Twenty years ago, when someone mentioned EDM everyone thought of Sinker EDM.

CNC Sinker EDM:
An extension of the basic sinker EDM which adds many advanced capabilities such as the ability to cut in any direction, to orbit and vector, to position and do multiple operations, and to have programmable, varied cutting settings. Although the basic cutting process is still the same, in actual use, CNC Sinker EDM is almost a totally different process, offering advantages of faster cutting, improved geometric accuracy, lower electrode wear, ability to hold tighter tolerances, improve surface finishes, and reduced cost through greater automation.

Wire EDM:
This is another specialized EDM machine that uses a continuously moving, or "traveling", brass wire, which is threaded through a start hole in a workpiece, typically a die plate, to cut very accurate openings with very little skilled labor or intervention required. High-pressure jets of deionized water are used to flush the dirt away, and some have the entire workpiece submerged under water for thermal stability and to enhance flushing in unusual workpiece configurations. Some machines have automatic wire threading, and some have four-axis capability, which can produce tapered openings and more complex shapes. As speed of cutting and ease of use have improved, these machines have become very popular. Today, when EDM is mentioned, most people think of Wire EDM.

Fast Hole EDM (a.k.a. Hole Popper):
A sinker EDM machine that has been optimized for a single purpose: to drill small, very deep holes as fast as possible. Originally used to produce wire start holes in hardened die plates, this amazing process has found many uses in production applications. Its main advantages are speed and large depth to diameter ratio through holes. Depth control for blind holes is difficult with this machine.

Tap-Buster:
A very crude and inexpensive apparatus used exclusively for eroding away broken drills and taps. Not used for controlled or precision applications.