+1 clive ... i hear ya ... but then you go down the whole road of tuning fork ... and defeat the whole purpose of having the rails move

im going with bearings moving , like i drew on the original concept, its much easier and i doubt there will be any loss by going with this

silyavski ... i have enough travel on the rails and screws ... its just the design now. I have about 230mm between gantry beam and table top. but only having 230mm of movement isnt enough , i need another 120mm minimum to pull out a long series bit

for example - if i were to machine a block of foam for a plug that is 160mm thick .... and lets say i needed to machine right to the bottom of the foam , so i would need a bit of 120mm long ( the router clears the piece for arguments sake)

then the travel required would be the piece 160mm + 120mm bit + clearance 50mm = 320mm right ? if the move the block up like Clive suggested by 90mm then i would have sufficient clearance

going to redraw bearings the other way and have a scrutinize