Wire EDM ductile iron

We have had no luck trying to wire EDM ductile iron. It is a 1 inch thick tooling plate with 100mm fixture holes and dowel holes. We can rough cut fine , but we can't touch it with skim cuts. Called Mits and they gave us some things to try, but I am almost convinced there are traces of something non conductive in the metal. Anyone have any tricks they would like to share? We have finished the job with our jig grinder, but I would like to know if it can be wired. RJT
www.progtool.net

Ductile iron has thousands of small spheroids of graphite. I am sure this graphite "conducts" but I can think of no mechanism that would make it "cut", no more than you can "cut" a graphite electrode with wire EDM.

John Oder

John
We wire EDM graphite electrodes regularly with no problems. The thing that has me puzzled is we could rough cut with the wire very well, but cannot skim cut. Maybe my terminology should be "erode" instead of "cut".
RJT

Could have fooled me on cutting the electrodes

John

Wire EDMing graphite is slow compared to steel, tough on filters, everything in your tank gets dirty, but is is very accurate and the finish is excellent. If we can mill ,turn, or grind the electrode, that is always our first choice. This ductile iron is the first thing that is concuctive that has ever given us a problem. Solid carbide included (again, just slow and hard on consumables).

I think what you are experiencing here is that there are inclusions in the material that are non conductive. They are very small so that when you are rough cutting the spark is Strong enough to jump the inclusion and keep the cut going. When finish cutting the power is reduced and the spark stops when it encounters one of these inclusions. I have had this thing happen to me on several pieces of tool steel over the years. On on occasion it was a punch for a die I was working on. I rough cut the block overnight but when I tried to do my finish passes it would go to one spot and stop. It just wouldn't cut past. I tried reversing directions but the wire would cut up to around an inch of the spot that was a problem and either break the wire or stop altogether with reduced power.
After hours of trying to save the punch I gave in and took it out to see what was up.
After a visual that showed nothing unusual to the naked eye I glass beaded the part and looked under a microscope. What was apparent was a bright silvery line in the center of the material that extended from the stop point of cutting to the stop point from the other direction. I am sure that this line was the problem and must have been some type of crystalline form in the steel that was non conductive. Since the material you are trying to cut is cast I would be willing to bet it is full of microscopic non conductive particles. Can you get by with a single pass cut and be within tolerance?

No, we ended up jig grinding to finish it. Your explination makes sense, and your story reminded me that we had the same thing happen once in a piece of tool steel. I've never experienced being able to rough , but not skim in 20+ years of wire EDM. RJT