[matridium]

Is it a bad idea to mount round linear rails like I have them in the picture? These will be used to make a CNC x y table.
One pair will be mounted by the support, and then another one attached by the blocks. This is no final design or spacing of blocks buy more or less for illustration purpose only.

[wizard]

If I understand your question correctly, your arrangement of the linear rails is pretty common. Probably more so for profile rails but I've seen machines (milling type machines) with round rails implemented this way. There is a problem with torque being applied to the rail in a less that optimal way, how much of a problem that will be depends upon what you are doing with the machine. In a mill type machine the cutter is always between the two bearing blocks instead of the reverse where the bearings are on the table and the rials on the saddle.

Now on a router type machine you may run into issues with this sort of design. I'm not even certain you are making a CNC router here actually as you say XY table. In the end you can do that for an XY table but that doesn't mean it is the best choice for what you are doing. A bit more info might help.

[matridium]

This will be for a milling machine type not router. I searched for this kind of config but could not find one like it. I posted here since many router machines use this kind of rail.
Thanks for the info so far.

[A_Camera]

Is it a bad idea to mount round linear rails like I have them in the picture? These will be used to make a CNC x y table.
One pair will be mounted by the support, and then another one attached by the blocks. This is no final design or spacing of blocks buy more or less for illustration purpose only.

This is THE best way of mounting them.

[matridium]

That good news. I figured thatitnwould be OK but wanted to see if there were any real issues. This will be my first CNC from scratch. My first was a modification of a X2 mini mill.
I'll probably post progress once I start the build.😁

[awerby]

This will be for a milling machine type not router. I searched for this kind of config but could not find one like it. I posted here since many router machines use this kind of rail.
Thanks for the info so far.

Milling machines don't use this kind of rail because it has too much slop. Machinists working in metal generally need to hold tighter tolerances than round rails like those can deliver. Routers can get away with it because they aren't expected to have as much precision. It's okay to mount them upside-down, but that won't help with their basic problems.