I am currently in college for a welding techniques program and by the end of april the course will be over. I have been using class time to make my own CWB tests using a simple jig provided by the college and filling it in with the mig. I have done 5 of these tests and they all failed due to the same defect. Once i machined the backing plate off and grinded it out flat I would always notice a hairline crack on the 90-degree side of the groove. It is clear that I am getting lack of fusion on my first pass down in the lower corner where the 90 plate meets the backing plate. I tried a couple machines, I tried running hot and every test i made has been grinded so i am not welding on any surface scale or other impuraties. On the last attempt I even tried welding it backhand. As far as the rest of the welds go its nearly perfect but once that backing plate is off there is a very thin line where the filler metal did not fuse in that bottom corner. I want to be able to pass this test with the mig everytime so I can go out and get a job, can anyone give me some pointers? thanks