Here's an alternative to sinking the build plate. I'm not claiming it's a *better* alternative in all cases...
Make up a C shaped structure to elevate the bearing that holds the lead screw above the mid plate. Have the open part of the C towards the build plate. There is a flat plate on the top of the C shape that the bearing mounts under. Think of it as a pipe supporting the bearing mounting plate with one side cut out of the pipe. The pipe supports the plate on three sides so it's not going anywhere. The side towards the build is fully open so you have no (major) clearance issue in that direction. There are a *lot* of ways you could build the structure. You would need a bit more than just the pipe to make it work (like some thread rod ...).
If the bearing is below the bottom of the hub arms, you lose no print area at all when the hub approaches the lead screw. You *could* set it higher and only loose the radius of the bearing past the lead screw.
If you want the arms to go level, that sets a max height on the bearing. Your nuts have to get down on the lead screw.
The motors would mount below the mid plate. If (for some odd reason) they don't have 6" shafts on them, you would need a shaft extension.
On the plus side:
This gets you back to a solid mid plate. The build plate would be up where you can see it and poke at it. Your hot end could have all sorts of junk hanging all over it with no major restrictions. Pulling full height parts out of the printer would not be an issue. Adjusting bearing location with it above the mid plate would be a bit easier.
On the minus side:
You have three gizmos to make up. I'm not sure that a printed part would be strong enough, but it might be. Certainly an aluminum plate (or three) and three big thread rods would make an adequate structure for each support. You have one more thing to get wrong / out of level / drift / mess up in the design.
Alternate:
You could make the pipe solid all the way around. That would not let you have a hot end with several inches of stuff hanging off of it. It would give you support on four sides. Moving the bearing around for alignment might be a bit more complex. You are pulling up on the pipe (pre load on the bearing) so having it whole probably keeps out one possible way for it to tip.
Truth in lending:
Yes, this is a way to get back to a design that fits an 18.6” build plate through a specific width door.
.
Make up a C shaped structure to elevate the bearing that holds the lead screw above the mid plate. Have the open part of the C towards the build plate. There is a flat plate on the top of the C shape that the bearing mounts under. Think of it as a pipe supporting the bearing mounting plate with one side cut out of the pipe. The pipe supports the plate on three sides so it's not going anywhere. The side towards the build is fully open so you have no (major) clearance issue in that direction. There are a *lot* of ways you could build the structure. You would need a bit more than just the pipe to make it work (like some thread rod ...).
If the bearing is below the bottom of the hub arms, you lose no print area at all when the hub approaches the lead screw. You *could* set it higher and only loose the radius of the bearing past the lead screw.
If you want the arms to go level, that sets a max height on the bearing. Your nuts have to get down on the lead screw.
The motors would mount below the mid plate. If (for some odd reason) they don't have 6" shafts on them, you would need a shaft extension.
On the plus side:
This gets you back to a solid mid plate. The build plate would be up where you can see it and poke at it. Your hot end could have all sorts of junk hanging all over it with no major restrictions. Pulling full height parts out of the printer would not be an issue. Adjusting bearing location with it above the mid plate would be a bit easier.
On the minus side:
You have three gizmos to make up. I'm not sure that a printed part would be strong enough, but it might be. Certainly an aluminum plate (or three) and three big thread rods would make an adequate structure for each support. You have one more thing to get wrong / out of level / drift / mess up in the design.
Alternate:
You could make the pipe solid all the way around. That would not let you have a hot end with several inches of stuff hanging off of it. It would give you support on four sides. Moving the bearing around for alignment might be a bit more complex. You are pulling up on the pipe (pre load on the bearing) so having it whole probably keeps out one possible way for it to tip.
Truth in lending:
Yes, this is a way to get back to a design that fits an 18.6” build plate through a specific width door.
.