I am designing a printer based on the Mendel90. I thought to use four bushings on the X carriage; one in each corner. The extra bushing would add weight but I will shave more than that other ways so didn't worry about it. All looked sound on the CAD drawing. Then I printed the parts and assembled them for a test fit and found one of the four bushings did not touch the table top when I placed it there.
Three bushings contacted the table but the fourth did not. The gap between the table and the offending bushing is less than 0.4mm and I'm sure by playing with it a bit (I printed the parts at different times with different PLA and slightly different settings so something could easily be out because of that) I could make all the corners the same height but that made me wonder if this condition is the reason for using three bushings mounted in a triangular arrangement? It would be easier to ensure a planar carriage with three mount points.
And if the above is true might this be the same reason the Mendel90 has the Y carriage mounted at three points as well as the X carriage?
I worried that pushing a slightly un-planar carriage onto the two smooth X rods would in turn cause a slight twisting deformation in them. This in turn would put twisting stresses onto the X ends. All in all a bad scenario. Better to keep everything planar from the start.
