I would look at a free program called flatcam, it imports gerber files and you can do isolation milling with it.
There are some videos of how to use it and a good instruction manual.
It's. 1000times better than coppercam.
The trick is with CNC pcb milling is getting the depth of cut correct. I use a 0.2mm bit and do 2 passes to give .4mm clearance. It's all about getting the cutting depth, the way I cracked mine was just to lay out A couple of dummy tracks different sizes side by side then used flatcam to prouduce the g-code with a set depth then cut them out and used a loupe to measure the track width. Say if the track was sized at 0.4mm and if it measured smaller than that then your cutting depth is to deep again if it came out lager then cutting depth was not deep enough.
I now use that setting I got for all my boards and get perfect results.