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These are the NSK specs on running parallelism....seems in theory I have about 16 microns for my 800mm spaced rails to play with....to be honest I think you can keep chasing numbers till you're red in the face and for a router that's going to be used for wood and plastics mostly with some aluminium then I'm not going to get my knickers in a twist about it.

I'm going to do a quick test as Jonathan did (I'm intreagued now) and then once the gantry is made bolt it to the carriages and see what they sound and feel like.