MIG 1/8" gap butt weld question.

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I don't have any pictures, but today we tacked up a butt weld with a 1/8" gap using 3/16" plate steel using .035 wire on the Lincoln powermig. The settings we used were 15.4 volts at about 158 amps (as opposed to the books suggested 110 amps and 19 volts) and although my welds were flush there was a little bit of undercut at that extended from the toes about 1/32".My question is, how can I eliminate this undercut and what is a good rule of thumb for the settings? Airgas says about 1 ampere for .001 thickness, meaning 187 amps for 3/16" plate. Is there a better rule of thumb to go by to get the voltages and amperes?I'm a perfectionist, so this little bit of undercut and dialing in the settings bothers me.
Reply:Hold longer at the edges to allow it to fully fill. Rule of thumb settings and such like those on the inside of most 110v "pocket-migs" are only intended to get you in the ball park. It is up to the operator to figure out where he she is going for a touchdown or a first down. The knobs buttons and dials were put there for [B]you[B] to play with. Try a joint at different settings, and remember at the part heats up you might have to dial back some on the machine to compensate.
Reply:Alright, thanks for the reply! My instructor says that the little bit of undercut is acceptable and would pass the test since there needs to be a cover pass over it. My problem with staying at the edges a bit longer though is that my bead usually ends up a little convex and I get a tad too much penetration. I'm assuming that this means that I would lower the voltage a little bit? At this moment we're just using preset settings defined by the instructor as this is to get used to the feel of welding, dialing the machine is coming later since it's only the 2nd class.
Reply:Don't do it for a living, and never been to a school(wish I had).Seems to me that any undercut isn't too good.  It means you aren't watching the puddle, and truly learning about the behavior of the filler.  Always push a little metal back into the toe(?) after you just cut into it.  You made a trench, and now you have to fill it.There won't be a cap on a single pass weld.  I think every weld ought to be good.  IMHO"Any day above ground is a good day"http://www.farmersamm.com/
Reply:What position were you doing this in and was the undercut on the backside or frontside?
Reply:Originally Posted by farmersammDon't do it for a living, and never been to a school(wish I had).Seems to me that any undercut isn't too good.  It means you aren't watching the puddle, and truly learning about the behavior of the filler.  Always push a little metal back into the toe(?) after you just cut into it.  You made a trench, and now you have to fill it.There won't be a cap on a single pass weld.  I think every weld ought to be good.  IMHO
Reply:Undercut is more a function of operator than heat.  Take some time, pause at the sides longer and see what happens. Think of it as putting on paint.   Put enough on so its about to run.David Real world weldin.  When I grow up I want to be a tig weldor.
Reply:Originally Posted by David RUndercut is more a function of operator than heat.  Take some time, pause at the sides longer and see what happens. Think of it as putting on paint.   Put enough on so its about to run.David
Reply:Originally Posted by farmersammDon't do it for a living, and never been to a school(wish I had).Seems to me that any undercut isn't too good.  It means you aren't watching the puddle, and truly learning about the behavior of the filler.  Always push a little metal back into the toe(?) after you just cut into it.  You made a trench, and now you have to fill it.There won't be a cap on a single pass weld.  I think every weld ought to be good.  IMHO
Reply:Originally Posted by Fat BastardMost codes allow some amount of undercut / incomplete fusion. Ask your instructor to show you some acceptable weld profiles. AWS D1.1 allows for some concavity as well as some convexity. Give it some time only the second class you have many hours of looking at that dang bright light before you need to worry about a small amount of nonfusion. Enjoy
Reply:Taking a look at my book once again (a hobart book) has me questioning it's reliability as far as the settings go when I compare them to other sources (Airgas and charts on welders).The book claims that for 3/16" steel I should be using .035 wire with 100% carbon dioxide, voltage set from 17-20, and the amps from 90-110. The other sources such as Airgas, the charts, and carious professional welders claim that the amps should be from 140-160 and the voltage from 19-22. What are your opinions on these settings since they seem to be around the same voltage? Would these settings only have an effect on travel speed and still be reliable? If so, then I'm guessing I should bump the wire speed down and focus on technique.
Reply:Originally Posted by espm1000Taking a look at my book once again (a hobart book) has me questioning it's reliability as far as the settings go when I compare them to other sources (Airgas and charts on welders).The book claims that for 3/16" steel I should be using .035 wire with 100% carbon dioxide, voltage set from 17-20, and the amps from 90-110. The other sources such as Airgas, the charts, and carious professional welders claim that the amps should be from 140-160 and the voltage from 19-22. What are your opinions on these settings since they seem to be around the same voltage? Would these settings only have an effect on travel speed and still be reliable? If so, then I'm guessing I should bump the wire speed down and focus on technique.