I have wanted to add linear rails to my SO3 for well over a year (I had 4x 600mm HG15 rails in a box since Nov of 2018 
). I just wasn’t satisfied with the HDZ, I was getting a lot of chatter still and broke a few v-wheels when cutting into 6061. Let alone having to do such shallow DOC, really wasn’t pleasing and depending on the LOC of the endmill using only 5% or less of it. Also, what else I got to do? I’m not very artistic, so I just resort to modding my toys 
Typically, I have only ever seen people that upgrade to linear rails, go all out with ball screws as well. I didn’t really want to do that, I felt the belt drive is still efficient for my needs and at the moment only have a problem with rigidity. Nor did I want to invest in the upfront costs of both linear rails and screws all at once.
Seeing @Vince.Fab’s X-Axis linear rail mod really simplified my design in my head, originally I was going to try to mount the X rails at the top and bottom of the extrusion (this would reduce Z clearance however) but with Vince slapping the rails on the face, sacrificing some Y workspace, really made sense. I sadly wasn’t one of few that got his adapter plate 
So I ended up doing the design of the X and Y plates from scratch, copying Vince’s idea for the X plate.
I 3D printed a few iterations to test measures, clearances, etc before manually milling the first prototype on a Little Machine Shop [Benchtop] Mini Mill. Which by the way, was the first time ever operating a mill, was very intimidating, even for how small it is.
I made several mistakes while milling out the first aluminum prototypes, broke a center drill, was in the wrong coordinate system on the DRO for a couple drilling ops, clogged and welded a 1/4" 3F, stalled the spindle with too large a bite (also caused the stock to shift… which I didn’t notice). Thankfully all that didn’t compromise the functionality … completely anyway 
The installation of the prototype #1 went pretty well, but did have some alignment and clearance issues. For one, the belt idlers M8 screws interfered with the carriage blocks. So I had to grind down the M8 screw heads (because ultra-low profile M8 screws would have cost about $10 each x4)
Second, the belt idlers themselves were not high enough and slowly ate into the SO3 extrusion. For some reason I didn’t think to put shims between the carriage and plates 
All these problems were on the Y-plate, which I had tackled first being the more complicated part. X-axis plate pretty much went without a hitch. I then fixed clearance issues and other odds 'n ends in my design. So I could make the second prototype of the Y-plates, with the SO3 using the first prototypes! This allowed me to add rounded corners and chamfering, as injuring my hands while handling the first ones was not uncommon.
It was very awesome seeing the huge difference in performance cutting on the SO3 and the wall finish was amazing compared to the chatter mess before. Note: the face surface finish was done on the Mini Mill
Prototype #2 plates turned out great and are now installed 
), I decided to open the designs to the public freely. Don’t hate me Julien 
