…code, cg, electronics, 3d printing…

I picked up some small but powerful cylindrical magnets, 8mm diameter by 3mm tall, to incorporate into a new multi-part 3d print:  They’ll help hold the whole thing together.  I print mainly in PLA, and while it shrinks less than ABS, it still shrinks, so I needed to make sure that the cylindrical holes I design in my 3d model will actually allow the magnets to fit, based on how my Makerbot Replicator (1) prints things.

Using my micrometer, I checked the size of the magnets:  They were within a few hundredths of a mm of their spec:  Negligible change.

I designed & printed a simple rectangular volume to place my test holes:  It measured 60x30x3 mm.  In it I placed three different holes with diameters of 7.5, 8.0, and 8.5 mm.  For the record its print specs are:  300 micron layer resolution, 2 shells, 10% infill, blue PLA extruded @ 220 deg, HBP off, on blue painters tape.  My print nozzle is .4mm.  Took about 12 minutes, and was firmly affixed to the build platform before removal with no curling.

You can see the results below:

So what were the results?  Checking with my micrometer….

• The outer dimensions of the printed rectangular volume were very close to the 3d model:  .1 mm larger (or less) on X, Y, & Z.
• The printed cylindrical holes were each nearly exactly .5mm smaller from the 3d model:  As you can see from the above image, the modeled 7.5mm hole ended up being printed @ 7.0 mm, the 8.0 hole @ 7.5mm, and the 8.5mm hole @ 8.0 mm.
• Because of this, the 8mm wide magnet fits snugly into the 8.5mm modeled hole, which ended up printing with a 8.0mm diameter.
• Another interesting side effect:  Even though the rectangular volume and magnet were both 3mm high, and the magnet fit in the whole snugly but easily, I was unable to push it all the way through without some force:  I believe this is because the first few layers probably ‘squish out’ more on the build platform, causing a slight lip to form on the bottom edge.  Nothing a drill or file (or obsessive-compulsive magnet pushing) couldn’t fix though.

So this raises the question:  Why does the outer-volume of the rectangle match the 3d model within .1 mm larger, but the interior cylinders are all .5 mm smaller?

Based on my previous post, I wanted to get better results from my sand-cast.  Things I learned from last time:

• Even though I had a split mold, the vertical edges caused the mold to tear-out during removal.  I have a feeling this is due to the ‘ridged’ sides the 3d-printing causes, making the walls ‘grip’ more.  Because of this, I adjusted my 3d model to have slightly tapered sides.
• The extruded text on my first model suffered the same problem as the above issue.  To resolve, I simply made the text extruded much less.
• The previous split mold only had one anchor point, in the middle, to join the two sides:  When connected it gave more yaw-play than I wanted.  The new version has two anchor points on either side.
• I watched a bunch of sand casting videos and read over multiple sites to make sure I got the sand cast itself created properly.

Based on all that, the end result turned out really nice:  Sprue cut off with saws-all, imperfections ground off with an angle-grinder, and polished with a Dremel:

Here’s a shot of it freshly pulled from the cast with the sprue still connected:

And here’s a shot of the 3d-printed two-part mold.  It took just under 4 hours to print on my Makerbot Replicator.  200 micron layer resolutoin, gray PLA, 220 deg C on a glass build plate covered in blue painters tape:

And finally, here’s a movie of the furnace just before it was time to pour:

I shot it with bare hands:  I got really hot, really fast 😉