First time build - Steel Frame CNC Router

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Thanks everyone for the comments, please see below revised design showing modifications are per your suggestions (I think ).

Attachment 17114Attachment 17115Attachment 17116

Still need to figure out some of the finer details for fittings, positions, etc but think it will be OK for initial review.

JAZZCNC - I checked and checked again and its definitely 50x50mm box section drawn in SolidWorks all to scale (although spindle might be out as this was a copy/paste job ). Might look off because of the machine being quite small compared to others (750 x 400 x 100 cutting area).
The ends on the gantry are welded and don't go all the way as I wasn't too sure how else to get access to the bolts. Gantry is all steel, would 4mm box section for this be OK also?
To confirm, as the size of the machine is fairly small, would it still need the 20mm pitch screws or larger motor for the X-Axis or can it do without? If needed, are they 2020 ball screws?

Clive S - Sorry I have never heard of the mesa route before and still after reading up on it I have no clue does the 7i76 replace the BoB and 5i25 act as some sort of motion controller?

RouterCNC - Simplest way I could think of pulling up the X Axis screws was just to stick it onto some box section and weld wherever it needed to be as well as using a 5mm aluminium spacer to help align things as suggested, will this do the job?

Thanks again for the comments and guidance guys.