I solely TIG at my job and last week I was basically doing a 90-degree weld of plate onto a piece of tubing where the plate had a hole in it and the tubing went through the hole. The material was stainless. The welds looked okay overall except that I was getting noticeable undercutting on the tubing. The only way I was able to get a significant decrease in it was to change the angle of the torch so that I was coming in almost parallel to the flat plate with the torch at nearly a 90-degree angle to the tubing. It actually lessened the problem a fair amount, but I've noticed that I've had this issue on more than one project at work even on plain steel, and though it's not always a concern with a given job, I'd really like to know what causes it and how I can eliminate it so that when the boss says, "No undercut on this. Period." I can do the job right.