Making a Toy Caterpillar Automata

This is a toy that looks complicated and seems like it demands real precision to work at all but honestly, it's pretty straight forward and there is a bit of wiggle room in the measurements. (Wiggle... caterpillar... never mind.)

The toy came together over several weekends of trial and error. Hubris and the occasional flying disk o' death off of the miter saw delayed me but I'm fairly confident that one of these could be made (but maybe not painted) in a day without too much grief.

So to begin at the beginning...

A few years ago I made my first caterpillar automata.  Ultimately, I  called it "The Very Hungry Caterpillar That Photographed Very Poorly." It was a gift for one of my nieces and it is still working today.


It had just six cams and I used five large beads for the body and a "doll's head" for, well, the head. There was nothing to prevent the body parts from rotating on their shafts, but that was fine.

I didn't have a set of plans for that one. It was just a proof of concept that became a finished toy. That was the same approach I took with this one; build and test as I went along.

I needed to make a toy for little one's school auction and for a friend's newborn. I wanted to try my hand with some more automata and I recently came across Woody Mammoth's version of a caterpillar toy on the web. I really liked the look of his, so it seemed like the way to go.

So... for those following along at home; here is Cam Terminology 101:

The cam is attached to a rod that runs through the pivot point. When the rod is rotated the cam turns pushing the follower up through the slide and then allowing it to fall as it continues to turn. So rotary motion gets turned into reciprocating motion. The follower is aligned over the pivot point and is always in contact with the cam.

Now on to the build.

The cams were cut from 1 1/4 pine dowel on the miter saw and are exactly 1/2" wide. They need 1/4" holes through their face that go all the way through. I used a jig that I usually use to drill offset holes in wooden wheels. The center of the hole is 5/16" from the edge.

I used a pine dowel for these. One batch seemed to have rougher edges that splintered a bit but a lot of factors can go into that. They cleaned up fine.

The case for the caterpillar was made with 1x3 (3/4" by 2 1/2" actual) pine boards. The case was 8 1/2" long (8" is fine as well). The holes on the top piece need to be right down the center and I used a 9/32" bit to allow clearance for the 1/4 dowels that will be the followers. I drilled the hole for the head at 2" in from the end and the first hole for the body at 3". After that, it was 8 more holes each 1/2" from the last one. ( I finally got smart and set up a template for this.)

The front and back pieces were 3" tall. They have a 9/32" hole right through their very center and need to line up since the rod need to go through both holes.

Once the holes were done I glued one end to the front of the top and the other to the back of the bottom to make two "L" shaped pieces. I put a coat of beeswax and mineral oil on them but was careful not to get any on the surfaces that will need glue during final assembly.

The followers are 2 1/2" long pieces of 1/4" oak dowels (I needed to make 10 of these.) The base of the followers were little pine blocks I cut that were 1" long (left to right looking from the front of the finished toy), 1/2" tall (up and down) and about 7/16" thick (front to back.) They each have a 1/4" flat bottomed hole drilled in them about 1/4"deep (using a Forstner bit.) I found that drilling the holes on either end of the stick already cut for thickness and height and then cutting to length on the band saw was much safer than cutting it to final size and trying to drill the holes.

I glued the followers together with their bases and checked that they moved freely in the holes on the top of the case. I hit the holes with a rat tail file to make sure nothing was too tight.

I took a 10" or 12" piece of 1/4" oak dowel and did a quick test to see where I could start safely lining up the cams. I glued the first one in place and then it was just a matter of lining up the next cam so it was a little off line from the previous one but glued to each other. I had a 1/4" difference in the rotation for each one.) The less the difference, the smoother the action. When finished it looks like a screw. I did double up on the cams for the head. I matched them for this one. On the other one I just cut one dowel 1" thick instead of 1/2" and drilled the offset hole in it. The thin disk at the front is used to help lock the rod in place so it doesn't move too far back and forth once the crank is attached.

