You spend hours designing an injection mold core or cavity, days CNC milling and grinding to get the size and shape obliged. The only thing remaining could be the EDM machining of the details. You have the graphite electrodes made up,which takes a couple more days, and finally everything is to in your CNC EDM machine.
You look into you job the next morning and inspect it while still in the machine, everything checks out, so you remove it, clean it up and create it for the mold polisher must have all shiny and marvelous. Soon, he comes in and tells you he needs to show you something.
DC arcs are primarily caused by one thing: bad flush. Even with central flushing or lateral flushing, there could be dead spots where EDM sludge builds up; the base of ribs happen to be a favorite hiding spot.
The sludge acts like part in the electrode and attracts sparks that arc across the dielectric fluid and faithfully reproduce their shape in the steel. If this continues to a length of time, you get a pit. More time it persists, the bigger the compare.
Sure enough, right at the base of the deepest part, a rib that will be there to generate the plastic part stronger, is really a pit. Another name for this pit is a DC arc, or zit, or some expletive is actually unprintable. Therefore that a contact small hole, or crater at the most inaccessible region of your mold that looks like exercises, diet tips bombed while you view it through a microscope.
Dirty oil is yet another common culprit to DC arcing. Occurs because the filters are full, or not fine enough to filter the EDM sludge and grit. It must be able to filter in order to a 1 micron size in order to ensure no pitting due to dirty lube.
Some manufacturers claim that the machines have software that prevents DC arcing, so you can a degree they do. Mostly this is by retracting the electrode out of your cut so flushing can take place. Some have a high speed oscillating effect that adds to the flushing as well. Then there are some that have improved cutting parameters permits adapt when the machine senses a DC arc.
Then you will find many, pits, not tall enough to scrap the part, but certainly big enough to make the polishers job much tough and possibly changing the dimensional integrity of the steel.
It may be like a complete waste of time to make sure you have adequate flushing, but in the long run, you'll save time and money. Even with the high end, new EDM machines, you needs to keep this as their intended purpose. Whether you deal with it through the circuitry or mechanics with the orbiting or the filter, don't forget to take care of the flushing.
All methods really get down to improving filtering. Even high tech, new EDM machines will pit. I am aware because I've had to polish many surfaces EDM'd by these units! You will find a nice looking surface that is not so in order to detect prior to you begin removing the first layer of recast.
EDM 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.