My sole issue with the design is making the rails adjustable, and not the bearing holders. Adjusting the rails and not the bearing holders depends on the bearing holders being identically spaced at both ends. This is fine if there's an eccentric bolt or similar mechanism to make that adjustment, but as is, it seems to me that a slight misalignment with the skate bearings at either end could result in a potential wobble in the gantry which would introduce tolerance errors.
I have built 3 of this design so far and haven't had any issue with the bearing being non-adjustable. When making the bearings I used spacers and scribe into the aluminum and then use an automatic center punch to place a divot to mark the hole position. After making all of the bearing assemblies, I place them on one of the rails and make sure the ends line up, matching up pairs that align, if any do not align, I make an additional bearing assemble that does line up. So far I have only had to make 1 additional bearing assembly on 2 of the machines.
I also use a spacer to ensure that the space between the mounted bearings is consistent. What I have found is that given that we are dealing with wood any spacing issues must be lost in slight flexing of the wood when adjusting the rail spacing.
I'm just gonna say, new here, but so far possibly the best plans I've seen for building without any weird impossible to get components or tooling needed. I hope I can start a build thread before the end of 2018.
A lot of work went into making the plans, and there are a few things I have learned helping build a few more of the machine that I need to add to the plans. I also have a variation in mind that might make the gantry a bit easier to build, but I need to test it out before I change the plans. I haven't been actively building much lately, hoping to change that soon.
Please let me know when you start building and I if I haven't updated the plans by then I will try to let you know what needs to change.