Quote Originally Posted by Jonathan View Post
For that reason it's best to mount them so the rail supports are facing each other. This evens out the deflection, so you effectively have medium deflection all round instead of large deflection when cutting one way and small deflection the other way.
I don't understand that - not saying you're wrong just that I don't understand. That said, we of course accept that bearings are not operating at their best when they're being pulled off the rail.

Mounting the rails and thereby bearings (on which the z-assembly moves along the y-gantry) top and bottom - with the bottom rail inverted - means (by my understanding) that the load is primarily carried by the top 2 (typically) bearings; the bottom 2 bearings preventing rotation of the z-axis assembly. With the rails/bearings mounted on their side the z-axis load is (again by my way of thinking) carried by all 4 bearings albeit with the force on the side of the bearing. So load-wise is 4 bearings carrying the weight on their side better or worse than 2 carrying the weight conventionally?

With regard to deflection (or moments), when the y-axis gantry moves in the x-direction, I rather assumed that where one pair of z-assembly bearings were effectively being pulled off the rail (tension) the other 2 would be under compression and vice-versa with direction of x+/x- travel.

This is interesting because I was planning to use the same design for my next larger (8' x 4') machine. I found this design (which I borrowed from another forum member's post) to create a low-profile y-gantry with easy mounting of rails and y-axis motor/ballscrew.

Now I'll have to get the pencil, paper and abacus out to calculate some moments; unless someone has an 'app' for that.

PS. Obviously, I accept the rails-on-their-side design must be wrong since most implementations don't use it - I still think it's quite a neat design though.

Thanks

Karl