Concerns

[Skeen]

Lots of good questions:

1. On printers using timing belts alike the GT2 belt, we sometimes have the issue of backlash. Using a rack and pinion approach to the X,Y axis must incur a lot of backlash, by the mechanical nature of the gears? - I also imagine the backlash is going to increase, as the gears wear?

Because the table is gravity loaded against the pinion gears, the backlash is inherently removed. As the racks on the build platform wear, the table simply lowers by some minute amount. Eventually (that is, after many many hours of use) the build table may need replacing, but it is just an injection molded part that will be fairly inexpensive. (Having a spare table is probably handy to have around anyway.)

I imagine a certain amount of backlash will always exist, due to the tolerances, in the interlocking between the teeth profiles of the to medias. This is even a slight problem for the threaded rods on the z axis of most printers.

Just for comparison, to timing belt driven systems; how much can one forcibly move the table, when holding the axises in place? - That is how many microns backlash is there; teeth to teeth.

2. How is rapid acceleration and deacceleration to be handled, without skipping teeth on the spur gear? (On the x,y axis) - I fear that printing operation speed will be quite limited by the acceleration limits, which follow from this design.

We've tested accelerations up to 5000mm/s^2, print speeds up to 100mm/s and rapid moves up to 200mm/sec and skipping teeth was not a problem. You actually have to run into something pretty hard before you skip teeth. We've never seen an instance of teeth skipping during normal printing. Check out this video of one of the early prototypes: http://youtu.be/eqscOkfifoE to get a rough idea of the speeds and accelerations.

The thing I dislike about this video, is that you seem to be running from end-to-end. If the system is indeed not relying on end stops of sorts, then this doesn't matter, however I'd love to see the system running a spiral or something non regular.

That being said, the actual speed is of no interest, if precision isn't there, and I'm quite frankly not completely convinced about this system yet.

3. Moving the entire printing bed, which is undoubtedly a lot heavier than the hotend, will limit printing speed quite immensely? - On the 100mm/s video it seems like the printer overshoots quite a bit?

The print bed only weighs a few ounces, less than most direct-drive extruders. This was part of the motivation for the design - minimize the moving mass. There is no overshoot evident in the print quality. If you print a pretty heavy object (say 250g), then the mass does become more of an issue, but we've tested the build platform with adding quite a bit more mass and still achieve decent accelerations.

This is something that bugs me; You have a conflicting design goal here; You want to maximize the weight of the table, to reduce backlash, decrease teeth skipping, and ensure reliable printing. While on the other hand you'll want to minimize it, to ease printing speed, acceleration and deacceleration, and to reduce wear?

If you did indeed find a good trade of, then I'd to congratulate you, it must have been quite some work.

What is decent? - 2500mm/s^2?