Offset Cam Automata (Caterpillar Mechanism)

What is it?

The caterpillar mechanism (the caterpillar toy) is a children's toy that resembles a moving caterpillar. The caterpillar toy is an example of engineering brilliance that captivates with its lifelike crawling motion. At the heart of this marvel are offset cams, discreetly hidden within its mechanism. These offset cams convert rotational energy into a smooth, vertical movement, mimicking the graceful motion of a real caterpillar. Through careful engineering, the cams ensure that each crawl is seamless and realistic, delighting both young and old with its mesmerising performance. 

This toy was made specially as a childrens' toy which can be used in schools, daycare stations, etc.

Tools:

-Center punch

-Drill press (6mm drills)

-Sandpaper+sanding board

-PVA/wood glue (depending on the material used)

-3D printer

-Hacksaw/junior hacksaw

-Hand drill+ drill bits (6.5mm bit + 6mm bit)

Supplies:

-10mm thick wood:

-2 planks of 150x80mm 

-2 planks of 100x80mm

-Hot glue gun/any adhesive

-6mm wood poles

-72cm long (doesn't have to be one piece, as long as can be cut into pieces of 9cm)

Take one plank of 150x80mm wood, drill eight 6mm holes each 10mm away from each other (using first the panel pin and centre punch then the drill press as seen in the first photo). Make sure to only do this to ONE of the 150x80mm planks.

Take the 100x80mm plank, drill a hole in the middle of the wood. I used measured it 4cm down and 4cm right to get my final hole location. Do this for both planks of 100x80mm wood.

Assemble the planks together with PVA or wood glue. Hold the structure together with rubber bands as it dries.

Design the offset cam on Fusion360.

Diameter of body of the cam: 35mm

Diameter of the hole in the cam: 6mm

Thickness of the cam: 1cm

I 3D printed 9 cams because my design needed 8 cams but I would always recommend to have an extra, just in case since hand drilling might be a little unreliable.

Next, create the body for the top thats going to the showing the moving result of the offset cam. If you refer back to the picture of the caterpillar toy at the top, this is going to be the actual caterpillars body in circles, on the top of the automata.

The body should be 1cm thick, and have a 6mm (diameter) wide hole on the breadth (thick part)of the side.

Create a follower with extended walls 2mm tall like in the picture. The thin walls are there to guide the rotating offset cams on the botton to make sure they follow through in the whole rotation.

Tip: make sure to test print all the followers once (just print one) to get started and test it out. See if there are any bumps- if yes, resize the walls. The orange follower was my test print, seeing as it worked/printed out fine, I decided to print the rest.

Cut eight poles with the length of 9cm from your 6mm wood poles. Sand it so that it is smooth and the 3D printed cams can spin smoothly on it. At this point you should have your followers, cams, and body printed out.

Take the body and hot glue one end of the stick to the body. it can be uneven if it doesn't do all the way it. If that happens, just sand down the rest of the body so that all the heights are uniformed. You can see the before and after here.

(If you can see that the colors of the cams are no coherent in all of the pictures it's because I had to do multiple trials to find the proper size that worked).

Cut another one of the 6mm wood poles into a 22cm long pole. This is the main pole that the offset cams will be glued onto and rotating on. Once you have that, place the pole through the bottom horizontal holes and slide in your cams. To make sure this mechanism actually works, you have to glue the cams in place so they are offset. This means that each cam is identical in shape to the other but it's positioning is just rotated around 20 degrees from the next. Once you have these rotated cams glued in place, it should look like this and should turn smoothly.

When gluing on the cams, its important to align carefully that each cams is directly under each holes on the top piece of wood.

To finish assembling the final product, insert the body+stick from the top and join it with your printed walled followers from the bottom. Glue it together with either hot glue or any adhesive. Be sure to align the follower directly on top of the rotating cams below.

If by any chance your cams and poles are not rotating smoothly and keep getting jammed, detach the cam and use a hand drill to widen the hole. Since the previous hole was 6mm, use a 6.5mm drill bit.

Another way around this is to sand down the poles using sandpaper, although I would not recommend this because this take more time as you have to sand this by hand.

If your cam is not following smoothly because the 3D printed parts are ridged and have uneven printing, Take a small piece of tape and put it where the ridged part is getting jammed to give it some extra cushion.