Removing rounded nuts. Okay, so not the best of opening lines but we've all been there. The nut has rounded off at the corners, spanner just slips, ring spanner not much beter, mole grips etc just making it worse, no burning tackle. If space and access allow then a Cold chisel and a hammer may be the answer. You need to spread the nut and free it from the thread of the bolt. Position the blade of the chisel in line with the bolt which puts it at 90deg down one of the faces of the nut. A few good blows with the hammer will spread the nut and hopefully release it enough for it to unscrew from the bolt. If the nut is very old then it may even split and come off altogether. This technique is really only for bigger dia bolts and very dependant on access but has worked well in the past for me. Watch your thumb with that hammer