Irving, thanks for that, I am unfortunately just a basic steel and ally basher and maths is not a strong point, I would appreciate it if you could help me out with the calculation, I currently use a rotozip for a spindle, I want to change this as soon as I can afford to, I think this is about 800w 20k rpm, I imagine the largest diameter router bit I would use would be 10mm and the worst material would be MDF which I understand needs a fairly quick feedrate.

I didnt have much spare time last night but quickly ran a test applying a force of 500N in the Y axis direction at the tip of the router with it at its lowest point produced a deflection of about 2.5mm !! much more than I remember although I have changed things a bit since the last run, I think that the channels on the side of the machine will be ok, but I need to change the X axis channel to something more substantial, I think that the 500N force is possibly over the top it is a stab in the dark !!