So then there is some test fitting to see where that locking disk needs to be placed to have the followers line up with the cams. By having the bases of the followers be a little less that 1/2" wide and the cams exactly 1/2" wide, it makes it run pretty smoothly. I hit a few of the "feet" of the followers on the beltsander once or twice just to give a little more room. Once I was happy with the alignment, I glued the locking disk in place and the top and the bottom together.

Now the the body...

I purchased a 1 1/4" diameter poplar dowel from the local old school hardware store. I used it for the 7 main body segments. I also cut one disk each from a 1 1/8" and a 1" dowel for segments toward the tail. Each segment needs to be just short of 1/2" thick. The poplar dowel splintered a lot less than the same sized pine dowel I used for the cams and had a smoother,denser end grain after it was cut and sanded.

I experimented with using my bad saw to cut the disks and I was very disappointed with the results. They weren't a total wreck but they required a fair amount of time on the belt sander to clean up and they were uneven. After a little slip cost me a part of a fingernail, I finally was like "Not worth it!" I cut a new set using my power miter saw with a stop block. Super easy and super precise. One note though...DO NOT lift the blade while it is still spinning! I spit three of those little disks across that shop at about Mach 4 before I learned that lesson.

The body segments need a 1/4" hole drilled in the center of their edge for the followers. I used the same jig as for the cams but turned up on its side. I used a Forstner bit (thank you once again Benjamin Forstner) to get a nice flat bottomed hole. I made the holes 3/8" deep. You could probably make this with 3/16 dowels but I find the birch dowels I get in that size to be pretty fragile. Your millage may vary.

I used a pre-turned craft store piece called a "doll's head" for the caterpillars head. (It is 35mm across, about 1 7/16"ish.) I plugged the hole of what would normally have been its base with a length of 3/16" dowel and then drilled a 1/4" hole for a follower to fit in. I used the same jig as with the other pieces and left the dowel long so I could use it as a handle while drilling. Once the hole for the follower was done, I trimmed the dowel flush and sanded that part flat.

The smart thing to do now is to test your body segments to make sure they don't rub against each other too much AND THEN paint and finish them even though they will be slightly thicker once painted. Just be sure to keep the segments in the same order you test fitted them. 1/16" one way of the other really makes a difference. You don't want too much wiggle room between the pieces though since it can allow the followers to get out of alignment and interfere with each other. Don't let this stress you out. It's a toy. Not the engine on an airplane. It will work fine.

So, with all the pieces cut and tested, I painted them and gave them a coat of spray acrylic. Yeah... it looked a little creepy but this setup let me paint the full disks and have them dry out of the way. Also let me use the spray acrylic on both sides at the same time. It was a big time saver.

One thing I added before the acrylic was a shapely rear end for the caterpillar. He just didn't look right with a round head and flat butt so I rounded off a 3/4" dowel, painted and glued it on to the 1" body segment. Now... baby got back!

Last thing in the crank. I used a 2" hole saw to cut a disk of 1/2" pine from a board, sanded it up and painted it to look like an orange. A little 1/4" dowel became the stem/handle for the crank. It goes in the 1/4" hole where the leaves are.

I tested the fit again and the glued the body pieces on to the followers. Almost done.

I trimmed the front and back of the rod and added the handle. I left a tiny bit over 1/2" of the rod protruding from the front to attach the crank on it. I made sure to leave a little space so it wasn't constantly hitting the case. I guess a washer is called for here, but I tend to stay away from metal in the toys if I can help it.

Okay then.. all set.

Crank

Head

The full critter

And here he is in action. I did the cams slightly different on this one with an extended cam in the tail (not needed) and a longer cam for the head that worked as the locking piece as well.


My guess is that most parents of the last 40 or so years are familiar with Eric Carle's "The Very Hungry Caterpillar." It's a great book and it has a very distinctive style. I could never match it but I figured I could capture the vibe by using some of the same colors and shapes.

People really like the pleasant wiggle and whimsical nature of this toy. It really has whet my appetite for simple automata. Stay tuned.