We got a question about inertia and the Bukobot's ability to move around the

We got a question about inertia and the Bukobot’s ability to move around the mass of large prints on the moving Y platform (the same applies to Mendel variants and most other Repraps). The message suggested printing “large lego blocks with a 50% fill volume (ABS 1g/cc => (20x20x20/2 cm^3)) the finished weight could be 4 kg.” The message also asked how this would compare to the snickerdoodle [sic] with its platform that only moves on Z. I was supposed to keep my response short, but that didn’t really happen…

This issue never comes up for several reasons:

1. There’s no reason to print at 50% fill. A part printed at 50% fill won’t be appreciably stronger than one printed at 20-25% fill. It will take longer and be heavier, but it’s just not worth doing.
2. A quick back-of-the-envelope calculation suggests that the absolute fastest you could print with our nozzles is about 1kg every 3 days, and this is assuming much higher than normal print speeds, constant extrusion (the extrusion actually starts and stops) and several other unrealistic circumstances. This means that your hypothetical 4kg print would take a minimum of 12 days of continuous printing.
3. The plastic normally comes on 1kg spools, and spools larger than 5lb are almost unheard-of (I happen to have the only 10lb spool I’ve ever seen on my desk, and its size is constantly causing jaws to drop). Any smaller spools would have to manually be changed several times during such a print. Also, at $100 US, I got a pretty good deal on this enormous spool.
4. Even if printed completely solid, I estimate that the shape of a lego block (they’re mostly hollow and open on the bottom, if you actually look at one) measuring 200x200x200mm would only have a volume measuring about 1/6 or less of that volume, and it’s hard to think of anything else that you would reasonably want to print within a 200x200x200mm volume that would take up that much of the total volume. You might use the maximum extent of the volume on one, or even two dimensions, but using the entire volume on all three is extremely unlikely. If you want a solid cubic brick of plastic 200mm on a side, there are faster, cheaper, and easier ways to get it.
5. The motors are over-spec in terms of strength, and the weight resulting in deflection of the linear rods will become an issue before inertia due to printed mass does.

A more realistic hypothetical maximum mass for a single print is closer to 1/3kg, which is well within the capabilities of our printer to move around on the Y axis (the platform moves on Y, and the extruder on X and Z, I think you got that confused). It is true, however, that as the print volume of a machine expands beyond the 200x200x200mm build volume (which is roughly the largest standard volume for desktop 3d printers), the moving platform does become a potential issue, and these hypothetical larger-format printers would benefit from not moving the platform on the X or Y axis (though you quickly get the to the point where you don’t want to move the platform on Z either due to weight).

For the record, I think that printers like the Bukobot and various Mendel variants could handle moving around a kilogram or two on their Y axes better than the cantilevered Z axis of a printer like the Solidoodle (you called it snickerdoodle) or Makerbot could handle the weight of the same amount of plastic.

The printers can move the mass but any slender, tall object dislodges itself on sharp acceleration way before that.
They aren’t exactly bolted to the platform.

@Marcus_Wolschon I know you see this issue on your ToM. Having used a cupcake and several different printers with a mendel-style axis arrangement, this issue seems to be many times more prevalent on machines with the platform moving on X AND Y as it is on those where it only moves on one of those. Of course, it’s still less of an issue on machines with the platform stationary on X and Y, but if it was even half the issue on machines that move the platform on Y that it is on the Cupcake and ToM, there would be no repraps doing it that way.

Im in the process of purchasing a Shopbot Buddy CNC and this thing has a unique (for cncs) moving y axis. Ill be interested to see how that works out but its moving a sheet of 24"x32" plywood around against a spindle cutting head. Of course with much bigger motors and all.

The only problem I’ve seen documented for a moving Y machine is the tall printerbot. The part was half a meter of makerbeam, IIRC. It came unstuck from the bed somewhere around half way through the print.

I’ve watched slender parts wobble near the Z limit of my MG Prusa, but only enough to result in a slightly poorer finish. Im not sure the wobble was the slender part, rather the heavy build platform moving against the springs of the adjuster screws. The tall part just made it easier to see the vibration. Still, even this is only a problem of implementation and not a fatal flaw of the arrangement.

Myself, I’d like to build a moving-Z machine to get a constant, minimized moving mass, and I want to build larger than the moving platform can comfortably accommodate. No, I don’t plan to print implausibly giant blocks of heavy plastic.