I purchased some cheap 8mm shafting from China and it all turned up bent (up to 0.5mm run out) so I came up with a simple but accurate way to straighten them quickly with low budget tools. I used a DTI (dial test indicator) and some skateboard wheels and bearings.
Results were as good as plus and minus 5 microns. The worst case was plus and minus 10 microns. Way good enough for a 3D printer.
See my video to show how it was done.
www.youtube.com/watch?v=mRtIxG2co5w
