Hlidskjalf, CoreXYU (dual X-carriage)

Currently a work in progress.

Started bolting stuff together in January 2017.
Began in June 2016 with the need for a larger printer than my Makerbot replicator 2, with varios mods.
Searched the internet in July for a 3d printer with a list of desired specs:
1. CoreXY kinematics.
2. 2 printheads (not dual extruder).
3. Linear guides for all 3 axis.
4. 300 x 300 x 400 HT build envelope, with heated bed.
5. Ballscrews for Z axis.
6. Easy part access. There's better materials than ABS that don't need an enclosure.
7. Fits on a 24" x 30" desk top.
8. 7" touchscreen, preferably with a handle jog wheel.
9. CNC capabilities for light cutting or engraving.
10. Under $3k.

By August had several candidates, none fulfilled all the specs. So did what everyone here does, innovate. In September posted renders on Thingiverse and started ordering the big ticket items.
Hlidskjalf (Odin's Chair) by Prot0typ1cal

October emptied my wallet into the build, and began 3d printing the bits and pieces on my Makerbot.
First was the awesome Toranado precision geared extruders finished on October 7th:
Prot0typ1cal 3D printed The Toranado Precision Geared 1.75mm Extruder - v2.1 by Toranado3D

November saw frame parts from Misumi USA, every week was like an early Christmas. Stepper motors, linear guides, ballscrews, 7" panel due, Arduino, RADDSv1.5 controller with extension, RAPS128 drivers, 1000W 24VDC PSU, plus little extras in between. Life was good.

December turned cold waiting for orders from China. Found out the Chinese linear guides were junk, as was most things ordered from eBay that shipped from China. Same with stuff from AliExpress and astonishingly, NewEgg. Overall, every Chinese supplier either shipped crap, or didn't ship at all. Sad, with round eyes, either repaired or re-purchased from U.S. suppliers.

That brings us back to the top of the page, January 2017.

If you have, or ever wanted, a CoreXY, you've probably stumbled across this site:
CoreXY | Cartesian Motion Platform

Well, this is how a CoreXYU works: