Effector platform for LISA and questions

Effector platform for LISA and questions

Postby Dejay » Tue May 27, 2014 12:40 pm

Hello! I really like the whole LISA design, especially the printed arms and simplicity of the movement. I want to build a large and precise 3D printer and I somehow like the idea of a static print bed. But have a couple of newbie questions, sorry if this has been discussed before!
NOTE: Ok I had a couple of newbie questions, but since reading through the whole reprap thread they mostly have been answered. I've learned a lot about screws, buts and bearings! I'll still post this, maybe this helps someone interested in the design, please correct me if I'm wrong! :)

Is it possible to create a larger "Kossel style" effector platform for LISA?
From what I understand from the pictures and design of Lisa Simpson the arms meet in the middle and rotate around one central axis, and one arm doesn't rotate to give the effector a fixed orientation. If you would attach the arms on a circle around the effector, and make one fixed as well, you would get kind of eccentric movements that might be hard to calculate. How hard would it be to calculate the IK for this for someone who never done inverse kinematics?
It would be great to mount quad extruders and fans, or add tool changing capability.
"I have just recently made an experimental hub piece that will allow you to use any of the Kossel hot end mounts." (Nicholas)

ANSWER: "LISA needs the concentric hub" (Nicholas). I can't quite visualize it in my head but probably you can't just attach the arms outside of the center of the hub either because the forces would get too big or it would tilt or something. This probably limits the design to bottom mounted tools. Possibly a quad extruder could be mounted though, sacrificing build height.

Is LISA a slow printer? Is it very precise?
I was wondering what was meant with "Cons: Speed (I have been able to go faster than my filament drive can go.)". Does this mean that the movement of Lisa is too FAST or too SLOW? I would think that the direct connection of the motors with the threaded rods would create rather slow movement compared to a belt drive.
How big an issue is this?
How fast in mm/s did you manage to go?
Would it be best to use different threaded rods with a different pitch to increase / decrease speed?

ANSWER: You need special lead screws with a very high lead value (1" or 25.6 mm with a 8 start vs. the usual 2 or 3 mm lead for the usual CNC lead screws.

How good is precison and backlash?
Wouldn't slow mean a very high precision?
And is backlash low? As there is no belt or string and the connections should be very ridgid.

ANSWER: Precision and backlash is probably better than normal kossels

Threaded rod kinematics
Another (stupid) little question about LISA, as the arms are rotating around the threaded rods they change height, so it's a bit different from the rostock kinematics. Even if one motor is not moving and the other two motors make the center rotate around that axis, the top of the arm would change height slightly. Has this been thought of in the firmware calculations or is this effect neglectable?
ANSWER: Of course this has been though of! :)

How loud / noisy is LISA?
In Nicholas "First Good Print" video it seems very loud and screeching but in dannydefe's video it seemed rather quite?
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Re: Effector platform for LISA and questions

Postby Nicholas Seward » Tue May 27, 2014 5:31 pm

This is one way you can have more than one extruder but it make the inverse kinematics and calibration more of a headache.

The noise is because I have these great resonators attached direction to the steppers. Some rubber washers and gaskets could make it much much quieter. (Now that I have all the screws tightened up and the machine has been broken in the noise is less than in the video. It is the loudest machine I have. I don't really worry about it since it isn't loud enough to keep me up at night when it is printing in the other room.)
Nicholas Seward
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