Orientation of Linear Guides along a Gantry

[Nachiket7]

I am designing a CNC to hopefully build soon, and in the design of the z axis, I was considering the orientation to mount the linear guides that I am planning to use (Picture attached). I drew up a small cartoon about why mounting the guides in a slightly unconventional way could be better for rigidity. The cartoon depicts a section view of the linear guides and where they would mount to the Z axis assembly. This relies on the assumption that most cutting loads will push the guides to the left or right.

I can't tell whether this is a better solution or not. If it is, I don't know why it isn't as common. The cartoon I made will help understanding this, and I would love to get some advice on this design / feedback on my assumption.


Thank you!

p.s. If I do choose to go with this design, I don't know for sure how I will mount the rails this way. I plan to use T slot extrusion to build the frame, and was wondering whether that was also the right choice for the gantry. I know very little on this topic, and I would love some advice. Thanks again

[pippin88]

I can't see any attachment

[Nachiket7]

Sorry about that.

Hopefully you can see this now

[awerby]

It will be a lot easier to align them if they're both on the same plane (as in your first picture) than if they're on separate planes (as in the second picture).

[pippin88]

Round rail bearings have unequal stiffness / ability to resist force.

They are worst when the bearing car is pulled up away from the rail.

Profile / square bearing like Hiwin/THK/NSK/many others/Chinese clones are soooooo much better than round rails. I strongly recommend not using round rails.

[Nachiket7]

Could you clarify a bit on why round rails aren't great? Wouldn't they still resist deflection better than just using rods for the rails?

I'm choosing to use round rail bearings because they are much cheaper to use than the regular linear rails, and I have a local supplier. My proposed build plate size is 50cm x 50cm so I don't think the loads that I will face are high enough to choose using conventional linear rails.

[peteeng]

Hi Nachi - Round rails are cheaper for a reason. Its also fine to select a component on cost if it achieves what you need it to do. Square rails are stiffer and support moments and they are more accurate. That's why they cost more. To match that extra accuracy in the build you have to have good surfaces to mount to. To answer your Qs:
1) Round rails are fine for what they are - you have to understand there limits
2) Unsupported round rails are not stiff. Round supported rails are stiffer then square rails are as stiff as their foundations
3) If your happy with round go round, if you can afford better use better. Peter

[pippin88]

Round rails don't resist rotation (around the axis of the rail). They are also weak when 'pulled up'

Round supported rails are much better than simple rods.

But for the time and effort and expense that goes into a machine, it is worth considering the cost of profile / square linear rails. There are cheap options from China.