I don’t generally like to post my plans in advance, because I don’t want to jinx it and I don’t want to feel obligated, but this I need to share.
I am considering a stacked version with basically two MPCNC machines stacked on top of each other, with the Z axis traveling through both. Only the lower gantry would have a motor and lead screw to drive the Z axis vertically.
The upper gantry would constrain the Z axis from twisting. Both sets of gantry rails would have motors and belts like the MPCNC, and each motor driver would drive four stepper motors in series instead of two. I can get a 24V power supply to make sure I’m not running out of voltage with so many motors in series.
To maximize stiffness, I was considering extra stiff rails for the lower gantry and the Z axis, shown in yellow. The upper gantry need not be quite as stiff. This depends on the height, which I’ll get to in a bit. It’s fairly easy to find 1" (25.4 mm OD) tube with thick or thin walls, so I was thinking the yellow rails can be thick-walled, while the blue rails are cheaper. The green outer frame has lower stiffness requirement, and I was considering using 3/4" EMT to minimize cost.
I will need to design corner pieces that allow the legs to pass through, which should be straightforward enough. If I go with mix-and-match tubing sizes I will need to make a roller piece that rolls on 3/4" EMT but accepts a 25.4mm gantry rail, which I was thinking I would just take the “C” design and bore out a 25.4mm cylinder digitally on the STL file. The center assemblies would be unmodified “J” design since all rails are the same size.
Side panels of wood or diagonal braces would support the machine from racking. Mid-span supports on the lower rails would minimize sagging, but one interesting possibility is using the upper assembly to support the weight of the tool and Z axis, either though a counter-weight or some elastic. It seems to me the upper rails could sag a lot before it has a negative effect on the system, so why not have the upper rails bear all the weight. Apart from speed, I would have much less penalty for heavy tools or rails.
The height of the upper portion presents an interesting choice. Let’s say the lower rails are 1 foot from the deck, and the upper rails are x feet above the lower rails. Then one kg of horizontal force on the tool at maximum extension (minimum Z) produces 1/x kg of force on the upper gantry (in the opposite direction), and 1+1/x kg of force on the lower gantry. (This neglects the center assembly’s stiffness against twisting.) The mathematically “ideal” height is perhaps very tall, resulting in the Mostly Printed Telephone Booth :). But there are diminishing returns, so for practical reasons I will probably choose an overall height of perhaps 4 feet. Horizontal dimensions are not yet decided but won’t be more than 4 feet. With some thought into the corner design, it should be possible to make the height adjustable without disassembling the machine.
Cost wise, this build is definitely a higher expense. Most parts you have to pay for twice, but the tool, the z-motor and leadscrew, and the electronics are paid for only once. I’m estimating about $160 for motors, belts, bearings, PLA, and conduit. I don’t have a good number for thick-walled tube or the 5/16" hardware but my rough estimates are about $600 all-in.
I have mentioned before that I don’t have any “real work” to put this machine to use. This is entirely academic/entertainment. A certain minimum Z height does matter for a tool changer, and the tool change capability also demands more x/y area because it gets consumed by the tools. I have not yet looked seriously into a 4th axis (or 5th, ha), but I am expecting that it could require a decent Z height to fit under the tool. Also MPCNC as 3D printer would require some Z height, although it is a pretty silly waste of stiffness.
I know this “just in case” for everything is not the smart way to achieve any one purpose for real work, but since my purpose is experimentation I think it will serve me well.