This is not quite a CNC issue but perhaps someone has had a similar problem. I have a Optimum OPTI F30 mill and I found that the clamping system (two long bolts) does not do such a great job in fixing the head in place as you move the head up and down the column.
The first issue was that the recesses cast into the head are very rough and they do not actually hold the bolt head in place when you try to tighten the opposite side to clamp the head. Once I shoved small bits of metal in there to stop the bolt heads turning it worked for a while. Now I have found that the nuts have almost siezed on thier threads and tightening or loosening them is very difficult, making clamping the head in place a difficult exercise.
I was thinking of making shafts that could be clamped in place using large solenoids or a similar electro-magnetic device.

Any ideas?

Jay