Question..... how do i weld a vertical groove weld without the undercut on my stringer beads?
Undercut is caused by either welding too hot, holding too long an arc, erratic movements, not pausing long enough on the toes of the weld, arc blow, or some combination of all the above. Welding vertical groove welds with a 7018 rod set on DCEP (direct current electrode positive aka reverse polarity) almost always seems to set up arc blow on the second half of a 7 or 8 inch test weld.
The first half goes great and that second rod just starts acting up and trying to burn unevenly t one side or another causing undercut. Sometimes moving the ground clamp before welding the second half of the plate helps. What always helps is switching to Alternating current. The problem with that is that most procedures call for using DECP- reverse polarity and you are locked in to using DCEP on a certification test so you need to practice with DCEP.
AWS D1.1 Standard vertical groove weld tests are often 3/8" thick plate set up with a 22 1/2 degree bevel ( 45 degree included angle) with a 1/4" gap and 1/4" thick backing plate.
That is kind of a tight groove to get a 1/8" 7018 rod into and you don't have a lot of wiggle room..you certainly don't want to have to deal with arc blow. I would say that moving the ground clamp periodically, Having the amperage set right (100-115) and holding a good close arc length are the 3 main things needed to prevent undercut on this type of weld joint.