Motorized Portal Illusion Sculpture

The portal illusion is similar to the barber pole illusion, where spinning a spiral shape gives the illusion of linear motion. The portal illusion strengthens this illusion by adding several spinning elements, all spinning at different speeds and directions. The design I shared here has 3 elements, spinning at 3 different speeds and 2 directions.

Ever since I saw the portal illusion desk toy, I wanted to make one. However, I did wonder how difficult it would be to motorize it. The driving nut used on the original is a simple solution, and the nut itself helps with the illusion, but it limits the stroke to the travel of the nut, and made it more a toy than a sculpture.

I had some false starts, but the solution I ended up with is simple, requires no special hardware like bearings, and can either be used manually, or be motorized. It does require a quite precise 3D printer, but nothing exotic.

You can find the original here: https://www.printables.com/model/414209-botanical-portal-desk-toy

Tools:

1. a 3D printer
2. Needle files
3. Hobby knife
4. Allen keys (2.5mm for the M3 screws)
5. Silicone lubricant (optional)
6. (Soldering iron)
7. (Dremel with a sanding drum (If you are too lazy to file))
8. Multimeter (optional)

For the main build:

1. Filament
2. 10x M3x12 socket head cap screws (DIN 912)
3. Glue suitable for plastic

For the motorized part:

1. N20 gearmotor (3-6V, slower is better. Something like 60rpm is already quite fast)
2. Wire
3. heatshrink
4. DC-DC converter
5. A trash USB cable
6. Fuse (0.2A-1.0A) (optional)

For the sculpture, some parts need to be 3D printed. The parts are all support free, the STL files are pre-oriented and split up into single objects. The parts do require quite a bit of precision, since the parts will need to work together. I have provided 0.2-0.3mm clearance on all moving parts, but depending on your printer, this may or may not be enough.

I printed my parts on a Prusa mini, and a Prusa Mk3.5 in PLA. PETG might also be nice for the sliding parts, but under normal circumstances PLA should be fine. I printed my base parts in white and matte black. The actual illusion parts are printed in bronze and silver. Filaments with particles, such as metallic should be avoided, since you might be able to see the parts spinning, giving away the illusion.

Most parts can be printed at normal settings, but the gear benefit from lower layer heights, and the actual spinning parts need to be as clean and smooth as possible. Any imperfection is going to show the spinning.

To make a full portal illusion, you will need:

1. 1x PI-P01-00 (the base)
2. 1x PI-P02-00 (driveshaft)
3. 1x PI-P03-00 (small gear)
4. 1x PI-P04-00 (medium gear)
5. 1x PI-P05-00 (large gear)
6. 1x PI-P06-00-1 (small spinner, body)
7. 1x PI-P06-00-2 (small spinner, top)
8. 1x PI-P07-00-1 (medium spinner, body)
9. 1x PI-P07-00-2 (medium spinner, top)
10. 1x PI-P08-00-1 (large spinner, body 1)
11. 1x PI-P08-00-1 (large spinner, body 2)
12. 1x PI-P08-00-3 (large spinner, top)
13. 1x PI-P09-00 (top)
14. 3x PI-P10-00 (pillar)
15. 3x PI-P11-00 (pillar cover)
16. 2x PI-P12-00 (driveshaft lock)
17. 1x PI-P13-00 (back leg)
18. 2x PI-P14-00 (foot)
19. 2x PI-P15-00 (bezel ring)

Printing the parts, some tricks can help getting the best out of the parts. While I work with Prusa printers, most features I use are present in the more popular slicers.

For the 3 spinners, I wanted a black base, and a colored spinner. To do this, Prusa allows you to change materials at a desired layer. You can slide the bar in the preview to the layer where you want to switch materials, and press the tiny plus on the right of the bar. This inserts a material change.

Some of the parts are quite tall. These parts run the risk of breaking of the bed. To prevent this, you can enable the brim, and if that fails, use a gluestick on the printbed to stick the part to the bed. The cheapest washable gluestick works best for this. This is essentially pure PVA in a convenient stick.

Some of the gears are quite small. Having lower layers here increases the resolution and makes the gears work better. In Prusa Slicer you can change the layer height throughout the part, but if this is not possible, simply lower the layer height for the whole part.

The gears might have stringing after the printing. The gears can be cleaned up using small files. If the stringing is heavy, you can also use a knife, but files are safer.

The spinners are designed to prevent stringing. My earliest versions had 2 and 3 arms in a single print, but this led to massive stringing and unusable parts. The small cross section of each of the arms on the spinner meant constant starts and stops. Still, there might be some cleanup on the spinners. Be careful not to scratch the surface of the spinners. Any mark that can be seen spinning will give away the illusion.

The gear, the base of the spinner and the top of the spinner can be glued together. On the top there are alignment features to help get a good alignment. On the gear there are not. Try and get all the parts as concentric as possible.

The diameter of both the gears and the spinners need to be made to fit to the base and top. There will be a seam that needs to be filed down on all the rings, and if you are unlucky (I was unlucky) the rings will still be off slightly. For this mechanism to work properly, the parts need to run extremely smooth. The gears push the spinners up, and any friction is enough to bind the spinners. When you fit the spinners in the base and top, they need to spin freely with no noticeably binding or excessive friction. If you are impatient you can use a Dremel with a sanding disc.

On the driveshaft there will be a seam on the front and back. On my parts, these bound in the base, and needed to be filed flush with the surface.

