The frame, if welded properly, will not warp or have enough stress to affect a router style setup. the end use product tolerences are not that close and there is not enough stress on the frame to be of any significance. Many have built metal frames for router, laser, and plasma. Its when you try it on a mill to machine metals that this come into play. Browse around and ask specific question to those who have built and they will have solutions to minimal warpage....i.e. if your are using linear rails, shimming or epoxy beds under the rail after assembly. Most using round rod have designed adjusting collars to make these tolerances doable.