ok, first attempt using sketchup 8

[WandrinAndy]

I'm itching to place an order with chai.

That makes at least two of us Dean! I'm finding that the tweaking, particularly around aligning multiple pulleys and belts, is taking quite long but reckon it's worthwhile getting this right up front in the virtual world.

Btw, your design is looking great to this newbie. It amazes me how many design permutations there are... Suppose it's a good thing as it provides more ideas for us all to crib!

[JAZZCNC]

Dean go with 1605 for the Z axis for the extra resolution. Speed won't be a problem over this short distance.
Also the plate on the rear of the Y axis isn't needed and just costing money and weight it adds very little to strength. Seen has you have room on the Y axis at each side then if you want to stiffen up the joint between bearings a front plate then just use a " L " bracket plate on the side. This will add much more strength for fraction of the cost.
Thou if you use decent sized bolts then even this isn't needed.!

Regards the Gantry design then again I urge you to have some form of gantry sides that ties into the bearing plates which fasten to the RSJ. This will help greatly with twist and resonance, if you don't then I think you will have some.?
Gantry sides that dropped would allow the screw bolt to the web of the RSJ and do away with stuck up in air screw.? Both will work but dropped its out the way of chips and being caught with material etc when loading.!