Back to the concept of a foldable RepRap using a scissor-like mechanism for the Z axis.
The main advantage is that this time the folding mechanism is part of the movement, and don't require any preparation for the machine to function... it will unfold as much as the part to print is tall. And makes for an even more lazy foldable RepRap : open the case, plug, print.
- tensioning the z-belt should be done in the upward direction, in order to tension against gravity
- change the aluminium flight case (not cheap) for a regular wooden flight case (easier to use and customize)
- move the linear rail 5mm closer to the pivot, in order for the end rail (printed part) to play its role of mechanical stop
- if the aluminium plate were made with a CNC instead of a laser-cutter, we could include the tapping of the holes (would save lot of time)
- the complete upper plate weight enough at lowest Z height to move the z-motor (if motor off) : caution when using M84, or use a higher gearbox ratio (14:1 is good), or a worm drive.
- creating a RepRap Firmware for the scissor Z axis, probably using a mathematic similar of a linear delta : solving a square triangle equation (will need help here)
- add LED lights
Determining the Z height at wich to print was tricky, a bit of play in the z-belt system allow something like 0.5mm of deviation, but here is a first test print. Without an adapted firmware or gcode post-processor to compensate for the non-linear z-axis, the 50mm test cube is 50x50x80mm (we can clearly see the gradually smaller progression in successive Z moves). Well, that was just to show it could be used and that I went as far as I could by myself :)
32 teeth pulley being bigger, it leave me not enough belt length to try, but the 14:1 motor is strong enough ! This time I can start from under the bed, as it is supposed to be parked here. But there is a little flex in the bottom plate (3mm alu) that still allow the belt to skip around the pulley sometimes (bad noise to hear).
Almost one year after... [blade-fuse changed], Duet2 works, and it moves ! But with the full weight multiplied by the scissors at the lowest height, the belt skip some tooth on the classic 18 teeth pulley and/or the motor does not have enough torque (5:1 reduction). I could use a bigger pulley to have more teeth in contact or a greater reduction for the motor.
once assembled there is more rigidity than what I feared to have, thanks to alternating the orientation of the scissors (good intuition here) the material works mostly in its horizontal plane, so even plastics seems fine (except that a scissor broke for pushing to far) - [video of a test move]
was imagined during the initial concept of the "FoldaRap2" mid-2012, then named "Mondrian" when it was not pursued at the time