King creaky is right, the gantry sides need to be wider at the base. it is too slender and will cause the bearings to bind under load. It looks like you are using on 100m block on each side? I would change that and use 2No. 50mm blocks and then space them at least 100mm apart. The 2 thirds rule is a good basis (300mm high so 200mm wide) You could reduce the spacing between the y axis rails and lower the assembly which would have the same effect.

The z axis could be improved by putting the rails on the z plate and the blocks on the y carriage. this would lower the c of g at full travel and more importantly make the z axis very strong in its top position. meaning that if you want to make cuts in tougher material you just need to raise the work piece on the bed to use the increased strength.

Using a large plate for the rail mounts is a waste and is just adding weight, this is were you can economise. 10mm plate could be used for the gantry sides providing you also used some triangulation, or added another pair of cross members behind the rail mounts to increase the lateral and torsional stability. If it was me I would use 10mm plate for the sides and then use 4 No. cross rails in ali extrusion,(the 2 back ones could be lightweight as the are for bracing not direct loading.


Finally I would swap the orientation of the Y axis lower rail so it is upside down and a mirror image of the top one. open bearings are weakest on the open side so as you have it the cutting force pushing up is acting against the weakest side of the of the top rail so it isn't being effectively used. By mounting them back to back they offer a better spread of load capacity and make the machine more rigid for the same components. I'm not say what you have done is wrong or wont work just offering a refinement that might make better use of your components.

Hope this helps and is just my opinion