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Arc marks

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Arc marks

Postby Warrenh » Thu Jul 13, 2017 7:53 am

I weld aluminum about 95 percent of the time. Anosized aluminum is about 75 percent of that. We use wooden tables to prevent scratches. Grounding is done inside the pipe with an insulator. There has been a discussion about aluminum shavings on the table causing arc marks on the workpiece. There are some marks, tiny pits. Is it possible to get arcs to metal that is not grounded and not in the circuit? I can't see how it's possible but others are convinced. Anyone have any experience with this. Remember this is a wood table.

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Re: Arc marks

Postby Bingo » Thu Jul 13, 2017 8:21 am

Warrenh wrote:I weld aluminum about 95 percent of the time. Anosized aluminum is about 75 percent of that. We use wooden tables to prevent scratches. Grounding is done inside the pipe with an insulator. There has been a discussion about aluminum shavings on the table causing arc marks on the workpiece. There are some marks, tiny pits. Is it possible to get arcs to metal that is not grounded and not in the circuit? I can't see how it's possible but others are convinced. Anyone have any experience with this. Remember this is a wood table.

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If the pipe is grounded only other vearable is your torch, you strike a arc you complete the circuit. Ok so at my work we tig weld pipes a lot (in power horses) so our machines could be up to 75 yards away and we usually ground our lead by the welding machine and only pull one lead to my work area. Everything is conducive there but only place that arcs is my tig rig.
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Re: Arc marks

Postby Warrenh » Thu Jul 13, 2017 10:12 am

That is my unferstanding, too. I cant see an arc to a piece of metal not in the circuit and not grounded. These pieces are in a wood table.

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Re: Arc marks

Postby dave powelson » Thu Jul 13, 2017 11:35 am

Warrenh wrote:That is my unferstanding, too. I cant see an arc to a piece of metal not in the circuit and not grounded. These pieces are in a wood table.

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Why not wipe the wood table clean of the swarf before starting?
(that's kinda of given thing to do-anyway).
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Re: Arc marks

Postby Skylineauto » Thu Jul 13, 2017 2:58 pm

I would say you need to make a better ground point inside the pipe, or clean every contact point in the grounding circuit outside the welder. Wood has moisture and conduct electricity. with the high voltage of the HF start I could see it being an issue. Maybe look at at getting a wood surface that will not conduct electricity, wont scratch your work and can handle the temperatures.
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Re: Arc marks

Postby Poland308 » Thu Jul 13, 2017 5:48 pm

Humidity is probably a factor. Go get some mfd sheet that's 1/4 inch thick. Overlay your table temporarily and see if the problem goes away.
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Re: Arc marks

Postby nelson » Sat Jul 15, 2017 1:37 am

Radio frequencies dance around the skin of a conductor, true, but I don't think the frequency or the current is high enough to cause visible damage.

What species of wood table and what's it's moisture content?

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Re: Arc marks

Postby Warrenh » Sat Jul 15, 2017 7:53 am

Its plain plywood and we have high humidity. I work about 3 miles from the gulf of mexico and 1 mile from a bay. The tables are cleaned before use and there are two layers of plywood above the steel frame. Anodized aluminum had a hard finish that is hard to weld through, also.

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Re: Arc marks

Postby MarkL » Sat Jul 15, 2017 9:15 am

Warrenh wrote:There has been a discussion about aluminum shavings on the table causing arc marks on the workpiece. There are some marks, tiny pits.

Interesting problem. Can you clarify if it's known whether the arc marks are in the same location where the pipe touches the shavings? If not, the arc marks are either caused by a discharge through the dampness in the wood or the moisture in the air itself. As a first step, you could place the parts of the workpiece that aren't directly grounded on a separate table but at about the same distance from the grounded portion. If there are still arc marks, then it can't be going through the table, it's going through the air. If you have a sheet of material that's a good insulator, like phenolic or even a clean plastic cutting board, you could put that on the plywood and that would tell you if the arcing is through the moisture in the plywood. I can imagine in that kind of humidity that it's possible for the HF to act like a lightning strike and find its way to anything in the room that's conductive, regardless of whether it's directly attached to the welder's ground connection.
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Re: Arc marks

Postby dave powelson » Sat Jul 15, 2017 3:26 pm

Presumably the OP is bump welding this anodized, which means repetative, arc on, arc off.
Needs a really good ground, still needs to clean the table.
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