Thanks for the comments Jonathan, I'll beef up the mounts and possibly look at pulleys. This router will be used for mainly balsa and ply so I cant see the need for the 20mm or 25mm rails, surely the 16 will be strong enough? The only Ali I'll be cutting on it will be thin stuff for horns, brackets etc. out of 1mm or 2mm thick.

If you look at the below router it's made out of 30x30 Ali and skate bearings and cuts balsa fine for making models! Yes it's not fast or particularly accurate but was fit for the purpose it was built for...

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I will be making a smaller stronger router/mill for doing Ali blocks/brackets etc at some stage in the future and this will be made with linear rail and all the other goodies!!