Wednesday, February 14, 2007

 

Certifying the z-axis positioning integrity

This evening I finished writing and debugging the positioning function for the z-axis on Tommelise. I taped a piece of corregated cardboard to the tempered glass surface of the xy positioning table which I hoped would absorb a bit of overrun should that be required.

I had misjudged the length of the extruder barrel by several millimeters and had to mount the Mk 2.1 under the mounting flange on the xz positioning table with a washer for shim instead of on top. That was annoying but minor. I had set the bottom limit just a few millimeters above the xy work table. I measured the distance to the surface with the z-axis measurement module at 6450 pulses to the glass + 3 mm and used the move module to determine that the surface of the cardboard was at 6200 pulses (51.25 mm/2.02 inches) from the top z-axis limit.


I then tested this measurement for reliability and repeatability at a variety of places on the xy positioning table. I am quite pleased with the result. The xz positioning table would just brush the surface of the cardboard. Using a simple spirit level on the xy positioning table during its construction got the work surface a lot flatter than I had expected that it might.

Now I have to port the coordinated xy positioning module over from the old firmware program and certify it and then port the Mk 2.1 control module over and integrate it with the coordinated xy positioning module and then test that ensemble. I should be laying down some plastic this weekend barring unexpected difficulties.

Comments:
Nice Forrest! I had a thought about verifying the levelness of the horizontal axes. Put a small micro-switch (oriented to turn on when the surface is pressing against it) on the extruder and have it monitored by the controller. You could then move to corners and centers and verify to within the repeatability of the switch's operation the relative level. Would be lots of other issues but seems like that would help leveling anyways.
 
More coolness!

Instead of a microswitch, how about a volt meter set to ohms with the beep function? The cardboard is replaced by a flat metal plate. Then you could actually hear the high spots as it moves about.

BTW, read about your difficulties extruding HDPE and HPP. That's too bad, back to the more pressure problem.... :(

Maybe a fatter feedrod, like 6mm, should be able to get a much better grip on that given a similiar drive mechanism, no?
 
What problems? You haven't been keeping up. I've got HDPE, HPP and ABS extruding nicely now. As well, HDPE adheres to my cardboard working surface nicely. This weekend I'm doing my first live printing exercises with HDPE. I'm not trying to print anything so much as debug my software/firmware/hardware interface and find out a few things like "will HDPE make strong bonds to itself when extruded onto HDPE in a room temperature environment?" (preliminary indications are that it will) and "will printed parts shrink and warp as I print them" (don't know about this one).

I report much more extensively at the Tommelise website, the Clanking Replicator Project than I do here these days.
 
Oh, okay, that's good then. I read that you were extruding HDPE & HPP a while back, but then crossed an older post where you mentioned problems!! I often wish the blogger system -dated- posts, that way I would not tangle them up now and then...

Right now I'm repairing a large panel made of ABS, I'm interested in that plastic. If it sticks hot on cold, you could extrude a modified section onto an existing oversized structure without glueing...

Okay, now back to work with ABS, epoxy and fiberglass repairs...
 
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