Fridge Gears!

Refrigerator Gears

I created three numbered gears to stick on the refrigerator:

• 150mm
• 75mm
• 50mm

The 150mm gear has a handle.  Turn it.

I find the action rather mesmerizing.  My 3-year-old granddaughter is obsessed. My 5-year-old granddaughter persists with endless questions while turning that crank.  Everyone has fun!

The numbers 1, 2, and 3 represent the number of revolutions each will make compared to the large wheel.

• The medium gear turns 2 times every time the large one turns
• The small gear makes 3 revolutions in that same amount of time.

A kid can learn a lot from this combination.

• Counting
• Odd/Even
• Multiplication
• Magnetism

This could be a nice learning experience getting to know your Prusa 3D printer.  I certainly learned a lot in the process.  Maybe you will too!  Some features you might be interested in:

• Pause to swap color
• Pause to insert magnets
• Press-fit bearings
• Settings to print a large flat thing that covers much of the build plate.

If you have any questions or thoughts on this, leave me a comment. I love comments!  If you make a set of gears for yourself, post a photo.  I'll be thrilled to see what you come up with!

I've included the Fusion 360 file.  Go ahead and make changes however you like.  Let me know if you come up with something better.  I'd love to hear about it!

Parts

These are the parts you need.

Printed Parts

• 150mm gear
• handle
• 75mm gear
• 50mm gear
• 1x large magnet holder
• 2x small magnet holder

Three variants

I posted three different folders so you have three different options depending on your preference:

• Tight Fit gives you undersized holes and oversized axles for a very tight fit.
• Loose Fit gives you parts that are sized exactly based on the specifications of the bearings.
• Separate Numbers gives you an option to print the numbers seperately.  You will need to glue into place during assembly.
  • You might choose this option, for instance, if you prefer to print the gears face-down on a smooth build plate to give the part an extra-smooth finish.

I've also included the Fusion 360 file to give you unlimited variations.

hardware

Bearings:

• 20mm Bore 32mm OD 7mm
• https://www.amazon.com/dp/B082PWB7R5

Magnets:

• Neodymium Disc Magnets
• 1.26 inch D x 1/8 inch
• (Metric: 29.5mm x 3mm)
• Do not use anything thicker! You could ruin your nozzle as it scrapes across the tall magnet.
• https://www.amazon.com/gp/product/B089CZG942 

STEM Fun

There are  ways to learn from these gear-magnets.

Rotations

Count the turns.

• The large gear labeled "1" rotates once for every two rotations of the medium gear labeled "2".
• The large gear labeled "1" rotates once for every three rotations of the small gear labeled "3".
• The medium gear labeled "2" rotates twice for every three rotations the small gear labeled "3".

Teeth

Count them.

Diameter

Measure it.

• The large gear labeled "1" is 2x larger than the medium gear labeled "2".
• The large gear labeled "1" is 3x larger than the small gear labeled "3".
• The medium gear labeled "2" is 2/3 larger than the small gear labeled "3".

Multiplication

Count the rotations while turning the crank:

Medium Gear

• If the big gear goes around twice. How many times does the medium gear go around:
• 2 x 2 = 4
• If the big gear goes three times:
• 3 x 2 = 6

Small gear

• If the big gear goes around twice. How many times does the small gear go around:
• 2 x 3 = 6
• If three times:
• 3 x 3 = 9
   

Odd Even

• If a gear rotates left, the gear next to it rotates right.
• If a gear rotates left, the gear connected two away also rotates left.
• If a gear rotates left, the gear connected three away rotates right.

Jam or no Jam

What happens if three gears are placed clustered so that each touches both the other two.

• See above: every gear that is three away must turn the opposite direction.
• Log jam!  (gear jam)

Can you place 5 gears in a circle? What happens.

• All connecting circles must have an even number of gears to spin.

Printing Instructions

Here are my thoughts based on my experience printing these parts.

Colors

Your imagination can take you anywhere.

But consider that black gears will not stand out on a black refrigerator. And white ones can blend into the background of a white washing machine.

One set that I printed, I varied the color of each gear size (blue, green orange), but made all three numbers the same color (tan).  And I matched the magnet holders with the numbers (tan).

