Cheers Eddy, i will try that tomorrow, i used to run at similar feed rates (2-3m/min) with my old machine, when i decreased feedrate it started overheating cutters due to decreasing the chip load, as i had to run the router at full speed to get full torque, so ill start with the numbers you suggested and check the cutter for over heating, im just a bit wary of running too slow feedrate/ too high spindle speed.

TBH i thought i was taking it gently, i used to take 6mm deep passes with a 6mm cutter with a wood router, and this spindle is more powerful... But ill try it shallower. Ive not used 4mm cutters before and yeah 12000 is a bit slow if there smaller then the cutting edge is not moving as fast as a 6mm cutter would, add that to me running too slow and i recon that could cause the problem.

Yeah your right, i do know this i work with birch ply a lot, just my brains a bit scrambled today...it does cut more cleanly on the surface when you cut down the grain though.

Thanks again

James