Dave,

As you've designed it you'll not be able to get to the bolts for the X axis linear bearings, they'll clash with the gantry uprights the same is true possibly on the Y axis where the Z axis rail mounting bolts might go. I had to re-design mine a few times because of this, it looked good until I realised I couldn't put it together. There's still a few things I'd do differently next time (yes there will be a next time!!) which I've found out from experience in building this one!! Your Z axis looks to be relying on the linear rails for strength? Why not use a nice slab of the 15mm plate you've got? The rails we'd normally suggest to go on the front plate of the Z axis as they add a bit more strength.

I'd also suggest getting 1610 ballscrews for the X and Y axis as they'll give you better speed, if you connect to the ball screw via pulleys and belts you'll a) reduce any possible resonance getting back to the stepper and also give yourself flexibility to cut wood at a decent speed and also gear down if you need the accuracy.

As Clive says, if you can stretch to profiled linear rails they you'll be upping the machine to a whole new ball game, they're quite cheap from China now, I got mine second hand off eBay from Korea. One thing you'll need to make absolutly sure of, as Clive says, is that your table is totally flat, i.e filling and scraping using a straight edge as reference to get a really flat surface, especially if you go with the profiled linear rails.