By Randy Hough
The scene is all too familiar to anyone connected with electrical discharge machining, or EDM. The scenario goes something like this:
You spend hours designing an injection mold core or cavity, days CNC milling and grinding to get the size and shape required. The only thing remaining is the EDM machining of the details. You have the graphite electrodes made up,which takes a couple more days, and finally everything is set up in your CNC EDM machine.
Typically, you might make 4 cavities, which will produce 4 exactly identical plastics parts when everything is finally finished. Thing are going well, you have run the first 6 different shaped electrodes through the process, now there is just one more. Only another 16 hours and you will be done and onto the next project.
So, you get it all set up to run all night, and you feel confident that everything is as it should be, you can sleep peacefully without waking up in a start, wondering if you did this or that before going home.
The next morning
You check on you job the next morning and inspect it while still in the machine, it all checks out, so you remove it, clean it up and give it to the mold polisher to make it all shiny and nice. Soon, he comes in and tells you he needs to show you something. Oh oh…
Sure enough, right at the bottom of the deepest part, a rib that is there to make the plastic part stronger, is a pit. Another name for this pit is a DC arc, or zit, or some expletive that is unprintable. What this means is that there is a small hole, or crater at the most inaccessible region of your mold that looks like it was bombed when you view it through a microscope.
Now you need to get creative and fix it, if you can. Usually there is some convoluted way to fix it by cutting out the bad part and making an insert to replace the pitted area. Sometimes though, it is just not possible to repair it and the entire piece must be scrapped!
How to prevent DC arcing
DC arcs are primarily caused by one thing: bad flushing. Even with central flushing or lateral flushing, there can be dead spots where EDM sludge builds up; the bottom of ribs are a favorite hiding spot.
The sludge acts like part of the electrode and attracts sparks that arc across the dielectric fluid and faithfully reproduce their shape in the steel. If this continues for a length of time, you get a pit. The longer it persists, the bigger the pit.
Dirty oil is also a common culprit to DC arcing. This happens because the filters are full, or not fine enough to filter out the EDM sludge and grit. It should be able to filter down to a 1 micron size in order to ensure no pitting due to dirty oil.
Some manufacturers claim that their machines have software that prevents DC arcing, and to a great extent they do. Mostly this is by retracting the electrode out of the cut so flushing can occur. Some have a high speed oscillating effect that improves the flushing as well. Then there are some that have improved cutting parameters that will adapt when the machine senses a DC arc.
All of these methods really get down to improving flushing. Even high tech, new EDM machines will pit. I know because I have had to polish many surfaces EDM’d by these machines! You will find a nice looking surface that is not so easy to detect until you begin removing the first layer of recast.
Then you will find many, pits, not big enough to scrap the part, but certainly big enough to make the polishers job much more difficult and possibly changing the dimensional integrity of the steel.
Save yourself time and money
It might seem like a waste of time to make sure you have adequate flushing, but in the long run, it will save you time and money. Even with the high end, new EDM machines, you should keep this in mind. Whether you deal with it through the circuitry or mechanics of the orbiting or the filter, make sure you take care of the flushing
Electrical discharge machining is an amazing machine tool with tremendous importance for manufacturing. It is possibly the least known and most used machine for producing the endless variety of plastic products we use and appreciate everyday. Just look around you!
About the Author
| Randy Hough | Randy Hough lived, traveled, and studied in Europe for 3 years after college at Colorado State University, where he was an art major. After marrying in Denmark, he and his wife settled in Wisconsin, and he graduated from the Winona Technical Institute in Minnesota in 1978. Randy then became a mold making apprentice with the state of Wisconsin. After the 4 year apprenticeship, he worked at several shops to broaden his experience and finally settled in the beautiful Upper Valley of Vermont, where he lives and works. His main emphasis has been CNC EDM and the hand work side of mold making. At present Randy is involved in web site work regarding plastic injection molding/mold making as a means to make use of some of his accumulated knowledge. |
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