Hi there !
This is my first post here, so bare with me…
I am in the planning and designing phase of my next to become 3d printer. All I got so far are the linear rails, 4 x IKO LWL12B, 390mm long, each with one carriage. Now, all the corexy printers I saw online with linear rails, use only one rail for the y gantry. I am going to try bowden setup first, but my plans are to use two rails for the Y, just like the designs for the round rods printers, thinking that maybe the extra rail will add to stiffness, if I decide to add direct extruders on top (and why not, maybe some micro spindle and make it a hybrid machine).
Those of you who have more experience and knowledge than me, let me know what you thing about my xy gantry design. I think I am going for the classical corexy with different planes for the belts. The weight of the y gantry will be around 650 grams with bowden hot-ends.
New sticky required. No more “Please bare with me”.
We should keep our clothes on in this Community. For decency’s sake!
I know it’s silly but this one keeps appearing. 
Every time you want to say this, think of the Big, Cuddly, Black or Brown things that inhabit various parts of the World. That’s right, Bear!
I like the design. Working on an MGN12 setup with a single rail for the X but may have to try this instead. Have to figure out how to utilize it in a CoreXY build.
The design look fine to me until you test it. It’s important in corexy how you are attached the belt to the gantry and how you tension them. That’s the key. In another not what material are you using for supporting the X to the Y
First: This is the X-Axis. It is like a silent agreement 
Second: Overkill. Use only one. I have a ±500mm long MGN12 for my X in my SkimmyV3 and I am sure, that even MGN9 would be enough. With this size, you !!!want!!! safe on weight at the X-Axis!
That will work, but it’s not the cheapest way to go or the most simple. I recommend flat plates at the ends and a single rail offset to the back upside down. It will be lighter weight, cheaper, simple and low cost.
If you use two rails this close together, the mechanics will be very over-constrained. The rails and carriage will have to be aligned JUST RIGHT to keep from binding up. That said, yes you can certainly do it, and it will perform well.
It is overconstrained in all directions for the y axis. If you are worried, then use 1 rail, but bigger.
A single flat plate spanning the distance would be better.
The reason why the round rod designs work is because the bearings can rotate radially (they’re not made for radial rotation, but are not restricted), in fact for this reason, 2 rods must be used.
Linear rails are limited only to the axis of movement, without play in the other axis.
As long as everything is kept loose when you are positioning the rails, everything should correctly space themselves when you go to tighten your rails down. If you are still worried about binding, just get long carriages or wider rails and you shouldn’t have a problem. Rails are incredibly resistant to binding. That’s why they were invented. However, they are prone to chattering if they are twisted to keep that in mind.
Like @Brook_Drumm said, go with flat plates. Using tubes will make it hard to mount your pulleys and route your belts.
They way your rails for your X-axis are positioned I don’t really think is optimised for what you are trying to accomplish though. You could use a single rail with a backplate to keep it from flexing. And the way you have your carriage set up makes it real easy to possible squeeze your rails together.
I suspect it will be fine. I did similar linear rail on my MendelMax 2 and I didn’t have any constraint issues. T slot extrusions have enough adjustment though I do think a big flat plate for the XY frame would be sturdier on top of better aligned. The double X rail probably isn’t necessary though.
Thanks everybody for your comments and advice.
Regarding the costs, this is no issue, as the rails are already purchased. My only concern was to get a working gantry with maximum stiffness, but as you guys pointed out, alignment and binding with this type of setup might become an issue. I might reconsider the mounting supports for the Y axis, it gives me headaches when trying to mount and route the belt. Indeed, having something flat simplifies things, but I would like to keep both rails in the setup and mounted vertical. Maybe I will go for 10mm thick aluminium plates. The square stock with 2mm walls seems a little bit wobbly.
Alignment can easily be adjusted with a test indicator. If you have ever used rails on router it’s a similar situation. Would suggest long blocks or even dual blocks. on one side.
