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.

The UHMW bearings ride on all four sides of the X axis guide tube.  The trick to making them was to get the thickness right- too thin and the fit will be sloppy, too tight and friction will be too high.  

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.