@Paul_Shaw , I agree that it is preferred to use the supported rail on the bottom or front side. In my design with Alu30 series, that made things too complex.
By the way, dust protection of the X-axis was considered, by simply mounting corner profiles on the sides, the rail and ball screw can be covered.
@Menno_de_Graaf @Paul_Shaw1 I’m half way through building a similar design to what you are both mentioning.
Originally I chose 30 series 8020 for the simple reason that the slot pattern is 30mm apart which matches the hole spacing for the supported linear rails. It is good to work with, rigid, and cuts easily on the table saw. But probably was not researched enough; I just wanted to get started and it made for an easy way to begin.
The frame of the machine raises the Y-axis rails up above the bed, similar to the pic that Paul added. As a result, I do not need vertical plates for my gantry, rather the gantry bolts directly to a horizontal plate that is also attached to the Y-axis linear bearings.
My gantry has the supported linear rails at the top and bottom, similar to Paul’s picture. I had not heard much commentary on that approach, but I wanted to reduce the amount of cantilever the Z-axis and spindle, out from the gantry. I’m hoping to reduce flex, by keeping it a bit closer to the gantry.
The gantry frame itself consists of a top extrusion (holding the upward facing linear rail) and a bottom extrusion (holding the bottom facing linear rail). Small, vertical members at each end provide for the height of the gantry rather than buying a bigger dimension 8020. And to stiffen the gantry given the relatively small extrusions placed horizontally, I have skinned the entire face of the gantry with a sheet of aluminum which is attached to the gantry extrusions the whole way around the perimeter. I plan to skin the Y-axis frame as well.
All in, I am currently tracking at about $1200 USD for everything. I’m probably about 50% done. Learning lots, and constantly finding myself tempted to improve the design even before I get it up and running on version 1.0. I keep telling myself get v1.0 working and then use it to build parts to improve upon itself…