Generating fast EDMs

Dec. 1, 2002
New generator technology propels wire EDM into high-speed.

New generator technology propels wire EDM into high-speed.

With the CleanCut-Generator, shops make fewer cuts to produce the same surface finishes of a conventional EDM generator.

In addition to its high-speed EDMing capabilities, Charmilles' CleanCut-Generator produces a thinner recast layer as compared to conventional wire-EDM generators.

After intensive machining, the thermal shield, a layer of zinc, on Charmilles' XCC wire virtually disappears in the direction of wire feed. This indicates that the layer has done its job protecting the wire core.

On titanium grain 5, used in the medical industry, a conventional EDM generator produces surface deposits and pollution (left). The CleanCut-Generator minimizes such deposits and leaves a homogeneous surface (right).

The pace is set at 37 in. 2 /hr. That's how fast two new wire EDMs cut tool steel to exceed a long-standing, industry speed barrier of 30 in. 2 /hr. Their secret is a special generator that not only makes for fast machining but also preserves part-surface integrity.

Called CleanCut (CC), the digital generator is from Charmilles Technologies and premiers on the company's Robofil 240cc and 440cc wire EDMs. It creates a different spark design, one with a maximum ignition voltage increased by 25% and a maximum amplitude tripled from 400 to 1,200 amp. While producing such energetic discharges accelerates cutting speed, it is usually at the cost of a rougher surface finish and thicker recast layer. However, the effect is just the opposite with the CCGenerator. It actually minimizes the recast layer and achieves finer surface finishes, so parts require fewer skim passes.

While the CC-Generator does form recast deposits after roughing, the layer is about 4 thinner as compared to conventional generators. And if shops rough at a speed slightly lower than the maximum, the amount of deposit drops to a few microns thick. This means the machined face, after roughing, maintains more of the physical and metallurgical properties of the base material. In addition, repeated finishing passes with the CC-Generator don't alter the machined surface as much. Also, the generator produces neither electrolysis nor electrochemical corrosion from roughing through to finishing.

Another important feature of the CC-Generator is its universality. Its programmable sparks adapt to a range of materials such as steel, tungsten carbide, boron carbide, titanium, aluminum, PCD, composite materials, and exotic materials. When machining carbide, for example, the CC-Generator maintains the material integrity by depleting less of its cobalt binder.

Typically, carbide contains 12% cobalt binder, and conventional generators may deplete up to 9% of that. With the CC-Generator, shops machining carbide maintain an 11.8% cobalt content, even at the EDM cutting edge. For cutting tool manufacturers, this means the EDM process does not compromise the life of the cutting tool made from the carbide.

Although the sparks or discharges of the CCGenerator are programmable, no two are the same due to the nature of electricity. For this reason, the generator, in real time, processes and acts upon each discharge through specific software and hardware to maintain its high-speed performance.

The CC-Generator uses a system called Pilot-Expert. This ultrafast data-acquisition system analyzes each discharge individually, processes the data, and orders the generator or other units of the machine to perform appropriate actions. Basically, it is the brain of the process, ensuring there's no wire breakage and preserving machined-surface integrity.

In addition to Pilot-Expert, Charmilles complements the CC-Generator's powerful and controlled discharges with a special copper-coated wire called XCC. It features a diffused layer of zinc, which is essentially where craters caused by the discharges develop. Because zinc has a low point of fusion and high heat evaporation, it absorbs a lot of the energy from crator formation. So only a small portion of the remaining energy transmitted diffuses into the wire's core.

The zinc layer basically forms a thermal shield. As a result, the wire core heats up less, allowing much higher discharge frequencies than does a wire without a zinc layer.

There is also an increase in speed when shops use the CC-Generator with brass wire. Running brass wire with the generator especially works well for tall and long workpieces such as punches and moveable parts in molds and plastic extrusions.

Meeting user demands

Charmilles engineers developed the CC-Generator to meet the demands of EDM users. These demands center around machining fast and usually fall into one of three categories. In the first, shops want fast machining but have no special demands in regards to surface finish. Shops with such a demand are often machining prototype parts, roughing dental implants and various medical prosthesis, or doing general mechanical engineering.

The second category involves shops that want to machine as fast as possible without surface deposits. When cutting electrodes for EDM diesinking machines or cutting work in general, toolmaking shops frequently wirecut tooling, such as dies or punches, with medium precision in a single pass. They typically want machined surfaces free of any deposits, so the tool is immediately usable.

Shops wanting to machine faster but with no alteration of the machined surface make up the third category. This demand is prevalent in such industries as carbide, for producing powder compacting dies and sintering dies; medical, for bio-compatibility reasons; mechanical engineering, for cutter sharpening; and electronics, for producing cutting tools for integrated circuits

The machines

The Robofil 240CC and 440CC wire EDMs feature Charmilles' CleanCut-Generator for high-speed burning.

In addition to sporting the new CC-Generator, the Robofil 240cc and 440cc wire EDMs feature Windows NT-based controls, high-speed threading systems, collision protection, and more. The control is said to drastically reduce the operator learning cycle by guiding them through drawings that illustrate available options. The system, which supports the Ethernet, is compatible with previous Charmilles systems, and it can link to other computers and robots.

ThermoCut is the high-speed wire threader on the 240cc/440cc machines. It stretches the wire and cuts it under the effect of heat, regardless of wire diameter and material. There are no parts subject to wear, so the threader is maintenance free. Because ThermoCut cuts wire burr-free and produces a tapered tip, wire easily passes through the wire guides.

Charmilles' collision protection safeguards the five axes of the machine during manual or unattended operation. It enhances machine performance and reduces maintenance costs in the event of programming or manual positioning errors.

To support heavy workloads, the fixed tables of the 240cc/440cc machines are thick stainless steel and rest on Rhenocast main-frame structures. These fixed tables prevent damage to the machine's X, Y, U, V, and Z-axis guidance and movement systems caused by accidental shock.

The 240cc and 440cc both deliver high-precision angles up to 30° irrespective of Z position. They do so across the entire length of Z-axis travel programmable from 0 to 15.75 in.