For another set, I did the opposite.  The gears are all black and the numbers: red, orange, blue. And I printed the magnet holders all black.

Matt finish looks best for my toy-colored gears.  Glossy finish looks a little better for my black pieces. I'm curious to see what you came up with!

Print Order

I suggest printing in this order:

• 25mm gear
• small magnet holder
  • try magnet-holder-small-tight.stl first
  • If that is too tight, try magnet-holder-small-loose.stl.
• handle
• 150mm gear
  • same color as handle
  • select “tight” or “loose” variant depending on findings of small magnet holder.
• large magnet holder
• 75mm gear
• small magnet holder

General Printing:

These suggestions apply to all printed parts

• Always clean your build plate between prints. Especially before printing the large 150mm gear.
• Wash with soap and water especially if you used glue.
• Then scrub with 91% Isopropyl Alcohol and paper towel.

Printing the Magnet Holders:

These suggestions apply to the printed pieces that hold the magnets.

• There are two variants.
  • Try the “tight” variant first. This one if oversized at 20.2 mm.
  • If you print the “loose” variant, and it is too small, you can wrap it in thin .1 mm tape for a tight fit.
• From the slicer, insert a (G-code) pause at the top of the magnet cavity. The printer will stop at that layer so you can place the magnets.
• Optionally, before inserting the magnets, apply a very tiny drop of ethyl 2-cyanoacrylate Super Glue to prevent rattling during use.
• No support needed. (You definitely don't want support inside the magnet holes.)
• Print as "structural" not "speed".
• I printed with PLA and 80% infill. I'm not sure if this density is absolutely necessary.  I suspect 20% would be fine too.  This setting is leftover from some failed prototypes that used a much smaller “skateboard” axle.
• Magnets stubbornly want to stick to the build plate.
• Do not add glue to the build plate before printing the magnetic holders. Piece removal is difficult enough without that glue.
• Make sure build plate is completely cooled off before removing magnets holders. You risk tearing away the thin plastic layer that hides the magnet that is hidden inside.
• Use a scraper to remove the pieces from build plate to also help avoid tearing out the magnets.

Printing the Gears

These suggestions apply to the gears.

• From the slicer, insert a (G-code) pause to change color at the base of the number sticking out from the top of the print.
• On my Prusa MK4S I had to slow down top layer all the way to 20mm/s. If I don't set that parameter then I get strange wavy artifacts rippling across the top of the print -- thus ruining the 150mm gear especially.  (To me, this setting seems unreasonably slow. If anyone has a better approach to solving this issue, please post a suggestion.)
• Printing as "structural" slows everything down. This may have also helped with solving the rippling surface issue I had.
• When paused before printing the number, if you are switching from a dark color to a light color (aka from red to white or something), inspect and purge extra if needed.  The number is very small using about 3mm of filament. So any discoloration in that first 3mm from the spool will be noticeable in the finished print.

Printing the Large 150mm gear:

These considerations apply to the large 150mm gear:

• Print the handle first if you want to match the gear color.  This will save you an extra step of swapping out your filament color.
• Do consider glueing the large 150mm gear to bed with Layerneer glue or Glue Stick. There is significant risk the outer edge can warp and separate from the plate during printing.
• I added a brim for this part to further help it stick at the edges.

Assembly

Assembling the parts is easy but consider this:

• Press the bearing into the hole in the back of the gear.
• Press the printed magnet piece into inner hole of the bearing.
• If it is not snug you can wrap with a 7mm strip of .1mm masking tape.
• Optional: glue the bearing into place with ethyl 2-cyanoacrylate Super Glue.
  • Warning: be very careful to only apply glue to the outer rim and inner rim of the bearing. Stay very much away from the moving parts. The gear won't spin smoothly or it could even seize up!

Play

Stick your gears to the refrigerator or:

• washing machine
• file cabinet
• magnetic whiteboard
• steel baking sheet
• or (mostly) anything you can think of that is made of metal.
  •  Feel  free to attempt aluminium or copper if you are doing magnetism experiments! 😊

Enjoy!