The bezels can be glued to the base and top. The bezels are only cosmetic, they obscure the rings, preventing you from seeing them spinning. This strengthens the illusion. The exact position of the bezels is not important, but try and get them concentric to the eye.

The mechanism uses a size type of screw, an M3x12 socket head cap screw. The DIN number, should you have trouble finding the part is DIN 912. Any screw of this size should work, but it is designed to work guaranteed with this one. With an allen key or hex screwdriver, you should be able to thread the screw into a 3D print directly. If you have trouble getting the screw started, see if there is an obstruction like an elephants foot at the base of the hole and remove this.

Place the driveshaft (PI-P02) in the hole on the side of the base (PI-P01), and check if it spins freely. If you are using a motor, remove the driveshaft and mount the (prewired!) motor in the base (PI-P01). Then mount the foot (PI-P14) to lock the motor in place using 2 screws. Place the driveshaft over the D shaft of the motor. Then use the 2 driveshaft locks (PI-P12) to lock the driveshaft in place. The locks are mounted using 2 more screws. In the top of the base, 3 sets of teeth from the gears should line up with the 3 slots.

The pillars (PI-P10) can be mounted to the top (PI-P09) with 3 screws. The pillar covers (PI-P11) can be inserted into the holes and can be secured with a tiny drop of glue.

The 3 spinners (PI-P06, P07, P08) with gears (PI-P03, P04, P05) can be placed in the base. You should be able to spin the driveshaft to see the spinners spin freely. If you have a motor installed, be careful not to force the motor too much. If the spinners bind or rise up, file the rings some more. When you are happy with the fit, you can optionally apply some silicone grease to increase the life. Do not overdo it, or the grease will provide too much stickiness and bind the spinners.

Now mount the top to the bottom. Align the 3 tops of the spinners in the top, and when all lines up, push the pillars in the base. If everything fits well, the spinners should still have some vertical play. Spinning the driveshaft should spin the 3 spinners without hitches. If everything moves correctly, secure the pillars with the remaining screws. If you have a motor, this will cover the back screw. You can omit this if you use the motor.

The sculpture is now assembled. Spinning the driveshaft should move the mechanism.

If you want the sculpture motorized, you can add a small N20 motor to the sculpture. In this step I will give instructions on how to make the sculpture USB powered.

Wire the N20 motor with some scrap wires. Polarity is not important in this step, and the exact gauge is also not that important. The motor should not draw much more than 100-200mA unless it binds. The motor itself is mounted prewired in the previous step. Around 10cm of wire should be enough for the motor.

Cut a USB cable. We want the USB A side (the rectangle) so we can power this project from a simple USB charger. Use a knife to carefully remove about 2-3cm of the cover. Most USB cables have a metal shielding. Fold this out of the way, and cut the excess shielding off.

You should be left with 4 wires, 2 of which are red and black (and might be slightly thicker). These are 5V and ground. The other wires (white and green) can be removed. Cut them at different lengths to prevent shorts.

If you use a fuse (I used a 300mA polyfuse) solder it to the red wire with some heatshrink. USB is usually pretty well protected, so this step can be omitted, but better safe than sorry.

Solder the 5V and ground pin to the input side of the DC-DC converter. The in and output are usually labeled on the back of the DC-DC converter. My DC-DC converter had some headers from a previous project, but you can directly solder to the pads.

If you have a multimeter you can now set the DC-DC converter with the potmeter. I set my converter to around 2V, but your motor may be different. You can adjust the voltage later if you have ever thing assembled.

Solder the motor wires to the output side DC-DC converter. Polarity does not matter. Check your wiring for shorts, and if you are certain that everything is good, plug in the USB to a charger. The sculpture should start to move. If it does not, check if you can try to start it by hand. Else try adjusting the voltage on the DC-DC converter. The voltage may be too low. If all else fails, unplug the USB and start checking for breaks in the wire. Leaving a motor powered but unable to move may burn out the motor.

Heatshrink can be used to cover all exposed pads and wires.

If the sculpture moves, you can decide if you are happy with the direction. If you want the sculpture to go the other direction, you can swap the motor wires on the DC-DC converter output side. This changes the direction of the motor.

You can adjust the speed of the sculpture by adjusting the voltage on the DC-DC converter. Lowering it lowers the speed, raising it raises the speed. Try not to lower the voltage too much that the sculpture might jam. A jammed motor can overhead and break.

Sadly, the design did not account for a wire in the base, and to keep the design clean without a motor, nothing is provided. A file can be used to make a small channel where you want the wire to exit. If you have it available you can also use a drill to make a hole for the wire.

The whole power assembly can be placed in one of the pockets in the base.

The whole portal sculpture turned out amazing. If you focus on it too much, you can see the individual elements spinning, but if you are further away, or squint a little, it looks like the whole core is moving continuously. While I thought it would be difficult to make this project motorized. I am quite pleased with the simplicity of this mechanism. The first few tries required a bunch of bearings, concentric shafts and many gears.The single driveshaft driving 3 gears at different speeds and different directions is quite elegant.

The included images give one of my old designs, with a bulky body and a set of gears. It also included an LED ring at the top and bottom, to give me the option to add a lighting portal to either side. This options was left out to keep the project simple.

This project was intended to be a quick one. However, challenges with printing the spinners and making everything move smoothly meant that it took over a week to get everything right. I printed around 5 spinners before I was happy with them, and was sanding for over an hour combined before I decided to start using the Dremel.