Choosing between alternative X axis bearings designs

[eci22]

Before choosing the bearing position you need to fix the gantry.

Separate tubes are weak.

You should have one large tube.

Your pros and cons are correct. There is no "best" for this design choice. They each have pros and cons. It partly depends on the other design compromises you make, for instance how much stick out the spindle will have due to other decisions.

If you choose good rigid gantry uprights, for example large tubes and not thin plates, you can get back the lost space under the gantry due to rails underneath.

For me, the hassle of trying to align rails in alternative B and the increased complexity and weight of the carriage make it not worthwhile. I prefer alternative A.

Thanks pippin88, I think am leading to the same conclusion as you. My diagram wasn't the best, the rails will be on the tube going across only (along the x axis), the diagram shows that it also goes across the small section of box that is going in the Y direction but this won't be the case since this space will be used to house a motor and leadscrew bearing support, so the rails only sit on one section of tube.

If you are going to weld up the gantry from steel box section then you can use the epoxy levelling method to get a flat surface to fit the rails to in option A, but you should make it more solid. My own gantry is made from two pieces of 100x50 box section welded together to give a 200mmm high face for the rails to fit on. As always with a DIY build, your best option depends very much on what tools or other workshop facilities you have access to. Option A makes the Z-axis assembly much simpler to build as well.

By "more solid" do you mean by adding bracing? This is something I will be doing for sure. Yes they will be welded, though I am considering welding the highest x-axis guide box to the top of the frame and bolting the one closest to the ground to make fine adjustment in reference to the welded one, haven't thought this completely through yet. Do you have pictures, or a build log ? it would be great to see your machine Thanks for your input