Sink or Ram EDM is a cavity type of EDM (Electrical Discharge Machining). Sink/Ram EDM involves submerging a part into oil (dielectric fluid) and applying power to the part, using an electrode to create a spark and cutting away at the base metal. These parts are usually of odd shapes, and Sink/Ram EDM is a primary process for creating molds (i.e, for tennis shoe soles). Ferrous alloys are usually used for electrodes in this process.

Wire EDM is also known as wire-cut EDM (Electrical Discharge Machining). The part is submerged in water and is usually cut using a brass cutting blade (wire) that holds a very tight tolerance (i.e., .004mm). EDM machines can also cut two parts at once and are programmable. Wire-cutting EDM is commonly used when low residual stresses are desired; therefore there is little change in mechanical properties.

An Interview With Xact

Q: Is EDM being used to machine progressively smaller parts? Why?

A: Yes, very much so! Several factors are involved ... The equipment has become much more refined over the years. In the past, realistically only the high-end machines consistently did a good job with small wires. But now, wire EDM is being used to machine smaller parts because most of the new machines are designed to better handle small wire diameters. Many small parts incorporate features that require the use of small diameter wire.

The wire transport systems on newer machines are better able to feed the small diameters under very stable tension. Additionally, the new machine generators are much more sensitive to conditions during machining and are therefore much less likely to break wires during machining. The knowledge base of various generator settings has expanded over the years reducing the amount of time it takes to get a good part. Auto-threaders on the new machines can thread the small diameter wire much more reliably than the old machines could.

Wire EDM produces virtually no burr, applies no cutting forces, and has no cutters to wear. The process is therefore extremely consistent. This is critical because small parts typically have tight tolerances and are fragile.

The unique capabilities and advantages of wire EDM are becoming better understood by many more designers and engineers who can then create parts based on the process. The process is no longer used only if there is no other way to make a part, but rather it is incorporated into the design of the part.

The demand for smaller components is increasing in many fields. Minimally invasive surgical instruments are shrinking in size every year. The electronics industry is always looking for a way to do more in a smaller space.

Q: What industries are the smaller parts for?

A: Most of the smaller parts that we see at Xact are for the medical field. We have seen smaller parts in many other industries including fiber optics, defense, and electronics.

Q: What is it replacing in terms of machining these small parts?

A: Many of the parts that we see could not be made any other way. They were designed “from the ground up” to use wire EDM.

Q: Are these complex geometry parts? How so?

A: No, in fact many of the parts have very simple geometries but they have a feature that lends itself to wire EDM. The part may have thin delicate walls, be made of a difficult to machine material, have small corner radiuses, or narrow slots. Many times we are simply cutting small round holes or even cutting in a straight line (i.e. putting slots in a part).

Q: What are the challenges here? Does the width of the wire limit how small you can go?

A: At Xact the smallest wire that we cut with is .004” diameter and the thinnest wall thickness we recommend on the part is then .004”. If the walls are thinner than that, they can be obliterated during the rough cut unless precautions are taken. The real challenges for us are not EDM related. The wire EDM is only one step of the process. You need to handle, fixture and inspect the part also. At Xact we have 5 video inspection systems that enable us to accurately check these small parts efficiently. We cut small parts every day on many of our 30 wire EDM’s so we have the experience and expertise necessary to fixture and handle these small parts.

Q: As the same time as EDM is being used to machine smaller parts, is it getting faster? What are the requirements to machine smaller parts at greater speed?

A: What are the limitations in terms of size and speed and what's being done to surmount these limitations? The cutting times for many small parts are often very short and cutting time is only one component of the cost. Many parts that we see require more time to load, clean, inspect, and package than they do to cut.

Detailed geometry machining to tight tolerances is best done at less than maximum speeds due to the physics of cutting with a thin wire. The more straight lines (or large arcs) and/or the larger the tolerances, the faster you can machine. The melting point and the conductivity of the material being machined also affects the speed.

Q: How does turn-and-burn capability factor into this? Does it just cut down on setup time and thereby contributing to increased part production or does it increase the speed of the machining action itself. Also, what exactly is turn-and-burn? Is it turning while burning or turning and then burning?

A: We do not do turn-and-burn but we do what we call auxiliary rotary spindle wire EDM. The part is constantly rotated (spinning) while being machined like you would on a lathe. True turn and burn allows you to tie the rotation of the part into the movement of the axes and therefore machine while rotating the part. We are currently looking into adding this capability.

How has the increasing cost of copper affected WEDM? Do customers buy less wire, WEDM cut less, or does it just cut into their bottom line? Does it make WEDMed parts less attractive to first time buyers?

The smaller the diameter of the wire the less sensitive it is to copper prices. Even with the price increases, wire is still an economical cutting tool and very often, the only way to produce the part. If the EDM process consistently makes good parts, this must be compared to a possibly less capable but “less costly” process.