The Physical Properties of Eroded Surfaces Encountered In Wire EDM – Part #2 Continued from “The Role of Metallurgy in EDM Wire”

It might be easiest to start this discussion by considering what are the potential physical surface properties which have been suggested to control the wire flushing efficiency in WEDM. Flushing in the WEDM application involves removing the solid debris created by each individual discharge which in fact is created one at a time in rapid succession but never two simultaneously. The discharge envelope collapses under the pressure of the liquid dielectric medium (D.I. Water in most cases) after the ON-time and OFF-time cycles are completed by quenching the debris converting it to solid particulate.

Those candidate surface properties controlling the wire’s contribution to flushing are:

1. “Vaporization Temperature” (represented by eroded surfaces’ Boiling Point) as suggested by the traditionally accepted theories of WEDM Heat of Sublimation as characterized by the energy per unit volume needed to transform the material affected by each individual discharge. If you will recall the Post of “Five Myths of EDM” the first candidate property cannot be the surfaces’ “vaporization temperature” because, as discussed in that Post, there is no such animal since metals vaporize over a very wide range of temperatures and not at some arbitrary fixed temperature.

So that leaves

2. the Heat of Sublimation as the only viable candidate. You need to know, as illustrated in the previous Post, that sublimation is defined as the process whereby a substance transitions directly from the solid to the gas state, without passing through the liquid state. The Heat of Sublimation is the heat required to accomplish that transition and has the correct units, e.g. KJ/cm3, which relate to the WEDM process where one is trying to remove unit volumes of the workpiece.

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These 5 Myths of WEDM all fit Webster’s definition of a myth as they can all be shown to be fictitious.

Myth 1. Wires with a low “Vaporization Temperature” run cooler and therefore break less frequently!

Myth 2. HIGHER tensile strength wire will always solve a Wire breakage problem!

Myth 3. If it costs twice as much, it better cut twice as fast!

Myth 4. FLUSHING FLUSHING & FLUSHING is all about dielectric water pressure!

Myth 5. This wire is best for All applications!

Myth 1. Wires with a low “Vaporization Temperature” run cooler and therefore break less frequently!

I would like to spend a few extra minutes discussing the first myth because it is an excellent example of how inaccurate information gets introduced into a technology, and when no one challenges it, it soon becomes an accepted part of the lore. This myth has inaccurate statement, stacked on top of inaccurate statement, but to this day it is repeated as the gospel in “authoritative” books. It can be traced back to the patent (U.S. Patent No. 4,287,404) that was issued for “A-Type” wires.

Therein lies part of the problem. You would think anything in an issued patent must have been reviewed by “experts” and therefore must be correct, right? Wrong! Inventors are not required to explain why their widget works better; they are only required to prove their widget works better. However, few can resist the temptation to explain why. Unfortunately, patent examiners rarely comment on these explanations, presumably because they have no need to.

This myth originated in the explanation as to why pure zinc works better on the surface of an EDM wire that was offered by the inventors of zinc coated wire. Let us examine it in detail. For starters, there is no such thing as “a vaporization temperature” of metals. Metals vaporize over a range of temperatures, although the amount vaporized at low temperatures may be minuscule. When you see someone list the “vaporization temperature” of metals, it most frequently represents a listing of their boiling points. A boiling point is defined as the temperature at which a liquid maintains a gaseous partial pressure of one atmosphere (760 torr) over its liquid phase. www.chem.purdue.edu/gchelp/liquids/boil.html - Referenced info from Purdue University.

At fifty degrees below the boiling point, there will still be quite significant vapor pressure present. The fact that metals do not posses a vaporization temperature is the first error in this myth. The speculation that by vaporizing the zinc, the wire will run cooler is highly suspect and might be considered the second error. In theory, the statement would be true under most circumstances. Indeed evaporative cooling is a well-known phenomena and was the theory behind the ablative heat shields used on re-entry vehicles of early Space Missions. Unfortunately in EDM, the wire is not in the vacuum of space but is rather immersed in dielectric fluid which will most likely cool the surface far more effectively by conduction than by evaporation. However, let’s not call this an error and concede the wire will run cooler, even though that is highly unlikely. Now we come to a clear error. The inventors speculated that since the wire ran cooler, it would be less likely to fail by thermal shock. Sorry, metals do not fail from thermal shock; ceramics fail from thermal shock. As I mentioned, you will still see this myth repeated time and again as the gospel. Do not be taken in by such techno-babble. This is not to say high performance wires do not work. They work very well, thank you, and I hope in time we all will have a good understanding as to why.

Myth 2. Higher tensile strength wire will always solve a wire breakage problem

The second myth has to do with tensile strength. If you think science has been dealt a cruel blow using the term “vaporization temperature”, wait till you see what they have done to tensile strength. It is probably the most misunderstood and abused concept in the WEDM literature.

I have seen it hopelessly confused with load carrying capacity (they are not the same thing), and no, higher tensile strength wire will not cure wire breakage problems.

Does that mean tensile strength is not important? Absolutely not! It has very little to do with wire breakage, but it has a lot to do with most automatic wire threading systems.

Kind of like SUPERMAN solves ALL the worlds problems – He Doesn’t, right? — Kind of like SUPERMAN solves *ALL* the worlds problems – He Doesn’t, right?

Myth 3. If it costs twice as much, it better cut twice as fast

The third myth is not one of technology, but rather one of economics. No, if it costs twice as much, it does not need to cut twice as fast to be quite valuable.

Time savings is time saved and Time is of course money that can be made by making more parts. Every advantage needs to be taken wither it is twice as fast or 50% faster.

There is still value in time saved!

The Speeds are for reference and are averages from our tests that we have performed on a Exectek Machine. These are high performance EDM wires that are available for purchase at our website online stores.

We feel confident that our wire offerings are the best in the industry and the speed can be increased as the .110” or 2.794mm per minute are not the highest speeds reached, they are averages that can be achieved in real life contour cuts. (Without Wire Breaks) Comparison of speed data without a clear definition of testing methodology and machine tool employed is a mistake. It is like judging a book by the cover!

Myth 4. FLUSHING, FLUSHING & FLUSHING is all about the Dielectric Water Pressure!

To me this is the one myth that really needs to be addressed.

Simply stated flushing is not all about high pressure water pumps and the machines artificial intelligence. (The Spark Gap monitoring)

The choice of wire types which offers a variety of wire constructions has a huge impact on flushing and therefore wire breaks. Because the wire coating proprieties will make smaller erosion debris, which is just as or more important than the water pressure. Stay tuned for a more complete discussion of wire constructions in follow-up editions of the WEDM Tech Wize.

Remember: WEDM is NOT Abrasive Water Jet Cutting!

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Myth 5. This wire is best for All applications!

Finally there is the need to understand where specific wire types can be used most effectively.

There is no “ideal” EDM wire type that can be all things to all applications. However by understanding the construction of wire types, one is better positioned to make intelligent choices. That is what we plan to accomplish with todays GIP High Performance EDM Wire.

Let’s start by considering the metallurgical properties we need to concentrate on in the NEXT edition of WEDM Tech Wize. “May your Sparks always be ON-TIME!”