"Filling" undercut?

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I was welding some 1/2" plate with 1/8th inch 7018 rod, at about 130 amps DC and getting some undercut.  I think I read somewhere that you can let the rod "fill" the undercut area before continuing on the bead, but I'm confused.  I thought undercut was from moving too slow or focusing the heat too much in one area?  Any advice would be appreciated!  Thanks!
Reply:Undercut is from moving too fast or too long of arc.Push the rod into the puddle as you weld,  Try a few more amps so you can't put the arc out.  I run 1/8" 7018 out of the oven at 145 or so amps on 1/2".DavidReal world weldin.  When I grow up I want to be a tig weldor.
Reply:Depends on position, generally it is because the welder is holding too long of an arc or an incorrect angle.(Direct the rod toward the undercut.
Reply:I wouldn't go by amperage settings as there could be wide variations.  For a starting reference, try to find a heat around where you encounter splatter about  1/16"-3/32"  diamater and a visable 'puddle' that appears to be about 2 to 2-1/2  times the length of your rod with flux.  Think smooth, no sudden jerky moves.
Reply:Your settings are good for what you are doing. You didnt indicate what position but always keep you and your rod square with your weld. Hold the edge a little longer and it will fill in. As you go and the pool gets hotter you will need to move a little quicker and hold time on the edges will be slightly shorter as well.
Reply:I would say slow down.  If you are experienced like you name fluxcore, then you are use to a faster progression.
Reply:Try using less of an angle, lots of times when a guy is starting out he will try to look straight onto the plate and hold the rod tilted right down so he can see, try doing the opposite making u pull a long arc. Put the rod at a 5 to 15 degree angle straight into the joint and watch the puddle from the side, once u get a puddle established and the correct size, drag it along maintaining the angles, lift ur hood up at the end of ur weld and watch the slag peel upOn a side note, if u get undercut you can always use another small pass to fill it up. if u were going too slow lets say on 1/4 inch id say that u could get some undercut and ur fillet is probly getting too largeNothing Ever Got Done By Quitting, Never Give Up.
Reply:I "ASSumed" it was a groove weld.  Could you help us out with the position and joint design?For me the heat is perfect when the rod is riding on the (not burnt yet) flux on the side away from the puddle.  Like dragging it at a small angle.Is your plate cooling between passes?  One pass for every 1/8" is a GUIDE, so 4 passes may be what you need.  How many are you taking ?A little more info and I will look it up in the book.David Last edited by David R; 01-26-2010 at 04:09 PM.Real world weldin.  When I grow up I want to be a tig weldor.
Reply:I was welding a 1/2" plate across the teeth of an excavator bucket, so the operator could use the bucket to scrape more than cut into the dirt.  I beveled the edge of the plate and ran some short beads about an inch long on each tooth.  The undercut was on the steel plate.  Thanks for all the advice this far, I will try decreasing my arc length and adjusting my speed a bit.  I've been arc welding for a year now, and for some reason I seem to only get undrecut when I'm welding thicker plate.  Anything smaller than 1/2" I'm pretty good with, with almost zero undercut.  Maybe its a mental thing
Reply:Could be heat at start?  Are you welding that bucket outdoors in teh cold?  If so, try a little pre-heating with a torch first.  Get the weld up to at least 75-100 F.  Plate's too cold, so puddle doesn't want to wet in normally, and this is throwing off your welding speed.  As others said, you may be accustomed to travelling faster with FCAW.  Watch the puddle, it'll tell you when you're ready to advance.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:I don't know exactly what you are doing, or how familiar you are with these sorts of jobs. A few thoughts from when we made one for a 3 foot bucket on our old trackhoe.We used a heavy piece of cutting edge from a loader instead of plate. The cutting egde was flat on one side, and the other side was tapered and the had a shelf  sort of like this /______. We flipped that so the flat was up, and the teeth were under the edge and tight to the "lip" where the taper ended and it stepped down. That way the teeth took most of the force, both digging and scraping. Also we only welded the 2 teeth that were just inside the corners, not all the teeth. That way all we had to do was knock out the 2 retainers and beat off the straight edge and attached teeth. Don't weld to the corner teeth! The shanks are usually set so that they usually spay out, so they are wider at the tips than the bucket sides. If you do you'll never get that edge off without cutting those teeth off. I don't have any picts of that edge unfortunately. We sold that machine a few years back and all  that stuff went with it. We've thought about making a wider straight edge (5'-6') to go on the new machine for grading, but he doesn't do that much finish grade work anymore..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Simply put, undercut is the lack of adequate filler metal. It can be caused by improperer rod angle, moving too fast, or having too long of an arch. Hope this helps.Threesleep