I was getting some flex in the Y Axis of my homemade cnc machine (believe it or not with those beefy aluminum extrusions) It was actually twisting... So I had to beef it up a bit with some 1/4" steel bar - here's how I grinded a 45 degree chamfer on the steel for my wheels to roll on!
Basically I mounted my angle grinder in my Z Axis and mounted it at a 45 degree angle. Then I made sure that the stel bar was exactly straight down the axis and then ran the grinder back and forth , lowering it little by little (rememeber I have some flexing) :)
What I ended up with was pretty good - not exactly perfect but it will work good for now - and there's not a bit of flex that I can produce in the rail at all now! :)
Here's a few pictures and a video...
The video is here (Right-click and "Save as")