Obtain the following components:
1) Rubber band - un-stretched but flattened length of 130mm, width 7mm. I purchased a bag of assorted sized beige rubber bands ("Jot" is the only information on the bag) from an office supply store and many of these were inside.
2) Jumbo paper clips - One cut and bent to shape as in photo, the second for use as a tool for pulling the rubber band through "Tube Rubber Band.stl".
3) Ball bearing - 14.25mm (9/16"). I purchased mine at a local hardware store.
4) Sewing machine thread - about 120mm. I obtained mine from my wife's sewing kit.
Print all the parts.
Assemble the rubber band tube. Start by forcing the rubber band into the slot in "Mount Rubber Band.stl", then with the slot facing away from the slotted end of "Tube Rubber Band.stl", insert the rubber band into "Tube Rubber Band.stl" via the slotted end. Using an unfolded jumbo paper clip as a hook, pull the rubber band through the tube then slide "Mount Rubber Band.stl" into the slots in "Tube Rubber Band.stl". Next, push the rubber band through the collar end hole (the larger of the holes) on "Ball Arm Mount.stl" passed and clear of the hole for "Arm.stl", then slide "Arm.stl" into "Ball Arm Mount.stl". The rubber band should now be stretched around "Mount Rubber Band.stl", then through "Tube Rubber Band.stl", then around "Arm.stl", and "Ball Arm Mount.stl" should sit snugly on top of "Tube Rubber Band.stl". Test the assembly by winding the rubber band using the arm about 10 turns, then release. If the arm doesn't turn freely, petroleum jelly can be used around the collar of "Ball Arm Mount.stl". When complete, press the slotted end of the assembly into the larger of the two holes in "Base.stl", this is a very tight fit so a small bit of sanding and/or trimming with a modeling knife may be needed.
Press "Rod.stl" into the smaller of the two holes in "Base.stl". This also is a very tight fit so a small bit of sanding and/or trimming with a modeling knife may be needed.
Press the ball bearing in "Ball Bearing Holder.stl". Depending on the color and machine settings, a little dish soap and light trimming of "Ball Bearing Holder.stl" with a modeling knife may work. However, I found using two ratchet wrench style sockets, the first being 1/2", and the second being 1/4", works best. Place the 1/2" socket standing on its end on a hard surface with the socket end facing up. Next, center "Ball Bearing Holder.stl" on the socket. Next, place the ball bearing on "Ball Bearing Holder.stl". Finally, using the 1/4" socket like a center punch, and a rubber mallet, gently tap the ball bearing into "Ball Bearing Holder.stl".
Bend the jumbo paperclip into the shape shown in the photo.
Using about 120mm of sewing machine thread, tie one end to the jumbo paperclip hook, and the second to the "Ball Bearing Holder.stl", leaving about 100mm between the hook and the holder. I used cyanoacrylate to secure the knots. Make sure the small hole at the end of "Arm.stl" is vertical, then insert the hook into the small hole.
To operate, unhook the ball bearing hook from the arm. Wind the arm about 8 to 10 turns, then release to see if that's enough winds for your rubber band to spin the arm about 8 times. If not, wind a few more times then try again. When successful, wind, hook the ball bearing to the arm, then holding the base securely with your hand, release the arm. I recommend safety glasses until you get the hang of it.