arcing to the fixture...

Jay Cee · Jun 29, 2007

Always had problems with aluminum parts arcing to fixtures if the surface of the part isn't perfect. I've had a lot of luck making the fixture out of the same material as the part. Now someone tells me to zinc coat the fixture and my problem will go away. Huh??? This guy comes from a non-EDM backround but says he knows this really good EDM dude who does this... Anyone heard of it?? Anyone have any other tricks they'd care to share?

BillDe · Jul 1, 2007

Never heard of that. No special tricks, I just try to get things fixtured as best I can, it's not a perfect world.

Bill

TheMetalDoctor · Jul 2, 2007

The arcing problem is extreme with aluminum. I never did understand why it happens when I have good ground contact with my part. Apparently there are stray electrical fields everywhere when edming. The problem is never noticed until you take the part out after 20 hours of erosion and noticed that it took a piece out of the zero plane in the machine. It's not nice. I guess the secret is to put ground straps everywhere you can.

TMD

customlll · Jul 2, 2007

I cut about 50% aluminum and never had a problem. I am cutting a 5" thick job now that is going to take 2 weeks of cutting. Am I missing something?? I have a Chamilles 310.

Jay Cee · Jul 2, 2007

I see it mostly on parts that get loaded and unloaded out of fixtures. Smaller parts seem to be most suspect. I've never seen it on a big burn or anything that's clamped directly to a machine rail.
The first few pieces will only have a little arcing mark on them. If you continue to run without sanding/stoning the fixture the arcing marks will get progressively worse.
It seems to me to be a result of any gap between the fixture and part being close to or equal to the spark gap. Once a few pits are in the fixture, that distance becomes more common resulting in more and more arcs.
I am really surprised to hear so many never having this problem. Maybe you guys don't cut to much fixtured small part aluminum??

MitsTech · Jul 3, 2007

Are you cleaning the aluminum surface going into the fixture immediately before loading? And I don't mean with a rag. Aluminum oxidizes almost immediately when exposed to air, and aluminum oxide is not conductive. That could be what is happening. If tolerances and part finish allow, scuff down the areas of the part to be clamped immediately before loading. Also keep your fixture 100% sterile clean between parts.

The other option is to put aluminum strips between your fixture and your part; although not knowing what your fixture looks like this may not be possible.

There was a somewhat similar thread about aluminum fixtures not long ago, but I couldn't find it in a search.

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arcing to the fixture...

Jay Cee

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customlll

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Jay Cee

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MitsTech

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