I’ve been over both docs a lot because I’m still in the planning stage, but I don’t think I’ve seen any reason beyond “It’s got a 3” to build the LR3 over the LR2, especially because the LR3 requires more table with that single guiding rail. The LR2 even looks more stable with the use of the SS tubes, even though I know that can probably be a cost and a point against the LR2.
So why is the LR3 an upgrade?
Much more stable, less flex in the middle, endstops are the things that immediately come to mind.
Well, the baked-in end stops is a bonus. Much better than the add-ons I did for my LR2. I could do better now, I think, but the levelling factor for the Z rails is excellent.
The beam structure for the X axis is FAR more rigid, the strut reinforcements make the X axis structure more reliable and rigid. Less possibility of flex and sag.
The YZ plates and linear rails are also more rigid. Though the LR3 also suffers some at greater Z heights from the table, there is much more meat to the connection, meaning less overall flex and loss of rigidity as Z height increases. A possible downside of this is a limit on overall Z travel, other than the tube and lead screw length. (Mine is 50mm over the standard though.)
The use of a rail increases the reliablility along the Y axis. This gives the machine something definite to track and reference the Y axis to. This can be done with the LR2 by use of strips to guide the wheels, so maybe this is more a “baked in” vs. “add on” thing.
Full tool engagement. To me, this might have been the biggest thing. The LR2 has the plate plus some clearance for the #6-32 screws holding the rail holders below the router base. The router, of course, is designed to never let the collet nut come below the shoe base. As a result, you lose at least the base plate thickness from the amount of tool thst can engage the work. With some of these tools only having 12mm or so of workable area, and only able to have a couple of mm between the collet nut and the cutting area (or risk not having enough in the collet itself. --I had more than one endmill pull out of the collet in the LR2.) This can mean trouble working with through cuts. The LR3 toom.mount design means that I can lower the tool tonwithin a hairs breadth of the collet nut again and engage the whole endmill with the stock, while being sure that I have full engagement inside the collet.
Ability to remove it from the table. The LR3 comes off of the table in seconds. Loosen a couple of M5 screws (Or don’t, some people can just pull the blocks loose) and then pull out the dovetailed blocks on the other end. The machine plus Y belts is now off the table and you can use it all (except for the Y rail) as a flat assembly table. The LR2 by comparison is a lot more work. (Full disclosure, I don’t do this myself.)
Yes, LR3 requires more table area, but it’s pretty much the same footprint in reality because nothing sticks over the edge, and I think that’s a great advantage.