Hi everyone, long time listener, first time poster.

I have been designing my own CNC rigid gantry machine to cut metal with. I've really latched onto the idea of making it out of granite surface plates as they have many benefits for machine rigidity and accuracy. I've already acquired the surface plates and the idea is make a couple cuts to have have 3 pieces for the uprights and gantry, and then attach them to the base.

The construction method is use epoxy to glue the granite slabs together and restrain them with screws. It will save me a lot of typing to just say that if you've seen Piotr Fox's videos on his granite machine, you'll understand the concept.

Let me get to the design in progress. I've had many iterations to get to this point and this is clearly unfinished but I need some guidance from here:




FIRST, let me say this: I am thinking I want two ballscrews on the Z table but what you see is two possible locations for them, so both would end up in the same position obviously. So why those positions and not one in the middle?

I'm trying to make the construction as simple as possible at this stage, and I'm not totally sure about having pulleys with long belts and their effect on accuracy, so I'm trying to keep belts as short as possible. What is not shown is the mounting plates for the motors but you can see that the motor, ballscrew mount, and end of the bottom granite slab all line up on the same plane and it would be easy to make a mounting plate to tie them all together.

If you have any thoughts on one location or the other for the Z table ballscrews, or on why I should just go for a single ballscrew in the middle with a long belt to the motor on the edge, then please share.

The second part I'm having a rough time with at this point is the top ballscrew for the X axis. From a construction standpoint this is the easiest solution but it does bother me that it is a single ballscrew and it's outside the two linear rails. I'd like some reassurance that it would be fine, but if it's likely to be a problem then I'll work to move it to it's proper location, but the compromise there would likely be that I'd have to move the X plate further out with some riser blocks to make clearance for the ballscrew underneath. I may be able to make a cutout in the 1 inch thick X plate to fit it though.

Here's a shotgun blast of info:
These are 16mm ballscrews and I'm a little worried they might be too small for their intended purpose. Furthermore those are 20mm linear rails, and Nema 23 motors (already have these also). The base slab is 36 x 24 x 4 inches. The side slabs are 24 x 7.5 x 3 inches. The gantry slab is 24 x 9 x 3 inches which gives me 7 inches of rails separation there. As it is, there is a little more than 8.5 inches of clearance between the bottom of the gantry to the table, 22 inches of X travel, and 18 inches of Y travel but I could accommodate more travel there if I let the plate go beyond the edges of the base slab.

This is my first custom build. Please be gentle!