Concerns

[NewMatter]

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.)

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.

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.

4. What is the printer displacement? - Possibly as a ratio, to the build volume, because the printing area seems super tiny, compares to the printer?

The print area is 150 x 100mm. With this design, by necessity, the footprint of the printer must be at least 2x the X and Y print dimensions. For a large print area, this becomes problematic, but for a smaller print area printer, this is manageable. For instance, the MOD-t is has only a 16% bigger footprint than the Makerbot Mini, but it has a 50% bigger build area.

5. It seems like the axises are controlled via servos, rather than steppers, which kind of position feedback is used? - hall sensors? - Also why did you go with servos?

The reason for using servos is that you never skip steps. Even if your motion profile exceeds the capabilities of the motors, you may get larger positioning errors, but you will never lose track of where you are altogether like with steppers. And the servos use high resolution optical encoders for position feedback.

6. The two z-axis smooth rods, seems to be positioned very closely, why is this? - In terms of stability, wouldn't it make more sense to separate them?

The smooth rods in the earlier prototypes were close together for minimizing the dimensions of the Z-axis carriage. They will be a little wider in the production units. If you get them too far apart, you run the risk of the entire Z carriage cocking. Also note that because you don't have the dynamic loads of an extruder moving back & forth on the Z carriage, the Z-axis structure does not have to be nearly as stiff as with a conventional printer design.

7. I imagine the printer being very sensitive to vibrations, alike, as the bed is only held down by gravity. Have you done any research in terms of how much vibration the printer can handle before skipping steps and thereby ruining prints? - I would hope it's able to handle a kid bumping into the table, while printing?

As mentioned above, it is actually quite difficult to dislodge the build platform. Whacking the printer with a hammer might do the trick, but a kid bumping into the table it sits on it won't be a problem (unless he knocks it over altogether!).

8. Are the parts going to be consumer replaceable, alike with the reprap printers? - Also is the printer going to be extensible in terms of alternative heads? - Possibility of 2 hot ends, for say, PVA support?

This printer is a commercial product with some complex injection molded parts. There may be a few things on it where you could print your own replacement parts, but many of the parts (like the build table) would be difficult to print yourself. Also, this is intended as an entry-level printer and we aren't planning on making it configurable.

9. I'm actually a little sceptical about the x,y drive. All attempts of the same thing, within the reprap community has failed, and your solutions does not seem to address the major inherent issues?

We shared your skepticism when we first came up with the X-Y pinion drive mechanism, but the simplicity and low cost prompted us to explore it further. The first prototype convinced us that it had some potential. Since then, we've been through 4 additional generations of prototypes and have done a lot of printing. We wouldn't have brought it this far if we weren't completely confident that it was a reliable design that produced good quality prints.

[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?