Narrower arc from blunt tungsten

[WillieB]

A fellow on Welding Web (I think he's in witness protection so I won't name him) posted a chart of tungsten shapes, and how they relate to arc width and penetration (or lack of it). The chart is without explanatory information. I don't know if AC, DC or if it matters.
It shows that a very blunt grind produces a pinpoint arc, while a needle like grind produces a wide arc with little penetration. I've been grinding to a needle point trying to focus the arc. It seems that was the wrong thing to do.
We have discussed why, My uninformed guess has to do with the surface current phenomenon and the fact that electrons seek to continue running straight. A current on a cylindrical object will be concentrated on the surface. This higher current will tend to steer the rest of the current. As the shoulder of the grind is farther away from the work piece arc will originate farther away, if from a long pointed tungsten. From a shorter point it will be easier for current to continue along its "current" path, ionizing, then arcing across the gap.

I'm guessing here. Is there merit to my theory, or are other forces at play?

[H80N]

would like to see the chart....

it isn't the stuff from section 4 and 5 of this guide is it..??

http://www.millerwelds.com/pdf/gtawbook.pdf

[griff01]

A fellow on Welding Web (I think he's in witness protection so I won't name him) posted a chart of tungsten shapes, and how they relate to arc width and penetration (or lack of it). The chart is without explanatory information. I don't know if AC, DC or if it matters.
It shows that a very blunt grind produces a pinpoint arc, while a needle like grind produces a wide arc with little penetration. I've been grinding to a needle point trying to focus the arc. It seems that was the wrong thing to do.
We have discussed why, My uninformed guess has to do with the surface current phenomenon and the fact that electrons seek to continue running straight. A current on a cylindrical object will be concentrated on the surface. This higher current will tend to steer the rest of the current. As the shoulder of the grind is farther away from the work piece arc will originate farther away, if from a long pointed tungsten. From a shorter point it will be easier for current to continue along its "current" path, ionizing, then arcing across the gap.

I'm guessing here. Is there merit to my theory, or are other forces at play?

Diamond Ground products shows the same information. I don't remember if they gave an explanation.

Griff

[Ltbadd]

AWS handbook has a similar chart showing this. Processes like orbital welding, the angle the tungsten is ground to is an important parameter in the process and must be kept the same for repeatability.

[WillieB]

No, I tried to copy and paste, no such luck. It came with no text, a series of black and white photographs of arc shapes at 200 amps. Scale is not specified, it appears to be 1/8" tungsten. They show arc, and cut and etched weld. Using a large variety of incrementally long points including square cut. The trend shows deep penetration, and narrow arc, and bead from blunt ground tungsten, and wide arc, shallow penetration from long pointed tungsten.

I've been grinding long points in an effort to narrow bead width in aluminum, envisioning it delivering a pinpoint arc that would minimize the size of the heat affected zone. If this new information is good, I can narrow the heat affected zone with a blunt "golf pencil point".

In the photos arc appears to originate from the shoulder where the tungsten begins to narrow, flaring out from there. In the extremely blunt grinds or balled from cylinder, arc appears to blow straight off the end of the tungsten.

[taylorkh]

The explanation as I recall is that the electrons tend to be admitted from the tungsten at a 90 degree angle to the surface of the tungsten. They then are curved towards the weldment by the electric field. If the electrode is ground to a very narrow angle the electrons are emitted almost sideways and thus require more time and effort to be pulled to the weldment. This allows them to move further from the electrode and thus a wider arc. If the electrode is blunt, an unsharpened electrode being the extreme case, they are already "aimed" more towards the weldment and thus produce a narrower arc.

Think of firing a shotgun at various angles from horizontal to vertical and envision where the pellets will land. Sort of the same concept.

Ken

[gnforge]

Narrower arc from blunt tungsten

Had small amount of steel Tig on Friday. Tried blunt tungsten as similar to what is du for Alm.
Can't say as this small test that I agree.
I've always sharpened to point for steel & just didn't seem as good with blunt style grind. My arc seemed wider & less penetration than with pencil sharp grind. But was small amount of welding so not fair test but not so sure I like it as well. Will try again next week.
Greg

[Burnt hands]

These pages from Tig Handbook may help.

I use a sharp point and 200 Hz, advanced squarewave, 75% balance when I tig aluminum and find it works to give me a tighter, narrower arc.

Also have been playing with pulsing on DC and have found that I get a more focused arc at 2000 pps than without pulsing.

[Ltbadd]

Examples

This is from the AWS Handbook, eighth edition

[H80N]

I have settled on running 2% Lanthanated with a pointed or just slightly blunted tip on my Dynasties... and adjusting arc width on AC Aluminum by the frequency... higher freq.. narrower the bead...

here is an interesting video from Jody.... maybe a little off topic but interesting and related..

http://www.youtube.com/watch?v=bpanERwagaU

[OscarJr]

me?

Not sure if you mean me, but I was one of the first to reference those pictures from the Miller handbooks, and from ArcZone, and another from Google Images.









Now that I think about it, it couldn't have been me since I'm in no way in witness protection, LOL. But I do know that once I posted those pictures, others started saving them and posting them up once the same topic was brought up again.

[WillieB]

Oscar,
It wasn't you who posted and I saw, but it is the item I spoke of. I've been away, haven't tried testing the theory. Anything that will help to put the heat in the joint, instead of out in the field should make for a straighter finished piece.

[OscarJr]

Oscar,
It wasn't you who posted and I saw, but it is the item I spoke of. I've been away, haven't tried testing the theory. Anything that will help to put the heat in the joint, instead of out in the field should make for a straighter finished piece.

You mean "hypothesis".

But anyways, no need to test anything. The test results are on the Miller diagram. Also "surface current", or "skin effect" as it is called, only really applies to AC electricity, well above the 100kHz range. For DC welding, and for all practical AC welding, current will travel well into the interior of the conductor (tungsten).

[H80N]

Do not see a reference to type of machine... it will make a difference if applied to an inverter with multiple waveforms and adjustable frequency...like a Dynasty..
I have a feeling this is for a Mag Amplifier or chopper supply..

[WillieB]

Can I hijack my own thread? Define chopper technology. I mistakenly it seems guessed it was the way a syncrowave made square wave AC. People corrected me on that.