Starting to design out my build, looking for critique

[AndyUK]

I want to add motors to both the Gantry for the Y-Axis and the Z-Axis. For the Y Axis I thought about mounting them in the sidepanel holding up the gantry. For the Z-Axis I thought about lengthening the top plate and hanging the motor there. Would this be a problem in terms of placing extra torque on the Gantry?

As in, hanging the motor off the back of the carriage, the opposite side to the spindle? Don't think it'll be a problem in terms of torque on the gantry (else you're not going to be cutting much!), infact it may even help to bring your centre of gravity closer to the middle of the X axis bearings, which is where it should ideally be. The only issue I see is space, you won't be able to place the CNC up against something without loosing travel on X.

I added the Z-Axis. Currently, due to the different heights of the Rails and the Screw-sleds, the plate holding the spindle has a weird U-Shape. I could cut into the Z-Carriage and sink the screw into the gantry a bit to make this flat, but thus sacrifice gantry rigidity. Any thoughts on this?

I'd certainly be putting a second pair of bearings on that Z axis and lengthening the rail to retain your travel. Remember that the moment on that axis is Force x Distance to pivot - currently the pivot is the centre of your single bearing, the forces are at either end of the same bearing. With two bearings, the pivot becomes the point between them, and the forces are acting on the bottom of the bottom one, and the top of the top one. Your deflection in that direction will be significantly reduced.

How thick should the different Alu plate parts, like the bearing plates, sidepanels and Z-Carriage be? Currently, I have all those set at 20mm thickness.

What about endstops? I have access to a 3D printer, can I use that to print mounts for the endstop sensors, or would those be too flimsy? I do not see them having to take much force, but maybe I am missing something here.

20mm Ali plates for the main structures sound reasonable. Theres a spreadsheet on the forum for calculating gantry deflections that you should be looking at, and it'll give you an idea of how to approximate the cutting forces involved, and what deflection you'll expect from different materials. Regards endstops, I wouldn't be worrying too much about them just yet, but a 3D printed mount for the limit switches sounds like a good idea. With any luck they'll cut the motion before they take too much force, but having them break rather than damaging the linear motion components or your gantry sounds good ;)

What would be the advantage using a single motor? I am guessing the, more or less, guaranteed synchronization of the ballscrews?

Spot on - once you've set the relationship between the two screws with the timing belt, you don't need to worry about the gantry racking, or homing the motors independently every time you use the CNC to ensure a perpendicular X and Y. If you miss steps, they're missed on both sides - which is probably safer. Then theres the reduced complexity in limit and homing switches, and you don't have to buy yourself an extra servo/stepper driver which is £50-100 right there. You can also use a 4 axis controller and still have a spare axis for development later on.

... basically, I'm mildly regretting going for dual motors. But I didn't want the timing belt across the entire machine, so I'm paying the price!

Regarding this, would it also work if I were to flip-down the part of the Z-Carriage in the back that currently is mounted to the Screw Sled? That way I could freely adjust the height. I would only probably have to do it behind the current place where the screw is mounted, since I would hardly be able to actually screw anything in the other way around. Also, regarding the washers, would I just simply place washers between the sled and what I want to mount? The question arises, since that would massively decrease the area of contact between the two.

That sounds like how I've designed mine (see the build log).

Washers? I'd use some shim material which maintained the surface area.