I'm working on a similar project, and what I've found is that our muscles are unreliable at assessing stiffness of structures meant for CNC machines. A frame that "feels" rigid can still flex by amounts many times greater than the tolerances we wish to hold. I've run many FEA analysis on my machine and keep having to add stiffness to reach my goals. One of the beauties of T-slot extrusion is it's easy to add rigidity by strategically adding members. A downside is it's not generally the most cost effective path to adding rigidity.
None of those extrusions are straight by the nature of how they are produced. You will notice that there will be height and spacing differences along the extrusion. As you tighten linear rails on uneven extrusion surface, they will bend along the surface. You have to figure out better ways to get straight surfaces out of them. There are companies that machine the extrusions to 1 mil tolerance along a 48 inch length. Get ready to buy a precision shim kit, because you will need it. Ball screws and linear rails will bind if they are not parallel to each other.
Nobody tell Tom about all the connectors Vslot is designed to use, just let him think drilling through-holes is how you're supposed to connect it in every instance, even in the prototyping phase....
Make sure your rails are perfectly parallel and level when you do the final assembly. Your end unit should glide more freely than that. I had the same issue when I built mine.
I am skeptical of that ball screw placement. Ideally you'd want to position it in a location that wouldn't create torque on the gantry, which in a non-loaded situation would be in-between the two linear rails. You could also position it on the underside of the carriage in a spot that would oppose the torquing force produced during cutting.
Don't fall for the straight linear rails. Over all, the aluminum extrusions are probably straighter than the linear rails ( they get stretched after extrusion). True reference can only be accomplished by using steel straightedges with the needed tolerances. I've built a router with a clamping area a bit over 1250 * 2500 mm. The X-Axis linear rail (same beefiness as yours) was off by ~3 mm (a 🍌). By rigorously comparing of different "straight" objects at hand and a laser (and reversing them) i managed to find out that the aluminum extrusion is only off arround < 0.5 mm max on 1500mm. So i managed to bend and fix the rail to the extrusion and getting it, ...so said, straight enough. Should be about +- 0.2 mm, at least i tell this to myself. It came ou that it often does not matter as much, because the flattened bed follows the inaccuracies and the usually plywood stock is fixed to it. On smaller peaces it is obviously way more accurate, according to my small precision straightedge. Good luck!
As I'm sure you've figured out by now, the trick with those linear rails is to pass the bolts through each hole, and start the Tee-nut onto the bolt, then slide the whole assembly onto the 8020 bar.
You're not going to get a stiff machine with bolted extrusions. Yes, you can site on it, but that means nothing. I can sit on a bean bag but I'm not going to use it to machine something. Here is a simple thing to try: put an indicator on the middle of the gantry touching the middle of one of the two support beams and push the gentry & pull the front beam and see how much deflection you get. That's going to be the maximum accuracy of your machine. I'm going to bet it will be in the realm of 0.5mm at best. That will break small endmills. Get a 10mm steel T plate cut to mount to the outside of each side and that will improve matters substantially, better than just doubling the vertical extrusion. Even better, make the vertical arm substantially wider and join across them with more steel plate behind the gantry, bolted (or better welded) to the vertical arms of those plates with as much crosswise bracing as you can. Best of all, join the side plates across the bottom with a couple of steel plates. The extrusion is fine for bolting rails to, but that's about all. As the aluminium and steel will move differentially with temperature you'll need to think through the mounting of the extrusion carefully.
I love that we're in the era of CNC machines that we used to be in with 3D printers. I remember the old days of plywood and other makeshift designs that users themselves would build, coming up with machines from scratch with little or no help from plans/etc online. Just a community of makers truly making. Nowadays we are seeing a similar renaissance-of-sorts but with CNC machines instead of FDM printers. A great journey to follow along with!
One technique for mounting linear rails is to tighten just the top rail (preferably against a straight edge) and loosen the bolts on the bearings and other rail. Then tighten the bolts on the bearing blocks that attach to that fixed rail. Next, tighten the bolts on one of the lower bearing blocks; that will set the distance between the two rails. While moving the carriage side to side to hold the two rails at the set distance apart, tighten the bolts on the second rail. And finally, tighten the second lower bearing block.
Pro Tip: you actually don't have to slide the nuts in from the side 😊. Their shape and the ball detent on the back allow them to be added anywhere on the track just by pushing them in sideways and then rotating them INSIDE the slot by 90°. This works perfect for the original item system - I assume it will be the same for other extrusion brands.
Degrease and regrease the linear rails to make them a LOT smoother. Often the stuff that comes in them aren’t great and also if there are any missing balls you can add some more. It makes a big difference to how smooth they move.
I am not sure if this has already been suggested, but filling the extrusion with epoxy granite or a mix of sand and epoxy would add a considerable amount of mass. This would help with resonance and keep things from shaking around as much. The vibrations from milling metal can be significant. I have an MR-1 CNC machine which is of a similar design. It weighs about 1000lb and still shakes around when milling.
