What worked from the CAD model
- Overall build size: Using all 500mm extrusion lengths made it a snap to assemble the frame.
- Nema 17 motor brackets: Yes, all the other brackets needed a redo, but those motor brackets sure are nice!
- CoreXY gantry layout: CoreXY is awesome. If you’re in doubt like I was, just build a machine and you’ll understand. The belts work together to keep the gantry from racking. Belt tensioning is also a snap, because both ends of the belt terminate on a mount.
- XY Exoslide setup: Securing one Y rail with two (or four) slides secures lateral and vertical movement, while securing the second Y rail only requires one slide to prevent rotation about the first rail.
Items that were improved upon with hands-on feedback
- XY belt brackets: It’s better to mount to the 2020 extrusion than the ExoSlides. Motion and belt loads are then kept separate, allowing for the ideal number of ExoSlides to be used.
- Z axis design: Originally, this was a three rail design to avoid over-constraint. I believe that is still the best design for a moving bed. With a moving gantry, the gantry edge with the single Z rail in the middle could be made sufficiently stiff, but if the corner of the gantry was pushed hard enough, due to friction in the mounting, it would settle 1mm either up or down. Four Z rails looks over-constrained. But the gantry is a flexible plane, so four rails allows the plane to be defined and rigid at all corners.
- Multiple Z motors: A single Z motor is nice in that the gantry (or bed) can’t come out of alignment when the printer is powered off. The issue is the friction in the idlers and anti-backlash nuts adds up to more than a single motor can power. The ideal solution is a Z motor for each rail, allowing the gantry to be auto-leveled (as in the Railcore II or Voron II), though this requires spendy electronics. A future solution to investigate perhaps...
- Z ExoSlide setup: It requires only four ExoSlides split over three Z rails to fully constrain (yet not over-constrain) the gantry. A picture explains this best, see below. Similar to the gantry bracket finding, it is best to separate the motion constraint in X and Y provided by the slides from the Z constraint provided by the Z rods where possible. The 20mm brackets secure to the Z mounts (doubled up for stiffness), and the slides act independently (on one side, the other side is still combined).
Areas still needing improvement
- Toothed idlers: Why is it that a properly engineered solution doesn’t exist for this? They have two miniature bearings that can’t be properly secured by a bolt as the inner races aren’t connected and easily exceed their axial load before the bolt has any real preload. The alternative is using two flanged bearings with the downside of the belt teeth riding on a non-conformed surface. If anyone has a better solution, let me know in the comments!
- Bed build surface: Still waiting on the TheKKiinngg PEI powder coated spring steel sheet in the 300 x 300 size due out at the end of June. I’ll then machine magnet pockets in the 1/4in MIC 6 bed plate and secure with a 3M adhesive sided mains silicone heater.
- Hotend mounted bed probe: Inductive M8 for the spring steel sheet.
- Electrical box: I wanted an electrical box that could pass a safety rating such IEC. That requires metal or special flame rated plastic (not 3d printed!) which led to a Bud Industries electrical box (~$25) with an aluminum plate to mount all components. The first take was not bad, but board placement to clean-up the wires still needs improvement.
As for the name of the printer, well the working title of ExoSlide Moving Gantry DIY Printer was getting to be a bit of a mouthful. I put out a call for suggestions on Twitter and Instagram, and Gabe Yoder's suggestion of ExoCube was simple and easy to remember. So ExoCube it is!
You can build this printer in a moving gantry or moving bed arrangement, though the moving gantry is the first setup documented. All documentation will be under: Guides > Projects > ExoCube Printer
Posted by Giles Medlicott on Jun 23, 2019