At the moment I am trying to build a stand for my pillar drill. I am making this out of 50x50x3 box. The corners are cut to 45 degrees and after tacking I welded the outside of the corners first. Moving from one corner across to the opposite side etc. After the outside of the corners I welded the top and bottom of each corner, while the other side was still warm. I.e. welded the bottom of a corner followed by the top of the corner straight after. Again, I welded them opposite as before. As last I welded the inside corners with the similar method as the outside corners. The workpiece is within a mm of square but it has warped by about 3mm. What have I done wrong and how can I avoid warping in the future? Many thanks.