what a pleasure to watch and read this idea  you are doing wonderful job I like the idea you can see some more similar ideas here https://bestgearly.com/

This is indeed very impressive.  Well done to all involved!  I think I'm about to embark on a granite CNC build of my own and all passive learning and the opportunity to build upon other people's knowledge is invaluable.  Thank you for sharing and I'm sure I'll be documenting my build if and when it moves from CAD to reality.

I'm going to give some feedback on your build.  You will know much of what I'm about to write (and more!) because of your experience so this might be of little help to you (the author) but might assist other readers considering this route.  My background of nearly 35 years has been in precision manufacturing and CNC.  I've ran high profile machine shops and even part-owned one previously.  I have a lot of experience to draw upon in design and precision manufacturing (assembly, hand finishing & inspection/calibration) but I'm not a know-it-all.

1.  You learned the hard way that granite worktop isn't flat.  So, if you want an engineered flat surface, start with an engineered flat surface.  Granite inspection tables or surface plates aren't massively expensive. Second-hand ones are available and they remain a sellable asset long after the build is finished.  Get what you can afford and base your design off the limitations of its work area.  A used table may have pits or scratches that you'll need to fill and polish back but this isn't difficult and can be done with the same epoxy you're about to cast the granite with.

2.  Don't mould against wooden panels for critical surfaces (ie where you're mounting guide rails).   Aluminium tooling plate would be better.  Google "MIC 6" or "5083 cast tooling plate".  It's likely though that you'll still have to stiffen these to prevent the weight of the 'concrete' from distorting it while casting.. unless you went crazy with the thickness, only have a light pour of concrete over the top, or if you could lay it face down on that granite table we just bought :)  Stiffness can be added quite cheaply by screwing good quality aluminium extrusion on the back.

There is a cheaper way than using alum tooling plate and to use wood.  It's possible to 'float' plywood on a bed of resin.  Smoothly apply mould release to the face of the granite table, pour resin over the face of the table and drop the plywood on.  Add any stiffness reinforcement (alum box section etc) to the back of the plywood first and you'll find that you can accurately transfer the smoothness and flatness of the granite table onto a (now very rigid) piece of ply.  The disadvantages are; time, mess, the risk of permanently bonding plywood to our new granite table (even with copious use of mould release, don't underestimate the amount of sheer force it takes to break this bond!) and of course wood is always moving.  I've used this technique and I recommend you start with small test pieces to prove you can do it.

Another option would have been to cast flat steel bars in place to carry the rails.  At least the steel faces can be worked to very high accuracy, even by hand, once the the concrete has set (if necessary).  I'm going this way with my build and intend to use "ground flat stock".  I'm hoping I can support it so that it doesn't distort while casting then I won't have to work the faces at all (to achieve the accuracy I'm aiming for anyway) but at least the option is there.

My final thoughts are just regarding the bulk of the machine.  We can see it's huge and massively heavy and currently it's complete overkill while coupled with C7 accuracy ball screws and that spindle.  You do however have the foundations to go on and upgrade it into a really accurate mill.  Getting it running is to be applauded and it's all yours to upgrade and improve over time - a route many people should take even if they're aiming for high accuracy and rigidity in their first build and I'm sure you've have had that in mind from day one.  But for the benefit of others, I'll say this; if all you want to do is push a 6mm cutter thru' aluminium, you probably don't need half a ton of granite to support that kind of cutting force.  If you have a cutter diameter, material and spindle in mind, concentrate on designing your frame around that, with the components to (even eventually) achieve the accuracy you're looking for.  Chances are there will be a number of CNC builds on youtube these days that'll come close to matching your requirements that you can take as inspiration.  In all credit to the author, most of my online research is coming from videos and build logs that are just a few years old at best, so there's been an explosion of information and ideas released in the last couple of years that he didn't have access to.  (Obviously not all information is good information but I'm finding it interesting to dissect all builds right now regardless.)

Looks like a long post.  I hope I haven't bored or offended anyone.

- Matt

PS.  Those slideway covers are beautiful :)

Awesome project. What's the total volume of epoxy granite? How much does it weigh?

Very cool project!

Thank you