interesting convo chaps. :-) ive just watched a vid Jonathan's doing test cuts in aluminium on the second pass you can here what i class as pretty bad resonance of the cutter, that alone is mainly down to the rigidity and mass of the machine, now i to have use them blue coated cutters on jobs in the past, they are "OK" as chinese and korean cutters go but there are better out there but at a cost, personally for me using a router of any sort to cut steel is asking bit, especially when spindle plays a big role in the quality, and time it takes to machine a job in steel. i would convert a mill to cnc for use on steel any day and not rely on a cutter to be the saving grace. i can't see any router thats been made on this forum that could hold to a few microns. My machine at work does that but that way 6.5 ton.