Saturday, November 3, 2018

An Update to the Sand Table Mechanism

After the MakerFaire I looked at the sand table mechanism and found one problem.  The magnet carriage was tilted badly because the spring under the magnet was putting downward force on one side of the carriage.  The magnet carriage, made entirely of printed ABS, was wearing out quickly from sliding on the 16 mm square, powder coated X axis tube.  Apparently, when ABS slides on powder coat, ABS loses.



I decided I needed to do something about it or the whole thing would not be very reliable.  The UHMW bearings on the Y axis are holding up well, so I decided that I need to figure out a way to put UHMW bearings on the X axis, too.

After much thought and many CAD models, I came up with what seems to be a pretty good idea.  I used a piece of 1" square aluminum tube that fits over the 16 mm square X axis tube.  I added UHMW bearings to fill up the space between the two.  I milled some slots in the tube to allow easy attachment of the belts.  The belts pull on the aluminum, not printed plastic, so there are no worries about plastic breaking due to the stress.

The new magnet carriage design.  Belts will attach at the vertical slots in the aluminum body of the carriage, and the magnet holder will attach using zip-ties.
It turns out the belt attachments are almost exactly where they need to be to keep the belts parallel to the X axis guide tube.

Making the UHMW bearings was the hardest part of all this.  UHMW is a little tricky to machine because it's very soft.  When I first tried to fit it all together, I couldn't get the X axis guide tube to fit inside the magnet carriage tube with the bearings in place.  A few minutes with some very coarse sand paper fixed that.

Here's the first test video with the new magnet carriage in place:

New magnet carriage design for the sand table. from Mark Rehorst on Vimeo.

I intend to sand-blast the powder coating off the X axis guide tube which should reduce the friction.

Here's the new carriage with the magnet and spring in place.  When the sand box is on top of the mechanism, only a few mm of the magnet protrudes above the blue magnet box.  The white bits at the center are the UHMW bearings.

UHMW bearings ride on all four surfaces of the X axis tube.





The spring fits in a circular hole in the magnet box, and the magnet is free to move up and down to follow the bottom surface of the sandbox.

Future updates may include switching to a Duet controller board and NEMA-17 motors to try to quiet things down a bit.  I think it will be a lot quieter if I run the motors at a lower speed, too, but then it won't be nearly as fun to watch it work.


10 comments:

  1. I saw your build on openbuilds. Thanks for linking sandify. I had a lot of fun making it.

    ReplyDelete
  2. Hey Guys, I'm absolutely fascinated with this table. I really want to make one now. I have most things to get it running, with the exception of the code. I have a little programming skills but do have a little experience with marlin (1.x) Can one just install marlin for 3d printers and be good to go? Sandify looks awesome!

    ReplyDelete
    Replies
    1. All you need is a 3D printer controller board. You don't have to do any programming. Sandify will generate the gcode.

      Delete
  3. Sorry Mark, but my question wasn't answered. I understand that sandify generates the code for the machine to draw, but I still need the firmware to install on the arduino. Can the Marlin firmware be installed to run the code that Sandify generates? Or is there something else that needs to be done?

    ReplyDelete
    Replies
    1. The gcode generated by Sandify uses only a couple commands (but you may want to add a couple more at start and end of pattern files) that represent a tiny subset of the commands understood by all 3D printer firmware. Marlin and any other 3D printer firmware you can think of should work just fine.

      Delete
  4. Great, thanks! With the spotify gcode, do you have a single gcode file with multiple patterns in it or is there a folder that is created to store individual pattern files into? Sry for the dumb questions, but thanks a bunch!

    ReplyDelete
    Replies
    1. That will depend on your controller board's firmware. I am using a Duet WiFi board that allows gcode macros to call other gcode macros, so I can put together a "playlist" file that just calls other individual pattern files. I don't know if you can do that with Marlin.

      If your controller's firmware doesn't allow macros, you can always copy and paste multiple pattern files into a single, long gcode file.

      Be sure you check my more recent posts on the sand table- I have switched from steppers to servomotors and the performance jump is incredible. At "normal" speeds the table runs almost silently, but it can be pushed up to 2000 mm/sec easily if more noise and the drama of throwing sand is acceptable/desirable.

      Delete
  5. I watched your video at 500mm. It was pretty amazing. Great job!

    ReplyDelete
    Replies
    1. Thanks, but 500 has been my target speed almost from the beginning. With the servomotors the machine can run at 1500 mm/sec without breaking a sweat. See: https://vimeo.com/439561856 and https://vimeo.com/423704831

      Delete

Leave comments or a questions here and I'll try to post a response as soon as I can.