A friend is a metal shop teacher in a local high school. He has an older Hobart TIG welder and he tells me that his tungsten electrodes seem to lose their points much too fast. What are some possible causes for this assuming that the current is not set too high and that he's using the proper shielding gas? Engineman
Correction, it's a Lincoln Dial-arc. Engineman
He isn't running reverse polarity is he ?
Mart> >> A friend is a metal shop teacher in a local high school.
engineman wrote in sci.engr.joining.welding on Wed, 9 Feb 2011 09:56:22 -0800 (PST):
Check polarity.
What is the proper polarity as defined by electrode positve or negative rather than stating forward or reverse polarity? This may be his problem. How can he determine it for sure? Engineman
What is the proper polarity as defined by electrode positve or negative rather than stating forward or reverse polarity? This may be his problem. How can he determine it for sure? Engineman
reply:
What helped me learn it FINALLY was USNavy, except substitute SSN. Straight Stinger Negative. That leaves only one other one, so that has to be positive.
If the rod is positive, +, more heat will be on it. If rod is negative, -, less heat will be on the rod. Reverse symbol + has two reversed legs to make the + sign. Straight just has one straight leg to make the - sign.
Make the piece you want the hottest to be the +.
Steve
That took me a long time to grasp.
That's why AWS has long recommended that the terms straight and reverse polarity be replaced with DCEN and DCEP (Electrode Positive or Negative).
I like to think of the direction of electron flow through the arc, from negative to positive, just like in a Cathode Ray Tube (CRT) where the negative electrons go from the cathode (electron gun) to the anode (screen) - just in case you forget that the cathode is negative anode positive. The energy of the electrons is deposited entirely in the positive side of the arc (while conducted and radiated heat travels in all directions), putting the most heat in the positive side. For a high penetration rod like 6011 the use of DCEN makes the electrons hit the puddle for a hotter and deeper puddle, for a fill electrode like 7018 DCEP will melt more electrode with less heat input to the workpiece. (In electron beam welding *all* of the heat is deposited by the electrons, since there is no arc to radiate and conduct - a DCEN process where the E is an Electron gun.)
Obviously the goal of TIG welding is to melt the workpiece not the electrode, so DCEN is used (ignoring all variants of AC). Electrode tip life is also affected by:
Tip angle, where a sharp angle like 60 degrees total across the section of the tip will result in a hotter and shorter lived tip than a 90 or 120 degree tip. I consider 90 degrees as "standard", rarely use 60 (good for brazing small parts) and move toward 120 for current over 150 amps or so. (tip angle also affects arc size and shape, with a 60 degree tip producing the broadest least concentrated arc and the 120 degree tip producing the smallest most concentrated arc - consider that the electrons tend to leave the electrode surface straight out perpendicular to the surface. so one must make a trade off between tip life and arc shape.)
Tip flat, the size of the flat disk ground square across the end where the sharp point was, typically around 5 or 10 percent the diameter of the electrode. You can TIG with a sharp point (no flat), but, unless you are using a very low current like under 10 amps (where the sharp point is best) then the sharp tip will quickly melt back a bit, to about where the flat should have been. The ground flat at the end electrode works better than the melted back electrode because a ground surface with lots of grooves and sharp edges on a microscopic scale emits electrons better than a smooth melted surface. Higher current needs a larger flat.
(The tip cone and flat are normally symmetrical about the center of the electrode but can be ground canted to one side where limited access makes it desirable to direct the arc a bit to the side.)
Electrode size, too small an electrode will guarantee short tip life.
Lincoln used to have some pretty good books on the subject, probably still does, recommended highly.
Glen
In TIG Electrode Negative covers everything except Aluminum and Magnesium. Aluminum and Magnesium use AC. Electrode Positive can be used to get a nicer ball on your tungsten before AC welding aluminum.
You can TIG weld aluminum using DC Electrode Positive, but your tungsten will get hammered hard. The limiting factor is that the tungsten has to be at least as think as your metal.
Other things that can cause the tungsten to erode are contaminated gas, a leak in the gas fittings somewhere can taint your argon with air, running the high freq. continuously in DC (should be start only),using the wrong type of tungsten for your polarity, and using an undersized tungsten for the job.
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.